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It's no secret that exercise is key to controlling type 2 diabetes — and many doctors already urge their diabetic patients to get active. But it's a vague directive: How much exercise is enough? How often? And what kind? The simple answer is that any is better than none — in sum, that's what a new study published in the Sept. 18 issue of the journal Annals of Internal Medicine found. But it also found that not all exercise is created equal and that the combination of aerobic exercise and weight training is significantly better for controlling blood sugar than either alone.

The elegantly designed study, led by researchers at the University of Calgary and the University of Ottawa, involved 251 patients aged 39 to 70, with type 2 diabetes. The patients, none of whom were regular exercisers, were randomized to one of four groups: aerobic exercise, resistance training, a combination of both, or none. For 22 weeks, the aerobic group worked out for 45 minutes three times a week on the treadmill or stationary bicycle; the resistance-training group spent an equal amount of time on weight machines. The combination group was at the gym twice as long as the other two exercise groups, doing the full aerobic plus weight-training regimens. "We built up gradually to 45 minutes, but it's certainly vigorous," says Dr. Ronald Sigal, lead author of the study and associate professor of medicine and cardiac sciences at the University of Calgary. "It's not sprinting or maximal exercise like a marathon trainer would do, but for someone who's middle-aged and older and very overweight, it's fairly strenuous."

Overall, researchers saw improvements in blood-sugar control in all the patients who worked out. Compared with controls, patients in the aerobic group had a reduction of .51% in their hemoglobin A1C values — a test that measures blood-sugar control over the previous two to three months (lower is better). The weight-training group had a .38% reduction compared with controls. But the combined exercise group showed further improvements: in those patients, the A1C values went down an additional .46% over the aerobic group, and .59% over the weight-training group. Compared to controls, the combo exercisers had a nearly 1% lower A1C reading.

The benefits of a 1% drop aren't small, and they go beyond blood-sugar control: That reduction translates to a 15% to 20% decrease in heart attack and stroke risk and a 25% to 40% lower risk of diabetes-related eye or kidney disease. "To envision the importance of exercise, imagine an inexpensive pill that could decrease the hemoglobin A1C value by 1 percentage point," write Dr. William Kraus of Duke University Medical Center and Dr. Benjamin Levine of the University of Texas Southwestern Medical Center at Dallas, in an accompanying editorial. "Diabetes experts would be quick to incorporate this pill into practice guidelines and performance measures for diabetes."

Across all three exercise groups, data suggested that working out could improve blood pressure, triglyceride and cholesterol levels in people with diabetes; however, there was no significant difference in the changes among the groups. Exercising also led to modest weight loss — even though patients were put on diets specifically designed to maintain weight — and a reduction in belly fat. What's more, CT scans of patients' muscles suggested that exercise could improve their internal structure and function. "So, even if you're not losing weight, don't get discouraged just because of that," says Sigal. "There's still additional value [of exercise] independent of weight loss."

Until a few years ago, says Sigal, American Diabetes Association guidelines recommended against weight training for diabetic people, particularly for older and longtime patients, "out of a fear that blood pressure may go too high and may cause problems — strokes or some kind of acute event — during exercise... That wasn't based on any real evidence." Today, the Association advises patients to exercise 30 minutes a day at least five days a week, and recommends routines similar to the ones Sigal studied: aerobic workouts (such as walking, swimming, biking), with weight training (with weights or bands) and practice in flexibility (gentle stretching to reduce the risk of exercise-related injuries). But before you hit the gym for the first time, Sigal cautions, see your doctor — particularly if you're overweight, middle-aged or older, or have any other health issues, like smoking, high cholesterol or high blood pressure. You should get a stress test and make sure you create a safe workout program geared to your abilities.

Sigal is currently studying the benefits of exercise in insulin-dependent patients with type 1 diabetes. Next year, he plans to launch another study on exercise and type 2 diabetes — to find a way to get "people into the gym and, perhaps more importantly, get them to continue doing it once they've started."

• Sora Song