In the first part of this series I looked at physical activity and weight loss in a historical perspective and found that prior to the 1950s, few overweight and obese individuals were recommended to exercise more. It’s often suggested that the increased energy expenditure during exercise is responsible for shedding the fat, but regular physical acitivty is also associated with several other changes that could affect bodyweight.
Aerobic exercise includes light-moderate intensity activities that depend on oxygen to meet energy demands during training. While aerobic activities such as jogging and cycling can be sustained for long periods of time, anaerobic, high-intensity activities such as sprints and weight-lifting recruit more fast twitch muscle fibers and involve shorter work periods. Most activities depend on both anaerobic and aerobic energy.
Both regular aerobic and anaerobic activity potentially influence bodyweight by several different mechanisms. Energy expenditure during exercise is often considered the main component in regards to fat loss, but research shows that other aspects of physical activity might be equally as important.
Energy expenditure and metabolic rate
Excess post-exercise oxygen consumption (EPOC) refers to the body’s increased rate of oxygen consumption after exercise in an attempt to return itself to the pre-exercise state. The magnitude and duration of EPOC depends on the intensity and duration of the exercise, and also training status and gender (1). EPOC and increased metabolic rate following both aerobic and anaerobic exercise further increase the total energy expenditure achieved from training (2,3).
Both regular aerobic and anaerobic physical activity can also influence “long-term” basal metabolic rate by several different mechanisms (4,5). Resistance training promotes muscle growth, and the increased muscle mass results in a higher resting energy expenditure. Increased muscle mass will also lead to a higher calorie expenditure in the hours after exercise, most likely due to the elevated protein synthesis.
Insulin, leptin and inflammation
Physical activity indirectly influences body composition by improving insulin sensitivity (6,7,8), reducing serum leptin levels (9,10,11) and preventing oxidative stress and inflammation (12,13). Exercise is also linked to other metabolic changes that could play an important role in weight regulation.
Insulin is a hormone that allows cells throughout the body to take up glucose from the blood. Insulin resistance occurs when cells don’t respond adequately to insulin, and it involves elevated levels of blood glucose. There’s a widely held belief that elevated insulin (hyperinsulinemia) and insulin resistance cause excess fat storage, but insulins role in overweight and obesity is highly debated (14,15,16). Insulin resistance seems to correlate with body mass index, but it’s hard to establish cause and effect as we know that weight gain in itself can lead to insulin resistance (17).
Leptin is produced by fat cells and allows for the brain to monitor the size of the fat stores and regulate them by varying appetite and energy expenditure. Leptin resistance is a condition where leptin produces a smaller response at the receptors in the brain. This is thought to trigger hunger and reduced energy expenditure, which is the bodys attempt to increase leptin production and overcome leptin resistance (18). Leptin resistance is often seen in overweight and obese individuals and essentially involves having a malfunctioning energy homeostasis (18,19,20).
Obesity is characterized by low-level chronic inflammation and oxidative stress. Inflammation sets the stage for insulin resistance, diabetes and obesity, and reducing inflammation can be helpful when trying to lose weight (20,21,22). While athletes exposed to high-intensity exercise often experience alterations in intestinal permeability, decreased gut barrier function and endotoxemia, it seems that a moderate amount of physical acitivity reduces chronic inflammation and that this effect is independent of the exercise-induced weight loss (23,24,25).
The human body is a complex system, and it’s clear that just looking at energy in versus energy out is a simplistic view on exercise and weight loss.
All posts in this series on exercise and weight loss
Part 1: Introduction and history
Part 2: How does physical activity affect bodyweight?
Part 3: Physical activity, increased appetite and food intake
Part 4: Exercise and weight gain
Part 5: How much do I lose?
Part 6: Summary, discussion and conclusions