Author: Er. Wasiya Farzana
What are they?
The term anti-nutrients refers to defence metabolites, having specific biological effects depending upon the structure of specific compounds which range from high molecular weight proteins to simple amino acids and oligosaccharides.
To avoid predation, sedentary species (plants, fungi and bacteria) synthesize a range of low and high molecular weight compounds. These secondary metabolites play a role in defence against herbivorous, insects, pathogens or adverse growing conditions
Plants commonly synthesize a range of secondary metabolites as part of their protection against attack by herbivorous, insects and pathogens or as means to survive in adverse growing conditions. If farm or domestic animals or humans consume these plants, these compounds may cause adverse physiological effects. The term anti-nutrients refers to defence metabolites, having specific biological effects depending upon the structure of specific compounds which range from high molecular weight proteins to simple amino acids and oligosaccharides. Legumes are rich source of anti-nutrients in human diet.
Anti-nutrient substances from nutritional point of view, interferes with normal growth, reproduction and health, when consumed regularly in amount existing in a normal component of diet therefore should be considered as harmful and toxic. A significant part of human population relies on legumes as staple food for subsistence, particularly in combination with cereals. They are unique foods because their rich nutrient content includes starch, protein, dietary fibre, oligosaccharides, phytochemicals (especially the isoflavones in soybean) and minerals. Their nutritional content contributes to many health benefits to humans. So, the knowledge regarding various anti-nutritional substances present in foods as well as techniques to reduce them in the diet is essential for health and wellbeing of the population.
Classification of the Anti-Nutritional Factors
The anti-nutritional factors in plants may be classified on the basis of their chemical structure, the specific actions they bring about or their biosynthetic origin (Aletor, 1999). Although this classification does not encompass all the known groups of anti-nutritional factors, it does present the list of those frequently found in human foods and animal feedstuffs.
The anti-nutritional factors may be divided into two major categories. They are: (1). Proteins (such as lectins and protease inhibitors) which are sensitive to normal processing temperatures. (2). Other substances which are stable or resistant to these temperatures and which include, among many others, polyphenolic compounds (mainly condensed tannins), non-protein amino acids and galactomannan gums (Osagie,1998). More often than not, a single plant may contain two or more toxic compounds, generally drawn from the two categories, which add to the difficulties of detoxification.
There are several antinutritional factors that are very significant in plants used for human foods and animal feeds. They are: (i) Enzyme inhibitors (trypsin and chymotrypsin inhibitors, plasmin inhibitors, elastase inhibitors), (ii) Haemaglutinnins (concanavalin A, ricin), (iii) Plant enzymes (urease, lipoxygenase), (iv) Cyanogenic glycosides (phaseolunatin, dhurrin, linamarin, lutaustralin), (v) Goitrogens (pro-goitrins and glucosinolates), (vi) Oestrogens (flavones and genistein), (vii) Saponins (soya sapogenin), (viii) Gossypol from Gossypium species e.g. cotton, (ix) Tannins (condensed and hydrolysable tannins), (x) Amino acid analogues (BOAA, DAP, mimosine, N-methyl-1-alanine), (xi) Alkaloids (solanine and chaconine), (xii) Anti-metals (phytates and oxalates), (xiii) Anti-vitamins (anti-vitamins A, D, E and B12) and (xiv) Favism factors.
Protease (trypsin) and amylase inhibitors
Compared with legumes, common anti-nutrients level in cereal are quite low.
Protease inhibitors are widely distributed within the plant kingdom, including the seeds of most cultivated legumes and cereals. Protease inhibitors are the most commonly encountered class of antinutritional factors of plant origin. Protease inhibitors have the ability to inhibit the activity of
proteolytic enzymes within the gastrointestinal tract of animals. Due to their particular protein nature, protease inhibitors may be easily denatured by heat processing although some residual activity may still remain in the commercially produced products. The antinutrient activity of protease inhibitors is associated with growth inhibition and pancreatic hypertrophy. Potential beneficial effects of protease inhibitors remain unclear, although lower incidences of pancreatic cancer have been observed in populations where the intake of soybean and its products is high . While protease inhibitors have been linked with pancreatic cancer in animal studies, they may also act as anticarcinogenic agents.
The Bowman-Birk inhibitors derived from soybean have been shown to inhibit or prevent the development of chemically-induced cancer of the liver, lung, colon, oral and esophagus . Trypsin inhibitor and chymotrypsin inhibitor are protease inhibitors occurring in raw legume seeds. Trypsin inhibitors that inhibit the activity of the enzymes trypsin and chymotrypsin in the gut, thus preventing protein digestion, are found in many plant species mainly in different grain legumes. Trypsin inhibitors are a unique class of proteins found in raw soybeans that inhibit protease enzymes in the digestive tract by forming indigestible complexes with
Mode of action:
Trypsin inhibitors are a unique class of proteins found in raw soybeans that inhibit protease enzymes in the digestive tract by forming indigestible complexes with dietary protein.
Effects:
Negative: The antinutrient activity of protease inhibitors is associated with growth inhibition and pancreatic hypertrophy.
Beneficial:
Lower incidences of pancreatic cancer. have been observed in populations where the intake of soybean and its products is high.
They may also act as anticarcinogenic agents.
The Bowman-Birk inhibitors derived from soybean have been shown to inhibit or prevent the development of chemically-induced cancer of the liver, lung, colon, oral and esophagus.
Haemagglutinins:
Haemmagglutinins are proteins in nature and are sometimes referred to as phytoagglutinins or lectins.
Source:
As in legumes, most cereals commonly consumed by human contain glycoprotein called lectins.
Mode of action:
Many lectins can bind to intestinal epithelial cells, where they may impair nutrient absorption and cause damage that may allow infiltration of bacteria into the blood stream.
Health effects:
If some types of beans are consumed raw, they may cause shock cramps.
Saponins:
Saponins are group of natural products possessing the property of producing lather or foam when shaken with water.
Saponins are a heterogenous group of naturally occurring foam producing triterpene or steroidal glycosides that occur in a wide range of plants, including pulses and oilseeds such as kidney bean, lentil, pea, chickpea, alfalfa, soybean, groundnut, lupin and sunflower. Saponins reduce the uptake of certain nutrients including glucose and cholesterol at the gut through intralumenal physicochemical interaction. Hence, it has been reported to have hypocholesterolemic effects
Chemistry:
These are glycosides of high molecular weight. These are diverse class of glycosides found mainly, but not exclusively in plants, they comprises a steroidal or triterpene aglycone linked to one, two or three saccharide chains of varying size and complexity via ester and or ether linkages.
Sources:
Saponins have been reported in soyabean, sword bean and jack bean. Saponins occurs in broad range of plants consumed in the human diet, including legumes (soy, peas and beans), root crops (potato, yam, asparagus and allium) as well as in oats, sugarbeet, tea and many medicinal herbs.
Health effects:
Negative: Toxic saponins cause nausea and vomiting.
Beneficial: Hypocholesterolemic effects in man.
Methods to overcome:
These toxins can be eliminated by soaking prior to cooking.
Phytates:
Phytate (is also known as Inositol hexakisphosphate (InsP6)) is the salt form of phytic acid, are found in plants, animals and soil. It is primarily present as a salt of the mono- and divalent cations K+, Mg2+, and Ca2+ and accumulates in the seeds during the ripening period. Phytate is regarded as the primary storage form of both phosphate and inositol in plant seeds and grains. In addition, phytate has been suggested to serve as a store of cations, of high energy phosphoryl groups, and, by chelating free iron, as a potent natural anti-oxidant .
Phytate is ubiquitous among plant seeds and grains, comprising 0.5 to 5 percent (w/w).
The phosphorus bound to phytate is not typically bio-available to any animal that is non-ruminant. Ruminant animals, such as cows and sheep, chew, swallow, and then regurgitate their food. This regurgitated food is known as cud and is chewed a second time. Due to an enzyme located in their first stomach chamber, the rumen, these animals are able to separate, and process the phosphorus in phytates. Humans and other non-ruminant animals are unable to do so.
Phytate works in a broad pH-region as a highly negatively charged ion, and therefore its presence in the diet has a negative impact on the bioavailability of divalent, and trivalent mineral ions such as Zn2+, Fe2+/3+, Ca2+, Mg2+, Mn2+, and Cu2+. Whether or not high levels of consumption of phytate-containing foods will result in mineral deficiency will depend on what else is being consumed. In
areas of the world where cereal proteins are a major and predominant dietary factor, the associated phytate intake is a cause for concern.
Chemistry:
Inositols with 4, 5 or 6 phosphate groups are common in the seed of many of our grain legume and can reach concentration higher than 10% of dry matter
Sources:
In monocotyledons such as wheat and rice, phytates is present in germ of corn and in the aleurone or bran layer allowing an easy separation by milling.
However, in diacotyledons seeds such as legumes, nuts and oilseeds, phytates are found closely associated with proteins and is often isolated or concentrated with protein fraction of these foods.
They can be regarded as stores for phosphate and mineral nutrients that are important for plant nutrition and especially vulnerable during germination.
Health effects:
Since, phytates contains complex zinc, iron, magnesium and calcium ions in the digestive tract, they can cause mineral ions deficiency in animals and human.
Methods to overcome:
Phytate contain of food can be lowered by addition of enzymes which hydrolyze them.
Oligosaccharides and Iso flavonoids:
Legume seeds are generally rich in oligosaccharides (up to 20%), such as stachyose and raffinose.
These compounds serve as carbon source during germination therefore, their contents can be reduced in legumes through germination which is common practice, e.g. in soyabeans.
Sources:
Iso flavonoids have been detected in soyabean , lupins and several other legumes. They are involved in plant defence against fungi, bacteria, viruses and nematodes (phytoalexins , phytoanticipins), act as signals in Legume-Rhizobium interaction and exhibit estrogenic activities.
Mode of action:
Recently it was found that the isoflavone genistein inhibits tyrosine kinase.
Health effects:
Since, these enzymes are often stimulated in cancer cells, the lower incidence of some kinds of cancers in people which ingest isoflavone rich food, such as soybean products, has stimulated the hypothesis, that some legumes rich in isoflavones can prevent cancer.
Cynogenic glycosidases:
The cyanogenic glycosides belong to the products of secondary metabolism, to the natural products of plants. These compounds are composed of an a-hydroxynitrile type aglycone and of a sugar moiety (mostly D-glucose). Cyanogenic glucosides (a-hydroxynitrile glucosides) are derived from the five protein amino acids Val, Ile, Leu, Phe and Tyr and from the nonproteinogenic amino acid cyclopentenyl glycine. Although derived from six different building blocks, they constitute a very small class with around 50 different known structures. A number of plant species produce hydrogen cyanide (HCN) from cyanogenic glycosides when they are consumed. These cyanogens are glycosides of a sugar, often glucose, which is combined with a cyanide containing aglycone.
Cyanogenic glucosides are classified as phytoanticipins. Their general function in plants is dependent on activation by b-glucosidases to release toxic volatile HCN as well as a ketones or aldehydes to fend off herbivore and pathogen attack.
Chemistry:
Cynogens are glycosides of 2-hydroxyl nitriles and widely distributed among plants,
e.g. the Rosaceae, Leguminosae (e.g., in Phaseolus lunatus and Vicia sativa), Graminae, and Araceae.
In case of emergency, when plants are wounded by herbivores or other organisms, the cellular compartmentation breaks down and cynogenic glycosides come into contact with active B-glucosidase, which hydrolyses them to yield 2-hydroxynitrile.
In addition to the toxic effects, cynogens can serve as mobile nitrogen storage compounds in seeds which are important during germination.
Source:
Cassava is a crop plant rich in cyanogenic glycosides.
Vicine and Convicine:
(Favism factors)
Favism is a hemolytic disease that is found in sensitive individuals with consumption of broad beans.
It is more widely found in people living in the Mediterranean countries.
Mode of action:
The structure of hemoglobin, which is the primary carrier of oxygen, is upset.
Health effect:
Dizziness, vomiting, feeling of tiredness and dark orange urine, which is the first symptom of blood transfusion, are symptoms of this disease.
The disease disappears soon but incidences of death may be encountered when the disease is prolonged.
This disease of hemolytic anemia is caused by favogens.
Favism also causes high fever and jaundice.
Lupin alkaloids :
Grain legume, high in protein is commonly used for livestock feed for Europe, Africa, Australia and Asia. It has been cultivated for over 2000 years. Over 500 species of the genus Lupinus are known. In its raw form, the mildly toxic lupin alkaloids present in plants causes a bitter taste, and used as defensive mechanism herbivorous.
Removal:
Alkaloids are commonly removed (or reduced) by soaking the raw seeds in water prior to use.
.Non-protein amino acids:
Hundreds of types of non-protein amino acids have been found in nature and they have multiple functions in living organisms.Microorganism and plants can produce uncommon amino acids.
Health effect:
In humans, non-protein amino acids also have biologically-important roles. Glycine, gamma-amino butyric acid and glutamate are neurotransmitter and many amino acids are used to synthesize other molecules, for e.g. Tryptophan is a precursor of the neurotransmitter serotonin, Glycine is a precursor of porphyrins such as heme, Arginine is a precursor of nitric oxide, Carnitine is used in lipid transport within a cell.
Thermal treatment:
Legume seeds are hardly been consumed raw, they are usually cooked and by this procedure, lectins and protease inhibitors are inactivated.
Low molecular weight compounds are leached out into cooking water, to be discarded afterwards
Today, a deeper knowledge of chemical structure of the anti-nutrients can help to devise technological strategies to process legumes seeds in order to obtain toxin free products.
Since a diversification will increase the economic value of overall crop plant, food technology and rational processing are an alternative to breeding of anti-nutritional factors free plants which can be more susceptible to pests and pathogens.
Followed by modern food technology (separation, filteration etc.), pure nutritionally valuable dietary products, such as protein, dietary fiber, oil and other fine chemicals can be generated. Because of higher costs these considerations are related to the use of legumes seeds which will be utilized for human consumption (an example are soyabeans for which a specialized industry has been developed).
These situations is different in case of feed industry (which is a major outlet of legume seeds in Europe) where pulses are simple ground or pelleted.
For this purpose we need to develop varieties which are low in heat stable nutritional factors (e.g. alkaloids, saponins, phytates, isoflavones and non-protein amino acids).
If heat labile compounds (protease inhibitors, lectins) can be denatured by heat treatment during grinding and pelleting these compounds might be maintained since they confer resistance to the plants.
Some of the heat labile anti-nutritional factors inactivated by thermal processing are given in Table.
| Anti-nutritional factor | Common food sources | Effects of anti-nutritional factors |
| Avidin Hemagglutinnins Lathyrogens Goitrogens a-Amylase inhibitors Trypsin inhibitors Thiaminases | Egg whites Red kidney beans, yellow wax beans Chick pea Sweet potatoes, beans, cabbage, turnips Cereal grains, peas, beans Legumes egg whites, potatoes Fish, shellfish, brussel sprouts, red cabbage | Binds biotin, making it biologically unavailable Induces red blood cell clumping Disrupts collagen structure Causes goitre by limiting iodine absorption Slows starch digestion Inhibits activity of trypsin Destroys thiamin |
Chemical Detoxification:
Deaminocanavanine is a well known untoxic deamination product of canavine.
The degradation of canavanine to deaminocanavanine under alkaline conditions occurs therefore a chemical strategy for the detoxification of this compound and has already been successfully employed for the processing of the canavanine containing seeds of C. ensiformis
The major anti-nutrients in the seeds of V. sativa (besides vicine) can be rendered inactive by mild hydrolysis. It is reasonable to propose that in principle post-harvest detoxification procedures can be developed for these anti-nutritional factors
Fermentation:
The various Indian household preparation techniques for their effectiveness in detoxifying L. sativus and their methods which included a fermentation step were the most effective in reducing ODPA levels, eliminating 95% of this toxin.
Further, improvement in detoxifying is likely to be made with selection for better ODAP degradation.
Such methods can, in principle, also be used for the post harvest detoxification of Vicia seeds, thus providing an alternative approach to the wider utilization of these grains without the need for genetic removal of their low molecular weight anti-nutritive and unpalatability factors.
The incorporation of fermentation processes into other simple food technologies also offer good prospects for a detoxification of food source while simultaneously giving flexibility in the manipulation of
flavour, texture and colour of the raw material.
Germination:
Pea and lentil sprouts have gained popularity in recent years. Traditionally, Mediterranean, grain legumes have not been used as sprouts.
The potential toxicity of beta-isoxazolin-5-one-alanine (BIA), the biosynthetic precursor for the lathyrism toxin beta-ODAP may be a risk factor if consumption increases during the germination of lentils and peas.
This kind of processing, however, which reduces the contents of oligosaccharides and of other N-containing ANFs, has a long history in Asia, where it has served to improve the palatability of soybeans.
Pea sprouts are a very recent addition to Chinese cuisine.
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About Author / Additional Info:
I am currently pursuing PhD in Agricultural Process and Food Engineering from Indira Gandhi Agricultural University