Question – Will HIIT Cause You to Lose Gains?
Answer – The short answer is yes if you mean size. No, if you mean strength or endurance or health.
Muscle size includes the stored glycogen. Glycogen is the stored form of sugar, and its presence holds water, too. If your gains are about sheer size of muscle, then you want to eat plenty of good carbs along with protein, drink lots of water, and get lots of sleep to build and keep muscle from being utilized for fuel. You want to do high-volume work outs and rest adequately between sets to recover your slow twitch fibers while burning out your fast twitch by the end of the work out.
If your gains are about strength or endurance or general metabolic health then the answer is no. HIIT will strengthen, condition, and lean you out because it burns up your glycogen stores. In fact draining your glycogen stores is the best part of HIIT.
HIIT is all about incomplete recovery. You DON’T want to rest long between exercises but recover partially and then hit the next work interval hard. By interval 5 or 6 your heart rate should be 90%. By the time you’re done, if you truly did high intensity, you will have emptied your muscles and your liver of their glycogen stores. That means your body will have to rebuild those stores for the next 12–24 hours. This is called EPOC (excess post-exercise oxygen consumption) which means your body is now burning fat (because you used all your stored glucose).
If you’re a body builder, HIIT can be a problem. For any other sport or general health, it’s a great value, not only due to up-regulating enzymes that burn fat but building more mitochondria and conditioning you to function in an anaerobic environment (at a high heart rate).