Type 2 diabetes is mainly characterised by a reduced ability of the hormone insulin to stimulate glucose uptake in body fat and muscles (insulin resistance) combined with insufficient insulin secretion that leads to increased blood glucose levels. It has been shown that weight reduction and an increase in daily energy expenditure decreases insulin resistance. There are some factors that are associated with an increased risk of type 2 diabetes: these are obesity, previous gestational diabetes, hypertension, family history of type 2 diabetes, dyslipidaemia and some ethnical groups are more at risk. Persons with "prediabetes" are also at high risk: they have abnormal blood glucose levels but not in the range of diabetes. Prediabetes often precedes the development of type 2 diabetes. We searched for trials that intended to prevent the development diabetes type 2 in the above mentioned at risk groups. We assessed the effects of increased physical activity alone or in combination with dietary interventions on diabetes incidence and other outcomes.
We included eight trials with 2241 participants randomised to exercise and diet intervention and 2509 participants to standard recommendation. Furthermore, 178 participants were randomised to an exercise only intervention and 167 participants to a diet only intervention. The duration of the interventions in the trials ranged from one year to six years. Interventions varied between studies but mainly consisted of caloric restriction if the person was overweight, low fat content (especially saturated fat), high carbohydrate content and the increase of fibre intake. Physical activity varied but on average at least 150 minutes each week of brisk walking or other activities such as cycling or jogging were recommended. Interventions were mainly delivered by frequent individual counselling by a physiotherapist, an exercise physiologist and a dietitian. Incidence of diabetes was reduced by 37% (relative risk reduction) with exercise and diet. This had favourable effects on body weight, waist circumference and blood pressure. More evidence is required on effects of exercise alone in the prevention of type 2 diabetes. No study reported relevant data on diabetes and cardiovascular related morbidity, all-cause mortality and quality of life.