If you are aiming to burn fat and build muscle, try short-term fasting. Not only has fasting been shown to improve make us live longer, it also appears to be of great value for athletes. 

When you fast, your body responds by secreting several key hormones. In particular, testosterone and growth hormone (GH) trigger fat burning and building of new muscle tissue. Testosterone also helps improve energy levels, mood and libido among other things. Exercising when fasting has shown to increase levels of these hormones at a magnified level. 

Working out on an empty stomach also has notable effects on our hearts and blood cholesterol levels. Because both fasting and working out cause stress, we release more cholesterol instead of glucose, meaning our bodies draw upon fat as a fuel source. The fewer fat cells our bodies contain, the less likely we will find ourselves losing sensitivity to insulin. Insulin is the key hormone that supports our ability to absorb the nutrients from the food we do eat and directs sugars into cellular storage for future energy. When we eat too much too often, we become can resistant to the effects of this hormone and losing body fat is much more difficult.