The exercise prescription

The guidelines recommend combining such cardiorespiratory (aerobic or endurance) exercise as brisk walking, stationary bicycling or swimming, with resistance training.

For aerobic exercise, the basic goal is a minimum energy expenditure of 1,000 kcal/week, which would translate to 150 minutes of moderate activity (e.g., 30 minutes of brisk walking or the equivalent at least five days a week). Vigorous exercise will accomplish as much in less time: the guidelines recommend at least 90 minutes/week.

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Exercise goals can be achieved by combining moderate and vigorous activities (e.g., 20 minutes of vigorous exercise on two days, and 30 minutes of moderate exercise on two days). Whatever the intensity, there should be at least three training days each week with no more than two consecutive days between sessions.

Although exercise intensity can be defined rigorously in terms of heart rate and oxygen uptake, perceived exertion provides a guide: up to 12-13 on a scale of 20 would be considered moderate; more than that qualifies as vigorous. A commonly used metric is the ability to converse. “Moderate exercise might mean walking at a brisk pace but slowly enough to carry on a conversation,” says Dr. Blumenthal. “A vigorous activity is one that makes it difficult to speak two full sentences at a time.”

The guideline authors emphasize that activity should be initiated gradually in previously sedentary individuals. Shorter, more frequent sessions (e.g., three 10-minute bouts instead of one of 30 minutes) may be better tolerated and offer equivalent benefits.

Resistance training also supports glycemic control; in some studies, combining weight training with aerobic training doubles improvement. The guidelines call for a series of resistance exercises that engage all the major muscle groups three times weekly. A reasonable protocol would involve two to four sets of eight to 10 repetitions of each exercise with one- to two-minute rest periods between sets.

Risk reduction

As discussed earlier, cardiac hazards of exercise training can be stratified in terms of symptoms and risk factors. These hazards are best addressed with appropriate testing and by modifying the prescription when indicated. Patients with T2DM should be knowledgeable about symptoms of myocardial ischemia and understand the importance of reporting them promptly to a care provider.

Hypoglycemia in the context of exercise is less common in T2DM than in type 1 diabetes but can occur in patients who use insulin or oral hypoglycemic medications. Risk is higher in disease of longer duration and among patients with lower BMI, low or variable glucose measurements, or impaired awareness of hypoglycemia.

To minimize the risk of hypoglycemia, reduce the dosage of insulin (by 20%-30%) just prior to exercise. With oral drugs, consider reducing dosage by up to 50% (or even skip altogether) the day of an exercise session. Dosage adjustments should be based on tightness of control, history of hypoglycemia, and self-monitoring of blood glucose.

Foot ulceration is, in theory, a risk associated with the repeated impact of some exercise activities. But in practice, active people are less prone to this complication, most likely because of improvements in blood flow and glycemic control.

The risk of foot ulceration may be reduced further by choosing lower-impact activities (e.g., stationary bicycling, swimming) and maintaining a consistent schedule of exercise rather than alternating frequent episodes with long periods of inactivity. Increasing exercise gradually will allow protective tissue to develop, according to Dr. Blumenthal.

Exercise Training for Type 2 Diabetes Mellitus: Impact on Cardiovascular Risk: A Scientific Statement From the American Heart Association was published in Circulation (2009;119:3244-3262) and is available online.

Mr. Sherman is a freelance medical writer in New York City.