Nutrition and Micronutrients

Protein quality assessment (PER)

Protein quality is judged by how well a protein supports growth and maintenance. It depends on both the digestibility of the protein and the pattern of its essential amino acids. The Protein Efficiency Ratio (PER) is the classical bioassay for this purpose. Young growing rats are fed a diet with a fixed, standard protein level for a defined period, and PER is the body weight gained divided by the protein consumed over the same period: \mathrm{PER}=\dfrac{\text{weight gain (g)}}{\text{protein intake (g)}}. The unit is grams of weight gained per gram of protein eaten. Casein is used as the reference protein under the same conditions. Because PER is measured on whole-animal growth, it reflects both digestibility and amino acid composition. This is why it differs from chemical scores and in vitro methods.

Do not confuse PER with related indices:

• Net Protein Ratio (NPR) — like PER, but the weight loss of a protein-free control group is added to the test group's gain, so it corrects for endogenous nitrogen loss.
• Net Protein Utilization (NPU) — the percentage of ingested nitrogen retained by the body, based on nitrogen balance, not on weight gain.
• Coefficient of protein digestibility — the fraction of ingested protein absorbed, calculated from faecal nitrogen.
• In vitro digestibility — protein breakdown measured with enzymes in the laboratory, not in an animal.

Example: if a rat gains 18 g while eating 60 g of protein during the assay, its PER is 18/60 = 0.3 g per g.

Anti-nutritional factors

Anti-nutritional factors are naturally occurring food constituents that reduce the availability of nutrients or impair digestion. They are distinct from beneficial phytochemicals, which have positive physiological effects. Many of them are inactivated by heating, soaking or germination, which is why legumes are cooked before eating.

Key factors and their actions:

• Phytate (phytic acid) — binds divalent cations such as \mathrm{Ca^{2+}}, \mathrm{Fe^{2+}} and \mathrm{Zn^{2+}}, forming insoluble complexes that lower their absorption.
• Trypsin inhibitors — proteins (for example, Kunitz-type and Bowman-Birk inhibitors in soybean) that block the digestive proteases trypsin and chymotrypsin, lowering protein digestibility.
• Lectins — proteins that bind carbohydrates on cell surfaces and cause hemagglutination, the clumping of red blood cells.
• Raffinose-family oligosaccharides (stachyose, raffinose) — humans lack the enzyme \alpha-galactosidase in the small intestine, so these sugars reach the colon, where bacterial fermentation produces gas and causes flatulence.

Distinction: isoflavones and resveratrol are phytochemicals that are generally regarded as health-promoting, so they are not classed as anti-nutritional factors in the usual sense.

Example: a diet rich in phytate may lower iron absorption even when total iron intake is adequate.

Fat-soluble vitamins

The fat-soluble vitamins are vitamins A, D, E and K. They are absorbed together with dietary lipid through micelle formation and incorporated into chylomicrons. They are stored mainly in the liver and adipose tissue, so excessive intake, especially of A and D, can cause toxicity, whereas water-soluble vitamins (the B-complex and vitamin C) are not stored to any large extent and are excreted in urine.

Major forms:

• Vitamin A — retinol and its derivatives; retinal is needed for rhodopsin, the visual pigment.
• Vitamin D — cholecalciferol (vitamin D3) and ergocalciferol (D2), which regulate calcium and phosphate absorption.
• Vitamin E — tocopherols and tocotrienols, of which \alpha-tocopherol is the most active antioxidant form in humans.
• Vitamin K — phylloquinone (K1) from plants, menaquinones (K2) from bacteria, and menadione (K3), a synthetic form.

Deficiency signs: vitamin A deficiency causes night blindness and xerophthalmia, with dryness and keratinisation of the conjunctiva and cornea. Thiamin, niacin and biotin are water-soluble B vitamins and do not belong to this group.

Example: a patient with fat malabsorption may develop vitamin A and E deficiency, even when water-soluble vitamins are adequate.

Bioactive compounds in soybean

Soybean contains several bioactive compounds besides protein and oil. The most important phytoestrogens are the isoflavones genistein and daidzein. In the raw bean they occur mainly as glycosides, namely genistin and daidzin, and they are released as aglycones by hydrolysis during digestion or food processing. Because their structures resemble oestradiol, they can bind oestrogen receptors weakly and produce oestrogen-like or anti-oestrogenic effects depending on the tissue.