Protein Metabolism During Infancy, edited by Niels C. R. Raiha. Nestle Nutrition Workshop Series. Vol. 33. Nestec Ltd., Vevey/ Raven Press. Ltd.. New York © 1994. International Recommendations on Protein Intakes in Infancy: Some Points for Discussion Brian A. Wharton Old Rectory, Belbroughton, Worcestershire, DY9 9TF, United Kingdom BACKGROUND Populations The definitive international report on protein requirements is the one published by the WHO in 1985 (1). It forms the basis of many other recommendations, including the American ones published in 1988 (2) and the recent British Dietary Reference Values (3). These reports also include energy requirements, which are obviously closely related to protein requirements. Foods In addition, there are other recommendations and in some cases legislation con- cerning the composition of infant formula, follow-on milk, and weaning foods. Since these foods are consumed by infants to meet their nutritional requirements, the re- ports are in effect recommendations concerning the infants' nutritional intake, or at least part of it. A few reports are "international" (e.g., Codex Alimentarius of WHO/ FAO, reports of the European Society of Pediatric Gastroenterology and Nutrition, and reports of the Committee on Food of the European Community). There is a plethora of national reports concerning infant foods. References will be given to individual reports at relevant points in the paper. There is little point in doing a line-by-line comparison of all these reports, whether for populations or foods. Instead, 1 shall discuss five areas where there is room for debate. NUMERATOR For numerical recommendations a numerator and a denominator are involved (e.g., protein per person per day). In this section the numerator is considered. 67 68 RECOMMENDATIONS ON PROTEIN INTAKES Multiples of Nitrogen The major problem with the numerator is its expression as weight or as a weight of nitrogen contained within the food. The use of nitrogen arises from the chemical analysis of foods, but it extends into other nutritional considerations when assessing balance data (e.g., amount of nitrogen in feces). In parenteral and enteral nutrition it has become the custom to express protein, peptides, and amino acids as nitrogen. The conversion of weight of nitrogen to weight of protein uses a conventional multiple of 6.25. For milk the more appropriate factor 6.38 is often used. For weaning foods containing cereal protein only, the appropriate figure is 5.7-6.0, depending on the cereal. In practice, the content of protein stated on a label of a weaning food is likely to be calculated from food composition tables rather than from direct chemical analyses, and these tables will have used multiples of analyzed nitrogen appropriate for the source of protein. Non-protein Nitrogen The multiple of nitrogen method does, of course, count non-protein nitrogen as protein and where this is amino nitrogen it is likely to be available for synthesis of protein by the baby. The problem of how to account for the non-protein nitrogen of breast milk is well known. In the British national "pooled sample" of breast milk (4), total nitrogen was 210 mg per 100 ml, of which non-protein non-amino-acid nitro- gen was 46 mg. If total nitrogen found in the milk is multiplied by 6.38, a figure of 1.3 g of protein per 100 ml is obtained, but when non-protein non-amino-acid nitrogen is excluded, the figure is 1.05 g. It is not clear, however, that non-protein nitrogen compounds, particularly urea, should be excluded. Urea reaching the colon either directly from the food or via diffusion from the blood is hydrolyzed by the gut flora releasing ammonia, which travels via the portal vein to the liver for production of non-essential amino acids. The concept of urea cycling in protein economy, particularly during rapid catch-up growth, has been developed by Jackson (5). We should beware of assuming that non- protein nitrogen is of no nutritional significance. To regard the protein and amino acid nitrogen content of breast milk as true protein and to use this as a basis for determining protein requirements may lead to an underestimate because it ignores urea. Heine has explored the bioavailability of urea nitrogen in breast milk (6). Further caution is necessary if we are tempted to deduct from this "true protein" the immunological proteins such as lactoferrin and secretory IgA (on the basis that they are not absorbed and are not nutritionally available), to give a final nutritionally available protein content in breast milk that is perhaps as low as 0.8 g per 100 ml. Two studies have found that relatively small amounts of the secretory IgA and lactoferrin ingested in breast milk appear in the feces of infants a few weeks old (7,8). This suggests that large amounts of these protective proteins are broken down and so become nutritionally available higher in the gut. Our own observations were that RECOMMENDATIONS ON PROTEIN INTAKES 69 only 1% of estimated lactoferrin intake appeared in the feces of breast-fed babies aged 2 weeks; the figure for bovine lactoferrin was around 0.5% (9). In summary, neither non-protein nitrogen nor the "immunological proteins" should be discarded when assessing the amount of protein available to and required by an infant. DENOMINATOR Individuals and Their Weight Recommendations on protein intake in children, which are part of national or international recommendations for all ages, use either an individual or his weight as the denominator; that is, the "safe level," "advisable intake," or "reference nutrient level" is expressed as grams per person at a particular age or as grams per kilogram of body weight. The WHO figures from which the others follow are based on the factorial method, or at least a modification of it, since the determination of obligatory nitrogen loss on very low protein diets is not relevant for a rapidly growing infant. A typical calculation for 0-4 months quoted in the WHO report (1) is an increase in tissue protein of 3.5 g per day plus the equivalent of 0.5 g each day lost from desquamating skin and sweat. At this age average retention is 45% of intake. Therefore, an intake of 8.9 g is necessary to retain 4 g; that is, for an average weight at this age of 5.25 kg, an intake of 1.7 g/kg is needed. Some factor may then be applied to allow for safety to give an advisable or safe intake of around 1.9 g/kg. Volume, Weight, and Energy of Food In recommendations for food, values expressed as grams per infant or grams per kilogram of infant clearly will not do. For infant formulas the choices of denominator are weight of powder, volume of feed, or energy content of the feed. Few people will think in terms of the weight of powder in a formula (about 13 g per 100 ml in most), and the volume fed is the obvious measurement. Furthermore, if nutrients are expressed per unit volume, this stresses the role of water as an essential nutrient, which is vital when considering the renal solute load of a diet and its effect on the concentration of urine and on water balance. This type of reasoning led to British recommendations on the composition of infant formulas being expressed per unit volume (10). However, volume is not a suitable denominator for a weaning food; the consistency of a cereal and sugar gruel, whether homemade or provided by a manufacturer, could vary considerably. In addition, as weaning progresses renal function, including concentrating ability, increases, so the renal solute load of a diet becomes less impor- tant unless there are very abnormal losses of water from lungs, skin, or gut. Of the possible denominators, therefore, only energy can be applied to food for both suckling 70 RECOMMENDATIONS ON PROTEIN INTAKES and weanling infants. Apart from this pragmatic reason there are also physiological reasons for the choice of energy. It recognizes the interrelationships between the requirements of protein and energy and the relatively large proportion of both energy and protein used for growth in the rapidly growing infant. Many international reports therefore express protein per unit energy, both for infant formulas [ESPGAN (11), Codex Alimentarius (12), European Community (13)] and for weaning foods (14). Protein/Energy Ratio in Physiological States Since there are physiological as well as pragmatic reasons for expressing protein content of foods in terms of energy, should this concept be extended to the recommen- dations for individuals? In other words, should the reference, safe, or advisable intakes of protein by groups of people be expressed not per person or per kilogram of body weight, but per 100 kcal of food ingested? Fomon (15) has argued in favor of this method of calculation and we have also used it as a way of expressing advisable intakes of many nutrients for children of various ages (see Table 1). Where recommendations for protein and energy are expressed separately, then calculation of the ratio from these figures is, in theory at least, invalid since the protein figure is chosen to meet the requirements of 95% of the population (that is, more than most people need) while the energy figure is the observed average intake of the population (that is, less than what 50% of the population consumes). The mathematics are complex since the ratio involves two factors, each with its own variance. The concept and statistical manipulations are discussed in some detail by Beaton & Swiss (17). Table 2 shows the protein/energy ratios calculated in this way for infants. The ratios are all above the ratio found in breast milk.