Food and nutrition.pdf

Function of Food 115. $ince pure crystalline vitamin A, which is' termed retinol al~ohol, isnow available, the vitamin A activity in foods is now widely .expressedand measured usi,ng retinol equivalents (RE) rather than the iI~ternationalunits (IU) previously used. One IU of vitamin A is equivalent to 0.31 f.lgretinol.Humans obtain vitamin A in food either as preformed vitamin A(retinol) or as carotenes which can be converted to retinol in the body.Beta-carotene is the most important in human diets and is better convertedto retinol than other carotenes. It has been determined that six moleculesof beta-carotene are needed to produce one molecule of retinol; thus ittakes 6 f.lg of carotene to make 1 f.lg of retinol, or 1 RE.Dietary SourcesVitamin A itself is found only in animal products; the main sourcesare butter, eggs, milk, meat (especially liver) and some fish. However,most people in developing countries rely mainly on beta..carotene for theirsupply of vitamin A. Carotene is contained in many plant foods. Darkgreen leaves such as those of amaranth, spinach, sweet potato and cassavaare much richer sources than paler leaves such as those of cabbage andlettuce. Various pigmented fruits and vegetables, such as mangoes,papayas and tomatoes, contain useful quantities. Carotene is also presentin yellow varieties of sweet potatoes and in yellow vegetables such aspumpkins. Carrots are rich sources. Yellow maize is the only cereal thatcontains carotene. In West Africa much carotene is obtained from red palmoil, which is widely used in cooking. The cultivation of the very valuableoil palm has spread to other tropical regions. In Malaysia it is widelycultivated as a cash crop, but its products are mainly exported rather thanconsumed locally.Both carotene and vitamin A withstand ordinary cookingtemperatures fairly well. However, a considerable amount of carotene islost when green leaves and other foods are dried in the sun. Sun-dryingis a traditional method of preserving wild leaves and vegetables oftenused in arid regions. Since serious disease from vitamin A deficiency iscommon in these areas, it is important that other methods of preservationbe established.Absorption and UtilizationThe conversion of beta-carotene into vitamin A takes place in the wallsof the intestines. Even the most efficient intestine can absorb and convertonly a portion of the beta-carotene in the diet; therefore 6 mg of betacarotenein food is equivalent to about 1 mg of retinol. If no animalproducts are consumed and the body must rely entirely on carotene for