Introduction

Human diet has been evolving since the dawn of civilization. Selection and consumption of food depend upon the availability of resources, climatic conditions, and socio-economic needs. Food not only provides the necessary calories and nutrition to the human body but is also a source of bioactive compounds that help us combating degenerative effects of toxins and preventing many health problems (Singh et al. 2016). Vegetables, fruits, and legume seeds provide carbohydrates, proteins, minerals, and vitamins to our body. Besides these, they are also a source of health-promoting biologically active compounds (Ezekiel et al. 2013; Singh et al. 2017a, b, c). Some food items provide many health benefits to our body along with basic nutrition and tomato is one such food present in the platter of people all over the world.

Tomato (Solanum lycopersicum L.) is a highly popular fruit crop grown and consumed by people across the globe. The worldwide production of tomato in 2014 was 170.75 Mt from 50,238.1 km2 areas under cultivation which was much higher as compared to that recorded in 2010 when the production was 151.89 Mt from 44,955.8 km2 areas under cultivation (FAOSTAT 2014). Tomato production is highest in China followed by India and USA and it is consumed more in Mediterranean countries. In India, the cultivation of tomato in 2014 was carried out on an area of 8820 km2 with a total production of 18.74 Mt while in 2010 the area was 6344 km2 and production was 12.43 Mt (FAOSTAT 2014). Cooked or raw tomato is consumed in the various parts of the world in different forms viz, curries, sauces, salads etc. Consumption of tomatoes exert positive effects on human health and is known for anti-inflammatory, anti-genotoxic, anti-mutagenic, anti-proliferative and chemopreventive activities (Rafi et al. 2007; Scolastici et al. 2007, 2008; Polívková et al. 2010; Feng et al. 2010).

Tomatoes are considered as part of healthy diet regime as they are low in fats and are without any harmful cholesterol. Nutrients like Vitamin A, ascorbic acid, potassium, and folate are present in significant concentrations in tomatoes. Non-nutritive phytochemicals like carotenoids (lycopene, phytoene, and β-carotene) and polyphenols (flavonoids, flavanones, and flavones) are also present in significant amount in tomato (Tan et al. 2010). The contents of nutrients present in raw, ripe and cooked tomatoes are given in Table 1. Phytochemicals are fortified with ripening and cooking of tomatoes. Carotene is present in high concentration in ripened red tomatoes and cooking fortifies the lycopene content in tomatoes. α-carotene is not present in the cooked tomatoes whereas it is present in significant amount in ripe red tomatoes. Lutein is also present in high concentration in ripe tomatoes while it is absent in unripe tomato. Epidemiological evidence has suggested the potential role of tomato phytochemicals in preventing blindness, respiratory disorders, cardiovascular diseases (CVD) and some forms of cancers (Agarwal and Rao 2000; Sesso et al. 2004; Tan et al. 2010). Also, the potential role of these phytochemicals has been observed in the prevention of mutations in DNA (Hazewindus et al. 2014). Tomato is a vegetable of great interest because of its high content of health benefiting compounds. The present review is an attempt to compile information about beneficial phytochemicals present in tomato, their bioactive potential and health benefits reported in various research findings.

Table 1 Nutrients present in different types of tomatoes, red, ripe and raw
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Historical background

Tomato is believed to have evolved from small green fruits present in the foothills of Andes (Peralta and Spooner 2007). In 700 AD, a species of yellow tomatoes of a size similar to present day tomato was cultivated in Central America. Domestication of tomatoes first started in Mexico. After colonization, the seeds of tomato were introduced to various parts of the world and in many places, monoculture was adopted (Peralta and Spooner 2007). After the independence of Mexico, by the development of transport system and some land reforms, the production of tomatoes increased (Saavedra et al. 2017). Documentary evidence suggests that in Europe during 1544 tomatoes were first used but were considered toxic at that time. Dietary inclusion of tomatoes in European cuisine was promoted during next two centuries. Later with the advent of the green revolution and use of better irrigation practices and agrochemicals, the production of tomatoes increased worldwide. Present day tomato has evolved largely with the increase in horticulture techniques. In the 1990s with the advancements in biotechnology and genetic modification techniques, the tomatoes having better color, taste, shelf life and nutrients were developed (Saavedra et al. 2017). Various techniques were adopted to inculcate desirable changes in appearance, size, and quality of the tomato fruit. A paradigm shift in crop improvement objectives for the enhancement of health benefits and disease resistance is observed in recent years (Tan et al. 2010).

Phytochemicals in tomato

Tomato is known as health stimulating fruit owing to the characteristic array of phytochemicals. Phenolics and carotenoids are the main bioactive compounds present in ripened tomatoes. The red color of a ripe tomato is because of a significant amount of lycopene (Martí et al. 2016; Perveen et al. 2015). The tomato fruit also contains β-carotene known for its provitamin A activity. The present section focuses on main phytochemicals (phenolics, carotenoids, vitamins, and glycoalkaloids) present in tomato. Chemical structures of important phytochemicals of tomato are given in Fig. 1.

Fig. 1
figure 1

Chemical structures of some bioactive compounds in tomato; a α-carotene; b lycopene; c lutein; d canthaxanthin; e β-carotene; f all-trans-retinoic acid; g all-trans-retinol; h 9-cis-retinoic acid; i 13-cis-retinoic acid; j naringenin; k kampferol; l myricetin; m quercetin; n α-tomatine

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Phenolic compounds

Phenolic compounds are the class of plant secondary metabolites that possess one or more hydroxyl groups attached to a benzene ring. Structurally, they vary from simple phenolics to complex polymers (polyphenols) on the basis of number and position of hydroxyl groups attached and structural elements that link phenolic rings (Singh et al. 2017b). Polyphenols are known to reduce the oxidative stress and thus counteract various health issues, including CVD and cancer (Singh et al. 2018). The phenolic compounds reported in tomato are phenolic acids (caffeic, chlorogenic, sinapic, p-coumaric and ferulic acids) and flavonoids (quercetin, rutin, kaempferol, and naringenin). Flavonoid accumulation occurs during maturation in tomatoes with a decrease in chlorophyll content and ripening of peels. Quercetin and chlorogenic acid are the most abundant flavonoids in tomato (Sharma et al. 2017). Tomas et al. (2017) reported contents of chlorogenic acid, rutin (quercetin-3-O-rutinoside), naringenin chalcone and naringenin as 17.9, 24.8, 2.45 and 0.12 mg/100 g DW, respectively in fresh tomato fruit. The chalconaringenin content decreases during post-harvest stage (15 mg/100 g at harvest decreased to 0.41 mg/100 g after 3 weeks of storage) of tomatoes.

Martí et al. (2016) summarized the literature by giving ranges of different polyphenols in ripened tomato fruits and enlisted naringenin chalcone as the major polyphenol with concentration range of 0.9–18.2 mg/100 g FW followed by rutin (0.5–4.5 mg/100 g FW), quercetin (0.7–4.4 mg/100 g FW), chlorogenic acid (1.4–3.3 mg/100 g FW), caffeic acid (0.1–1.3 mg/100 g FW) and naringenin (0–1.3 mg/100 g FW). Total phenolic content (TPC) in tomato varies with cultivar and is greatly influenced by variation in solar UV radiation (Sharma et al. 2017). Kaur et al. (2013) analyzed TPC in different commercial and wild cultivars of tomato. The wild cultivars found to have highest TPC (141.98 mg/100 g FW) and quercetin content (56 µg/g FW).

Carotenoids

Carotenoids are the major classes of bioactive compounds present in tomatoes. These plant pigments are produced by isoprenoid biosynthetic pathway with main roles as antioxidants and harvesting of light in plants (Singh et al. 2016). The main carotenoids present in tomatoes are lycopene, α-carotene, β-carotene, γ-carotene, δ-carotene, phytoene, phytofluene, neurosporene, and lutein. Martí et al. (2016) reviewed the literature and documented ranges of different carotenoids viz, lycopene (7.8–18.1 mg/100 g FW), α-carotene (0–0.002 mg/100 g FW), β-carotene (0.1–1.2 mg/100 g FW), γ-carotene (0.05–0.3 mg/100 g FW), δ-carotene (0–0.2 mg/100 g FW) and phytoene (1.0–2.9 mg/100 g FW) in tomatoes. Lycopene, phytoene, neurosporene, phytofluene, β-Carotene and lutein content in raw tomato is reported as 9.25, 1.86, 1.18, 0.80, 0.41 and 0.07 mg/100 g, respectively (Perveen et al. 2015). Lycopene is a major carotenoid responsible for characteristic red color and special antioxidant properties of tomatoes (Martí et al. 2016; Rafi et al. 2007). Tomatoes contain 8.8–42.0 µg/g of lycopene and provide nearly about 85% of total lycopene in the human diet (Rao and Rao 2007).

Vitamins

Tomato fruit is considered as the rich source of Vit C and also contains vitamin A, B and E. Studies have revealed that Vit C content at first increases with the maturation of fruit and then decreases as the fruit ripens (Watada et al. 1976). Organic farming increases the level of Vit C and this increase depends on the cultivar and site of cultivation (Martí et al. 2018). Vitamin A is present in the form of carotenoids. Vitamin B is present as thiamine, niacin, vitamin B6, and folates. Vitamin E is present in lesser quantities than other vitamins and is present as alpha and beta tocopherols. The vitamin content in tomato gets affected by the type of cultivar, time of harvest and ethylene supplementation (Watada et al. 1976).

Glycoalkaloids

Glycoalkaloids are a group of alkaloids in which sugar is attached. These compounds provide protection against the pathogenic attack as they are toxic (Friedman et al. 2009). The glycoalkaloids in tomato are present in the form of tomatine. Tomatine is a mixture of two glycoalkaloids; α-tomatine and dehydroxytomatine which are known to be present in both tomato leaves as well as fruits (Friedman 2013). In green fruit, the content of alpha-tomatine is high (500 mg/kg) as compared to ripe red fruit (5 mg/kg) (Friedman 2013). These compounds are known to provide protection against human pathogens like bacteria, viruses, and fungi. The high content of tomatine in green fruit makes it taste bitter and undesirable to eat. The content of glycoalkaloids is also affected by cultivar type and time of harvest.

Bioactivities

Tomato contains phytochemicals with anti-oxidative, anti-proliferative, anti-carcinogenic, anti-tumorigenic, anti-inflammatory, anti-mutagenic and anti-atherogenic properties. Tables 2 and 3 summarizes studies showing the bioactive potential of tomato and Fig. 2 demonstrates the bioactivities of tomato reported in various studies. Reactive oxygen species (ROS) induced oxidative stress is the chief cause of cancer and CVD. The ROS cause oxidative damage to crucial cellular biomolecules such as proteins, lipids, and nucleic acid. Antioxidative defense system provides protective effects against ROS. Antioxidants such as catalases, glutathione peroxidases, and superoxide dismutases are present within human cells whereas polyphenols, carotenoids, vitamin C and vitamin E can be obtained from food (Agarwal and Rao 2000). Lycopene is a potent natural antioxidant (with singlet-oxygen quenching potential higher than β-carotene and α-tocopherol) available in tomato with notable anti-cancerous and anti-atherogenic properties. Dietary lycopene enhances the level of lycopene in the body and elevates the overall antioxidant potential by trapping ROS, therefore reducing the oxidative damage to biomolecules (Agarwal and Rao 2000).

Table 2 Summary of literature on effect of tomato products/active constituents on different bioactivities in vitro
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Table 3 Summary of literature on protective action of tomato products/active constituents against various mutagens
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Fig. 2
figure 2

Potential bioactivities of tomato demonstrated in different studies

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Numerous in vitro studies have shown protective effects of lycopene obtained from tomato against different types of cancer including mammary gland (Levy et al. 1995, Karas et al. 2000, Gloria et al. 2014), lung (Levy et al. 1995), colon (Salman et al. 2007, Tang et al. 2008), endometrial (Levy et al. 1995), leukemia (Amir et al. 1999), liver (Hwang and Lee 2006) and prostate (Palozza et al. 2010a, b; Pastori et al. 1998). Lycopene and other bioactive compounds cause cell cycle arrest and inhibit the growth of many cancerous cell lines in a dose-dependent manner (Table 2). Lycopene supplementation (2.5–10 µM) also causes a reduction in total cholesterol by decreasing HMG-CoA reductase expression (Palozza et al. 2010a, b). Studies reported dose-dependent anti-proliferative and inhibitory effects of lycopene on various cancer cell lines viz. K562, HuCC, Lymphoma and Mammary cancer cell lines (Salman et al. 2007; Uppala et al. 2013; Teodoro et al. 2012). Antiproliferative effect was also observed by the action of all-trans β-carotene (0.5–10 µM) on mammary cancer cell lines i.e., MCF-7, MDA-MB-235 and MDA-MB-231 (Gloria et al. 2014). The antioxidant potential of lycopene is associated with the ability to scavenge reactive oxygen species and modulation of phase I and II enzymes thus showing protective effects against various types of cancer. Tomatine present in green tomato extract is known to inhibit the growth of human breast, colon, liver, and stomach cancer cell lines. But, it was observed that tomatine can also inhibit normal human liver cell line (Friedman et al. 2009). Livny et al. (2002) compared anti-carcinogenicity of two carotenoids; lycopene and β-carotene at same concentrations (3–7 µmol/l), and reported that lycopene was a more effective inhibitor of KB-1 cell growth by increasing connexion-43 expression.

Human body encounters the attack of various toxins in the course of life. The exposure to these mutagens/xenobiotics is being increased as the civilization is progressing. Implications of exposure to these toxic compounds are evident in many ways like chromosomal aberrations, increase in ROS and induction of tumor. Some recent literature on protective effects of tomato against the action of various mutagens is summarized in Table 3. Treatment of different cell lines with various chemical mutagens like LPS (lipopolysaccharide), H2O2 (hydrogen peroxide), MMS (methyl methanesulphonate), 4-NQO (4-nitroquinoline-1-oxide), DEN (n-nitrosodiethylamine), AMVN (2,2′-azobis [2,4-dimethylvaleronitrile]) etc. triggered inflammatory responses. However, the addition of bioactive compounds from tomato along with these mutagens exhibited protective actions against them (Feng et al. 2010; Palozza et al. 2011; Lin et al. 2014). Srinivasan et al. (2007) treated rat hepatocytes with γ-radiations along with lycopene at three different concentrations (1.86, 9.31 and 18.62 µM) and reported lycopene as a radio-protectant as it reduces γ-radiation-induced DNA damage.

Health benefits

Carotenoids (lycopene) and vitamins (ascorbic acid and α-tocopherol) of tomato have a role in reducing oxidative stress and minimizing the risk of cancer and CVD (Tables 2, 3). Lycopene content in blood is known to be inversely proportional to the incidence of heart diseases (Sesso et al. 2004). The consumption of tomatoes is inversely correlated with the risk of inflammatory disorders such as atherosclerosis (Hazewindus et al. 2014). Oxidative modulation of low-density lipoprotein (LDL) plays a major role in protection against atherosclerosis and CVD. Modulation of atherogenic processes in endothelial cells by the action of lipophilic compounds of tomato on LDLs prevents CVD (Viuda-Martos et al. 2014). Studies have shown the protective effects of tomato products against various cancers, including prostate and lung cancer (Hwang and Bowen 2004; Palozza et al. 2010a, b; Tan et al. 2010). Polyphenols and carotenoids of tomato are known to obstruct tumor formation by interfering with initiation, promotion or progression of cancer (Martí et al. 2016). Quercetin helps in the remodeling of chromatin, thus inhibits epigenetic alterations during cancer progression (Martí et al. 2016).

Tomato is rich in carotenoids and high carotenoid intake in the human diet is known to be associated with low risk of chronic diseases. Carotenoids modulate the immune response, stimulate intercellular signaling (gap junction) pathways, possess pro-vitamin A activity, regulate cell cycle and apoptosis, and modulate many physiological processes, thus provide resistance to various diseases (Rao and Rao 2007). α and β-carotene and β-cryptoxanthin act as precursors to vitamin A and decrease in the content of these carotenoids in the blood lead to vitamin A deficiency (Fernández-García et al. 2012). Availability, absorption, breakdown, and storage of carotenoids are influenced by a number of factors. Mainly type, amount and association of carotenoids with other compounds influence their bioavailability in the human body. Lycopene in tomato occurs in microcrystalline form making it difficult for the absorption as compared to other carotenoids. Studies have revealed that heating food items leads to disruption of the cell wall and thus making the easy release of lycopene (Fernández-García et al. 2012). Also, the factors like gender, human health and age influence the carotenoids absorption. Alternation of fat absorption and presence of some drugs like aspirin in human body directly influences the carotenoid absorption. Carotenoids like β-Carotene and Lutein also interact and compete with each other during absorption. Considering health benefits of tomato, various breeding strategies to increase the level of beneficial phytochemicals in tomato have been carried out throughout the world (Saavedra et al. 2017). Improving the content of bioactive compounds would have commercial benefits in the production of drug supplements from tomato. This review suggested that tomatoes are carriers of compounds beneficial in managing and preventing many important health problems.

Conclusion and future perspective

The complex matrix of compounds present in tomato pose many health benefits for the human race, it is difficult to mention one particular constituent. It is the complex formulation in nature which provides all these benefits. Various studies have established the opposing link between intake of tomatoes/tomato products against the incidence of diseases. There is a need to understand the possible mechanism of action against various diseases. Whole fruit has more protective effects as compared to the derived compounds. Bioavailability of lycopene increases after cooking of the tomatoes. There is need to understand various pathways of action of bioactive compounds of tomato and their role in preventing invasion and metastasis of cancer.