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P. L.PELLETT Problems and Pitfalls in the Assessment of Human Nutritional Status MORE THAN 40 SEPARATE ELEMENTS OR COMPOUNDS HAVE NOW been identified as necessary for life, together with at least 40 grams per day of amino acids as protein and more than 200 grams per day of a mixture of carbohydrate , protein, and fat that can be metabolized for energy. Although all these materials are required for daily metabolism to proceed, it is not necessary that they all be provided daily in the diet. The ability of the body to draw on reserves or stores allows varying amounts of time to elapse before deficiencies may be recognized. Oxygen (if that be a nutrient) is needed continuously, water deprivation cannot last for more than a few days; total food deprivation can last for weeks to months, depending on fat reserves; whereas vitamin A stores may last for more than a year on essentially zero intake (Hume and Krebs 1949). Daily requirements for nutrients also vary enormously, ranging from microgram quantities for Vitamin B12, milligram quantities for many minerals and most vitamins, gram quantities for individual essential amino acids, calcium and phosphorus, and, of course, several-hundred-gram quantities of energyproviding materials. The distinction between required and non-required nutrients is not always clear-cut. Fiber is necessary in the diet for optimal health, yet it is not a nutrient ; its function, indeed, is not to be absorbed. Several mineral elementsfor example, zinc, copper, and selenium-have only relatively recently been demonstrated as essential nutrients, but nickel, tin, and vanadium remain in the uncertain category. Of the essential amino acids, histidine is now known to be required by adults, but the demonstration of deficiencies can take months, since the amino acid can be synthesized in the body, but only at a rate somewhat below requirements. Normal individuals can manufacture as much cystine and tyrosine, as they need; thus, these two amino acids are termed non-essential (non-essential amino acids are so essential that the body has learned to synthesize them!). Under conditions of liver damage, however, cystine and 163 III. Nutritional and Biopsychological Constraints tyrosine can no longer be synthesized fast enough from methionine and phenylalanine respectively, so that they become essential and must be supplied in the diet (Horowitz et al. 1981). With some vitamins, synthesis may occur (invalidating some of our definitions): the well-known example of nicotinic acid, synthesized from the amino acid tryptophan, complicated the search for the cause of pellagra (Horwitt et al. 1955). Vitamin D (cholecalciferol) can be synthesized in the skin, and it functions more as a hormone than as a vitamin (DeLuca 1974). Even vitamin C, the first discovered if not first named of the vitamins, whose lack has probably influenced human history more than any other deficiency, may be synthesized to a very limited degree in pregnancy and lactation (Rajalakshmi et al. 1965). Diets (and most individual foods) are mixtures of nutrients, in varying proportions , along with other compounds such as fiber (of various types), phytates , cholesterol, natural toxicants, and trans-fatty acids, which may be harmful or beneficial. No single food can supply all our needs for long, and the lack of intake of a single food mayor may not affect the intake of nutrients. A safe rule of thumb is that the more components there are in a dietary, the greater the probability of balanced intake. The corollary of this is that monotonous dietaries with few components are more likely not only to show deficiencies , but to be seriously affected by dietary avoidances. The assignment of credit or blame to dietary intake for the development or prevention of malnutrition is much less straightforward than many assume, both inside and outside the field of nutrition. Nutritional status and health status overlap; adaptation to high or low intakes may occur; nutritional requirements, although based on scientific facts, depend on informed judgments and are subject to a wide range of individual variability; and, finally, estimates of nutrient intakes are only approximate except under controlled metabolic conditions that are hardly nonnal. It is thus not surprising that the role of food preferences and food aversions in nutrition must often remain anecdotal and equivocal. It is now increasingly recognized that malnutrition may be caused less by nutrient deficiency as such and more by many interrelated social, political, and economic factors Oohnsson 1981; Pellett 1983); the widespread prevalence of malnutrition is usually a symptom of a very sick society (Maletnlema 1980). Because of this...

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