How Exercise Helps

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Q: My doctor keeps telling me exercise is extra important for people with diabetes. If I lose weight by dieting, do I still need to exercise?

A: In a word, yes. Exercise is good for your whole body, inside and out. To help you understand better, let's get into the cellular science of exercise.

Let's begin with glucose, the primary energy source for all living things on earth since life first appeared on the planet. All organisms, starting with the most primitive ones, had to work hard for their food. Glucose was always scarce. So the problem was how to survive with limited resources and how to be prepared to fight or escape from larger or stronger organisms looking for their own next glucose meal.

Producing Energy

Fight or flight. Both reactions require a quick glucose boost and rapid energy production. This energy surge is powered by tiny "energy factories" in our cells called mitochondria, which use the glucose flooding the muscle cells to make high-energy fuel (known as ATP, which is short for adenosine triphosphate).

Glucose usually enters cells with the help of insulin and specialized "glucose transporters." Glucose transporters are protein rings, which float inside the cell. When an insulin molecule attaches to the cell's surface membrane, it causes the glucose transporter protein to merge with the surface membrane. This produces a glucose channel, or pore, that allows glucose to cross into the cell.

In the past 20 years, research has shown that when muscles contract at a moderately intense level, the contractions themselves cause more glucose transporters to merge with the cell membranes. Exercise also increases the number of mitochondria in muscle cells (each cell in our body has thousands of mitochondria), as well as the number of blood vessels in muscles.

Increased blood flow to the muscles during exercise also increases the production of nitric oxide, causing blood vessels to expand, allowing more blood, and thus more glucose and oxygen, to get to the muscles.

If there is no further exercise, the newly activated transporters and extra mitochondria gradually "melt" away over the next two to five days. This "use it or lose it" process has been refined by evolution over several hundred million years and has been clearly demonstrated in rats and humans.

Why We Need to Exercise

High blood glucose levels cause cardiovascular system changes that are the opposite of those caused by exercise. They cause blood vessels to constrict. They cause the mitochondria to produce toxic "free radicals" (which may damage the inner lining of blood vessels and heart muscle). High blood glucose levels also trigger the production of other substances that may damage blood vessels and cause muscle cells to die.

Several recently published clinical studies have shown that people with diabetes who exercise for at least one-half-hour a day have better glucose control and a much lower risk of complications from cardiovascular disease than those who do not exercise. Exercise increases the levels of active glucose transporters and mitochondria in cells, and the amount of nitric oxide in blood vessels. This helps improve glucose control and blood vessel function. Exercise also lowers total cholesterol, raises HDL ("good") cholesterol, lowers triglycerides, and reduces blood pressure.

All of which are pretty good reasons to get cracking on your workout.

Sheldon H. Gottlieb, MD, FACC, is a cardiologist at Johns Hopkins Bayview Medical Center, Department of Cardiology, in Baltimore, Md. He also directs the Diabetes–Heart Failure Program at Johns Hopkins HealthCare, LLC.