In part 3 in the series of posts on exercise and weight loss I looked into the compensatory mechanisms to exercise, such as the increased appetite and food intake that often accompany physical activity. While compensatory mechanisms largely influence whether someone loses or gains weight from exercise, there are also some other factors that could explain why some people lose a lot of weight from exercising more while others find that their bodyweight remains the same or even increases.
Resistance training promotes muscle growth
Heavy resistance training promotes muscle growth, and weight gain will be determined by energy intake. A common practise is to consume excess calories beyond what is necessary to build muscle. This “bulking”-phase results in both muscle growth and fat gain, and total bodyweight increases. It’s also possible to gain lean body mass (the mass of the body minus the fat (storage lipid)) while still losing weight, and the increase in lean body mass seems to correlate inversely with the rate of the weight reduction (1,2).
Because resistance training is linked to added muscle weight, it’s best to estimate exercise-induced weight loss by looking at changes in fat mass.
Some people gain weight from aerobic exercise
Aerobic exercise rarely promotes weight gain, but a review by King et al. (2012) shows that a small percentage of people gain weight when they are adhering to a training program with a lot of aerobic activity (3). It could be that these individuals compensate for the increased training effort by being less active during the rest of the day and/or eating more food, but differences in fat loss between participants in calorie restricted studies suggest that some people also respond better to exercise than others. The exact reasons why this happens are still unclear, but differences in metabolism, microbiome, gut permeability and inflammation could play key roles.
Leaky gut, microbiome and inflammation
Obesity is characterized by increased intestinal permeability and low-level chronic inflammation. Obese individuals also have an “obese microbiota” with different species of microbes compared to lean indviduals (4,5,6). Inflammation is a main component in most chronic diseases, and alterations in the human microbiome and a “leaky gut” are important proinflammatory conditions present also in many non-obese individuals.
The effect exercise has on the gut microbiota is largely unknown, but one study found that male rats who had free access to exercise experienced a significant increase in the number of Lactobacillus, Bifidobacterium and B. coccoides–E. rectale group compared to rats that didn’t exercise (7). Lactobacillus and bifidobacterium are two of the most well-studied types of bacteria that are beneficial to human health. It’s also possible that the initial compositon of the microbiome affects how you respond to exercise in terms of weight loss or weight gain,
Although physical activity has been shown to be anti-inflammatory (8,9), the stress and heat during prolonged and high-intensity exercise might disrupt intestinal epithelial cell tight junction protein (10,11). It’s likely that individuals who already have an inflammatory gut microbiota (e.g., with increased numbers of LPS-containing bacteria) and decreased gut barrier function are more susceptible to the effects of prolonged, strenuous exercise.
In general, it seems that compensatory mechanisms and physiological differences (e.g., intestinal permeability, metabolism, microbiome, inflammation) partially explain individual differences in exercise-induced 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