The myth about burning muscle.
Because this topic has come up many times, I decided to do some research on it and get the scientific answer.
The order in which your body uses fuel is: glucose --> glycogen --> fat --> protein. Your body uses glucose and glycogen as the main energy sources. You use fat as the next energy source. The body rarely, if ever, breaks down protein and uses it for energy.
As far as “burning muscle”, I found the below. This is highly simplified, but I found many references that support it. This is what happens if you’re fasting; if you are NOT eating at all.
Basically, for energy the body uses glucose as its primary fuel. It gets glucose from breaking down carbohydrates. Any glucose not used immediately is stored in the liver as glycogen. Your glycogen reserves last about 6 hours.
After you run out of glycogen, your body starts breaking down fats for energy. It will do this for about 2-3 days.
Starting around the 4th day of fasting, your body begins to synthesize ketone bodies from fatty acid breakdown. This primarily fuels the brain and not the body (about 70%).
DEPENDING ON HOW MUCH STORED FAT YOU HAVE (your body fat percentage), once you’ve been fasting for approximately 5 days to 3 weeks, and the ketone bodies are depleted, your body starts to break down protein, which is converted into glucose by the liver. This protein will come from your muscles and your body organs.
Keep in mind, the above is if you’re NOT eating at all. If you have a good nutrition plan, even if it’s a calorie-deficit one, you’re not going to “burn” muscle.
EDITED TO ADD IN THIS MORE SCIENTIFIC EXPLANATION BY MEDX:Your body doesn't really store protein, it uses it to build body tissues. There is a huge amount of misinformation about "burning muscle/protein" as a fuel source. The truth according to current physiology and biochemistry research is that the body spares and recycles protein whenever possible.
As part of its normal regeneration process, your body is constantly breaking down damaged cells into amino acids which are then reutilized by the creation of new peptide bonds to make new proteins for construction of new tissue. This cell turnover is very necessary to maintain healthy tissues. The slowing of this process is one of the components that causes physiological decline associated with aging.
When carbohydrate reserves are exhausted, the liver can convert proteins into fatty acids, ketone bodies, or glucose. Before amino acids (the building blocks of protein) can be catabolized, they must be converted into substances that can enter the Krebs cycle for oxydation or converted to glucose/glycogen. The process of converting amino acids into carbohydrate (glucose) is called gluconeogenesis. It is actually rather difficult for the body to convert protein to carbohydrate in order to burn it. It only happens in significant quantities in two situations, during extreme high energy demand when glycogen and blood glucose are exhausted but before fat stores (lipids) can be converted into glyceraldehyde and then to glucose, or, in periods of starvation when both carbohydrate and lipid stores are exhausted.* The first proteins to be utilized are the free proteins and amino acids in the blood. The body will not cannibalize (notice the distiction from catabolization) healthy muscle or vital organ tissue exept in extreme circumstances.
Another cause of muscle loss is atrophy due to disuse, such as is seen when muscles are immobilized in a cast for an extended period. Some illnesses can also cause muscle wasting. Overtraining without adequate recovery also interferes with muscle growth and repair.
So, as long as you have decent nutrition with normal glycogen and glucose stores, have some non-essential bodyfat (above 5% or so in adult males) and adequate dietary protein, you will not "burn" significant amounts of muscle tissue as the result of resistance training or aerobic exercise. You still need to allow adequate recovery time based on your own physiology to avoid overtraining.
*Source: Advanced Principles of Anatomy and Physiology by Tortora and Grabowski, Harper Collins Press
