Effects of Fasting on Muscle Growth

Doctors at the National Institutes of Health investigated blood sugar (glucose) availability affected the growth of muscle stem cells (myoblasts) into new muscle cells. At first, low glucose levels impaired the ability of myoblasts to develop into new muscle cells. But the low glucose level also stimulated AMP-activated protein kinase (AMPK), which in turn stimulated the expression of NAD+ enzyme Nampt. NAD+ is known to affect SIRT1, which has been implicated in the differentiation of skeletal muscle cells, as well as longevity factors.

The report notes that:

Importantly, inhibition of AMPK, Nampt or SIRT1 resulted in skeletal muscle cells that were oblivious to a nutrient poor environment and were able to differentiate under conditions that otherwise would not be suitable.

But conditions under which the body self-inhibits AMPK, Nampt or SIRT1 were not noted, other than in the abundance of glucose availability, so it not clear what conditions prompt muscle development under nutrient poor situations. They do observe, however, that when nutrition is again available, and the glucose level rises again, the muscle-building resumes. Thus it seems that this 'pathway' from AMPK to Nampt to SIRT1 is activated when glucose levels drop, in order to save the body from expending scarce energy resources on muscle-building.

As with other studies of this kind, these results support the suggestion that people wishing to add muscle mass while undertaking a fasting regime, should choose one of the intermittent fasting protocols, and exercise during or just after feeding periods, rather than during fasting times.

From: Skeletal Muscle Development Responds To Nutrient Availability reported in ScienceDaily (May 12, 2008). The researchers include Marcella Fulco, Yana Cen, Po Zhao, Eric P. Hoffman, Michael W. McBurney, Anthony A. Sauve, and Vittorio Sartorelli.