Another misconception about HIIT is that it will cause muscle loss. This is just not true either. This myth got started because a higher amount of calories burned during HIIT will come from stored amino acids (muscle tissue) when compared to lower intensity cardio. As long as HIIT sessions are kept to a short duration muscle loss not be a problem. In fact, muscle growth and muscle retention are increased due to the effects HIIT has on anabolic hormones. Just one 10-15 minute session of HIIT can increase testosterone and growth hormone levels for hours after the workout has ended.
This may explain why the fat-burning effects of eating more protein were confirmed in a study published in the American Journal of Physiology. One group was fed a high-protein diet (just over 1 gram per pound of body weight per day) while the second group consumed an amount closer to the lower recommendation of the RDA (recommended dietary allowance). The group eating the higher-protein diet burned the most fat.