Controversy About Dietary Protein Requirements and "Nitrogen Balance"

These are some articles that discuss flaws in the reasoning and methodologies that go into estimates of "nitrogen balance" and protein requirements for different groups of people [Kurpad and Young, 2003: (http://jn.nutrition.org/cgi/content/full/133/4/1227) (http://www.ncbi.nlm.nih.gov/pubmed/12672948?dopt=Abstract); Fuller and Garlick, 1994: (http://www.ncbi.nlm.nih.gov/pubmed/7946519); Millward, 1998: (http://jn.nutrition.org/cgi/content/full/128/12/2563S) (http://www.ncbi.nlm.nih.gov/pubmed/9868206?dopt=Abstract)]. The articles by Millward (1998) and Fuller and Garlick (1994) are pretty devastating critiques of the experimental methods that go into measuring amino acid metabolism and even of the concept of nitrogen balance. The concept of "nitrogen balance" has never seemed very useful to me, and these articles highlight a lot of the flaws with the concept.

I think one has to look at these requirements or "recommendations" for protein intake and ask oneself if the numbers seem reasonable. It just doesn't make sense to me that a protein intake of 0.6 g/kg/d, a number that one commonly sees and that these articles discuss, would be adequate. How could it be? That's 42 grams of protein for a 70-kg person (154 lbs). That seems just bizarre to me. A lot of these articles on protein are really difficult to follow, but the concept of nitrogen balance has always seemed to me to be like something related to animal feed that's left over from the 1940s. Maybe that's too harsh an assessment, but the concept seems barely scientific to me. If I invented a concept such as "CO2 balance," based on some variation of the measurement of the volume of CO2 expired (a procedure that is not called "CO2 balance" but that is used in some research looking at overall metabolic rates, in closed, experimental "chambers," for example), that wouldn't allow me to draw sweeping conclusions about the details of intracellular metabolism. But the concept of nitrogen balance attempts to do that, to some extent, and there are some significant issues with the concept.

It's become clear that some types of physical exercise do increase protein requirements, but the changes in the requirements vary with the intensity and duration and type of exercise. One major reason that protein requirements are higher for athletes is that branched-chain amino acids are oxidized for fuel in the skeletal muscles. There will probably continue to be a lot of controversy about the question of protein "requirements," but I've learned that the assumptions that go into a lot of these rigid rules, about nutrient requirements, can be seriously flawed. There's evidence that low dietary protein intakes reduce the mitochondrial DNA contents in cells in the liver and muscles, contribute to anemia in elderly people, in particular, etc. There's also research showing that high-protein diets increase the vitamin B6 requirement substantially, and I'm sure there's lots of similar research. The branched-chain alpha-keto acid dehydrogenase complex, which is the major multienzyme complex that regulates branched-chain amino acid oxidation during exercise, is, incidentally, a thiamine-derived-cofactor-dependent enzyme complex (the overall activity of the enzyme complex is vitamin B1-dependent).