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Nutritional requirement

Each individual uses or loses a certain amount of each nutrient daily; this amount must, there- fore, be made available to the tissues either from the daily diet or from the body stores of that nutrient. If the nutrient is taken from body stores, it must be replaced at a later stage, other- wise the stores will gradually become depleted and the person will be totally reliant on their daily intake. Eventually, a deficiency state might develop, if the intake is insufficient.

The amount of each nutrient used daily is the physiological requirement. It is defined as the

amount of a nutrient required by an individual to prevent signs of clinical deficiency. This amount varies between individuals; it could dif- fer from day to day due to different levels of energy expenditure. It may also alter with the composition of the diet owing to changes in effi- ciency of absorption or utilization of nutrients.

There are, however, a number of inherent problems with this definition. First, it is argued that this approach, based as it is on the very least amount needed to survive without develop- ing a deficiency, leaves no margin of safety. Consideration could be given to the provision of a nutrient store to act as a reserve in time of physiological stress or reduced intake. Second, it gives no guidance on how to determine the requirement for nutrients for which there is currently no recognized clinical deficiency. This applies to fats (except essential fatty acids) and sugars. Third, there are no universally agreed criteria of when clinical deficiency exists. This is because a clinical deficiency reflects one end of a continuum, making it difficult to define precisely, as indicated in Figure 3.1.

It is cumbersome to obtain individual values for each nutrient requirement. One solution is to look at the average requirements of groups of similar people and to define a reasonable

Figure 3.1The stages of development of a clinical deficiency. This is a general guide to the progression from adequacy to deficiency. In some cases, the biochemical end- point may be very difficult to identify or there may be no specific signs associated with deficiency.

minimum level. The age of the child is taken as a basis for defining ‘similar’ children; for pregnant women, the stage of pregnancy is taken as the common basis; for other groups of the population, age and gender are common criteria. This is the approach used by the Panel on Dietary Reference Values of the Committee on Medical Aspects of Food Policy, which produced the most recent set of data for the UK in 1991 (DoH, 1991). This Panel derived information about nutritional require- ments in a number of ways. These included:

■ measures of the actual intakes of particular nutrients in populations that are apparently healthy;

■ the intakes of nutrients that are required to maintain balance in the body;

■ amounts of a nutrient needed to reverse a deficiency state;

■ amounts needed for tissue saturation, nor- mal biochemical function or an appropriate level of a specific biological marker. The appropriate method has to be selected for each nutrient, taking into account its meta- bolic activity, mode of excretion, storage in the body and the availability of suitable biochemical indicators. None of the criteria used in deter- mining the level of requirement is deemed per- fect, but is the best available with the current state of knowledge.

Distribution of nutritional requirements in a population When measurements of requirements are obtained from a sufficiently large population, the results are assumed to follow a typical ‘nor- mal’ distribution curve. This indicates that, for the majority, the requirement is around the mean for the group, but some have higher require- ments and some lower values. If the group is sufficiently large, then half will fall above the sample mean and half below it; this is simply a property of the distribution and not something peculiar to nutrition or requirements.

From requirements to dietary reference values Having established the range of nutritional requirements for a particular nutrient, it is

necessary to define more precisely what would be an adequate level of intake to meet these requirements. Several options might be available (Figure 3.2). Setting the level at a point A, which is above the range of individual requirements, would ensure that everyone’s needs were met but might pose a risk in terms of excessive intakes, if the nutrient was harmful in large amounts. There would also be cost implications – should people be encouraged to buy so much food to meet this high level? An alternative might be to set the level at point B, which is the mean. By definition, this would imply that this level of intake would be sufficient for half of the population, but would be inadequate for the other half. This would not be satisfactory for most nutrients. However, point B, which is defined as the estimated average requirement (EAR), is used as the reference value for energy intakes. This is because it would clearly be undesirable to advise people to consume a level of energy that was above the needs of most of the population. In addition, the reference val- ues are actually intended for use by groups. Within a group, there will be some whose energy needs are above and some below the EAR. If the food provided, or consumed, contains an amount of energy that reaches the EAR, and the individu- als eat to appetite, then one can assume that their energy needs are being met. If the mean energy provided or consumed lies below the EAR, this suggests that some of the group may not be

Figure 3.2The normal distribution curve of nutrient requirements in a population and levels used for setting dietary recommendations.

FROM REQUIREMENTS TO DIETARY REFERENCE VALUES ❚

reaching their EAR and, conversely, a mean intake above the EAR implies an excessive intake of energy amongst some members of the group. However, judgements about individuals cannot be made by comparison with the EAR figure, as this is a group mean.

In practice, for the majority of nutrients, the Panel followed the pattern of previous commit- tees and used a point that is towards the upper end of the distribution curve of nutritional requirements, at the mean 2 standard devi- ations. Because of the particular properties of this type of distribution curve, this point (C) covers the requirement figures for 97.5 per cent of the population. It could be argued that this leaves 2.5 per cent of the population outside the limits and, therefore, at risk of an inadequate intake. However, in practice, it was felt that an individual would not have extremely high requirements for all nutrients, and it was thus unlikely that anyone would consistently fail to meet requirements across the range. Eating to satisfy appetite would be likely to ensure ade- quate intake.

Therefore, to summarize, point C was identi- fied as the reference nutrient intake (RNI). In addition, the Panel identified point D, at the lower end of the requirement range. This repre- sents the mean 2 standard deviations, and covers the requirements of only 2.5 per cent of the population, who fall below this level. Again, it is possible that there are some people who have nutritional requirements consistently below this point and who may, therefore, meet their needs at this level of intake. However, it is more probable that, if someone is consuming an intake as low as this, they are not meeting their nutritional requirement. This point has been called the lower reference nutrient intake (LRNI). It effectively represents the lowest level that might be compatible with an adequate intake.

The dietary reference value (DRV) tables (published by the UK Department of Health as Report 41; DoH, 1991), therefore, provide three distinct figures for the majority of nutrients: the LRNI, EAR and RNI, which can be used as a yard- stick to give a guide on the adequacy of diets. The Panel chose a new name for these figures, moving from the recommended daily amount (RDA), which was used previously. It was felt that

this name had been too prescriptive, suggesting that the amounts given referred to what individ- uals must consume. The corollary of this was that intakes that fell below the RDA were deemed to be deficient.

In setting the dietary reference values with a range of figures, the Panel intend the range to be used and, therefore, to provide more flexibil- ity in assessing dietary adequacy. In addition, the DRV tables contain other data on dietary requirements for fats and carbohydrates, and some micronutrients for which little informa- tion was available.

Fats and carbohydrates

The approach to dietary requirements based on deficiency is not appropriate for nutrients hav- ing no specific clinical deficiency. Consequently, in the past, no RDA figures were set for fats and carbohydrates. There is now considerable public health interest in fat and carbohydrate intakes, and a desire for guidance on intake levels. The DRV Panel used their judgement, therefore, based on research evidence of health risks at particular levels of intake, to arrive at popula- tion average figures for the components of dietary fats and carbohydrates as well as non- starch polysaccharides. Rather than giving absolute figures, the DRV values are expressed in terms of the percentage of total energy, which ideally should come from the various compon- ents (Table 3.1). To provide further guidance, individual minimum and maximum values are cited for some of the components.

Some micronutrients

In the case of some of the micronutrients, insuf- ficient data were available to establish a normal distribution of requirements and thence derive values for LRNI and RNI. In these cases, the Panel, wishing to give guidance, have provided a ‘safe intake’ figure, which is considered to be sufficient to fulfil needs, but is not so high that there is a risk of undesirable effects.

In the USA, a similar approach has been adopted in the revision of the Recommended Daily Allowances that had been set in 1989. Since 1998, the National Academy of Sciences has constituted a number of expert panels to prepare a comprehensive set of reference values

for nutrient intakes for healthy US and Canadian populations. A number of reports have presented Dietary Reference Intakes (DRI) for vitamins and some minerals. These parallel the DRV figures published by DoH (1991) in the UK. The DRI includes a number of values as follows.

■ Recommended dietary allowance (RDA), which covers the needs of most individuals in a particular life stage and gender group.

■ Estimated average requirement (EAR), which represents the mean requirement of the indi- viduals in a population.

■ Adequate intake (AI) represents an amount for a nutrient that is believed to satisfy needs, but for which there is insufficient evidence to describe the full range of refer- ence intakes.

■ Tolerable upper intake level (UL) – the high- est level of a nutrient intake that is likely to pose no risk of adverse health effects for almost all individuals in the general popula- tion. As intake increases above UL, the risk of adverse effects increases.

Although the figures presented in the US tables are not always the same as those in the UK, they provide similar guidance on the range of nutrient intakes compatible with health.

If the UK tables are compared with those pro- duced by the Food and Agriculture Organization/

World Health Organization (FAO/WHO) or by other countries, differences both in the range of nutrients listed and the amounts advised are found. This implies two things. First, there are differences in needs between peoples of certain countries, as a result of differing lifestyles and perhaps different genetic make-up of the popu- lation. Therefore, when tables of reference val- ues are used, they should be those relating to the country in question. Second, opinions differ between the committees drawing up the tables in different countries as to the safety margins that should be added to the figures. As a result, final figures also differ. This does not mean that some are more ‘correct’ than others; it reflects differences in emphasis and serves to underline the uncertainty surrounding such figures. It is important to remember that these figures are never an ‘absolute’ when their uses are con- sidered. They are basically the ‘best judgement’ based on the physiological and nutritional data available at the time.