If you have been hanging out at the gym long enough, you have probably heard contradictory opinions on the proper timing of aerobic exercise relative to strength training. One person says that aerobic exercise should happen before weight training in order to warm up the muscles. Another says the benefits of aerobic exercise are best experienced after weight training. Still another says to separate aerobic days from strength days altogether. Which is correct? If burning fat is your goal, the last choice - doing strength training and aerobic training on different days - is the most effective.

Furthermore, aerobic exercise is most effective at burning fat if undertaken first thing in the morning on an empty stomach. If your main goal is cardiovascular health, however, and weight loss is a secondary consideration, then aerobic exercise is most beneficial after strength training.

Let's examine how the body uses and stores energy to understand why this is the case. Let us say that you eat dinner at 7:00 P.M. and don't eat anything else that night. Your body will convert the carbohydrates that you ate into glucose (we will ignore protein and fat for the moment). Your body will use the newly created glucose for its most immediate energy requirements. Any extra glucose is converted by the liver to glycogen (about a pound) as an energy storage unit. Additionally, your muscles will absorb glucose and store it as glycogen - but only in the presence of an insulin molecule that has attached temporarily to the muscle cell. The pancreas releases insulin when glucose is detected circulating in the bloodstream. The liver then takes any additional glucose that has not been converted to glycogen and converts it into a triglyceride, storing it as fat in the adipose tissues (tissues that store fat - fat cells) of the body.

The protein components of your meal are split into amino acid subunits and absorbed into the bloodstream. Cells in your body use amino acids to build other proteins. However, if you eat more protein than the body needs, the excess will be broken down into other compounds and either used as energy if needed or stored as fat.

Finally, the fat portion of your meal is used as energy if needed. Cells absorb circulating fat and convert the fatty acids to fragments that provide the body with energy. Of course, any circulating fat not required by your cells for immediate energy are stored as body fat in the adipose tissues.

Now, let's assume that you ate just the right amount of food, and that nothing was stored as fat. Nevertheless, glycogen was stored in the liver and in the muscles. Approximately three hours after your meal, the glucose level in your bloodstream drops to a negligible amount. Your pancreas detects this and releases a hormone called glucagon, signaling to your liver to begin converting some of its stored glycogen back to glucose for the body to use. Throughout the night, the glucose converted from the liver-stored glycogen becomes your body's main source of energy. When insulin levels in the blood are low, the adipose tissues release some of their fat to be used as energy.

When you awake the next morning, your body has essentially depleted its glycogen stores. If you do not eat anything, your fat cells will begin releasing fat to be used as energy. At this point, if you hop on the treadmill before breakfast, your body needs energy to move, and since there is so little glucose circulating in your bloodstream, your body will use the fat from your fat cells. Hence, pre-breakfast, morning exercise is the most direct and effective way to burn fat.

If your schedule doesn't permit early hour aerobic exercise, then your next best shot at burning fat is immediately after strength training. The intensity of strength training depletes your glycogen reserves, and when you follow it with aerobic exercise your body will start to use fat from your fat cells. Your body does also use some fat as an energy source when glycogen is available, but it prefers the glucose from the converted glycogen.

Optimally, if time permits, do your aerobic exercise in the morning on an empty stomach and complete a strength training routine at a time later in the day, after you have eaten a meal.