REVIEW

Anti-nutrients

Puneet Kumar

 
Introduction

Anti-nutrients are defined as natural or synthetic compounds that reduce the bio-availability of vitamins, minerals, protein and other nutrients either by inactivating them or by interfering with their digestion and/ or absorption. Natural anti-nutrients are widely present in plant kingdom: in foliage and seeds of virtually every plant we eat. From evolutionary point of view, they are considered to be natural defense mechanisms of plants/ seeds against predator animals/ insects and parasites/ bacteria/ fungi. While some food items like grains, legumes, beans and nuts are typically high in anti-nutrient content (4), others like seeds of Cucurbitaceae family like pumpkin and watermelon are very low in anti-nutrient content (22). In addition, there are many substances like pesticides, heavy metals and “improvers” that enter the food item through cultivation or processing/ manufacture and decrease the nutritive value of food. These are synthetic anti-nutrients.

Major natural anti-nutrients present in plant kingdom are phytates, oxalates,cyanogenicglycosides, goitrogens, lectins (phytohaemagglutinins), saponins, tannins, lectins, protease inhibitors and toxic amino acids. Avidin is an anti-nutrient in duck eggs (1, 13).

2.0 Phytates

Phytates are hexa-phosphates of inositol. It is the major storage form of phosphates in cereals, legumes and oil seeds. As phytate accumulates in storage sites in seeds, other minerals chelate to it forming the complex salt phytate. In oilseeds, which contain little or no endosperm phytates are distributed throughout the kernel found within subcellular inclusions called aleurone grains or protein bodies.

Phytate is a strong chelator: It forms mineral-phytate complexes in the gut that adversely affects bioavailability of minerals like zinc, iron, calcium, magnesium, and manganese leading to deficiencies and sometimes even serious disorders like classical dwarf syndrome (due to calcium and zinc deficiency) that is common in Egypt (20). Bioavailability of proteins is also reduced in a similar manner. In addition, phytates acts as inhibitor of various enzymes like trypsin, tyrosinase, pepsin, lipase and amylase: further affecting protein bioavailability.

The phosphate in phytates is also not bioavailable because of absence of phytase in human gut.

In order to increase the nutrient value of food, various strategies have been used to reduce the phytate content of the food-items. Table-1 enlists these strategies. However, it is also important to understand that in certain situations, phytates in food play a positive role also. For example, food with higher phytate content have, in general lower glycemic index. Refined cereals have lower phytate content but have higher glycemic index. Another example is that of tofu. In tofu, presence of phytate helps as it chelates calcium and prevents formation of calcium oxalate crystals.

Table-1. Strategies to reduce phytate content of food items (3).

Strategy	How it works	Comments
Milling of cereals	By reducing the bran content that has maximum phytate content	It also removes some fiber and minerals
Soaking	It activates endogenous phytases: leads to 67% reduction of phytates	Some water-soluble vitamins, proteins and minerals also decreased.
Cooking	Some degradation of phytate	Only minor reduction in phytate content, since phytate is heat-resistant; also causes some reduction of heat-labile nutrients
Germination	By activation of endogenous phytases. Degree of dephytinisation varies with species.	One of the reasons why germinated cereals are more nutritious.
Malting	By activation of endogenous phytases	Makes the cereal more nutritious, just like germination
Development of low phytate mutants of crops	Biosynthesis of phytate is blocked by interfering with genes involved with production of phytates using genetic engineering	Often reduces the yield of crops, hence not economically viable as of now. However, such a mutant of maize (Maize lpa1 mutant) has shown promising results.
Introduction of phytase gene in cereals	By potentiating action of endogenous phytase	Introduction of AspergillusPhytase phyA gene has been attempted in wheat and barley with some success; further studies are needed (15)
Exogenous phytases in food	Enzymatic degradation of phytate in food	Being extensively studied as a novel approach

3.0 Oxalates

Oxalate is a simple di‐carboxylic acid [(COO)₂²⁻] and is present in small amount in a wide variety of plants, but is in large amounts in vegetables, especially in spinach, amaranth, beetroot, sweet potato and rhubarb. It appears that leafy tropical plants have higher oxalate content as compared to temperate plants. Horsegram, sesame seeds, cocoa and nuts also have high oxalate content. They are the end-product of metabolism in plants and are present as crystals in protein-storage vacuoles. The oxalate content of plants increases as the plant ages and become over-ripe. In general, oxalates are present in higher concentration in leaves as compared to other parts of plant, but it varies with species.

In humans also, oxalates are end-products of metabolism and thus are excreted. However, consumption of high-oxalate foods adds to the oxalate load and it predisposes to formation of stones in kidneys and urinary tract. Moreover, being negatively charged molecules, they combine with calcium ions in gut and interfere with absorption of calcium. In fact, out of all anti-nutrients that affect calcium bioavailability (oxalates, phytates, tannins and excess fiber), oxalate is the one that affects most (2). The adverse effect of oxalates is greater if oxalate: calcium ratio exceeds 9:4, since foods with higher ratio interfere with calcium absorption from other foods with calcium that are consumed at the same time. The foods with oxalate: calcium ratio of < one do not interfere that way and thus have much lower demineralization effect. Bioavailability of other cations like Magnesium and Iron is also affected by oxalates, but Zinc bioavailability is unaffected. Ingestion of extremely high amount of oxalates (5-15 grams) can even cause oxalate poisoning and death (18).

Soaking, cooking (specially boiling), germination, malting and fermentation reduce the oxalate content of food. Blanching [a cooking method wherein the food is scalded in boiling water, removed after a brief, timed interval, and finally plunged into iced water or placed under cold running water (shocking or refreshing) to halt the cooking process] is also reportedly helpful in reducing oxalate content, of spinach for example. In addition to these food processing methods, some other measures can also be used to tackle adverse effects of oxalates in food. First, there has to be sufficient calcium in the diet. Decreased calcium intake increases oxalate: calcium ratio, thus potentiating adverse effects of oxalate in diet (This is the reason why calcium is never restricted in diet of patients with stones in urinary tract). Second, pyridoxine (vitamin B₆) in therapeutic doses also counteracts the adverse effects of oxalates, while deficiency of this vitamin accentuates absorption of oxalates from the gut. Third, ascorbate (vitamin C) is a precursor of endogenous oxalate. Hence, excess of ascorbate should be avoided as it also adds to oxalate load ultimately (18).

Just like phytates, oxalates also have positive role in human health in certain situations. For example, consumption of oxalate-rich foods like rhubarb can protect teeth from acid erosion caused by cola drinks and lemon juice (18).

4.0 Cyanogenic Glycosides

Some legumes like linseed (flax), lima bean, kidney bean, red gram and some varieties of tapioca (cassava), white clover and young bamboo shoots contain cyanogenic glycosides. Kernels and fruits of certain plants also contain small amount of cyanogenic glycosides. For example, Prunus  spp., such as almonds, apricots, cherries, and peaches, "Loquat" (Eriobotrya, "cherry laurel" (Prunus laurocerasus) and even apple pips (Malus spp.) have cyanogenic glycosides.Kernels must be chewed to release their cyanide and large amounts are required to cause poisoning.

Structurally, they are glycosides of 2-hydroxynitriles that can be hydrolyzed by the enzyme beta-glucosidase into cyanohydrin. This is unstable and dissociates to hydrocyanic acid.

Maceration of fresh plant material as in chewing allows enzyme(s) and cyanogenic glycosides to come together, releasing hydrogen cyanide (HCN) gas. Hydrogen cyanide is very toxic even at low concentrations. It inhibits the oxidative processes of cells causing them to die very quickly. Because the body rapidly detoxifies cyanide, an adult human can withstand 50-60 ppm for an hour without serious consequences. However, exposure to concentrations of 200-500 ppm for 30 minutes is usually fatal. Aside from death, acute cyanide toxicity at small doses can cause headache, tightness in throat and chest, and muscle weakness. The effects of chronic (long-term) exposure to cyanide are less well known (16), although it is thought to be involved in tropical ataxic neuropathy in Nigeria. In other sub-Saharan African countries and less commonly in South America, consumption of cassava root (Manihot esculenta, also known as tapioca is a staple food in many tropical countries) together with a diet deficient in sulphur amino acids is apparently responsible for an endemic neurological disease known as Konzo (5), many other neurological disorders, glucose intolerance/ diabetes, growth retardation and goiter/ cretinism.

Processing methods, such as peeling, drying, grinding, soaking, boiling or cooking, soaking and fermentation have been reported by several studies to cause significant reduction in the cyanogenic glycosides of processed foods. These processes have been applied to food crops such as roots, tubers, cereals, and leaves, to cause significant reduction in the cyanogen contents of the crops. Food‐processing methods generally disintegrate cyanogens contents of plants, and this leads to the production of cyanide. Since cyanide is volatile, further processing techniques, such as roasting and drying, will volatilize the remaining cyanide to low level. "Garification" is a process whereby fermented and dried cassava mash is simultaneously cooked and dried in a shallow wok, results in a 90– 93% reduction in total cyanide content (25). Inadequate processing of Cassava roots during the times of drought or war has resulted in epidemics of the disease Konzo in countries like Mozambique (5). Although cyanide content of bamboo shoot is much higher than that of cassava root, the cyanide content in bamboo shoots decreases substantially following harvesting and processing (26). Another way to tackle this anti-nutrient is via plant breeding. Newer varieties of white clover have lower cyanide content than older ones. Similarly, work is underway to develop varieties of lima beans and cassava with lower cyanide content.

5.0 Goitrogens

Enlargement of thyroid gland is called goiter and any substance that lead to development of goiter is called as goitrogen. In the context of this chapter, this term “goitrogen” would be used for naturally occurring goitrogenic anti-nutrients in food items. Chemically, there are several of them including thiocyanates, isothiocyanates, glucoinolates, etc. They are most commonly present in vegetables of Brassica family (Cruciferous vegetables) like cabbage, cauliflower, broccoli, radish, soybeans, peanuts, etc. (11). However, the amount of these vegetables that needs to be eaten for significant goitrogenic effect is huge. Further, thiocyanate is a metabolite produced during detoxification of cyanide. Hence, foods rich in cyanogenic glycosides (vide supra, 4.0) are also goitrogenic when taken for a long time.

Goitrogens block the uptake of iodine by thyroid gland that leads to interruption in synthesis of thyroid hormones and ultimately goiter. Goitrogens can have a role in exaggeration, persistence, and development of goiter. Continuous exposure to dietary cyanide from insufficiently processed cassava products is responsible for endemic cretinism in African countries where cassava is a staple food and dietary iodine intake is also less (<100mcg/ day) (26).

Cooking reduces the amount of goitrogens by one-third. Washing and soaking also reduces the goitrogenic effect to some extent. If one intends to consume any of the goitrogenous food in raw state (raw spinach in a smoothie for example), then blanching is a good way to minimize the goitrogenic effect. But the best way to tackle this anti-nutrient is to have sufficient iodine in food and the best way to have sufficient iodine in every meal is to use iodized salt. Iodine supplement nullifies the goitrogenic effect of an average cooked meal.

It is interesting to note that thiocyanate in low concentration has antioxidant properties and might have health-promoting effects (7). It is especially helpful in patients of sickle cell anemia (20).

6.0 Tannins

Tannins are water soluble polyphenolic compounds with a molecular weight of more than 500 daltons. Legumes, millets, nuts (specially walnuts and hazelnuts), tea, coffee, cranberry, strawberry, blueberry, etc. are rich in tannins.

Tannins bind with protein, iron and vitamin B₁₂ and reduce their bioavailability (1, 12). They bind with enzymes like trypsin and amylase in gut that further leads to indigestion. Tannin-protein complexes involve both hydrogen-bonding and hydrophobic interactions. The precipitation of the protein-tannin complex depends upon pH, ionic strength and molecular size of tannins (1). Hence, the ultimate effect of tannins on nutrition is variable. There is recent evidence that tannins of lower molecular weight are absorbed from the gut and interfere with protein metabolism, and this effect might be more important than interference with absorption of proteins from the gut (6). These absorbed tannins have also been reported to produce other physiological effects such as immune response, hepatotoxicity and lipid metabolism (8).

Germination and dehulling reduce the tannin content significantly, while cooking leads to only modest reduction (2, 17). Addition of milk to tea/coffee reduces its tannin content (13).

On positive side, tannins are known to possess some anti-oxidative, antimicrobial and even anti-carcinogenic properties. However, its significance in human health is not yet established (9).

7.0 Protease inhibitors

Protease inhibitors are widely distributed within the plant kingdom, including the seeds of most cultivated legumes and soybean. They are the most commonly encountered class of anti-nutritional factors of plant origin (1). Ovomucoid is a trypsin inhibitor that is present in egg-white (13).

Most common protease inhibitors are trypsin and chymotrypsin inhibitors. Structurally, trypsin inhibitors are polypeptides that form well characterized stable complexes with trypsin on a one-to-one molar ratio. This blocks the enzymaticaction leading to indigestion of proteins (1).

Protease inhibitors are heat labile and are readily inactivated by heat (especially moist heat), i.e., by cooking. Soaking and germination also reduces their content drastically. Peas, which are sometimes consumed in raw state, have significant content of trypsin and chymotrypsin inhibitors. Efforts are underway to develop a variety that has low content of these anti-nutrients (10).

8.0 Lectins (phytohaemagglutinins)

Phytohaemagglutinins or lectins are carbohydrate binding glycoproteins that are widely distributed in legumes, soybean, certain oilseeds and tubers like potatoes among the food-items. They have specific affinity to carbohydrate moiety of glycoproteins present in cell membranes. Lectins in food bind with the glycoproteins in the cell membranes of intestinal mucosal cells (enterocytes) and interfere with the absorption/ transportation of various nutrients, specially carbohydrates (1). Diet very high in lectins can damage intestinal mucosal barrier, allowing the intact proteins to be absorbed into the bloodstream undigested. This, in turn has been linked to development of host of disorders that result from immune dysregulation like diabetes mellitus type-1, rheumatoid arthritis, etc. Damage to intestinal mucosa is also linked with development of peptic ulcer disease (14). However, nothing of these is substantiated as of now (28), thus the concept of "lectin-free diet" that is getting popular, thanks to pseudoscience websites these days is just a "fad" (24). The major flaw in most of the lectin studies is that they have been conducted with isolated lectins and not as a part of food in the form that it is eaten (24, 28). Since lectins are heat labile, cooking reduces lectin content significantly (1). Germination and fermentation is also known to reduce lectin content. The only problem that is substantiated being associated with lectins is "food poisoning like symptoms" (nausea, vomiting, abdominal cramps and diarrhea) associated with ingestion of raw/ undercooked food with high lectin content like kidney beans.

9.0 Saponins

Saponins comprise a large family of structurally related compounds containing a steroid or triterpenoid aglycone (sapogenin) linked to one or more oligosaccharide moieties. They are saponins are widely distributed in the plant kingdom, only a small number of such plants are actually toxic to humans. Further, they are mostly present in plants that are used as herbs (e.g., alfalfa) and spices that are usually used in such small quantities in the diet. Thus, their anti-nutritional effect is low in general (25). The staple foods that contain saponins are kidney bean, soybean, groundnut, chickpeas and quinoa (Chenopodium quinoa, a food crop in Andes) (11).

Saponins are surface active agents and interact with bile acids and cholesterol and interfere with absorption of lipids, vitamin A and vitamin E from the gut (1, 11). The negative effect of the saponins can be reversed, however, by the inclusion of dietary cholesterol, which interferes with the absorption of saponins by forming an insoluble complex with  saponins. By the same token, dietary saponins may exert a positive effect by reducing cholesterol levels in the tissue and serum of experimental animals (25).

Soaking, washing, rubbing and milling reduces the saponin content. For plants like alfalfa and quinoa, breeders have developed varieties with low saponin content (25).

10.0 Toxic Amino Acids

A wide range of toxic, non-protein amino acids occur in foliage in seeds of plants. Over generations, humans have learned to avoid plants that have high contents of toxic compounds. However, some wild species are inadvertently harvested along with crop species, while other enter the food chain through intermediary animals. There are some plants like alfalfa and Lathyrus species are consumed by some human communities and have some toxic amino acids/ peptides (19).

Most of these compounds have low general toxicities but prolonged administration of them can result in toxicity. Some of these inhibit protein synthesis, while others are incorporated into proteins of human tissues with toxic effects. Basic processes such as urea synthesis and neurotransmission may be disrupted (3).

Some of such toxic amino acids are enumerated below:

10.1 Beta-(N)-oxalyl-amino-L-alanine acid (BOAA): It is an analogue of glutamate and is present in chickling pea ("Kesari dal": Lathyrus sativus, and certain related species). Excessive consumption of this legume over long period causes lathyrism in which there is irreversible paralysis. Lathyrism is oldest neurotoxic disease known to man. Once prevalent throughout Europe, N. Africa, Middle East and parts of the Far East, the disease is presently restricted to India, Bangladesh and Ethiopia, where this legume is still consumed (27). Although the best prevention is to avoid the consumption of this legume, but there are ways to reduce the neurotoxin: Parboiling of the seeds helps in getting rid of this anti-nutrient (11); use of metallic cooking utensils rather than traditional clay pots to avoid accrued toxicity from iron-induced oxidation, addition of anti-oxidant seasonings, soaking seeds in lemon-water and avoiding unripe seeds is also reported to reduce the toxicity (29).

10.2 Canavanine: It is structurally related to amino acid arginine. Canavanine competes with arginine, resulting in the synthesis of dysfunctional proteins. Seeds and sprouts of alfalfa (Medicago sativa) have this anti-nutrient and its prolonged ingestion (especially of raw, unsprouted seeds) leads to certain autoimmune disorders like systemic lupus erythematosus (SLE). Stopping consumption often reverses the symptoms.

10.3 Azetidine-2-carboxylic acid (Aze): It is a non-protein amino acid contained in beets (Beta vulgaris) that is used to produce sucrose. It is suggested that it replaces proline in myelin sheath leading to multiple sclerosis. Sugar that is produced from beet is considered to be aze-free, but the byproducts of sugar industry are thought to enter food chain through animal fodder (23).

11.0 Avidin

Avidin is a tetrameric biotin-binding protein. It is an anti-nutrient present in egg-white. It contains four identical subunits (homotetramer), each of which can bind to biotin (Vitamin B₇, vitamin H) with a high degree of affinity and specificity. The avidin-biotin bonding is the strongest known non-covalent bonding. This way, it acts as a biotin-antagonist. Biotin is indispensable for human health as it acts as a cofactor for five carboxylases that are critical for fatty acid, glucose, and amino acid metabolism. It also has a role in adaptive immunity, particularly in NK and T cells.

It is significant only when raw egg while is consumed, as heat (>85C) deactivates it rapidly. Complete deactivation occurs if eggs are boiled for >4 minutes (12).

12.0 Conclusion

Anti-nutrients, the compounds that reduce the bio-availability of vitamins, minerals, protein and other nutrients either by inactivating them or by interfering with their digestion and/ or absorption, are ubiquitous in nature, especially in plant kingdom. Being present in almost every food-item we eat, it is practically impossible to avoid foods with anti-nutrients. However, appropriate strategies of processing of food (like soaking, cooking, fermentation etc.) decrease the anti-nutrients significantly. The most important strategy however, as elsewhere in nutrition, is to have variety in diet: Deleterious effect of anti-nutrient is expectedly more when one kind of food item is consumed on a regular basis. Further, small amounts of many of these anti-nutrients can even help in subtle ways; so-much-so that many of these are increasingly being recognized as functional foods or "nutraceuticals"!

- Dr. Puneet Kumar, Owner-Consultant, Kumar Child Clinic, S-19, Pankaj Central Plaza, Pocket 5, Sector 12, KM Chowk, Dwarka, New Delhi-110078. www.KumarChildclinic.com

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