Although we established in part 2 of this series that physical activity could affect bodyweight through several different mechanisms, both scientific studies and anecdotal reports suggest that some trainees are unable to lose weight by exercising more. The main reason this happens is because of something called compensatory mechanisms, which basically means that a lot of people compensate for the increased energy expenditure during exercise by eating more and/or being less active during the rest of the day.
The common belief is that physical activity promotes weight loss by increasing energy expenditure and thereby creating an imbalanced energy equation. However, sceptics argue that energy intake is adjusted according to energy expenditure and that more physical activity results in hunger and increased food consumption. In other words, “building up an appetite” from exercise might lead to more foods eaten the following hours or days after training. It’s also possible that individuals who exercise get “fatigued” or satisfied with todays efforts and end up being less active during the day.
People respond differently to exercise
A newly published review by King et al. (2012) looked at how physical activity influences eating behaviour and food intake. Studies show varying results, but the general message is that acute exercise-induced energy deficit results in no or only partial calorie compensation. Individuals respond differently, and while some people respond to exercise by eating more, others actually eat less (1).
There are several physiological, neural and psychological factors influencing the compensatory mechanisms to exercise. The type of exercise performed is also important, and aerobic exercise seems to suppress hunger more than resistance exercise (2,3).
Some individuals feel that they are allowed to eat unhealthy foods and increase their calorie intake because they exercise more. This approach easily leads to overeating since most people tend to overestimate the amount of calories burned during exercise (4). However, this can also happen the other way around when the trainee tries to change his or her lifestyle completely by exercising more and eating healthier. It’s likely that the mental state of the trainee strongly determines compensatory mechanisms to exercise, and that someone who’s motivated and have specific goals will be less likely to compensate for the training effort.
While most previous studies have focused on appetite hormones and exercise, new research shows that physical activity alters the food-reward network in the brain; thereby influencing how we respond to food. Some individuals experience increased reward value from food after exercise, while others display less interest in food following physical activity (5,6).
Is more exercise always better?
While a certain amount of physical activity is necessary to produce statistically significant changes, one newly published study shows that a moderate amount of aerobic exercise (300 kcal/day) results in the same fat loss as a high amount of aerobic exercise (600 kcal/day). The monitor sensors showed that the group who exercised for 60 minutes each day, compared to 30, had less energy during the rest of the day; thereby expending fewer calories outside the gym. The food diaries were also self-reported, and the group who exercised for 60 minutes each day could have eaten more than they wrote down. More studies are needed to show if these results are reproducable and if they are maintained long-term (7).
Compensatory mechanisms could explain differences in exercise-induced weight loss
More long-term studies on the activity in the food reward system, hormones and appetite in the hours and days after exercise are needed to get a better understanding of compensatory mechanisms.
In general, it seems that people respond differently to exercise and that these differences in compensatory mechanisms could partly explain why some people find that exercise is effective for losing weight, while others lose little or no weight from being more active.
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