A 65-year-old woman with new-onset type 2 diabetes and an A1C level of 8 percent wants to know if she can control her diabetes without taking medications or insulin.
What should physicians tell patients with type 2 diabetes about the role of exercise?
Regular exercise reduces A1C levels, adiposity, and triglyceride levels. However, no research has definitively proved a benefit of exercise on patient-oriented outcomes such as diabetes-related morbidity and mortality.1
Exercise, diet control, smoking cessation, and reduction of alcohol use are recommended lifestyle modifications for persons with type 2 diabetes.2 The American Diabetes Association recommends that persons with diabetes get at least 150 minutes of aerobic physical activity per week for improvement of glycemic control, and advocates performance of resistance exercises targeting all major muscle groups at least three times per week.3 Although some patients may be highly motivated to pursue lifestyle interventions to avoid the use of insulin or medications, others may see lifestyle changes as a burden that adds to the complexities of managing type 2 diabetes. Furthermore, there may be skepticism among physicians and patients about whether exercise can have a clinically meaningful impact on type 2 diabetes compared with any number of diabetes drugs available.
This Cochrane review provides strong evidence that exercise programs ranging in duration from eight weeks to one year reduce A1C levels by approximately 0.6 percent, as well as reducing adipose tissue and triglyceride levels.1 However, the studies reviewed did not provide definitive evidence of improvement in quality of life, morbidity, or mortality. The durations were insufficient to evaluate changes in mortality rates and there was minimal reporting of quality-of-life measures.
The clinical significance of a 0.6 percent reduction in A1C levels resulting from exercise might be extrapolated by a comparison with the average 1 to 2 percent reduction achieved with most classes of oral hypoglycemic agents.4 In the United Kingdom Prospective Diabetes Study, a 1 percent lower A1C level was associated with a 21 percent lower risk of death related to diabetes, a 14 percent lower risk of myocardial infarction, and a 37 percent lower risk of microvascular complications.5 However, this study did not demonstrate that therapies targeted to reduce A1C levels improved patient-oriented outcomes.6 Therefore, although it has been proven that among persons with diabetes who have lower A1C levels there are fewer deaths, heart attacks, and microvascular complications, there is not yet conclusive evidence that lowering A1C levels through exercise leads to a corresponding risk reduction in patient-oriented outcomes (a benefit that has been demonstrated for tight control of blood pressure and lipid levels in patients with diabetes).
Although common sense seems to support the importance of exercise for type 2 diabetes, there are several issues to consider when applying the evidence to patient care. First, despite evidence that exercise reduces adiposity in patients with type 2 diabetes, exercise should not be recommended as a primary means for weight loss (particularly short-term weight loss) because of the lack of a short-term net reduction in body mass index. Second, although the studies reviewed showed drops in rates of exercise participation, patients who continued to exercise maintained their A1C level reductions—thus, although long-term exercise may not be for everyone, it will benefit those who pursue it. Third, reductions in A1C levels were noted across a wide range of activity levels—from low-intensity, once-weekly qigong to high-intensity progressive resistance training.
Background: Exercise is generally recommended for persons with type 2 diabetes. However, some studies evaluate an exercise intervention including diet or behavior modification or both, and the effects of diet and exercise are not differentiated. Some exercise studies involve low participant numbers, lacking power to show significant differences that may appear in larger trials.
Objectives: To assess the effects of exercise in type 2 diabetes.
Search Strategy: Trials were identified through the Cochrane Central Register of Controlled Trials (CENTRAL), Medline, EMBASE, and manual searches of bibliographies. Date of last search was March 3, 2005.
Selection Criteria: All randomized controlled trials comparing any type of well-documented aerobic, fitness, or progressive resistance-training exercise with no exercise in persons with type 2 diabetes.
Data Collection and Analysis: Two authors1 independently selected trials, assessed trial quality, and extracted data. Study authors were contacted for additional information. Any information on adverse effects was collected from the trials.
Primary Results: Fourteen randomized controlled trials comparing exercise against no exercise in type 2 diabetes were identified, involving 377 participants. Trial durations ranged from eight weeks to 12 months. Compared with the control, the exercise intervention significantly improved glycemic control as indicated by a decrease in A1C levels of 0.6 percent (−0.6; 95% confidence interval [CI], −0.9 to −0.3;P < .05). This result is statistically and clinically significant. There was no significant difference between groups in whole body mass, probably because of an increase in fat-free mass (muscle) with exercise, as reported in one trial (13.9 lb [6.3 kg]; 95% CI, 0.0 to 27.8 [0.0 to 12.6]). There was a reduction in visceral adipose tissue with exercise (−7.1 square in [−45.5 cm2]; 95% CI, −9.9 to −4.2 [−63.8 to −27.3]), and subcutaneous adipose tissue also decreased. No study reported adverse effects in the exercise group or complications of diabetes. The exercise intervention significantly increased insulin response (131 area under the curve; 95% CI, 20 to 242; one trial), and decreased plasma triglycerides (−22.1 mg per dL [−0.25 mmol per L]; 95% CI, −42.5 to −1.8 [−0.48 to −0.02]). No significant difference was found between groups in quality of life (one trial), plasma cholesterol, or blood pressure.
Reviewers' Conclusions: This meta-analysis shows that exercise significantly improves glycemic control and reduces visceral adipose tissue and plasma triglycerides, but not cholesterol, in persons with type 2 diabetes, even without weight loss.
These summaries have been derived from Cochrane reviews published in the Cochrane Database of SystematicReviews in the Cochrane Library. Their content has, as far as possible, been checked with the authors of the originalreviews, but the summaries should not be regarded as an official product of the Cochrane Collaboration; minorediting changes have been made to the text (http://www.cochrane.org)