Impact on metabolic control
The positive gains from physical exercise for patients with type 2 diabetes are very well documented and there is an international consensus that physical exercise is one of the three cornerstones in the treatment of diabetes, along with diet and medication (Joslin et al., 1959; Albright et al., 2000; American Diabetes Association, 2002).
Several reviews (Sigal et al., 2004; Zanuso et al., 2010) and meta-analyses (Boule et al., 2001; Snowling & Hopkins, 2006; Thomas et al., 2006; Umpierre et al., 2011) report that increased physical exercise produce a significant improvement in glucose control in people with type 2 diabetes, yielding an average improvement in hemoglobin A1c (HbA1c) of between −0.4% and −0.6%.
A 2006 Cochrane Review, which includes 14 randomized controlled trials with a total of 377 patients with type 2 diabetes, compares the independent effect of training with no training (Thomas et al., 2006). The training interventions were 8–10 months in length and consisted of progressive aerobic training, strength training or a combination of the two, with typically three training sessions per week. Compared to the control group, the training interventions showed a significant improvement in glycemic control in the form of a reduction in HbA1c (glycated hemoglobin) of 0.6% (−0.6% HbA1c), 95% CI: −0.9 to 0.3; P < 0.05). By comparison, intensive glycemic control using metformin showed a reduction in HbAc1 of 0.6%, and a risk reduction of 32% for diabetes-related complications and of 42% for diabetes-related mortality (UK Prospective Diabetes Study (UKPDS) Group, 1998).
Despite the clear effect of exercise training on metabolic control, there was no significant effect on body weight. The reason for this is presumably that the exercise group reduced fat mass but increased muscle mass. One of the studies in the meta-analysis reported an increase in fat-free mass of 6.3 kg (95% CI: 0.0–12.6), measured by dual energy X-ray absorptiometry (DXA) scanning, and a reduction in visceral fat volume, measured in by magnetic resonance imaging (MRI) scanning, of −45.5 cm2 (95% CI: −63.8 to 27.3). No adverse effects of physical exercise were reported.
Physical exercise significantly reduced insulin response as an expression of increased insulin sensitivity and triglyceride levels. This Cochrane Review found no significant difference with regard to quality of life, plasma cholesterol, or blood pressure (Thomas et al., 2006). The findings from the Cochrane Review (Thomas et al., 2006) agree with the conclusions from a 2001 meta-analysis, which also evaluated the impact of a minimum 8-week training program on glycemic control (Boule et al., 2001). Training was found to have no effect on body weight (Boule et al., 2001). There are several possible explanations for this: the training period was relatively short, the patients over-compensated for their loss of energy by eating more, or patients lost fat but their volume of fat-free mass increased. There is reason to assume that the final explanation is the most significant one. It is well-known that physically inactive people who start to exercise increase their fat-free mass (Brooks et al., 1995; Fox & Keteyian, 1998). Only one of the studies included in the meta-analysis assessed abdominal obesity using MRI scanning (Mourier et al., 1997). The aerobic training program (55 min three times a week over 10 weeks) resulted in a reduction of abdominal subcutaneous fat, measured using MRI scanning (227.3–186.7 cm2, P < 0.05) and visceral fat (156.1–80.4 cm2, P < 0.05). The same study did not identify any effect from exercise on body weight.
A 2007 meta-analysis assessed the effect of self-management interventions with a view to increasing physical activity levels in patients with type 2 diabetes. The analysis involved 103 trials with 10 455 subjects. Self-management training was found to have a significant effect of 0.45% on HbA1c.
Interventions that included several different lifestyle recommendations such as diet and physical activity had less effect than interventions that included physical exercise only. Basic levels of HbA1c and BMI were not related to metabolic effect. The overall conclusion is that self-management interventions that include physical activity increase metabolic control (Conn et al., 2007).
A 2009 systematic review included nine studies with 372 patients with type 2 diabetes. Progressive resistance training vs no training induced a statistically significant reduction in HbA1c of 0.3%. There was no difference between resistance training and aerobic training as far as the effect on changes in HbA1c was concerned. Progressive resistance training resulted in large improvements in strength compared to aerobic training or no training. No significant effect on body composition was found (Irvine & Taylor, 2009).
A meta-analysis from 2013 found that exercise lowers postprandial glucose but not fasting glucose in type 2 diabetes (MacLeod et al., 2013). A 2007 meta-analysis evaluated the effect of aerobic physical training for a minimum of 8 weeks on lipids and lipoproteins in patients with type 2 diabetes. The analysis included seven trials with 220 men and women, of which 112 were in a training group and 108 in a control group. A statistically significant reduction of approximately 5% in LDL cholesterol was found but there was no significant effect with regard to triglycerides, HDL cholesterol, or total cholesterol (Kelley & Kelley, 2007).
A 2011 meta-analysis concluded that structured exercise training that consists of aerobic exercise, resistance training, or both combined is associated with HbA1c reduction in patients with type 2 diabetes. Structured exercise training of more than 150 min/week is associated with greater HbA1c declines than that of 150 min or less per week. Physical activity advice is associated with lower HbA1c, but only when combined with dietary advice (Umpierre et al., 2011).
A systematic review and meta-analysis from 2014 compared resistance exercise and aerobic exercise and concluded that there was no evidence that resistance exercise differs from aerobic exercise in impact on glucose control, cardiovascular risk markers or safety. Using one or the other type of exercise for type 2 diabetes may be less important than doing some form of physical activity (Yang et al., 2014).
Measures of fasting glucose and HbA1c do not accurately represent glycemic control because they do not reflect what occurs after meals and throughout the day in the free-living condition (Kearney & Thyfault, 2015). An accumulating body of evidence now suggests that postprandial glucose fluctuations are more tightly correlated with microvascular and macrovascular morbidities and cardiovascular mortality than HbA1c or fasting glucose, stagnant measure of glycemia. It is therefore important that unlike medications, which generally have a poor effect at improving postprandial glucose, exercise has been proven effective in reducing postprandial glycemic excursions in as little as a few days (MacLeod et al., 2013; Kearney & Thyfault, 2015).