A number of problems that have arisen in the application of earlier reports can be attributed to incomplete understanding of the meaning of requirement estimates and of the conceptual framework linking energy and protein requirements. Some principles underlying the application of these concepts to practical situations are discussed in sections 10 and 11.
As defined and used in this report, estimates of requirements relate to the maintenance of health in already healthy individuals. Health is understood to include patterns of activity that are judged to be consistent with satisfactory physiological and social function.
The requirements for energy and protein of an individual are defined in the following terms:
Energy. The energy requirement of an individual is the level of energy intake from food that will balance energy expenditure when the individual has a body size and composition, and level of physical activity, consistent with long-term good health; and that will allow for the maintenance of economically necessary and socially desirable physical activity. In children and pregnant or lactating women the energy requirement includes the energy needs associated with the deposition of tissues or the secretion of milk at rates consistent with good health.
Protein. The protein requirement of an individual is defined as the lowest level of dietary protein intake that will balance the losses of nitrogen from the body in persons maintaining energy balance at modest levels of physical activity. In children and pregnant or lactating women, the protein requirement is taken to include the needs associated with the deposition of tissues or the secretion of milk at rates consistent with good health.
All requirement estimates refer to needs persisting over moderate periods of time. The corresponding intakes may be referred to as “habitual” or “usual”, to distinguish them from intakes on a particular day. However, as a matter of convention and convenience they are expressed as daily rates (of intake). However, there is no implication that these amounts must be consumed each day.
The way in which requirement estimates should be applied when health or reasonable freedom from infection cannot be assumed, is considered in section 9.
There are important physiological differences between the requirements for energy and protein, as defined above. For energy, it is usually considered that once the level of body weight and physical activity has been fixed and the appropriate growth rate defined, there is only one level of intake at which energy balance can be achieved; in consequence, this becomes that individual's requirement for energy. Even if some degree of adaptation is possible, as discussed in section 4, it is likely that this range is fairly narrow. If the intake is either above or below the requirement, defined in this way, a change in body energy stores is to be expected unless energy expenditure is correspondingly altered. If such changes in expenditure do not occur, the energy store, mainly in the form of adipose tissue, will increase when the intake exceeds requirement and decrease when it is below requirement. It is clearly contrary to experience to suppose that for each individual there is one fixed setpoint for body weight and adipose tissue mass compatible with health. In fact, we recognize that for any individual there is probably a range of acceptable body weights, and this also applies to the individuals in a group (section 3.5). However, if the imbalance is too great, or continues over long periods, the resulting changes in body weight and composition can be detrimental to function and health. Consequently, there may be risks associated with intakes either above or below actual requirements.
For protein, the requirements of individuals are also expressed in terms of the amount of dietary protein needed to prevent losses of body protein and to allow, where appropriate, for desirable rates of deposition of protein during growth and pregnancy. In this respect, the requirements of the individual for both protein and energy are analogous. However, in contrast to energy, if more protein is ingested than is needed for metabolic purposes, essentially all the excess is metabolized and the end-products are excreted, since protein is not stored in the body in the way that energy is stored in adipose tissue. Furthermore, again in contrast to energy, no detrimental effect has been identified with intakes of protein moderately above the actual requirement. For an individual, the range between the intake that is just sufficient to compensate for losses (or permit growth) and intakes that are associated with harmful effects is therefore wide. The individual's requirement is thus defined as the lower end of this range (see section 5 for a more complete discussion). As will be seen in the next section, this fundamental biological difference between energy and protein has important consequences for describing the distribution of requirements among the individuals of a group.
After defining the requirements of an individual, the next step is to extend this definition to those of a group. Estimates of requirements are derived from measurements on individuals. Actual measurements on people of the same sex and of similar age, body size, and physical activity are in practice grouped together to give the average energy or protein requirement of that set of people, together with a measure of their variability. These results are then used to predict the requirements of other individuals or collections of individuals who have the same characteristics, but on whom measurements have not been made. Such a collection of similar individuals may be referred to as a class.
The characteristics of the class are that obvious factors that may affect requirements—age, sex, weight, etc.—have been matched. However, in spite of the matching there remain many unknown factors producing variation between individuals, so that there is a distribution of requirements within the class. Changes in the variables that characterize the class will involve a change in the average requirement and therefore a change in the position of the distribution.
These concepts apply to both energy and protein requirements. In tabulating estimates of requirements for a particular class of individuals, it is convenient to describe their distribution by a single statistic, the descriptor, which differs for energy and protein for the reasons outlined in the previous section.
For a class of similar individuals, the descriptor of energy requirements is the average of the individual requirements, without specific provision for the known individual variation in requirement.
The descriptor of the protein requirement of a class of similar individuals is the safe level of intake, an amount that will meet or exceed the requirements of practically all the individuals in the group, explicitly taking into account individual variation in requirement. Following the lead of the 1971 Committee (1), the safe level is defined as the average requirement + 2 standard deviations.
Fig. 1. Comparison of average requirement for energy and safe level of intake for proteina
a It is assumed in each case that individual requirements are randomly distributed about the mean requirement for the class of individual and that the distribution is Gaussian.
The contrast between the two descriptors is illustrated in Fig. 1, in which it is assumed that the statistical distributions are Gaussian, although the principles hold for other types of distribution.
Fig. 2 shows the proportion of randomly selected individuals whose requirement would not be met when a particular level of intake is provided. The curve describes the probability that the intake would or would not meet an individual's requirement.
Fig. 2. Derivation of a probability statement from a knowledge of the requirement distribution
The curve represents the cumulative distribution of requirements. Intake refers to an assigned level of intake for a randomly selected individual and it is assumed that there is no correlation between intake and requirement among similar individuals. If the individual selects food to meet nutrient or energy requirement, the probability statement must be modified as discussed in the text.
In a similar way, one can consider the probability that a high intake will be associated with detrimental effects for a randomly selected individual. As noted above, there is a major biological difference between energy and protein in this respect. For energy, potentially detrimental effects are associated with long-term intakes only slightly above the individual's requirement. In this case, there are two probability curves (meeting of requirements and causing adverse effects) that overlap, as shown in Fig. 3(A). For each individual there is a range of protein intakes above requirement at which no detrimental effect is known to occur. The two probability curves are thus separated, as shown in Fig. 3(B).
From these curves it will be apparent that if all individuals consumed protein at levels equal to, or moderately greater than, the “safe level of intake”, there would be very little chance that any would have inadequate intakes. At the same time, unless the intakes were considerably above this level, there would be very little probability of harmful effects. The 1971 Committee (1) chose to identify this lower point as the single descriptor of the distribution of requirements for protein. In 1971 it was called the “safe level of intake”; in earlier reports it had been called the “recommended intake”. The present Consultation chose to retain the term “safe level of protein intake”.
From Fig. 3 it is apparent that a similar approach could not be followed for energy. The level of energy intake that assures a low probability of inadequate intake (average requirement + 2 standard deviations) is the same level that implies a high probability of a harmfully high intake for most people. In agreement with earlier reports, this Consultation concluded that the only descriptor that could be safely adopted is the estimated average requirement of a group of any class of individuals. This is appropriate in another respect. Most people have the ability to select their food intake in accordance with their energy requirement over the long-term, since it is believed that regulatory mechanisms operate to maintain a balance between energy intake and energy requirement over long periods of time. This implies that one would expect there to be a correlation between energy intake and energy requirement among individuals if sufficient food is available in the absence of interfering factors. In the examples cited above (Fig. 3), food intake was taken as fixed at each level, and the probabilities of inadequacy or excess at that level for a random individual were considered. If self-selection is allowed to operate, it is to be expected that individuals will make selections according to energy need and the probability of inadequacy or excess will be low across the whole range shown in Fig. 3. If the average energy intake of a class were equal to the average requirement of the class, almost all individuals would be at low risk because of processes regulating energy balance and the resultant correlation between intake and requirement. Thus, when one is considering the requirements of classes, the estimate of average requirement is an appropriate descriptor for the distribution of requirement.
Fig. 3. Probability that a particular intake is inadequate or excessive for a randomly selected individual with regard to energy intake (A) and protein intake (B)
In the case of energy, the two probability curves overlap. In the case of protein the curves are separated by a “safe range of intake” that will be associated with low probabilities of either inadequacy or excess for almost all individuals.
For protein, there is very little evidence of a correlation between intake and requirement among individuals consuming self-selected diets. Thus, the probability descriptions presented in Fig. 3 apply whether the random individual consumes a self-selected diet or is given an arbitrarily selected amount of protein. Consequently, if the average protein intake of a group is equal to the average protein requirement of that group, it cannot be assumed that the probability of inadequacy will be low for most individuals. On the contrary, this would be an “unsafe” situation.
These considerations influenced the 1971 Committee (1) in selecting the particular descriptors of the requirement distributions for energy (average) and for protein (safe level). The bases for the decisions were discussed in abbreviated form in that report, although the implications may not have been sufficiently emphasized for all of its users.