Pregnancy and the first two years of life are periods of rapid growth and yet the knowledge of requirements for protein and dietary indispensable amino acids is very limited. The development of carbon oxidation methods opens the way to studies that should fill these important gaps in knowledge.
Currently protein and amino-acid requirements in vulnerable periods, such as pregnancy and during the first two years of life, are poorly understood. The growth of infants who are solely breast fed during the first 6 months of life and their ingestion of protein, total nitrogen and dietary indispensable amino acids provide a robust estimate of requirements. Conversely, the data during pregnancy and from 6 to 24 months are very limited.1 The classical method of nitrogen balance is unsuitable, as it means restricting protein or a dietary indispensable amino acid for ⩾7 days to permit adequate adaptation to the experimental diet.1, 2 It also lacks precision, as balance is derived from the subtraction of two large numbers, and is systematically overestimated.1, 2
Various carbon oxidation methods have been developed,2, 3 the most accurate of which is the 7-day-adapted, 24-h-indicator amino-acid balance method,4 but the most convenient of which is the minimally invasive indicator amino-acid oxidation (IAAO) technique, which requires adaptation for only a matter of hours.5 This allows the application of more complex approaches in children.6 Using this approach, a subject only needs to ingest a limited intake of protein or amino acid for a single day to study a particular intake level. Ideally a subject is studied over six or more intake levels from below to well above the predicted intake level, which permits accurate determination of the population mean requirement as well as the variation around that mean.
Based on a reanalysis of the nitrogen balance literature or a theoretical model of obligatory amino-acid loss, it has been suggested that adult protein and amino-acid requirements had been significantly underestimated by the N balance method and its assumptions.7, 8 It was reassuring that these higher recalculated estimates were supported by the IAAO, which is a totally independent method.5, 9 Further support for a higher protein requirement came from a biochemical functional method, which showed that while it is possible for subjects to adapt to a lower protein intake of 0.75 g/kg/day in terms of their nitrogen balance, nevertheless their ability to synthesize the major antioxidant glutathione was impaired.9 More recently IAAO has been applied to the determination of protein requirements in 6–8-year-old children and suggests that they too have been significantly underestimated.10 The observation that the protein needs of school-aged children have been underestimated is important in light of widespread stunting in the Developing World, where on the one hand poor sanitation and subclinical infections can impose their additional demands on requirements, while on the other hand adaptation to lower-than-normal intakes could exist. Clearly, further studies with simple non-invasive methods are required.
The current requirements of protein during pregnancy are much higher than thought earlier.1 The recommended additional safe level of protein intakes during pregnancy of 0.7, 9.6 and 31.2 g/day at mid-first, second and third trimester was based on mean protein deposition estimated from total body potassium measurements in normal healthy pregnant women gaining an average of 13.8 kg during pregnancy. An efficiency of utilization of 42% was assumed (this is an area for further research, as this may be higher in developing countries), a maintenance cost of 0.66 g/kg/day for weight gain and a CV of 12%. For a 50-kg woman with a habitual intake of 1 g/kg/day who gains 13.8 kg, this translates to an intake of 91 g/day or 1.42 g/kg/day, which is remarkably close to the 1.4 g/kg/day observed in adolescent mothers in the Montreal Diet Dispensary study.11 However, using the weekly increase in weight and protein composition of a 50th centile fetus in the third trimester,12 one can calculate a rate of protein accretion requiring an increase in fetal amino-acid requirements of 10 g/day. Based on this requirement, a maternal maintenance requirement of 6.6 g (assuming 11 kg gain at mid-third trimester), an efficiency of utilization of 42% and a CV of 12%, our estimation of extra protein needs at mid-trimester 3 is 44.2 g/day, which is 42% higher than what is recommended.1 Equally, while adaptation improving the efficiency of utilization and lowering the requirement may occur, this safe-level intake may be even higher in more challenging environments in the developing world. For example, we have recently observed that Indian women living in Bangalore had about half the gut absorptive capacity compared with American and Jamaican women (unpublished data). Hence, there is a need to conduct studies in different populations in their peculiar environment, and recommend that protein/amino-acid intakes during pregnancy be re-visited using non-invasive stable isotope techniques to re-estimate requirements in women living in their unique environment.
Nitrogen balance and carbon oxidation techniques are short-term studies, which may or may not predict function and health. Pregnancy offers a functional outcome, namely birth weight, low birth weight rates and perinatal mortality.13 In infancy inadequate protein, based on animal studies, would be expected to affect growth of lean mass and length/height, and in adults functional studies such as those measuring glutathione homeostasis are relevant.9 Even so, there are currently limited short-term data on protein and amino-acid requirements during the important 1000 days. Equally, the intake of too much protein in early life appears to predispose to an adverse body composition at 2 years of age.14 It was therefore the view of the authors that the non-invasive stable isotope methods including the minimally invasive IAAO model should be applied to strengthen the current sparse data,1 after which functional studies should be considered.
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The authors declare no conflict of interest.
This commentary is based on discussions during a Consultants’ Meeting at the International Atomic Energy Agency, Vienna, 12–14 November 2013.
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Pencharz, P., Jahoor, F., Kurpad, A. et al. Current issues in determining dietary protein and amino-acid requirements. Eur J Clin Nutr 68, 285–286 (2014). https://doi.org/10.1038/ejcn.2013.297
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