Low-Carb Diet & Protein Turnover

In this month's Journal of Clinical Endocrinology & Metabolism, researchers from the University of Michigan and Veterans Affairs Medical Center report their findings in Effects of Dietary Carbohydrate Restriction with High Protein Intake on Protein Metabolism and the Somatotropic Axis.

In this particular study, healthy individuals were recruited to participate in a controlled environment study that required them to remain hospitalized during the study. At the start of the study, participants were fed a standard western diet of 60% carbohydrate, 10% protein and 30% fat for two days. The diet was then modified to provide 5% carbohydrate, 35% protein and 60% fat for seven days to assess and measure the skeletal muscle fractional synthetic rate, whole-body proteolysis, and plasma concentrations of insulin, GH, and IGF-I.

The results surprised the researchers, who like many of their colleagues, believed that low carbohydrate diets cause muscle loss. Never mind that study after study has shown that carbohydrate restriction with adequate protein intake has a muscle sparing effect - old notions die hard!

In this study, researchers measured muscle breakdown, called proteolysis, as the rate of appearance of the amino acid leucine into the plasma. This is a much more sophisticated approach than simply measuring nitrogen balance, which has been measured in previous studies. Here protein synthesis was measured by the incorporation of labeled leucine into muscle tissue through a muscle tissue biopsy.

The data showed that during the low-carb diet, protein was being broken down and synthesized at the same time. The researchers concluded that the increased protein in the diet led to the increased muscle synthesis, while the lowered insulin caused an increased protein breakdown.

The major finding of the present study was that muscle protein synthesis increased despite strict carbohydrate restriction and a marked reduction in the daily exposure to insulin. This increase in protein synthesis was accompanied by an increase in whole-body proteolysis.

Problem is, the researchers didn't bother to measure the net effect and only showed that both mechanisms were simultaneously in action in the body. Without this critical data, the researchers position that the sum of the two determines whether one is in overall muscle building or muscle wasting mode is unfounded due to lack of supporting data. Now, granted, that does sound logical - but don't you think it would be better if it were based on actual data measures? I do!

Based on other research data available, we know that the body builds and repairs muscle while also breaking down protein to clear cellular debris, much of which is protein. This study shows that these dual mechanisms in the body were at work while participants followed the low-carb diet. From a metabolic point-of-view this is a good thing - it showed that the body is not exclusively catabolizing - or breaking down - muscle, but is also busy building new muscle.

As I already noted, the researchers did not measure the net effect, which is too bad since previous data suggests low-carb approaches actually have a muscle-sparing net effect on the body. Perhaps in the next study this critical data will be gathered to allow for a greater understanding of the net effect low-carb approaches have on lean body mass?