1 Introduction

Nutriments and low food availability prevail as a significant challenge worldwide [1], evidenced by common famines and malnutrition among much of humankind despite increased production. Estimates of basic needs, particularly food, were 27.7% for Africa, 7% for Asia, 1.2% for North America and Europe, and 6.4% for South America [2]. To address sustainability, the United Nations (UN) developed a class of targets/objectives known as ‘Sustainable Development Goals’ (SDGs) [3]. Its SDG 2 aims to institute a world free of hunger by 2030. A global outline reviewing the situation of nutriments indicated that approximately 828 million people on the whole earth are short of food and appropriate nutrition with varying levels, from chronic to acute [4]. About two billion communities are now vulnerable to low access to primary and minor nutrients. One of the dietary inadequacies that continue to abide in most rural homes is invisible starvation, with minimum discernible indicators [5, 6], and the consequence of this scenario is associated with poor health outcomes [7, 8]. One of the most common challenges of these unnoticed food shortages is malnourishment, a disorder resulting from a lack of appropriate nutrition. The most exposed groups of people are minors (usually under the age of 5 years), expecting females, and infants growing and residing in developing countries where poverty is the order of the day. Most of the upcoming nations’ economies reckon gloomily on agriculture; with this scenario, the agriculture sector is under pressure to generate 70% of the food needed to cater to the increasing inhabitants by 2050 or prior [9]. The world needs to adopt a more organized and revised agriculture system. Stakeholders can reduce food insecurity through crop production diversification and the inclusion of underutilized, forgotten, and climate-smart crops such as amaranth, teff (Eragrostis tef), quinoa (Chenopodium quinoa), and spider plant (Cleome gynandra) among others.

Most underutilized food plants are drought-tolerant, highly nutritious, disease- and pest-tolerant, and can produce better yields with minimal inputs. To date, just thirty (30) of the one hundred fifty (150) flora kingdoms that humans have domesticated are utilized to supply the world’s nutriments needs [10, 11], with staple grains [maize (Zea mays), wheat (Triticum aestivum L.), rice (Oryza)] making up the bulky; despite being prone to recurring environmental changes [12] and other abiotic stresses [13]. Underutilized crops refer to a collection of natural and cultivated flora species with meager worldwide outlets (markets) [14, 15]. They can sometimes described as under-used, minor, promising, or orphan crops [16]. Typically, these crops are nourishing [15] and offer noteworthy well-being good [17]. They also pose a distinctive portrait of well-off in nutrients with affordable protein substitutes to support peasant farmers’ lives [18], preserve the earth, and slow down carbon emissions [19].

One of the underused and usually undermined crop species is amaranth (Amaranthus), commonly recognized as pigweed, has over 70 breeds categorized as amaranth vegetables (leafy), grainy, weedy, as well as ornamental [20, 21]. Most amaranths are native to America, and domestication started in the 6000–8000 period. However, today, it is a universally propagated type [22, 23], with only 15 species originating in Asia, Africa, Australia, and Europe [24]. Amaranthus has been reclaimed in the middle of hardly any eatable crops with versatile, invaluable benefits, deserving quality attention, and no doubt it will be on the tables of many in this modern world due to its reasonable nourishment and low input production [20]. Research shows that amaranth production suits low-fertile areas and holds notable prospects for additional orientation and occurrence due to its standard divergence in genetic and physical traits [25,26,27]. It is plausible to thrive under the utmost warmth, rainlessness, and lousy soil conditions, including pests and other disorders. Farmers can successfully attain grain amaranth cultivation as leafy and seedy in some domains and periods where other floras hardly grow [28,29,30]. It has an innate strapping merchandise and production capacity yet to be prettily mined [31, 32]. It is a promising and resilient plant credit promotion. The current weather outlook has significantly given rise to undernutrition, starvation, and low productivity in the agriculture sector in southern Africa and beyond, and the scenario now calls for a newer approach, paying much attention to forgotten nutritious, health-beneficial, and drought-tolerant crops. Navigating Amaranthus collections may reveal their unrecognized potential and elevate the crop’s adoption into the central cropping system and usage. Aderibigbe et al. [57] analyzed the value chain of amaranth for food and nutrition, Peiretti [33] outlined its usage in animals, Ruth et al. [35] explored the underutilization of Amaranthus versus nutritional and nutraceutical potential, and Alegbejo [122] offered an overview of the plant’s nutritional value and consumption. Therefore, the recent review quests to sift the wholesome benefits, production practices, and remunerative prospects of versatile, underused, and orphan crop grain amaranth for global food and nutrient security, particularly for vulnerable regions.

2 Amaranth botanical description

Amaranthus belongs to the family Amaranthaceous, previously the Chenopodium [34], order Caryophyllales, which has about 70 kinds of angiosperms as members and includes over 400 varieties. It is suitable for production in the tropical and world zones of average annual rainfall of the universe [24]. Amaranthus exists as dioecious (the two types of reproductive organs are found on segregate plants) and homothallic (the two types of reproductive organs are located on the same plant) [35]. The dioecious are aboriginal of North America, while the monoecious are extensively spread across different continents [36]. Diecious amaranth plants have a restricted distribution and are bushy in appearance. Amaranth expresses itself as a fixation of C4 carbon, which is usually observable in grasses rather than dicotyledons [37]. Plants are between 0.4 and 1.5 m tall [38] and 0.3 to 3.0 m in height [27]. Leaves possess egg shape and are spherical with color variations of pale to deep green, depending on the genetic constitution of the species, with some species presenting red all over. Figure 1 depicts some color variations of amaranth leaves.

Fig. 1
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Amaranths growing in experimental fields in the 2021/2022 season with different leaf colors

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Blossoms are wine-colored, orange, plum, maroon, and lemony, with the principal cane ending in a well-distinguished flower [39], with the blossoms developed on branched clusters or glomerules. Projected are the well-defined flowers, colorful, glowing, and concluding. Figure 2 shows amaranth growing in experimental fields at North-west University, South Africa.

Fig. 2
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Amaranth variation in inflorescence colors: 2021/2022 season in the experimental field

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The microspore of the homothallic of the amaranth has a golf ball-like opening in the center of its convex structure. The broadness of seeds is about 1 mm in diameter, although they can range between 0.9 and 1.7 mm in magnitude, and heaviness varies between 0.6 and 1.3 mg each. The small diameter makes amaranth seeds 30–70 times microscale in size compared to conventional wheat grain [40]. The grain seed color varies with species, with creamy, white, brown, gold, and pink. Figure 3 shows different colors of amaranth seeds harvested in the 2021–2022 growing season.

Fig. 3
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Amaranth seed colors

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Different extension patterns, including sprawling, upright, forked, and branchless, are seen in amaranths. Native identifications for amaranth (English) includes ‘pigweed, misbredie, morogo, tepe, thepe, serepelele, theepe, umfino, imbuia or utyutu’ [28]. As for nutrient level and environmental tolerance, the species vary as well.

3 Geographic distribution of amaranth

The Aztecs and the previous American cultures depended heavily on amaranth as a staple food source [41]. Amaranth, as a crop, nearly perished in the Americas after the Spanish Conquistadors’ occupation of Mexico at the start of the 1500s, only to be revived through U.S. research in the 1970s [42]. Due to the scaled U.S. research, there was a notable amaranth seed production in the United States through the late 1970s, even though the production was limited to a few acres annually. The production of seed amaranth later travelled over the globe and was well adopted in nations like India, Africa, and Nepal for food in either leaves or seeds. More and more farmers from China, Russia, Italy, Greece, Australia, and South America initiated its cultivation [43]. It has built a comeback as a crop in Mexico over 20 years ago.

Commonly cultivated species in East Africa, China, and India are A. hybridus, A. blitum, and A. dubius. South America uses A. caudatus as a grain, and in the Southwestern United States. A. dubius is mainly cultivated for its leaf in China and the Caribbean. A. hybridus is a dual crop in Mexico, Nepal, Indonesia, Malaysia, Thailand, and the Philippines [44, 45]. Southeast Asia and China are ranked highly for famous classic greens, and interestingly, amaranth comes first in rank, seconded by Chinese spinach [44]. Amaranth species are at one’s disposal during the drier, warmer summer rainfall periods in southern Africa. Literature suggests that amaranth can adapt acceptably to the climates of Sub-Saharan localities, and people can observe its growth along the roadsides and many fallow landscapes [46]. The cultivation of amaranth is rare in South Africa and the rest of southern Africa; however, people find it in unmanaged lands and pick it up following the earliest precipitation [47]. Leafy amaranth (vegetable) usually grows as a wild vegetable. It is substantially consumed as a leafy vegetable by poor people living in the rural areas of Limpopo, Eastern Cape, Mpumalanga, Kwazulu-Natal, and Northwest [48] (parts of southern Africa). The cultivation of grain amaranth is mainly at the scientific level by the local universities in South Africa [49] and Zimbabwe, Botswana, and Cameroon [50]. In East Africa, the cultivation of amaranth leafy vegetables has gained momentum. The adoption and commercialization of grain amaranth in Africa, excluding Kenya, is hardly prevailing, and this can also be an indicator of why the production of grain amaranth is not lined up with other grains or cereals in the FAO statistics on production [51].

4 Amaranthus species

Amaranthus mainly contains 400 species [27], with a handful currently under cultivation. The total number of species established to date still needs to be determined [52]. The genus comprises leafy vegetable amaranth, grain amaranth, ornamental, and weedy amaranth. Alemayehu et al. [53] outlined three major seed amaranth types: A. caudatus L., A. hypochondriacus L., and A. cruentus, with A. tricolor grouped as an ornamental. A. blitum (slender amaranth), A. lividus, A. dubius (spleen amaranth), A. spinosus, A. thunbergia, and A. graecizans are produced for their leaves (leafy amaranth) [54]. Typical types common in Sub-Sahara are A. graezianus & thumbergii [53]. The three principal species cultivated for their seed use include:

4.1 A. cruentus

This is one of the species mainly significantly produced for its use as grain seed, even though some people harvest the tender leaves for use as relish, thereby rendering it a dual-purpose amaranth. However, it is known as a grain species in America at the outset. A. cruentus is popularly considered for its leaves in Africa and Asia to a small extent [55]. In all tropical nations, A. cruentus is an example of traditional green. In Benin, Togo, and Sierra Leone, it is the primary leafy vegetable and is also a major leafy vegetable in India, Bangladesh, Sri Lanka, and the Caribbean [55]. It is thought to be the most versatile species of amaranth, blooming in a broader range of day length than others [56]. In Nigeria, A. cruentus is also grown for a specific purpose as an ornamental [57]. It can reach a height of 2 m and generates one dominating middle root (taproot) with a ribbed and red-dyed stem. Simple, stipule-free, spirally organized leaves range from oval to rhombic-ovate in shape. The seeds vary in color, from yellow and white to pale brown. This species is usually considered for cultivation conditions, primarily in mountain areas in India, because of its hardiness to adverse climatic conditions [58]. Figure 4 shows A. cruentus growing in the experimental field during the 2021/2022 growing season.

Fig. 4
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Amaranth grain species: A. cruentus

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4.2 A. hypochondriacus

It is a species commonly grown for its grain in South America and Asia. It is the main domesticated species, and its production is mainly concentrated in the warm areas of Morelos, Puebla, and Guerrero states with a notable harvest in Mexico [59]. It is an ornamental in Africa, except at trial sites in Zimbabwe and South Africa [54]. It is a vigorous, upright plant that typically grows up to 40–200 cm in height [55]. It is ordinally red-tinted throughout the plant, distinguished by forked stems that are plump, angled, balloon-like, or well spread to relatively thickly fit out, coupled with multicellular hairs. The cotyledons (leaves) are lobed or unlobed and not broken into leaflets. Leaf style is alternate (one per node along the stem) with entire edges. The flowers of A. hypochondriacus mostly have a notable edge, are upright, often with minor or little projections observed at the end part of the axils, and have distinct deep red, purple, or beetroot red blooms; however, yellowish, and green blossoms are hardly present in these grain species. The hypochondriacus seeds are characterized by variations in seed color, including white, ivory, pinkish white, or black to dark reddish brown [60]. Seed sizes of 1–1.4 mm and smooth shiny surfaces are standard [24]. It matures early 75–90 days [61], is a mid-length season of 60–120 days [62], and is day-neutral.

4.3 A. caudatus (A. edulis)

It is commonly called lovelies, bleeding amaranth, or tassel flower and gets its name from its tiny blood-red, petal-less flowers. Caudatus has the shortest season, with 45 to 75 days to maturity [62]. In Africa, it matures in 60 to 70 days, depending on altitude, with the fastest maturity at lower altitudes. Oval-shaped, light green leaves characterize it. Cultivars with yellow–green flowers are available. This plant is everlasting, and its flowers keep good color when dried for arrangement. In South America, it is popularly utilized as a cereal [46] and cultivated to use the seeds in parts of Argentina, Peru, and Bolivia [57]. This type produces flour. The leaves vary in color, but purple or green are most common. The stems are purple and green, with purple roots. The inflorescences are purple with no spikes [63]. Seeds are black, cream, or pink. Figure 5 shows A. caudatus growing in the experimental field at North-West University, South Africa, during the 2021–2022 season.

Fig. 5
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A. caundatus

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5 Amaranthus seed production

Amaranth is a very tiny-seeded cereal, but this does not limit the use of conventional seed machinery in its cultivation. Farmers should be cautious to maintain a planting depth of 3–5 mm. Any planting depth exceeding 5 mm will affect crop emergence. It is an extensively adaptable species that can thrive in various environments and conditions between 0 and 2400 m above sea level. Although adapts to many poor conditions, significant yields are realized when production is done on good soils. Farmers can successfully cultivate amaranth in mixed or single-production systems. If one considers mixed cultivation, maize is the best match [64].

5.1 Land preparation

The seeds of grain amaranth are very tiny in size and, therefore, require a fine tilth for enhanced germination. The plowing of the land at a depth of 250–300 mm, followed by discing and harrowing on virgin land. Land preparation through discing previously cultivated land is practicable, although this method of land preparation gives rise to weed challenges as compared to conventional plowing. Tilling can be done using tractors, ox-drawn ploughs, or hand hoes, depending on the production scale. Inorganic or organic fertilizers can be used as basal dressing. Organic fertilizers are applied during the dry period (August–September). Heaps of organic manure are made in the field, spread, and followed by plowing. Application of organic manures is usually as per soil analysis results, with an application rate of 4046.856 square meters (acre) [27].

5.2 Planting

Farmers can distribute amaranth seeds in well-prepared land with a fine tilth. We usually do not recommend no-till as amaranth is a poor competitor to weeds during the early weeks after emergence. You should rinse the soil well. Seeds are planted directly in the field by scattering or hill placement. By broadcasting, you can scatter seeds in rows or over the plot using a hand or a planter. This approach is not cost-effective. Direct planting is possible when seedbanks are abundant. It is also known as sowing in situ, meaning no seedlings movement will take place. This method gives amaranth seedlings the advantage of early take-off and a good crop stand if the seedling emergence is good. Seed trays provide an alternative option for sowing seeds individually. This method can be tiring as the amaranth seeds are tiny and might be difficult or impossible to hold individually. A 1.5–3 kg/ha seed rate is recommended [65]. Mixing seeds with sand ensures even distribution. Row spacing of grain amaranth varies from 45 to 75 cm, and plant spacing is 30–45 cm. Wider rows give high yields and permit using row cultivators for weed control.

5.3 Nursery management

Apply water frequently, and it would be best not to overwater the seedlings as over-application can lead to damping off disease. Transplant after 4–5 weeks and hardening off should start a week before transplanting. The most appropriate time for transplanting is in the morning or late afternoon for best results. Apply adequate moisture to the new transplants.

5.4 Climatic and soil requirements

Amaranth can grow in temperate, tropical, and subtropical climates [27, 66]. It grows well between the temperature range of 18–35 °C [27], and at planting, the temperature should be at least 18 °C [58]. Warm and sunny conditions enhance an outstanding germination rate. Despite receiving only 500 mm of rainfall per year, amaranth still manages to produce a satisfactory yield. Amaranth produces a satisfactory yield despite receiving only 500 mm of yearly rain. Irrigation supplements are desirable during the dry season. Amaranth does well in medium–heavy, loose, and friable soils. Soil pH ranges from 5.0 to 8.0 [27]. Selecting soils that are low in clay will help reduce crusting problems, resulting in poor seedling emergence. It is sensitive to compacted soils [67]. Amaranth is a hardy species that withstand unfavorable conditions like full sun, inadequate moisture, and raised temperatures [26].

5.5 Crop management

5.5.1 Pest and disease management

Amaranth seed production experiences minimal disorders and pests that result in notable crop depletion. Amaranth seed production experiences minimal disorders and pests that result in notable crops. The usually notable prevailing diseases are damping off and seedling wilts due to pythium [68]. Diseases are managed through cultural practices, as no fungicides are labelled for the amaranth use [69]. Grain amaranth can tolerate considerable insect leaf feeding without affecting yields. On the other hand, leafy amaranth may have contrasting characteristics in this context, in which researchers have observed an increased incidence of insects and pathogens [70]. Grain amaranth farmers in Brazil recorded a few challenges of Lepidoptera larvae causing falling over of plants preceding the seed-filling stage in sprinkled fields [71]; besides this, the seed amaranth is ordinarily resilient to pests’ destruction, including nematodes.

Production of amaranth can be a challenge due to the lack of approved weed killers. Farmers can employ cultural practices such as cover crops and crop rotations to manage the difficulty of weeds, providing crops with the advantage of early take-off [43]. Mechanical weeding, 2–3 rounds in a growing season, can be carried out until plants form large canopies to shadow weeds. There are numerous benefits of keeping the area under production and the surroundings weed-free through hoeing, hand pulling, and using row crop cultivators. Once amaranth gets to 15–25 cm in height, the growth rate will be fast and will be able to resist the impacts of weeds by out-competing the later weeds [72]

5.5.2 Harvesting

Farmers should commence crop disposal early when the panicles (heads) color changes from green to gold, and the seeds shade with a light touch as grain amaranth readily breaks. The administration of the produce is the most pivotal phase of the production of grain amaranth; beyond attentive harvesting tactics, very high seed loss is possible. Delays in crop disposal may result in a percentage loss of up to 50% or higher [61]. The crop can be reaped manually using hand-cutting tools or a combined harvester.

6 Uses of amaranth

Amaranth is the only pseudo-cereal with the whole plant used for food and other uses. There are numerous techniques to treat the grain for ingestion, with expanded grain being the most famous. The seeds of amaranth are popped or boiled and eaten as rice [73]. The grains’ flaking, extruded, and ground-to-flour shapes have given them certain functional advantages, hence they earned the name ‘pseudocereal’ [74]. Flour is the most critical by-product of amaranth seeds. The flour is used in the baking industry [20]. It is the main component of a gluten diet with other pseudocereals such as quinoa and teff. Amaranth does not contain gluten; this trait makes its flour excellent for manufacturing foods for people with gluten allergies and celiac diseases [75].

Flour from amaranth seeds is traditionally used to produce snack bars or snake cakes in Mexico and other nations by putting together honey and popping amaranth [27]. In India, laddoos are typically made from grain amaranth and added to rice meals [76,77,78]. Farmers who used livestock feed obtained from seed amaranth noted increased digestibility [79]. The seeds are used for making chaqa (a local beverage) in Ethiopia [53]. An alcoholic drink known as ‘borde’, ‘kita’ (a yeast-free bread), and ‘atmit’ (a thin porridge) for breastfeeding women and their young ones is also made from grain amaranth. It is predominantly given to these mothers after giving birth [26]. It is crushed and combined with fish, flour obtained from maize, sorghum, or millet to process porridge for children [80]. Due to its ability to prevent and treat chronic degenerative disorders, the beverage sector utilizes amaranth grain as a raw ingredient [57]. In South Africa, grain amaranth has a lengthy history of use in folklore, local medicine, and as a source of dye [81]. Oil-containing squalene from grain amaranth species, hypochondriacus has since impacted the food, cosmetic, and pharmaceutical industries [82, 83]. According to reports, the squalene content of the amaranths is the highest of any plant species [84, 85]. It is applied to enhance protein, including critical nutrients of the terminating diets [86, 87]. People in Peru sprout and malt the seed amaranth to obtain a traditional chicha beer, while in Asia, it is an alternative for soy in producing a soy soup called ‘shoyu’ [30]. Uji, a type of porridge usually taken for breakfast in Kenya, is produced from flour made from amaranth seeds. Amaranth extracts addressed a variety of illnesses, including diabetes, diarrhea, and skeleton fractures, a long time ago [21, 88]. Research has evaluated amaranth’s therapeutic benefits in treating numerous chronic conditions [89,90,91]. The inflorescences are used as decorations, and the entire top portion of the plant can used as animal feed.

7 Amaranth as a versatile crop

During the 1970s, there was recovered attention on amaranth, which inspired farmer in Mexico to rediscover amaranth from wild cultivars because of the crop’s critical role in their way of life. What promoted this resurgence was the identified crop’s ability to resist bad weather and its taste. This was when the US considered resilient and undeveloped plant species that could increase the globe’s nourishment output. After being brought to the fore as a product with ‘superiority and agriculture potential’ by the US National Academy in 1975, the crop experienced an outpouring of adoration [92]. Unlike today’s one-purpose staples, Amaranth is extensively adaptable, and all portions of the plant are ingested. The adjustment capacity of amaranth to numerous climatic situations is one of the significant drivers behind its resurgence in present-day farming. Delving studies have reaffirmed this old cognition, conceding the crop’s distinctive capacity to live in hilly terrain and open plains, including its good stand for drought, raised salt media, and ice [93]. Like other crops that demand complicated techniques to survive, it is a crop that is typically flexible for eliminating the effects of increasing seasonal uncertainty. A universal rotational crop, amaranth is also adaptable to a range of geographic elevations, giving farmers from locales alternatives. Amaranth is a suitable match for the nitrogen-lack soils of southern Africa since it also uses C4 photosynthesis, a system that permits plants to have a higher photosynthetic rate. A remarkable nutritional status and climate adaptability are two outstanding benefits of amaranth. The Aztecs may have believed it granted them ‘supernatural power’ since it is a nutritional powerhouse. The grain has most industrial, medical, and domestic uses. Snake bites and foot and mouth disease in cattle are all treated with it [73]. The grains cure measles in adolescents by decreasing the body temperature when they sleep over grain spread on the bed [73]. Meat and apples are placed inside the grain chaff/thrash in boxes and noted to extend the longevity of the products by one and a half months for beef and apples 6 months [73]. Amaranth is suitable for many families in southern Africa due to these characteristics, where power outages are frequent.

8 Grain amaranth a climate-smart crop

The new re-establishment of amaranth in modern agriculture can be chiefly accredited to its remarkable resilience across manifold climate conditions. Upcoming research has corroborated prehistoric cognition about the crop’s unique hardness, highlighting its capacity to flourish in both hilly terrains and open plains. Amaranth demonstrates a high tolerance for challenging factors such as drought, saline soils, and frost [93]. A key factor underpinning amaranths’s resilience is its utilization of the C4 photosynthetic pathway. The Hatch and Slack pathway allows the plant to achieve greater photosynthetic efficiency and to flourish even in nitrogen-poor soils reducing its reliance on irrigation except in exceedingly waterless regions. Amaranth is a member of the NAD-ME subtype of C4 plants, it utilizes NAD+ as a cofactor in its decarboxylation process [94]. The NAD-ME subtype manifests lower water use efficiency under drought conditions compared to the NADP-ME subtype. This is due to differences in leaf structure and leaf curling rates in the NAD-ME subtype. In contrast, the NADP-ME subtype displays improved nitrogen efficiency under similar moisture deficit conditions [94]. Amaranth plants display higher transpiration rates compared to typical C3 photosynthetic plants. This allows amaranths to continue transpiring and assimilating CO2 even when experiencing early-stage drought stress conditions. Maize and sorghum may start to restrict transpiration and CO2 uptake more quickly in response to limited soil moisture [95]. However, Amaranth can maintain its gas exchange and carbon assimilation processes until the drought becomes critically limiting.

In summary, the physiological adaptations give amaranths an advantage in coping with intermittent or moderate drought episodes, as they can keep growing and accumulating biomass for a longer period before shutting down their photosynthetic machinery. These attributes position amaranth as a valuable solution for combating the challenges advanced by progressively seasonal unreliability and enhancing overall agricultural productivity.

9 Nutritional value of amaranth

The dual-purpose plant, nutrient-rich grains, and leaves give a very delicious relish and soup, utilized across the universe as nourishment to both people and fauna and are gaining momentum to be a superior nutrient powerhouse because of their attributes. Carbohydrates, dietary fiber, lipids, significant amino acids, as well as a variety of other crucial constituents like squalene, tocopherols, phenolic compounds, flavonoids, phytates, vitamins, and minerals, are available in this wonder crop [96, 97]. The seeds come with simple protein 13.1–21.0% and albumins, which digest quickly, including globulins, the greater part of their composition [38]. Protein content differs between amaranth species and varieties [98, 99]. The field-managed species were discovered to carry less protein and amino acids throughout other nutrients compared to the wild species [100, 101]. Baraniak and Kania-Dobrowolska [102] noted that climate, soil conditions, and the method of fertilization also influence protein content. The leaves are also a source of affordable, well-to-do protein and other crucial compounds such as carotenoids, vitamin C, dietary fiber, calcium, zinc, and magnesium [103, 104]. Compared with the main staples, specifically rice, amaranth poses a high value in nutrient composition, with three times more in calcium quantity than the main grains [105]. Amaranth’s wealthy composition warrants it to be most convenient for humankind dwelling in the communal areas of the less developed societies for bodybuilding and prevention of nutrients malfunctioning [106, 107]. To ascertain this fact [108], outlined the high sulfur present in amaranth compared with a legume. Although amaranth’s nutritional profile is valuable, on the other side, it lacks a variety of amino acids such as leucine, isoleucine, and valine, which are not much of a challenge to living organisms’ diets, as they are available in large amounts in amaranth compliments (millet, maize, cassava, etc.) [109].

Amaranth has a cholesterol-lowering effect [110, 111]. In addition, it carries vitamins required for daily uptake, which helps minimize vitamin shortfalls [112]. Besides, it also comes with riboflavin (vitamin B2) and vitamin C, much higher compared to cereals, coupled with vitamin E essential for delaying aging [113]. It also carries reasonable ash content in the leaves and less in the seeds, which helps to estimate the mineral composition [114]. Table 1 below shows the average nutritional composition of Amaranthus, other potential crops, and major food crops.

Table 1 Average nutritional composition of principal and potential crops (per 100 g edible portion).
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10 Health benefits of grain amaranth

Grain amaranth is a nourishing ancient pseudocereal attained remarkable consciousness in recent years due to its important health good. This multiple-purpose grain, grown and consumed for ages by many cultures is constituted of critical nutrients, antioxidants, and bioactive compounds that help the gross welfare of animals and humankind [117]. The various nutrients in amaranth play crucial roles in many bodily functions, such as energy production, red blood cell formation, and maintaining healthy bones and teeth. Consuming amaranth seeds contributes to heart health in some ways. Seeds of amaranth are naturally low in saturated fat and cholesterol, making them heart-friendly by lowering cholesterol levels and regulating blood pressure [118]. Squalene, found in grain amaranth is attributed to cholesterol level reduction and prevents the building up of arterial plaque [117]. Regular inclusion of amaranth seeds prevents constipation due to their fiber content. The existence of resistant starch helps to nourish important bacteria in the gut and promote their growth. The presence of bacteria in the gut supports improved digestion, nutrient absorption, and overall gut health. The antioxidants in amaranth, vitamin E, phenolic compounds, and flavonoids help neutralize detrimental free radicals in the body thereby lowering the danger of cancer, diabetes, and stroke [118]. Eating amaranth grains helps to reduce inflammation by slowing the body’s production of Immunoglobulin E. Grain amaranth meal serves as a substitute for wheat, barley, and rye in diverse recipes, facilitating those with gluten limitations alternative meals [119]. In some regions of Africa, seeds of amaranth have been traditionally employed for curing HIV/AIDS in human population [120]. Grain amaranth meal is mixed with moringa (Moringa oleifera) leaf powder in a ratio of one (1) cup of grain amaranth to 1 tablespoon of moringa leaf powder [121]. The moringa–amaranth mixture is believed to minimize the side effects of the ARV [120]. Although amaranth seeds contribute to many health interests, users need to understand that they should be taken at moderate levels as excess amaranth eating can cause health complications.

11 Antinutrients in amaranth

Antinutrients (ANF) are compounds produced in naturally existing food products through a variety of procedures, such as the dismissal of certain nutrients, reduction in the digestive process, or a metabolic utilization of feed, that have an unfavourable outcome upon required condition [122]. The term ‘anti-nutritional factors’ relates to organically operative chemical substances that repress the accessibility of nutrients to the body of a living organism and obstruct normal metabolic operations [123, 124]. The subsidiary biogenic substances found in amaranth include saponins, tannins, trypsin inhibitors, nitrates, oxalates, phytates, alkaloids, and cyanogenic chemicals [125]. Phytates and oxalates are particularly problematic [126]. Phytic and phytate work as anti-nutrients through reactions with minerals and trace elements like zinc, iron, calcium, and magnesium. They interrupt the assimilation of these minerals in the stomach, thereby minimizing their potency [125, 127]. Young children are more vulnerable to this risk of this chelation as their digestive systems are more sensitive compared to adults; therefore, they need more of these nutrients for growth [128]. Saponins can potentially damage the cell lining of the intestines, and if they are in access, they can cause holes in the intestines, allowing toxins and bacteria to enter the bloodstream [129]. Furthermore, they disturb the utility of the digestive enzymes leading to dyspepsia-linked health issues and lessening the absorption of numerous nutrients [130, 131]. One of the setbacks with these underutilized promising crops like amaranth is the existence of antinutritional elements, which typically demand operations prior to incorporation in the dietary formulation. It is possible to minimize anti-nutritional elements in the leaves and seeds by employing a variety of techniques. Some of these techniques are used at the household level, while others are used extensively in the manufacturing sector, and they include alkali treatment (chemical treatment), cooking, popping, roasting, and extrusion (heat treatment).

12 Grain amaranth production in southern Africa

Southern Africa, a southern region of the African continent, comprises Zimbabwe, South Africa, Zambia, Malawi, Botswana, Lesotho, Mozambique, Swaziland, Namibia, and Angola. Seasonal climates in Africa range from semiarid to arid and from temperate to tropical [132]. Seasonality plays a significant role in regulating plant growth and food availability. In a larger portion of the region, droughts are frequent. Poverty is also rampant as the region is one of the upcoming nations, resulting in a lot of deficits specifically starvation and nutritional insecurity. Over the past two decades, the number of individuals living in poverty has increased [133], and the area has some of the greatest levels of income variations in the world. Over the past 10 years, the average annual growth rate for agriculture in the region has been only 2.6% [134]. To ensure food security, a variety of natural resources that are abundant in the area must be used as only 48,653,300 ha are under cultivation, with just around 25% of the land being arable, and about 80% of the inhabitants engaging in subsistence farming [135]. Agriculture plays a pivotal role in eradicating food insecurity. The region needs to widen its crop base to include potential crops such as amaranth.

Amaranth produced for its seed use is very rare in the region. Production of the superfood grain is mainly at the research level, with some try-outs in Zimbabwe and South Africa. A. hypochondriacus L. is reported to have been introduced to the two wards of Binga district in Zimbabwe (Manjolo and Sikalenge), as a method of increasing the diversity of crops that are adapted to climate change [136]. The production was at a very minimal scale as it was comprised of only 47 women growers. The grain amaranth grown in South Africa is observed in institutions for scientific purposes, with patch productions of amaranth leafy by smallholder farmers, and a great portion of the amaranth leafy grew in the wild. He tested the crop on 8000 m2. Amaranth seed has not been produced commercially, hence there are no records of its output [137].

13 Prospects of grain amaranth in southern Africa

Recently, much attention has been directed to orphan crops, resulting in hundreds of crop types identified, and undoubtedly, amaranth is top in preference [127] and has been acknowledged as one of the most promising future plant species to feed the worldwide inhabitants [138]. The adoption of grain amaranth production seems promising since amaranth is drought tolerant and can produce an acceptable yield under stress environments. Amaranth is a niche crop with lucrative markets in Europe [139]. The smallholder farmers in Southern regions can produce and export amaranth, thereby eradicating poverty through earning foreign currency. Farmers in southern Africa can exploit this advantage and adopt amaranth production to eradicate hunger and malnutrition. Mono cropping is high in southern Africa, and amaranth can be used in many rotations. In addition, amaranth processing is more straightforward than other small grains (finger millet and sorghum).

In Europe, pseudo cereals are a specialized market with rising yearly requests. North America, Latin America, the Middle East, Europe, Africa, and the Asia Pacific region are the seed amaranth’s primary trade routes. The global amaranth market is anticipated to grow with a compound annual growth rate (CAGR) of 10.4% through the era [140]. Europe makes up about 65% of the world's market dominance [141]. The Confederation of British Industry revealed, that importing from developing nations 6000 tonnes of specialty cereals, the bulky of which were amaranth seeds, and the report also outlined that Germany is the primary consumer market of amaranth seeds [142]. Other industrialized nations like the UK, the Netherlands, Sweden, Belgium, and France are getting more and more familiar with this product, although amaranth is mostly used as a health food, and it can be purchased in tiny packages (raw, popped or milled) holding up to 500 g of the commodity, or it can be used as the primary product in breakfast cereals, baked goods, or healthy snacks. This gives southern Africa a great opportunity to take advantage of the prospects presented by the amaranth market at a global scale since the economies of many of the countries in this region are built on agriculture. The markets for amaranth products in Africa are small and undeveloped because of the scarcity of some goods, short of standard demand, and poor policies. Smallholder farmers who grow grain amaranth have low sales potential without significant connections to consumers.

African kingpins and other significant business sector players can create solid market ties with European nations and offer readily valuable data on markets with substantial insistence on amaranth seeds. This will increase income creation and lessen poverty in the southern African region. Mexico, Russia, China, India, Peru, and Kenya are some of the significant nations that cultivate amaranth for seed and hold market dominance [143]. Kenya has outperformed most African nations outside southern Africa. Amaranth is more prevalent in its food systems and is farmed on a commercial basis, marketed in metropolitan markets, and even traded outside of the country [144]; making it the only African country to export seed amaranth.

14 Constraints to production

Besides the positive attributes of the food crop, grain amaranth, the crop’s adoption into the main cropping system and acceptance is faced with uncertainties along its value chain. The principal constraints are related to some physical characteristics of the amaranth grain, propagation, refinement, and distribution of the grain and its derivatives [145]. Amaranth’s texture, color, and size can limit its acceptance and production [44]. The dimensions, and appearance of quinoa make it a simple substitute for rice. Cultivation of grain amaranth is challenged by the absence of endorsed seeds (formal) and the unavailability of transmissible and biological quality seeds [52].

15 Conclusion

In this review paper, some details of amaranth including their botany, species, production practices, nutrition, health, and anti-nutrient values were explored. Grain amaranth has been called a nutrient powerhouse and is gaining global attention as an important plant species due to its high nutritional value, health-boosting, resilience to diverse negative production conditions, ease of cultivation, short maturation period, and easy grain processing compared to other small grains. The nutritional profile of amaranth seeds, with their valued protein composition, appraised amino acids constitutions, and rich mineral and vitamin content, positions it as a valuable source of critical nutrients. Amaranth poses unique bioactive compounds such as squalene and flavonoids which contribute to its superior health benefits that include cardiovascular protection, anti-inflammatory properties, and immune system support. Many scholarly works have shown that amaranth is a highly adjustable crop species in the face of climatic change. Its ability to survive in insignificant lands with minimum water and fertilizer requirements further ascertains its suitability for sustainable farming practices. Furthermore, many amaranth species show a wide range of genetic diversity, promoting opportunities for improvement and adaptation to specific environments.

Production practices of grain amaranth have been examined, including planting techniques, pest, disease, weed management, and harvesting procedures. These practices, when deployed appropriately, can increase yield and quality, ensuring the standard output of nutritious grains. Although grain amaranth gives appraisable nutritional merits, it is essential to acknowledge the presence of anti-nutrients in its make-up. Appropriate processing techniques, such as cooking, fermentation, and soaking, can effectively reduce the antinutritional content and improve the bioavailability of nutrients, rendering optimal utilization by the human body. It also demonstrated its climate-smart attributes through its low carbon footprint and efficient use of resources. Grain amaranth’s ability to stand up to bad climatic conditions, such as deficit moisture and heat stress, makes it an appealing option for zones prone to inconsistent rainfall patterns. In addition, its low water requirements and high seed productivity contribute to water and land conservation, supporting the principles of precision agriculture. Lastly, grain amaranth provides a significant opportunity to address the impacts of climate change. Tapping the potential of this versatile and climate-smart crop requires further research, technological advancements, production practices awareness, and stakeholder and policy support. Adoption of grain amaranth as a stable food and incorporating it into main cropping systems have the potential to change the state of food and nutrition as well as promote human and environmental sustainability.