Methods used to determine requirements and set dietary recommendations
In order to derive the most accurate and appropriate dietary recommendations, committees of experts are established that look at the scientific evidence and use their judgment to decide which nutrients to consider and then, for each nutrient, make decisions in respect of the:
● criterion by which to define adequacy
● estimation of the average amount required to meet that criterion of adequacy
● estimated standard deviation of requirement in the population under consideration (i.e., the shape of the frequency distribution over the range of require-ments: broad, narrow, skewed, etc.).
The problem of different committees identifying different criteria of adequacy is illustrated by vitamin C (ascorbic acid). Experimental evidence (the Shef-field and Iowa studies) has shown that an intake of approximately 10 mg/day is required to prevent the deficiency disease scurvy in adult men. At intakes below 30 mg/day, serum levels are negligible, rising steeply with intakes of between 30 and 70 mg/day, after which they begin to plateau (and urinary excre-tion of the unmetabolized vitamin increases). The question facing the committees drafting dietary refer-ence values is whether to choose a level of intake that allows some storage of the vitamin in the body pool (e.g., EU AR 30 mg/day for adults), or one that more nearly maximizes plasma and body pool levels (e.g., US EAR 60 and 75 mg/day for women and men, respectively). Similarly, variations in calcium recom-mendations exist because some committees choose to use zero calcium balance as the criterion of adequacy, while others use maximum skeletal calcium reserves.
In some cases, one recommending body will include a nutrient among its dietary recommendations while others will not; for example, vitamin E, the require-ment for which depends directly on the dietary intake and tissue levels of PUFAs, which are highly skewed. The vitamin E requirement corresponding to the highest levels of PUFA intake would be much higher than that needed by those with much lower (but ade-quate) intakes. To set the high value as the recom-mendation might suggest to those with lower polyunsaturate intakes that they should increase their intake of vitamin E (unnecessarily). Thus, in Britain and Europe, only “safe and adequate” intakes have been set, based on actual intakes in healthy popula-tions, which should be at least 3 mg/day for women and 4 mg/day for men. In contrast, the US RDA (DRI) has been raised to 15 mg/day as α-tocopherol, based on induced vitamin E deficiency studies in humans and measures of lipid peroxidation.
There are even some examples of dietary compo-nents that have not traditionally been regarded as essential nutrients having recommendations set for them, as in the case of choline. The US DRI defines an adequate intake for choline (of 450 and 550 mg/ day for women and men, respectively), on the basis that endogenous synthesis of this compound is not always adequate to meet the demand for it (for the synthesis of acetylcholine, phospholipids, and betaine). Dietary intake data for choline and the sci-entific evidence for inadequacy are limited; thus, dose–response studies would need to be done before an average requirement could be derived. It is proba-ble that further dietary components will be included in dietary recommendations as research data accu-mulate. Potential candidates include the flavonoids and some other antioxidant compounds.