Non-Nutritive Factors That Interfere with Nutrient Absorption

5.3.2 Non-Nutritive Factors That Interfere with Nutrient Absorption

Non-nutritive factors, or anti-nutritional factors (ANFs), are biological compounds present in human or animal foods that reduce nutrient utilisation or food intake, and so contribute to impaired gastrointestinal and metabolic performance. The main ones are:

1. Phytates — abundant in unrefined cereals and millets. Phytates bind iron, zinc, calcium and magnesium and make these nutrients unavailable for digestion. Their content is reduced on germination.
2. Tannins — present in legumes; millets such as bajra and ragi; spices; tamarind; tea; turmeric; and certain vegetables and fruits. Tannins interfere with the absorption of iron and protein.
3. Trypsin inhibitors — these inhibit the activity of the enzyme trypsin in the gut and so interfere with the digestibility of dietary proteins, reducing their utilisation. They occur in soya bean and in the white of duck egg, and are inactivated by heat treatment.
4. Oxalates — present in green leafy vegetables and some legumes; they interfere with calcium absorption.
5. Goitrogens — also known as anti-thyroid substances, because they interfere with the uptake of iodine by the thyroid gland and may contribute to iodine-deficiency disorders when iodine intake is marginal. They are present in cabbage, cauliflower, turnips, soybean, bajra, peanuts and lentils.

How these factors can be reduced in everyday cooking. Most anti-nutritional factors are not a fixed, permanent property of a food — common kitchen practices lower them substantially. Soaking pulses and cereals before cooking leaches out some phytate and tannin; germinating or sprouting whole grains and legumes markedly reduces phytate content while raising vitamin C; fermentation, as in idli or dosa batter, breaks phytates down further; and ordinary cooking heat destroys trypsin inhibitors almost completely. Simply combining foods thoughtfully also helps — eating a vitamin-C-rich food such as citrus fruit, amla or guava alongside an iron-rich plant food like green leafy vegetables or whole pulses counteracts much of the inhibitory effect tannins and phytates would otherwise have on iron absorption. …