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 high as to be unattainable at population level.
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 all of the population. (b) The mean; the midpoint of the population’s requirement. (c) The mean plus a notional 2 SDs; the intake that is adequate for nearly all of the population. Note that, in practice, because insufficient data exist to establish reliable means and SDs for many nutrient requirements, the reference intakes describing the points a and c on the curve are generally set, in the case of a, at the level that is judged to prevent the appearance of signs of deficiency (biochemical or clinical), and, in the case of c, at the level above which all individuals appear to be adequately supplied. Thus, it is unlikely that even 2.5% of the popula-tion would not achieve adequacy at intake level c.
If the standard were set at the point of the average of all individual require-ments, then half the population would have require-ments in excess of the standard. In a normal distribu-tion, some 2.5% of points lie at the upper and lower tails outside that range of the mean plus or minus 2 SDs.
Thus, by setting the RDA to this point of the mean plus 2 SDs, we are setting the standard for 97.5% of the population. The consumption of most nutrients at levels somewhat greater than actually required is generally not harmful; hence, setting rec-ommendations at the population average require-ment plus a notional 2 SDs is logical if the aim is to describe an intake that is adequate for almost every-one. However, this is spectacularly inappropriate in the case of recommendations for energy intake, since even relatively small imbalances in energy intake over expenditure will lead, over time, to overweight and ultimately obesity, an increasing problem in most populations. Recommendations for energy intake are therefore given only as the estimated population average requirement.
Thus, for almost half a century, these were the terms used and the underlying conceptual approaches.
Two basic changes occurred with regard to terminol-ogy. The first was that the term “recommended dietary allowance” was altered and the second was that new terms were introduced so that the adequacy of diets could be evaluated from several perspectives. The reason for changing the terminology was in effect to re-emphasize some of the basic concepts underlying the term RDA. “Recommended” has a prescriptive air about it and there were concerns that consumers might see this as something that had to be met daily and met precisely. The term “allowance” reinforces the perception of a prescriptive approach. Thus, the UK adopted the term dietary reference value (DRV), the EU introduced the term population reference intake (PRI), the USA and Canada introduced the term dietary reference intake (DRI), and Australia and New Zealand now use the term nutrient intake value (NIV). All are precisely equivalent to the origi-nal concept of the RDA, a term that many countries prefer to continue to use.
Two new terms were introduced: a minimum requirement and a safe upper level. The minimum requirement represents the average requirement minus 2 SDs (point a in Figure 7.1). A definition describing this point is given in Figure 7.1 along with the various terms used to define this point. The concept of an upper safe limit of intake has gained importance in view of the increased opportu-nity for people to consume high levels of nutrients from fortified foods or supplements. The recently revised US DRI set “tolerable upper intake” levels (ULs) that are judged to be the highest level of nutri-ent intake that is likely to pose no risk of adverse health effects in almost all individuals in a group. The current European and UK recommendations also address this concern in the case of those nutrients for which toxic levels have been reported. The terms used by different recommending bodies to describe the various points on the distribution of individual requirements for a nutrient., while precise definitions may be found in the relevant pub-lications referred to.
The World Health Organization (WHO) has taken a rather different approach, defining population safe ranges of intake. “Normative requirement” is now used to describe the population mean normative requirement (which would allow the maintenance of, or a desirable, body store or reserve); “maximum” to refer to the upper limit of safe ranges of population mean intakes; and “basal” for the lower such limit, below which clinically detectable signs of inadequacy would be expected to appear. These WHO require-ments are revised in groups of nutrients at different times (see Further reading), and in those that date from 1974 the term “recommended intake” or “rec-ommended nutrient intake” is used to describe the average requirement plus an amount that takes into account interindividual variability and hence is con-sidered to be sufficient for the maintenance of health in nearly all people.
More recently, the United Nations University (UNU) has published a suggested harmonized approach and methodologies for developing nutrient recommendations, together with proposed terminology, that could be used worldwide to promote objectivity, transparency, and consistency among those setting and using nutrient recommend dations. Their preferred term, nutrient intake value (NIV), refers to dietary intake recommendations based on research data; the term “nutrient” was chosen in order to distinguish these from dietary compo-nents such as cereals, and the term “value” is intended to emphasize the potential usefulness for both assess-ing dietary adequacy (and hence dietary planning) and policy-making. The individual nutrient level (INLx) is flexible, in that x refers to the chosen per-centile of the population for whom this intake is suf-ficient; for example 98% (mean or median requirement + 2 SDs), written as INL98, but it could be set lower in the case of certain nutrients.
When a committee sits to make a recommendation for a standard in relation to nutrient intakes, it begins with a distribution of requirements. In the past, although the choice of criteria for requirement might vary between committees, the orientation was always the same: requirements were set at a level that should prevent deficiency symptoms. More recently, the concern for health promotion through diet has led to the introduction of the concept of optimal nutrition, in which the optimal intake of a nutrient could be defined as that intake that maximizes physiological and mental function and minimizes the development of degenerative diseases. It should be borne in mind that, although this may appear simple enough to define in the case of single nutrients, things clearly become more complex when considering all nutrients together, in all possible physiological situations. Genetic variability may also, increasingly, be taken into account; for example, the requirement for folate of those carrying certain variants of the MTHFR gene (around 10% of the population tested thus far) might, arguably, need to be set higher than for the rest of the population.
It is now recognized that there are several levels for considering the concept of optimal nutrition, i.e., the level that:
●prevents deficiency symptoms, traditionally used to establish reference nutrient intakes
●optimizes body stores of a nutrient
●optimizes some biochemical or physiological function
●minimizes a risk factor for some chronic disease
●minimizes the incidence of a disease.
● In the USA, the reference value for calcium is based on optimizing bone calcium levels, which is a move away from the traditional approach of focusing on preventing deficiency symptoms. An example of attempts to set the reference standard for optimizing a biochemical function is a level of folic acid that would minimize the plasma levels of homocysteine, a potential risk factor for cardiovascular disease. Another might be the level of zinc to optimize cell-mediated immunity. An example of a possible reference stan-dard to optimize a risk factor for a disease is the level of sodium that would minimize hypertension or the level of n-3 polyunsaturated fatty acids (PUFAs) to lower plasma triacylglycerols (TAGs). The amount of folic acid to minimize the population burden of neural tube defect would be an example of a reference value to minimize the incidence of a disease. At present, there is much debate as to the best approach to choos-ing criteria for setting reference standards for minerals and vitamins, and this is an area that is likely to con-tinue to court controversy. An important point to note in this respect is that, while minimizing frank defi-ciency symptoms of micronutrients is an acute issue in many developing countries, any evolution of our concepts of desirable or optimal nutrient require-ments must lead to a revision of the estimate of the numbers of those with inadequate nutrition.