Medical brain notes 38

  Dietary Reference Standards Introduction The first attempt to set standards for nutrient intakes was by the Food and Nutrition Board of the National Research Council of the USA in 1941, which pub-lished recommended daily allowances (RDAs) in 1943 to “provide standards to serve as a goal for good nutrition.” The first UK RDAs followed in 1950, pub-lished by the British Medical Association, and many other countries and international agencies now publish dietary standards that are intended to allow the adequacy of the nutrient intakes of groups or populations to be assessed by comparison with the standards.   As the amount known about human requirements and nutrient functions has increased, so too has the size of the documents describing the recommenda-tions, from a mere six pages dealing with 10 nutrients in 1943 to the series of weighty books, each dealing with the dietary reference intakes (DRIs) of only a few of more than 30 nutrients, published by the Institute of Medicine...

  Terminology and conceptual approaches to setting nutrient recommendations From the time of their first issue in the 1940s and throughout the next 50 years, the concepts and ter-minology of RDAs remained unchanged. The basis on which these RDAs were built was the statistical distri-bution of individual requirements to prevent defi-ciency criteria for the target nutrient. The peak of the curve of the Gaussian distributions of such require-ments is the “average requirement,” with half the population having requirements above this value and the other half having lower requirements. The RDA was taken to be a point on that distribution that was equal to the mean or “average requirements” plus 2 standard deviations (SDs) (Figure 7.1). By setting the recommendation close to the upper end of the distri-bution of individual requirements, the needs of most of the population would be met. If the standard were set to meet the apparent needs of almost everyone, the resultant value would be so ...

  Interpretation and uses of dietary recommendations When using dietary recommendations, several important points need to be considered.   The nutrient levels recommended are per person per day. However, in practice this will usually be achieved as an average over a period of time (days, weeks, or months) owing to daily fluctuations in the diet. As stated above, the setting of a range of dietary recommendations should encourage appropriate interpretation of dietary intake data, rather than the inappropriate assumption that the value identified to meet the needs of practically all healthy people is a minimum requirement for individuals. If an individ-ual’s nutrient intake can be averaged over a sufficient period then this improves the validity of the compari-son with dietary recommendations. However, in the case of energy intakes, such a comparison is still inap-propriate: dietary reference values for energy are intended only for use with groups, and it is more useful to compar...

  The use of reference values to assess the adequacy of the nutrient intakes of population groups Ideally, this is accomplished by discovering the distri-bution of intakes of a nutrient in the population group (e.g., by carrying out a dietary survey), and comparing these intakes with the distribution of requirements for that nutrient within the same population. In practice, reliable data with which to plot the second of these distributions have rarely been col-lected, and therefore what must be used is an estima-tion of the average requirement together with an esti-mation of the variance in that requirement, i.e., the standard deviation (based on whatever scientific evi-dence is available), that is used to plot the population distribution of requirements as shown in Figure 7.1.   Figure 7.1  Frequency distribution of individual requirements for a   nutrient. (a) The mean minus a notional 2 standard deviations (SDs); intakes below this will be inadequate for nearly al...

  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 ...

  Methods used to determine requirements Deprivation studies This is the most direct method and involves removing the nutrient from the diet, observing the symptoms of deficiency, and then adding back the nutrient until the symptoms are cured or prevented. Difficulties with this approach are as follows. First, that the exper-iment may need to continue for several years owing to the presence of body stores of the nutrient, and often requires a very limited and therefore boring dietary regimen. Second, unpredicted long-term adverse consequences may result. Third, such experi-ments are not ethical in vulnerable groups such as children (often the most relevant for study). In some cases, epidemiological data may be available; for example, the deficiency disease beriberi occurs in populations whose average thiamin intake falls below 0.2 mg/4.2 MJ (1000 kcal). Radioactive tracer studies This approach makes use of a known amount of the radioactively labeled nutrient, which is assumed to di...