Human Nutrition and Anti-nutritional Factors

Anti-nutritional factors

Anti-nutritional factors (ANFs) are compounds naturally present in foods, especially plant foods, that lower the nutritional value of a food by reducing nutrient utilisation or protein digestibility. Many of them are inactivated by heat, soaking, germination or fermentation, which is why processing is an essential part of preparing legumes and cereals.

Phytate (phytic acid) is the classic mineral-binding ANF. Its phosphate groups chelate divalent cations such as Ca^{2+}, Fe^{2+} and Zn^{2+}, forming insoluble complexes that reduce mineral bioavailability. Trypsin inhibitors, found in soybean and other legumes, block trypsin and thereby reduce protein digestion. They are heat-labile, so cooking removes most of their activity.

Each ANF is best learnt together with its characteristic physiological effect. Lectins are glycoproteins that agglutinate erythrocytes and therefore cause hemagglutination. Raffinose-family oligosaccharides such as stachyose are not hydrolysed by small-intestinal enzymes; colonic bacteria ferment them to gas, which causes flatulence. Kunitz-type inhibitors are protease inhibitors acting on trypsin and chymotrypsin.

Do not confuse ANFs with beneficial bioactive compounds. Isoflavones are generally regarded as beneficial phytoestrogens and are not classed as principal ANFs, and resveratrol is an antioxidant stilbene that does not impair digestion or absorption.

Fat-soluble vitamins

The fat-soluble vitamins are A, D, E and K. They are absorbed together with dietary lipids through mixed micelles, transported in chylomicrons, and stored mainly in the liver and adipose tissue. Because they are stored, they can accumulate to toxic levels, unlike the water-soluble B-complex vitamins and vitamin C, which are excreted in excess.

The main members are retinol (vitamin A), cholecalciferol (vitamin D3), \alpha-tocopherol (vitamin E) and the K vitamins. Menadione is a synthetic form of vitamin K. Thiamine, niacin and biotin, although they are vitamins, are water-soluble B-complex vitamins and must not be placed in the fat-soluble group.

Deficiency of a fat-soluble vitamin produces specific disorders. Vitamin A deficiency causes xerophthalmia, which begins with dryness of the conjunctiva and cornea and progresses to night blindness and keratomalacia if untreated. Retinol is needed for the synthesis of rhodopsin and for the maintenance of epithelial tissue, and this deficiency remains a leading cause of preventable childhood blindness. Do not confuse xerophthalmia with other deficiency syndromes: thiamin deficiency causes beriberi, pantothenic acid deficiency causes burning feet syndrome, and vitamin C deficiency causes scurvy.

Protein quality indices (PER)

Protein quality is assessed by indices that measure how well a protein supports growth or maintenance. The Protein Efficiency Ratio (PER) is a bioassay based on the growth of young, growing animals, conventionally rats, fed a diet with a fixed level of protein.

PER is defined as the body weight gain in grams divided by the grams of protein consumed during the test period. A higher PER means that the protein supports growth more effectively. For example, a rat that gains 2.5 g while consuming 10 g of protein has a PER of 0.25.

Do not confuse PER with other protein measures. The percentage of absorbed nitrogen retained by the body describes nitrogen retention, or biological value, rather than PER. Percent in vitro digestibility measures how well a protein is hydrolysed by digestive enzymes in the laboratory, and an amino acid ratio is a compositional measure, not a bioassay.

Soy isoflavones (phytoestrogens)

Phytoestrogens are plant-derived compounds whose structures resemble oestradiol, so they bind weakly to oestrogen receptors and can produce oestrogen-like or oestrogen-modulating effects. Soybean is the most important dietary source of these compounds, which belong mainly to the isoflavone class.