The Science of Exercise
Is physical activity the best medicine?
It's free. Most people can't get enough of it. Entreaties to do more of it abound. Yes, it's the crux of healthy living: exercise. And while just about everyone is better off working out regularly, exercise is, in some sense, the perfect drug for diabetes. Not only can it improve blood glucose control—which in itself reduces the risk of diabetes complications—but research suggests it may combat heart disease, weight gain, depression, and more.
Glucose on the Go
Muscle contractions have a powerful effect on how the body processes glucose, the original biofuel. The muscles are the major consumer of glucose during exercise. It's not surprising since they do most of the work. In each cell, muscles store dense packets of glucose, accounting for around 2,000 calories worth of energy throughout the body, according to Sheri Colberg-Ochs, PhD, a professor of exercise science at Old Dominion University in Norfolk, Va. "[This energy] just stays there unless you contract the muscle."
During exercise, the muscles deplete their individual glucose reserves. To help restock their glucose supplies, the muscles change in two important ways: They become more sensitive to insulin—a hormone that escorts glucose from the bloodstream into body cells—and they also start to absorb glucose on their own, independently of insulin.
This second pathway created during exercise is a boon for anyone with type 2 diabetes, which is marked by insulin resistance. "When the body is at rest, it has one mechanism for getting glucose out of the bloodstream. That way is insulin," says Colberg-Ochs. "What's so good about exercise is that even if the muscles are insulin resistant at rest, that's irrelevant with exercise."
Exercise's effect on glucose use occurs not just in people with type 2 but in almost everyone, including those with type 1 and pre-diabetes. A large study found that, in people with pre-diabetes, lifestyle changes that included 150 minutes a week of moderate-intensity exercise reduced the risk of progression to full-blown type 2 diabetes by 58 percent.
Exercise makes a little insulin go a long way. That's generally a good thing. But people who get their insulin from a shot or a pump can end up with too much insulin in the body. That can cause blood glucose to go too low (hypoglycemia), particularly when glucose is diverted to the muscle cells during exercise.
"That's why some type 1s shy away from exercise," says Colberg-Ochs. But they needn't. "If you plan ahead and reduce insulin intake, you can go through exercise without swings in blood glucose." People with type 2 can also suffer hypoglycemia with exercise because of medication, but the risk isn't as great.
Exercise's short-term effects can last anywhere from two hours to three days, according to Colberg-Ochs. "That's generally why, when we talk about how often people should exercise, we say that they should do it at least every other day, though probably every day is better." Other benefits of regular physical activity can last a lifetime.
If you keep at it and incorporate resistance training into your routine, exercise can provide additional opportunities to help with blood glucose control by building muscle. "Over the long term, there can be a change in muscle composition that can favor getting glucose into the cells," says Ronald Sigal, MD, MPH, FRCPC, a professor of medicine, kinesiology, cardiac sciences, and community health at the University of Calgary in Canada. "Also, over time, there are changes in body composition that can help glucose uptake, such as an increase in muscle mass."
Weight is made up of more than just pounds; the relative amounts of muscle and fat, how much fat is inside muscle, and where fat is found on the body are far more important fitness criteria than simply weight alone. Muscles can use fat or glucose for energy; as fat tissue in the body is decreased, the muscles become
better at using glucose for their energy needs.
Exercise can tip the scale toward a healthier body composition. The best way to build muscle is through resistance training. It involves working the muscles against an opposing force by, for example, lifting weights or one's own body weight (as with push-ups, sit-ups, or yoga). One study of older men with type 2 diabetes found that a four-month resistance training program skimmed off some of the visceral fat from deep inside their abdomens, which is known to be particularly bad for the heart.
Every ounce of muscle is a glucose-burning powerhouse, so the more muscle a body has, the better it will be able to control blood glucose levels. "You can only fit a certain amount of [glucose] in muscle, but if you have more muscle, then you can store more," says Colberg-Ochs.
So, what type of exercise is best for people with diabetes? A 2007 study in the Annals of Internal Medicine compared four different approaches to fitness: aerobic exercise, resistance training, both aerobic and resistance sessions, or no exercise at all. In the six-month study, previously inactive participants with type 2 diabetes who were from 39 to 70 years old worked out three times a week.
All exercising groups lowered their A1C (a measure of the average blood glucose over the previous two to three months) but those who combined aerobic and resistance training saw the greatest improvements in blood glucose control. To get the most from exercise, says Sigal, who led the study, you should do both: get your heart rate up and build muscle.
Raising the Pulse
Better blood glucose control would be reason enough to exercise, but the benefits of regular workouts stretch further, into heart health. This is of particular importance to people with diabetes, for this reason: Adults with diabetes are two to four times as likely to die from heart disease as those without the disease.
The major risk factors for heart attack and stroke are abnormal levels of blood fat—including cholesterol and triglycerides—and high blood pressure. "Exercise helps with fat metabolism," says Colberg-Ochs. "One of the responses to training is that muscles can better utilize fat as a fuel. It gets fat out of the bloodstream and anywhere else it shouldn't be."
While individual studies have produced conflicting results on exercise's effects on blood fats, an analysis of several high-quality trials suggested that physical activity can reduce the levels of LDL ("bad") cholesterol by 5 percent. Other research suggests exercise raises HDL ("good") cholesterol independently of weight loss. Its effect on blood pressure is less clear, but the overall picture is that exercise yields modest reductions.
Some studies have found that exercise lessens markers of inflammation, which is thought to contribute to atherosclerosis, the accumulation of fat and cholesterol in artery walls. "Exercise tends to improve the function of the lining of blood vessels," says Sigal. "Over time, you get reductions in inflammation, which is associated with decreased cardiac risk."
Many studies suggest that people who are more fit have less risk of dying from heart attacks or strokes. Yet it remains to be proved that exercise causes people to have healthier hearts.
How much exercise is enough to reap its benefits? The consensus from studies on people with diabetes is that 150 minutes of moderate aerobic activity each week, plus two or three strength training sessions, is adequate.
If one's goal includes losing weight, picking the appropriate exercise program gets more complicated: The effect of exercise on body weight is one of the most controversial areas of research. In principle, weight loss is simply a matter of energy balance: Burn more calories than you eat, and you'll lose weight; eat more calories than you burn, and you'll gain. So it would seem to make sense that shedding calories via workouts should spur weight loss, but research hasn't consistently supported that conclusion.
To lose weight, research suggests that up to an hour a day may be necessary, far more than the 150 minutes a week of moderate exercise that has been shown to improve blood glucose levels. "The amount a person has to exercise to lose the amount they want to lose may be considerable," says Sigal.
Exercise alone doesn't seem to help people drop much weight unless they also limit how many calories they consume. The problem may be that exercise increases hunger—and it's all too easy to offset the calories burned in an hourlong sweat session with a single muffin.
Yet, most successful long-term weight-loss programs have an exercise component. This suggests exercise is a key part of weight maintenance. In a large national study, 90 percent of participants who had lost 30 pounds or more and kept the weight off for over a year exercised an average of seven hours a week.
Blood glucose, heart health, and weight maintenance aren't the only areas where exercise seems to improve the outlook for people with diabetes. One study of people with type 2 diabetes, known as Look AHEAD (for Action for Health in Diabetes), found that participants who got more fit had fewer symptoms of depression and a better quality of life than those who stayed on the couch. This is important because research shows inactive adults with type 2 are almost twice as likely to be depressed as those who are physically active.
It's also been shown that exercise can ease anxiety, boost mood, improve self-esteem, and make sleep more restful. However, these mental benefits tend to be more dramatic in those who are less physically active or psychologically healthy before they start exercising, and they generally peak after weeks of regular aerobic exercise. Scientists are also avidly exploring whether exercise can stave off some types of cancer or help people recover from cancer. "In people with breast cancer or prostate cancer, regular exercise reduces fatigue," says Sigal. And there's some evidence that physical activity helps people learn, as well as keeping the mind intact in old age.
Exercise research has provided some fascinating insights on what movement can do. And though not all the science has been worked out, the proven benefits of exercise for people with diabetes provide a clear incentive to get moving now.