The thyroid hormone helps to regulate the amount of calories your body burns daily. It also impacts muscle growth by either speeding up or slowing down protein synthesis. Dieting will cause your thyroid hormone levels to decrease which slows down the calorie-burning process. In essence, your body will become less efficient at burning bodyfat as your metabolic rate decreases due to the reduction in caloric intake.
Testosterone declines when caloric intake decreases. Muscle growth is negatively affected when testosterone levels drop. If you want to hold on to your muscle mass, this is the worst situation to be in. In addition, the fat burning process is also reduced when testosterone levels decrease.
The immune system becomes impaired when over-dieting occurs. A weakened immune system will negatively affect your body's ability to hold on to muscle mass. Cortisol levels will increase due to the stress of a weakened immune system, causing catabolization or the tearing down of muscle tissue. Insulin resistance increases under these circumstances, increasing the likelihood that bodyfat will be stored rather than burned.
Leptin, a hormone which helps to regulate your body weight, declines when dieting occurs. The result: a craving for more food and therefore increased caloric intake and a decrease in metabolic rate, making it more difficult to shed bodyfat.
Instead of extreme dieting, consider the following instead:
- Reduce carbs gradually by no more than 30% of your caloric intake
- Continue to eat simple sugars to help regulate insulin levels
- Eat less carbs on non-training days
- Restrict cardio as too much can negatively affect hormone levels
- Have a cheat meal once per week
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."