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In all discussions on protein requirements it is
assumed that the needs for energy and micronutrients are met. In the breast-fed infant
this must indeed be the case, and it is therefore appropriate that the definition of the
protein requirements of infants should be based on the 'biological reality' of the
breast-fed baby. Discussion centered initially on some of the components of the factorial
estimates put forward by Dewey in her paper
Efficiency of utilization of nitrogen for maintenance and for growth
Two options were initially proposed for the table of factorial estimates (Table 6): 70% and 90%. It was agreed that 90% is too high, at least for infants over 4 months of age, and that 70% is more appropriate. Higher efficiencies have indeed been found in children who have reached reference weight-for-height after recovering from malnutrition, but this is not a normal situation, as these children may still be depleted of protein (Scrimshaw).
The question was considered, whether the utilization of protein in infant formulae is less good than in breast milk. Although there are inevitably differences in composition, the evidence is that there are no major differences in efficiency: slope of N balance studies with formula is about 0.74 (Reeds). One factor that may affect the utilization of N is the much higher content of NPN, particularly of urea, in breast milk. Dewey had proposed a value of 17% for the utilization of urea-N, derived from the work of Donovan et al (1990) and of Fomon et al (1987) (see also Darling et al (1993)). There are difficulties in the interpretation of these isotopic measurements; several participants (Young, Millward, Waterlow) considered that the figure of 17% for urea, leading to a value of 46% for total NPN, is too low. It was argued (Reeds) that in breastfed children there may be direct benefit from the utilization of urea-N because breast milk tends to be low in non-essential amino acids. Other components of the NPN glutathione, creatine, taurine and nucleotides (mainly pyrimidines) -may be used with 100% efficiency because they spare the metabolism of their precursor amino acids. It was suggested that a range of utilization of total NPN (4661%) be used for the estimates of protein intake of breastfed infants.
Allowance for irregular (saltatory) growth
The arbitrary figure of 50% by which the growth requirement was multiplied in the 1985 report was considered unsatisfactory by most participants. The alternative proposed in the paper is to use the observed CV of weight gain over 3-month intervals in the calculation of 'safe levels', but not in determining mean requirement. The CV over 1-month intervals would have been far too large. However, according to the validation analysis (Section 2.2.8), the CV of weight gain does not have a large influence on the calculated percentage of breastfed infants with 'inadequate' intakes. Moreover, since the physiological concern is with day-to-day variations, a correction derived from these longer intervals is also arbitrary. It may be, however, that there is an automatic adjustment of intake: when the baby needs more milk, the mother is able to produce it (Butte). To translate this in terms of requirement, it would be necessary to know the day-to-day variation in breast milk intake.
The point was also made that, in the 1985 report, growth of the infant was considered entirely in terms of weight gain, and no account was taken of a possible specific requirement for growth in length, through deposition of protein at the ends of the long bones. This requirement may be quantitatively small but qualitatively important, since long bone growth is a major stimulus for skeletal muscle growth (Millward) (see also below, under 'catch-up').
Amino acid requirements of the infant
There is remarkably little information on the amino acid composition of human milk in different populations (Reeds). All the emphasis has been on the amino acids in the NPN fraction. However, since the amino acid composition of most milks is dominated by two proteins, b-casein and a-lactalbumin, there is little room for significant changes (Reeds). Traditionally, it has been assumed that if an infant is growing adequately on a certain amount of breast milk or its equivalent, then the amino acid requirements will be met; but the actual intakes may be much greater than the minimum requirements.
In fact we do not have enough information about the SDs of amino acid intakes or requirements to enable us to arrive at a safe level. Young raised the question of how, for example, the specific value for the lysine requirement had been calculated. Reeds explained that there is no problem about the growth component, which is based on the lysine content of tissue protein. The protein requirement for maintenance can be calculated in various ways, but the results are not very different. Two assumptions were then made: first, that the total requirement for indispensable amino acids (IAAs) is 30% of the maintenance protein requirement; secondly, that the pattern of IAA requirements is the MIT pattern derived by Young and coworkers (see JCW's paper). The results of these calculations, which assume 100% availability, are shown in Tables 16-18. It is important to emphasize that these are not tables of requirements. The object of the exercise is to show that, with normal milk intake, regardless of the method of calculation, the IAA content of the milk more than covers requirements, even with a fairly large standard deviation. It is interesting also that the values in Table 18 are quite similar to those obtained by Torun for the safe levels of IAA intakes by pre-school children, using a completely different approach.
The values in the tables are compared with the amino acids in 800 ml of breast milk. The point was raised that this figure may over-estimate the intakes in developing countries, but both Prentice and Dewey have found that they are very similar to intakes in industrialized countries.
Older infants and children; the factorial vs operational (P/E) approach
It was agreed that the factorial approach for estimating the needs of older children is satisfactory. It is logical to use the adult figure of 100 mg N/kg/d for maintenance; both recent and earlier studies on children give similar results. However, it should not be implied that maintenance in a child is physiologically the same as in an adult (Young). It may be quite artificial to separate the maintenance and growth requirements.
Waterlow suggested that an alternative to the factorial approach is to look at the P/E ratios in the diets of children who are no longer exclusively breast-fed, starting from the assumption that the P/E ratio in breast milk represents a safe level (Section 2.4). This is quite different from the approach in the 1985 report, based on the ratio of requirements for protein and energy. The P/E ratio in breast milk is about eight; as growth falls off the safe level of P/E will decrease to about six. In Whitehead's studies in The Gambia and Uganda, P/E ratios were measured in the diets of individual children (Whitehead et al, 1977). In The Gambia, where diets are cereal-based, the ratios were very similar in the diets of all children, with an average of about eight. In Uganda there was a much wider scatter, and about 10% of children had diets with a ratio of five or less, which indicated a risk of protein deficiency. It is interesting that, at least at that time, kwashiorkor was commoner in Uganda than in The Gambia.
This approach does not require any assumptions about the extent to which protein and energy requirements are correlated. It was agreed that the P/E ratio provides a good way of looking at the adequacy of intakes, but it cannot be used to define requirements. Since, after infancy, protein requirements per kg are more or less fixed, while energy requirements per kg fall, the safe P/E ratio for adults is higher than that for children. It was pointed out that there are very few family diets around the world which provide less than 10% protein calories (Scrimshaw). However, although this may be so for adults, it is not necessarily so for children (Waterlow).
Protein digestibility and quality
When breast milk is no longer the sole source of protein, the digestibility and quality of the food protein become important. In adults differences in the amounts of protein required in the diets of different populations could be entirely accounted for by differences in digestibility rather than in amino acid patterns (Rand et al, 1984). It was agreed that more information is needed on digestibility in humans, and that it is not enough to rely on conventional rat assays. These studies should be made with different diets, not just types of protein. Since there is no evidence of any difference in digestibility between adults and children, it would be easier to do these studies in adults. More attention should be paid to variations in digestibility, which may be related as much to the subject as to the food, since there is evidence that in populations accustomed to a particular diet its digestibility may improve (Scrimshaw).
Measurements of faecal N (apparent digestibility) may be satisfactory for estimating total protein requirement by nitrogen balance, but they are not appropriate for determining the availability of individual amino acids. A distinction has to be made between endogenous and exogenous faecal N. since studies with 15N have shown that at least 50% of faecal N is endogenous, with a different amino acid composition from that of the food (Gannon et al, Shulman et al, de Lange et al).
More studies are also needed on the protein quality of children's diets by balance measurements, to check estimates obtained from amino acid scores. Previous studies have been done with isolated proteins; they should now be carried out with whole foods at a low level of N intake (200 mg/kg/d) to maximize differences in quality (Waterlow).
Catch-up growth and infection
Dewey, introducing this subject, recalled that in the 1985 report it was calculated that an extra intake of 0.23 g protein was needed per gram tissue gain, based on a tissue protein content of 16% and 70% efficiency of deposition. This recommendation would apply to a stunted child; a wasted child would need to deposit more fat and therefore theoretically would need less extra protein and more energy per gram gained, leading to a different P/E ratio of the food (Table 21). In practice, however, it might be unrealistic to make such a distinction, particularly since wasting and stunting are often combined.
The main difference between the two conditions lies in the rate of weight gain, which strongly affects the required P/E ratio. Wasted children can achieve very large rates of gain, and even under field conditions may gain at several times the normal rate. The required P/E is very sensitive to the rate of weight gain (Table 21). Therefore in a wasted child the lower ratio resulting from a higher fat content of the tissue gained would be compensated by the higher ratio needed for a more rapid rate of gain. It was also pointed out (Reeds) that in early catch-up there is a preponderance of protein in the tissue gained and in the later stages a preponderance of fat. However, it would be too complicated to make allowance for this in field situations.
For catch-up in stunted children the aim is to achieve and maintain normal body composition; the rate of weight gain is then limited by the relatively smaller increase that can be achieved in the rate of linear growth. For a child of 2 years to recover 3 s.d. units of height in 6 months would be an achievable target. This would require an increase of less than 5% in energy intake but a 37% increase in protein intake (Table 22). To give more energy would only result in obesity. Studies at INCAP have shown that children getting more protein during rehabilitation showed greater growth in length, but it is not clear whether this was related to the protein or to some associated factor (Town).
It is possible that stunting results from lack of a specific amino acid (see Waterlow & Schürch 1994), so that protein quality would be important as well as quantity. There is a need to investigate the relative efficiency of different sources of protein in promoting catch-up. Lipid metabolism may also be altered by an increased intake of animal protein, so the observed effects could be combined ones, not due to protein alone. Millward pointed out that in rat experiments intakes of protein greater than those needed for maximal weight gain can stimulate long bone growth. This means that amounts of protein should be recommended even greater than those needed to exert regulatory influences in terms of the Millward-Rivers model. However, he does not agree that stunting is primarily caused by protein deficiency; more probably it results from a combination of infection and micro-nutrient deficiencies, or all three (Scrimshaw).
A point that should not be overlooked in any discussion of catch-up is the extra needs of low birth weight babies, who may gain 20 g/kg/d when appropriately fed (see Section 2.2.3).
Rehabilitation after infection is a particular example of catch-up. Unfortunately, there are no good measurements of N losses during infection in children. They are likely to be particularly high during gastroenteritis. Studies in adults have shown an average loss of 0.6 g protein kg/d. If the recovery period is two to three times as long as the depletion period (Scrimshaw), it would mean that a 3050% increase in the daily protein intake would be necessary. Calculations based on data from The Gambia and Guatemala on the average time infected and the weight loss per day infected lead to similar results. Field data from Bangladesh showed that on average it took a child about 6 weeks to regain weight after an episode of diarrhoea. In Mexico it appeared that in children aged 7 years only a small proportion of their deficit in growth could be attributed to infection. The conclusion was that catch-up from the effects of infection occurred when the food supply was adequate (Waterlow). The various studies referred to above are summarized in Waterlow (1992).
A further question that needs investigation is how far the nitrogen loss during infection involves losses of specific amino acids. Reed's studies on the composition of the acute-phase proteins suggest that this could well be the case.
Summary of issues to be resolved (as presented by Dewey):
(1) Is the breast-fed infant a suitable model?
(2) What value should be given for the utilization of NPN in human milk ?
(3) Do requirements differ for formula-fed infants?
(4) What should be taken as the requirement for maintenance? 90 mg N/kg/d in infants and 100 in older children ?
(5) What value should be used for the efficiency of utilization of protein; 70%?
(6) How should we take into account intraindividual variability of growth?
(7) How should inter-individual variability in growth be calculated and over what time interval?
(8) Should the operational approach (P/E) be used to estimate safe levels rather than factorial + 2 s.d.
(9) Should amino acid requirements for infants still be based on the composition of human milk? They may be lower.
(10) Are data on the amino acid requirements of healthy children sufficient? Probably not.
(11) How should needs for protein and amino acids after infection be calculated ?
(12) Are estimates needed for the combination of catch-up growth + recovery from infection?
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