Many popular diet and nutrition books create the impression that the body's require ments for the essential nutrient vitamin A can be exclusively met with plant foods like carrots, squash, green leafy vegetables and orange coloured fruits. The low fa
Under optimal conditions, humans convert carotenes to vitamin A in the upper intestinal tract by the action of bile salts and fat splitting enzymes. Of the entire family of carotenes, beta carotene is most easily converted into vitamin A. Early studies indicated that four units of beta carotene were needed to produce one unit of vitamin A. This ratio was later revised to six to one, and recent research suggests an even higher ratio (Nutrition Review, Springer Verlag, New York, July 1993). This means that you have to eat an awful lot of vegetables and fruits to obtain even the daily minimal requirements of vitamin A, assuming optimal conversion.
But the transformation of carotene to retinol is rarely optimal. Diabetics and those with poor thyroid function a group that includes at least half the adult US population cannot make the conversion at all. Children make the conversion very poorly and infants not at all they obtain their precious stores of vitamin A from animal fats yet the low fat diet is often recommended for children (Jennings, LW, Vitamins in Endocrine Metabolism, Charles C Thomas, Springfield, Illinois).
Strenuous physical exercise, excessive consumption of alcohol, excessive consumption of iron (especially from "fortified" white flour and breakfast cereals), use of a number of popular drugs, excessive consumption of polyunsaturated fatty acids, zinc deficiency and even cold weather can hinder the conversion of carotenes to vitamin A (Dunne, Lavon J, Nutrition Almanac, Third edition, McGraw-Hill, 1990), as does a low fat diet.
Carotenes are converted by the action of bile salts, and very little bile reaches the intestine when a meal is low in fat. Butterfat stimulates the secretion of bile needed to convert carotenes from vegetables into vitamin A, and at the same time supplies very easily absorbed true vitamin A.
Nutritional pioneer Weston Price considered the fat soluble vitamins, especially vitamin A, to be the catalysts on which all other biological processes depend. Efficient mineral uptake and utilization of water soluble vitamins require sufficient vitamin A in the diet. He discovered that healthy primitives especially valued vitamin A rich foods for growing children and pregnant mothers. Working in the 1930s he found that their diets contained 10 times more vitamin A than the typical American diet of the time.
Supplies of vitamin A are so vital to the human organism that mankind is able to store large quantities of it in the liver and other organs. Thus it is possible to subsist on a fat free diet for a considerable period of time before overt symptoms of deficiency appear. But during times of stress, vitamin A stores are rapidly depleted. Strenuous physical exercise, periods of physical growth, pregnancy, lactation and infection are stresses that quickly deplete vitamin A stores.
One aspect of vitamin A that deserves more emphasis is its role in protein utilization. The disease kwashiorkor is as much a disease of vitamin A deficiency, leading to impaired protein absorption, as it is a result of absence of protein in the diet. High protein, low fat diets in children induce rapid growth along with depletion of vitamin A supplies. Growing children actually benefit from a diet that contains more calories as fat than as protein (Personal Communications, Mary G Enig, Ph.D).
A recent New York Times article noted that vitamin A rich foods like liver, egg yolk, cream and shellfish confer resistance to infectious diseases in children and prevent cancer in adults (New York Times, March 10, 1992).
Studies show that the best and most easily absorbed source of vitamin A is butterfat (Texas Agricultural Bulletin, No 560, February 1938), a food relished by young and old alike. So use butter and cream liberally for good taste and wise nutritional practice.
Sally Fallon is editor of the Price Pottenger Nutrition Foundation Health Journal.