Tracing metabolic flux to assess optimal dietary protein and amino acid consumption

There is a general consensus that a dietary protein intake of 0.8 g protein/kg/day will prevent symptoms of protein deficiency in young, healthy individuals. However, individuals in many physiological circumstances may benefit from higher rates of dietary protein intake. Stable isotope tracer methodology enables a variety of approaches to assessing the optimal dietary protein intake in humans. In this paper, we present an overview of a variety of tracer methods, with a discussion of necessary assumptions, as well as the clinical circumstances in which different methods may be preferable. Although we discuss the nontracer method of nitrogen balance, which has historically been used to estimate dietary protein requirements, this paper primarily focuses on tracer methods for estimating dietary protein and essential amino acid requirements under different physiological conditions. We will explain the following approaches: isotopic measurement of urea production; the arterial-venous tracer balance method; measurement of the fractional synthetic and breakdown rates of muscle protein; the indicator and the direct amino acid oxidation methods; and different approaches to measuring whole-body protein synthesis and breakdown. The advantages and limitations of each method are discussed in the context of the optimal approaches for use under different circumstances. Nutrition: Optimising monitoring of dietary protein intake Both clinical and practical considerations should inform how dietary protein intake is tracked in individuals. Little is known about optimal dietary protein intake in humans under different physiological conditions including disease states, prompting a review of current monitoring methods by Robert Wolfe at the University of Arkansas, Little Rock, USA, and co-workers. Monitoring nitrogen balance provides estimates of dietary protein and essential amino acid (EAA) levels, but requires several days of strict diet control. Various techniques monitor the uptake or fate of a specific tracer molecule in the body, but some are very invasive. The less invasive single EAA infusion methods provide rapid results and are valuable for monitoring immediate response to a meal, or in studies comparing protein intakes to a person’s baseline over time. Using two methods concurrently could improve accuracy of results.