Whenever you move, whether you’re doing an endurance exercise like running or just taking a walk around the block, your muscles need fuel to burn. The fuels that muscles use are glucose and fat. If you’ve ever checked your blood glucose level before and after a bout of exercise or a brisk walk, you know that it usually drops, because the muscles use the glucose in the blood for fuel. Fat is used for fuel by the muscles in much the same way. Here’s how it works, step by step:
1. You take a brisk walk.
2. In response to the movement, certain hormones, including epinephrine (adrenaline) and norepinephrine, get released from the adrenal glands, and the blood levels of these hormones start to rise.
3. The hormones circulate around the body and attach to special hormone receptors on the adipocytes.
4. Once attached to the receptors on the adipocytes, a complex chain of biochemical events takes place in the cell, signaling the adipocyte that the body needs fuel.
5. Being the clever devil that it is, the adipocyte receives the signal and processes the stored fat, mobilizes it, and sends it out of storage and into the bloodstream.
6. Once in the bloodstream, the fat gets wrapped up again in chylomicrons so that it can travel in the bloodstream to the muscles that need it.
7. Once the package of fat and protein arrives at the muscle, another complex chain of events occurs, stimulated by an enzyme called lipoprotein lipase and, lo and behold, the fat enters the muscle.
8. Once in the muscle, a truly biochemical magic act occurs. What happens is that fat gets broken down to its smallest elements (carbon, hydrogen, and oxygen), and inside a part of the muscle cell called the mitochondria, known as the power house of the cell, enormous amounts of energy are produced, which allow the muscles to move.
Dozens of complex biochemical reactions have to occur to produce the energy for our muscles to move, but the good news is that you don’t have to remember how to do any of it. All you have to do is move. The body takes care of all the rest.
The physical activity recommendation for improving health is to accumulate 30 minutes of moderate-intensity physical activity on most, if not all, days of the week. The key words in this recommendation are “accumulate,” which means you can do it in three bouts of 10 minutes, two bouts of 15 minutes, or one bout of 30 minutes, and “moderate-intensity,” which means the work should leave you feeling warm and slightly out of breath but not exhausted. Walking, biking, dancing, weight lifting, swimming, climbing stairs, hiking, gardening, housework, and many other activities of daily living such as walking your dog, washing your car by hand, and mowing your lawn with a push mower will all do the trick. In most studies of physical activity, even modest amounts of activity help the body reduce fat, if not all over the body, then certainly in the abdomen and deeper in the visceral fat where it counts most for good health.
Building muscle—through weight lifting or other resistance exercises—will help, too. Muscle burns calories and helps you maintain your metabolic rate. The more muscle you have, the bigger your body’s engine, and the more likely you will be to burn fat.
Once adipocytes get the signal from hormones and release fat into the bloodstream, they shrink just like a balloon that you let air out of. When they shrink, so does your body fat. But if you eat excess fat once you’ve shrunk your adipocytes, chances are it will find its way right back to the adipocyte, and once again you’ll gain fat.
No matter how much physical activity you do, adipocytes never shrink so much that they disappear entirely. Like a balloon that you let all the air out of, you’re always left with some remnant. The only way to totally remove adipocytes from your body is with a surgical procedure such as liposuction or excision. But even with these procedures, if you go back to eating excess fat, you’ll put all the fat back on.