Exercise training-induced effects on brown and white adipose tissue metabolism in humans : positron emission tomography studies in health and insulin resistance

Abstract

White adipose tissue (WAT) not only serves as a passive energy storage but also has an endocrine role releasing hormones that play a major role in the regulation of whole-body glucose homeostasis and insulin sensitivity. Active brown adipose tissue (BAT) is able to consume lipids and glucose to generate heat. The dysregulation of BAT and WAT may predispose a person to become obese and insulin resistant. Exercise training is established to reduce fat mass and insulin resistance. Some of the exercise-induced benefits may be dose-specific. However, only a few studies exist examining the effects of training on BAT metabolism in humans that are cross-sectional in nature and the results are contradictory. There are no controlled prospective intervention studies that have investigated exercise-induced effects on BAT metabolism directly in humans. Furthermore, there is no clear evidence that exercise improves WAT metabolism.

The aim of this thesis was to investigate the effects of short-term (2wks) exercise training with either, sprint interval training (SIT) or moderate intensity continuous training (MICT) on BAT and WAT metabolism in middle-aged sedentary healthy (BMI 26.1±2.4; age 48±5) and insulin resistant (IR) subjects (BMI 30.1±2.5; age 49±4). Further, the effects of longer term (6wks) progressive endurance and resistance exercise training on cold-induced BAT metabolism in healthy men (BMI 23±0.9; age31±7) were studied. BAT and WAT glucose and free fatty acid was determined using positron emission tomography (PET).

The results show that modifications after exercise training are not only adipose tissue depot-specific but also the type of exercise (SIT vs MICT) induces different responses. Training decreased insulin stimulated BAT glucose uptake but had no effect on cold stimulated BAT glucose uptake in healthy subjects. At baseline IR had impaired WAT GU compared to healthy subjects which normalized after training. SIT improves WAT insulin resistance while MICT decreases WAT free fatty acid metabolism in IR. This suggests that different adipose tissue depots respond differently to the metabolic demands of exercise training. Moreover, intensity affects different substrate uptake from WAT. This data suggests that changes in adipose tissue metabolism may help whole body insulin action. Overall, exercise-induced BAT and WAT adaptations provide potential therapeutic targets for obesity and type 2 diabetes.