Protein requirements and aging

Dear Sir: The recent article involving the use of the indicator amino acid oxidation (IAAO) method to assess protein requirements of octogenarian women (1) represents yet another attempt to show that there is an increase in protein requirements with age, a debate that has existed for decades. One reason for lack of resolution of this debate is that protein and amino acid metabolism is by far the most elaborate of any nutrient. Assuming that the protein requirement is an intake that allows maintenance of an acceptable body composition phenotype and associated normal function, we know that this can occur in population groups exposed to a wide range of habitual protein intakes, through metabolically complex adaptations. Evaluating exactly how adaptations to variation in protein intakes occur, at what cost, if any, and the lower and upper limits of protein intakes at which successful adaptation can occur is extremely challenging. In the absence of functional indicators of protein status of the adult population, all methods to date have been based on some measure of protein balance, either nitrogen balance or amino acid balance, through measures of amino acid oxidation by using stable isotopes. Concern for inadequacies of the nitrogen balance approach has resulted in investigators adopting acute postprandial studies, such as the IAAO method (1), to evaluate the response to protein or amino acid intakes as a proxy for the “requirement.” Assuming here that such studies can show useful information, it is certainly necessary that investigators adopting postprandial experimental protocols fully understand the metabolic complexity of the response of protein metabolism to protein intake and show that the model assumptions inherent in their studies are correct and that any metabolic response or endpoint does indeed directly relate to the “requirement.” The principal advocates of the IAAO method, Pencharz and Ball (2), have always argued that the change in the oxidation rate, the breakpoint, of a nonlimiting indicator amino acid ([1-13C]phenylalanine) in response to graded intakes of a test amino acid or of protein shows the intake that maximizes protein synthesis and minimizes indicator oxidation: ie, their definition of the “requirement.” In response to a study of the protein requirement of healthy school-aged children determined by the IAAO method (3), Millward and Jackson (4) argued that the use of the IAAO method to assess protein requirements, as opposed to requirements for amino acids, was invalid. This is because, in this specific case, the [13C]phenylalanine indicator does become limiting and this limitation determines the breakpoint. The experimental design of this approach measures [13C]phenylalanine oxidation in response to meals containing increasing amounts of protein (as an amino acid mixture based on egg protein) containing a fixed amount of phenylalanine. This is shown in Figure 1, which plots phenylalanine content of protein meals at each amount of “protein” intake expressed as the content relative to the amount that would have been present if the amino acid mixture was balanced. Thus, intakes of phenylalanine are in excess at the 2 lowest intakes and are deficient in the 4 highest intakes. Because of this, the indicator oxidation rate, shown in Figure 1, reflects the excess or deficiency of the indicator, not the amount of protein intake. The authors refer to the criticism by Millward and Jackson (4) of their approach in their article (1) and argue that “regardless of the protein intake, the total aromatic amino acid concentration is always 70 mg · kg−1 · d−1, which is higher than the aromatic amino acid requirement and thus could not be ‘balanced’ by increasing protein intake. With sufficient tyrosine, the concept of the IAAO approach is to keep the phenylalanine content constant and sufficient at any protein (amino acid mixture) level to reflect the protein oxidation rate.” This statement shows a lack of understanding of the postprandial response to varying protein intakes. In fact, with this study design, the protein oxidation rate, which is not measured, will be the opposite of the observed phenylalanine oxidation rate. As pointed out previously (4), on the basis of many published tracer studies of the feeding response, it can be confidently predicted that at low amounts of intake the meal protein will be fully used with low levels of overall amino acid oxidation but with high levels of [13C]phenylalanine oxidation because of its excess. However, as the intake of the amino acid mixture exceeds 0.6 g · kg−1 · d−1, overall utilization of the amino acid mixture for net protein synthesis will decrease as it becomes limited by the relative availability of phenylalanine. In consequence, overall amino acid oxidation will increase and [13C]phenylalanine oxidation will decrease. The validity of this argument is easily tested by measuring the response of blood concentrations of phenylalanine (predicted to be high in the excess intake range and low in the deficient range), changes that will be the opposite of other amino acids (eg, leucine). The argument that the total aromatic amino acid concentration always exceeds the aromatic amino acid requirement is irrelevant because in such acute feeding studies the “requirement” for phenylalanine is that which allows efficient utilization of the meal amino acid mixture for net protein deposition. As shown in Figure 1, the meals are phenylalanine deficient at intakes >0.6 g · kg−1 · d−1. Thus, these studies tell us nothing about the protein requirement of octogenarian women. FIGURE 1. Concentrations of the ‘‘indicator’’ amino acid (phenylalanine) in meal protein intakes relative to those of a balanced intake at each protein amount. Values shown are calculated from studies in 6 octogenarian women (1) ... It is the case that previous studies by the lead author of this article showed no difference with age in the protein requirements of adults as measured by both nitrogen balance (5) or by [1-13C]leucine balance (6). These 2 reports are separate publications from the same study that, together, comprise the most comprehensive study in the literature on the protein requirements of healthy adults. The study shows quite clearly no effect of age and sex, similar to our own findings (7), with the authors concluding that “there are no compelling data that the dietary protein needs of old people are different from those of young people when expressed per kg body weight.” It is a puzzle, therefore, that in this most recent study (1), the [1-13C]leucine balance article (6) is not quoted at all and the nitrogen balance arm of the study (5) is only briefly mentioned together with a list of reports arguing for an increased protein requirement, none of which include any unequivocal evidence. One would expect experienced investigators to have a consistent message in their published work or at the least explain why they have changed their view. My understanding of the literature in terms of well-conducted nitrogen balance or 13C oxidation studies is that the experimental evidence to date shows that requirement values do not change significantly with advancing age. As indicated in an editorial about this recent article (8), what is really needed are studies that show that incremental increases in protein intake make a difference—ie, that they do affect clinically important outcomes. Sarcopenia has been widely discussed as a potential consequence of inadequate protein intake, although there is very little, if any, unequivocal evidence that the loss of muscle mass and function with age can be influenced by protein intake (9). In the absence of clinical outcomes from well-conducted randomized controlled trials, the identification of a suitable experimental approach that could be adopted by different investigators could allow the requisite much larger numbers of volunteers to be studied and might settle the debate if agreement could be reached on a suitable method. It is quite clear to me that the IAAO method could not serve such a purpose.