When dieting, loss of fat is a given. However muscle mass loss can and usually does occur.
A study lead by John Hawley and Jose Areta showed that dieting reduces MPS (muscle protein synthesis).
Weight training prevented this decrease and supplementing 15-30g of protein prevented this rate dropping even further.
The study claims to show that training hard and supplementing with protein may prevent muscle breakdown during dieting.
Taken from American Journal of Physiology Endocrinology Metabolism, 2014.
