Introduction

Plants and their products have always played a substantial role in human welfare by satisfying various essential needs ranging from food to medicines. Currently, functional foods or the nutraceuticals of plant origin have gained in popularity and constitute a major share of the health care market (Bhat and Sridhar 2008; Bhat and Karim 2009). Today, consumers are quite practical and choosy in their approach to diet; they are interested in incorporating high nutrient levels with adequate amount of essential minerals into their normal diet, preferably with sources from plant origin. The expansion of the mineral supplement industry is a direct indication of the gaining importance of minerals to consumers. In this regard, interest has grown in finding traditionally consumed plant products (e.g. herbs, spices) that might not only have culinary and medicinal properties but that also are abundant in essential micronutrients. Most of the minerals, even at threshold levels, contribute significantly to normal growth and play a pivotal role in biochemical functions and essential enzyme systems.

Plant products play a significant role in traditional medicine systems, and the majority of consumers have the misconception that these products are inherently safe for consumption. However, there is concern among researchers that plants might accumulate heavy metals at significantly high levels when grown in polluted soil (Kalǎc and Svoboda 2000). If true, this will reflect the failure of good agricultural and manufacturing practices and quality assurance. Purity issues of plant-based drugs are also of high concern (Mazzanti et al. 2008). Hence, studying the presence of unwanted heavy metals and their concentrations in plant species used as traditional medicines and recommended to be incorporated in the normal food is a necessity.

Of late, numerous efforts have been made to determine the mineral and heavy metal contents of herbal, medicinal, and aromatic plants, vegetables, and even wines from various parts of the world (Aberoumand and Deokule 2009; Galgano et al. 2008; Lavilla et al. 1999; Miller-Cebert et al. 2009; Tuzen et al. 2007). Trace elements and heavy metals have certain health benefits at lower concentrations, but at higher levels, they can be toxic and pose health risks (Schumacher et al. 1991); thus, it is important to determine the levels of these compounds in common, popular, and widely used herbal plants (or as medicinal foods).

Detection of mineral and heavy metal concentrations in various foods has been conducted mainly using atomic absorption spectrophotometry (AAS) with flame atomization and a graphite furnace. However, other alternative, more reliable, and accurate methods are available, such as: inductively coupled plasma mass spectrometry, neutron activation analysis, and X-ray fluorescence methods (Knapp 1991). In this study, inductively coupled plasma optical emission spectrometry (ICP-OES) was used as this is a multi-element technique which allows fast and accurate analysis of nearly 75 elements of the periodic table.

The major objective of this study was to evaluate and provide baseline information on the mineral composition and heavy metal contents of some popular and commercially valuable plants from the Indian market that possess high nutraceutical value. If at least a few of the plants were found to have high mineral contents, they might have the potential to be used for biofortification and development of new food products.

Materials and Methods

Sample Collection and Identification

Certain common and popular plant materials that have therapeutic and nutraceutical value and are used in indigenous ayurvedic medicines were purchased from a leading local market dealer (from Mysore, India). The samples purchased were freshly harvested ones (less than 2 weeks) with no apparent physical damage. The materials (a total of 23 plants consisting of arial parts, leaves, bark, stem, root, rhizome, dried berries, seeds) were air dried at room temperature (25 ± 1 °C) and ground into fine powder in a mortar and pestle to avoid chromium contamination from stainless steel utensils. The powder was placed temporarily in air-tight polyethylene pouches until further analysis. The identity of the plant material was confirmed with assistance from local practitioners and botanists. Table 1 lists the common name, scientific name, family, parts used, and the medicinal/health-promoting effects of the plant materials used in this study.

Table 1 Plants and their parts used in the present studya
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Preparation and Analysis

The minerals and heavy metal [aluminum (Al), arsenic (As), barium (Ba), calcium (Ca), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg), manganese (Mn), molybdenum (Mo), mercury (Hg), potassium (K), sodium (Na), nickel (Ni), and zinc (Zn)] contents of the powdered samples were determined using the AOAC (1990) method. Briefly, a known amount of the sample was digested with a mixture of concentrated nitric acid, sulfuric acid, and perchloric acid (10:0.5:2, v/v), and the analysis was conducted using the ICP-OES (Varian Inc., Walnut Creek, CA, USA). Table 2 lists the conditions and parameters used. All the glassware were cleaned by soaking overnight in a 10% nitric acid solution and then rinsing three times with deionized water.

Table 2 Operating conditions of the ICP-OES
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Statistical Analysis

Analysis of minerals and heavy metals were performed in replicates, and data are presented as mean ± SD (n = 3) with one-way analysis of variance (significance level at P < 0.05) was performed with statistical software (ORIGIN®, version 6.0, Microcal Software Inc., Northampton, MA).

Results and Discussion

Table 3 depicts the mineral and heavy metal concentrations of the samples analyzed in this study. Values varied widely among samples, but in general, most samples were rich in the following essential minerals: Cu, Fe, K, Mg, Mn, Na, and Zn. These minerals serve as structural components of tissues, function in cellular and basal metabolism, and help maintain water and acid–base balance (Smith 1988). The abundances of K, Mg, and Ca agree with earlier data from many of the plant specimens (Lavilla et al. 1999).

Table 3 Minerals and heavy metal contents of plants (mg/100 g, dry wt. basis)
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Artemisia vulgaris had the highest levels of Ca (3,441.8 mg/100 g), whereas Annona squamosa had the lowest (3.72 mg/100 g). Ca is a major component of bone, assists in tooth development, helps regulate endo- and exo-enzymes, and plays a significant role in regulating blood pressure (Brody 1994). Using plant-based Ca as a nutritional supplement might be advantageous for consumers who are allergic to animal-based food sources or who practice strict vegetarianism.

Cu was the highest in Berberis aristata (134.96 mg/100 g) and lowest in Sida cordifolia (0.05 mg/100 g). Vitiveria zizinalis had the highest concentrations of Fe and Zn (6,413.3 and 5.93 mg/100 g, respectively). Deficiency of zinc and iron in the diet is a widespread problem and a matter of great concern, especially in developing countries where people rely more on vegetarian diets. These essential trace elements are involved with vital immune system (Zn) and metabolic functions and are intrinsic components of hemoglobin, myoglobin, and cytochrome (Fe) (Hemalatha et al. 2007). Hence, further study of V. zizinalis might prove this species to be a useful dietary supplement to provide Zn and Fe, especially for vegetarians.

Piper longum had the highest potassium content (1,352.3 mg/100 g), Anethum sowa had the highest sodium concentration (170.69 mg/100 g), and Rauvolfia serpentina had the highest Mn concentration (856.4 mg/100 g). Perry (1972) reported a correlation between therapeutic properties of Cr and Mn in plants with diabetic and cardiovascular diseases. Plant materials with high concentrations of the above-mentioned micronutrient elements will definitely play an important role in maintenance of human health when taken at recommended levels.

Earlier research on humans and livestock has shown that optimal intakes of elements such as Na, K, Mg, Ca, Mn, Cu, and Zn can reduce individual risk factors for health problems such as cardiovascular disease (Mertz 1982; Sanchez-Castillo et al. 1998). Mayer and Vyklicky (1989) reported that these elements play a significant role in neurochemical transmission and also serve as constituents of biological molecules as a cofactor for various enzymes and in a variety of different metabolic processes. Minerals like Fe, Zn, and Mn are also recognized to be potential antioxidants (Talwar et al. 1989), which are involved in strengthening the immune system. Similarly, Mg, and Zn are known to prevent cardiomyopathy, muscle degeneration, growth retardation, alopecia, dermatitis, immunological dysfunction, gonadal atrophy, impaired spermatogenesis, congenital malformations, and bleeding disorders (Chaturvedi et al. 2004).

Monitoring the presence and concentrations of heavy metals in plants/herbs used for medicinal and health-promoting effects is crucial because these levels can be used as an indicator of the surrounding pollution and because they can have ill effects when consumed directly. According to Schilcher et al. (1987), the presence of toxic metals in the environment is quite varied and depends on various factors, such as pollution from industrial and traffic emissions, the use of purification mud and agricultural expedients such as Cd-containing dung and the use of organic Hg fungicides and insecticides containing lead arsenate.

In the present study, the heavy metals Al, Ba, Cd, and Mo were not detectable, and lead was present but below detection levels in all samples analyzed. In contrast, As, Co, Cr, and Ni were detected in almost all samples, but they were present in low concentrations (Table 3). The concentrations of As and Co were highest in V. zizinalis, (53.1 and 10.2 mg/100 g, respectively), and this was the only plant material that was positive for the presence of Hg (3.6 mg/100 g). Arsenic is considered to be one of the most toxic elements, and the provisional tolerance (weekly intake) is 15 mg/kg body weight (FAO/WHO 1989). In an earlier work, Liang et al. (1998) measured various metallic elements in commercial Chinese medicines using AAS and reported approximately 10 mg/100 g of arsenic in the samples analyzed. However, the results on Hg (Table 3) were contradictory to an earlier report by Dahanukar et al. (1998) who found high levels of Hg (in six samples) that exceeded the permissible amounts (1–7.5 ppm). Contamination by Hg occurs mainly via its usage in dental amalgam, electric switches, and batteries, which then finds its way into soil. Hg poisoning causes inflammation of the mouth, stomach, and colon and acute abdominal pain, and it affects the kidneys.

Chromium is one of the trace metal nutrients that are essential to humans and animals. Its principal role appears to be in helping to maintain normal glucose tolerance in the body (Strickland et al. 1972). Chromium was highest in V. zizinalis (6.74 mg/100 g), followed by Bacopa monnieri (2.88 mg/100 g), and it was lowest in Chlorophytum borivilianum (0.08 mg/100 g). The main source of Cr contamination occurs via the use of plating metal, as an alloy of stainless steel, and in tanning of hides. Because Cr is not known to have any toxic effects, no maximum intake limit has been set. However, in the UK, the Committee on Medical Aspects of Food Policy has recommended that Cr intake should be >25 µg/kg body weight for adults and between 0.1 and 1.0 µg/kg body weight for children and adolescents (COMA 1991).

Momardica chasintia contained the highest levels of Ni (3.86 mg/100 g), while S. cordifolia (0.01 mg/100 g) had the lowest. Ni is exclusively used as an alloy in stainless steel, in coins, in metal plating (electric water heaters), and in batteries. It has been reported to cause allergies and eczema of skin (Jorhem et al. 1996). However, no maximum limit has been set for Nickel in food.

The presence of heavy metals in plant samples can be directly attributed to the surrounding pollution in the environment (e.g., atmospheric and water pollution). Wong et al. (1993) reported the presence of heavy metals (cadmium, cobalt, copper, iron, manganese, nickel, lead, zinc, and mercury) in 42 Chinese herbal medicines and attributed their presence to contamination occurring during air-drying and preservation. Soil condition (e.g., heavily polluted) also plays a significant role in determining the heavy metal concentrations of plant materials used for medicinal purposes (Kabelitz 1998). Januz et al. (1994) ascertained the contents of heavy metals to be correlated to industrialized regions, and they concluded that the plants grown in an industrialized region had higher contents of heavy metals than plants grown in a second less-industrialized region.

The mineral and heavy metal concentrations reported herein might not be on par with some of the earlier reports on medicinal plants. The differences observed might be due to different growth conditions, genetic factors, geographical variations in the level of soil fertility, efficiency of mineral uptake, and the analytical procedures employed (Özcan and Akgül 1998).

From the study, we conclude that measuring minerals and heavy metal concentrations in nutraceutically valued plant produce is highly significant not only from nutritional point of view but also to assess the quality when used as herbal drugs. The result of the present study provides vital information on the mineral composition for a few of the popular and nutraceutically valued plants of India, which need to be explored further to be used as a food supplement or for food biofortification to exploit the potential health-promoting properties.