N° 5 | December 2015

Derive maximum nutritional benefits from plant foods

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In India the prevalence of iron and vitamin A deficiencies is widespread. Almost 79% of children between 6 and 35 months and women between 15 and 49 years are anemic and 60 % of preschool children are suffering from subclinical defi ciencies of vitamin A. In addition to their low dietary intakes, this could be also explained by the lowest bioavailability of minerals of plant-based foods as well as the culinary practices, which can reduce the vitamin and mineral contents.
All the nutrients we consume through our diets are not fully available to the body for absorption, depending on other components present in the meal. However, there are ways to overcome this problem by judiciously selecting food combinations and processing them wisely.

Loss of nutrients during food processing

Although domestic food processing improves the taste, the fl avour and the palatability of food, food practices could result in a considerable loss of nutrients, particularly vitamins and minerals. For example, processing of grains by dehusking, milling, polishing and pre-cooking processes, leads to a partial loss of nutrients. Moreover, heat sensitive vitamins (vitamin B1, C) are more susceptible to degradation during heat processing but these losses can also occur by the leaching of water-soluble vitamins into the cooking water (vitamin B1, B9, B12), and by the destruction of unstable vitamins due to oxidation (ß-carotene, vitamins C and E).

Nutrient Bioavailability

The bioavailability of proteins, iron, zinc, calcium and the provitaminic ß-carotene is infl uenced by several components, which can reduce or increase their absorption:

  • Lectins, present in legumes, by direct binding, inhibit the activity of the protein hydrolyzing enzymes. Therefore, the presence of lectins leads to the incomplete digestion of proteins and lowers their bioavailability. However, legume cooking inactivates these protease inhibitors and hence improves the bioavailability of dietary proteins.
  • Organic acids present in food acidulants such as lime or amchur, enhance the absorption of non-heme iron from plant foods (from 30 to 86%) by reducing ferric iron to ferrous iron. These acids also increase the intestinal absorption of dietary zinc from 11 to 44%. Ascorbic acid present in fruit and vegetables is the most potent promoter of trace metal absorption. Thus, consumption of citrus fruit after a meal should help in the absorption of trace minerals, while drinking tea or coffee rich in polyphenols immediately after a meal, will be disadvantageous.
  • Phytates and dietary fi bre present in cereals and millets, and oxalates present in some green leafy vegetables, are chelators of cations, such as calcium, iron and zinc, and hinder their absorption.

The right food combination for a better nutrition

Even if some nutrients are lost during food processing, cooking of plant food generally improves the bioavailability of nutrients. Heat treatment of green leafy and yellow-orange vegetables by pressure-cooking, stir-frying and open-pan boiling, lead to an increase of the bioaccessibility (in vitro) of ß-carotene from 21 to 84%, 67 to 191%, and 23 to 36%, respectively.
Sprouting and malting of grains generally enhances iron absorption due to elevated vitamin-C content and reduced tannin or phytate content. These processes activate endogenous phytases, which in turn hydrolyse phytates, rendering iron and zinc more available. Fermentation of grains also improves mineral bioavailability by reducing phytate content.

Plant food matrix influences ß-carotene absorption, as mild cooking releases this carotenoid from the food matrix and facilitates its bioavailability. Furthermore, ß-carotene from the yellow-orange fruits such as mango and papaya can be absorbed to a greater extent, from 12 to 56% and 19% to 38% respectively, if these fruits are consumed as a blend along with milk.

Recently new enhancers of micronutrient bioaccessibility were identifi ed. Common to Indian culinary ingredients, they include sulphur compound-rich Allium spices (onion and garlic), ß-carotene-rich vegetables (amaranth), and pungent spices (pepper and ginger). Awareness of their beneficial influence would help in devising dietary strategies to improve the bioavailability of essential nutrients.

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