20 March 2022
Research update:
Is exercise a good way to lose weight?
Recently I learned about an interesting new concept related to physical activity, energy expenditure and weight loss, the so-called constrained Total Energy Expenditure model. Our Total Energy Expenditure (TEE) corresponds to the combined energy we spend during physical activity and at rest during maintenance of all our organ systems. Intuitively, we would assume that the more we are physically active, the more energy we would spend. In other words, common sense tells us that the more we exercise, the more fuel we burn and the more weight we will lose.
However, based on research by Herman Pontzer and others, this does not seem to be the case. His work and that of his colleagues suggests that there is no linear relationship between increased physical activity and energy expenditure. In fact, people who are highly physically active often do not have an increased Total Energy Expenditure compared to people who do not exercise at all.
For instance, Pontzer and colleagues measured Total Energy Expenditure in adult Hadza people of Tanzania, which are traditional hunter-gatherers with high levels of physical activity, and compared it with a sample of European and American adults. Despite walking more in a day than many of the Europeans and Americans in a week, the Hadza had the same Total Energy Expenditure (after controlling for for fat-free body mass, which is a major determinant of our energy expenditure rate).
There are many other studies that show similar results, which led Pontzer to conclude that only sedentary people who take up exercise experience an increase in their Total Energy Expenditure. If they increase their exercise regimen further, the increase in physical activity related energy expenditure is balanced out by a decrease in energy spent on non-physical activity related activities (=constrained TEE model; see figure below).
Both physical activity and non-physical activity-related metabolic activity contribute to the total energy expenditure (TEE). In the additive TEE model (top), the non-physical activity is constant. Engaging in physical activity increases the TEE linearly, resulting in increased burning of fuel and weight loss (if the food intake remains constant). In the constrained TEE model, TEE is homeostatically maintained within a narrow range. Increasing physical activity in sedentary individuals initially causes a slight increase in TEE. However, further increases in physical activity are not associated with significant changes in TEE. Instead, increased physical activity leads to a reduction in non-physical activity-related metabolic activity. As a result, the TEE is kept near a constant set point. In the constrained TEE model, engaging in physical activity would only lead to weight loss initially when taking up exercise, but not as the exercise intensity is increased. Adapted from Herman Pontzer (2015).
As Pontzer points out, the constrained TEE model makes sense from an evolutionary point. During evolution, physical activity is mostly dependent on food availability. For our ancestors, a lack of food meant that they had to become physically active to gather food or go hunting. In times of food scarcity, it would not be advantageous to increase their total energy expenditure even more. Instead, it would make sense to downregulate other energy demanding processes, e.g. via reduced stress and sex hormone production, allowing the individual to allocate most of their energy to activities that are essential for survival.
The similar Total Energy Expenditure of sedentary and highly active people means that people who do not exercise must be spending more energy on other things. According to the research by Pontzer, one major factor contributing to our energy expenditure is stress. Indeed, stress can increase our energy expenditure markedly. For instance, this article describes an experiment where a healthy individual was subjected to a math test or stress-inducing questions, which resulted in 40% more energy per minute spent during the math test and 30% more during the interview. Hence, one likely reason for the similar Total Energy Expenditure of sedentary and active people is that individuals who exercise regularly spend less energy on mental stress-related activities. In other words, they experience less stress when doing challenging tasks. It is indeed easy to imagine that traditional hunter-gatherers use less energy on stress than a high achieving office worker in the industrialized world.
Another response that is downregulated during high levels of physical activity are reproductive functions. This does not only include ovary function, which is well known to be downregulated during high levels of exercise. It may also include behavioral changes, such as less aggressive behavior of males (as an expression of male-male competition). Interestingly, Hadza hunter-gatherers were found to have morning salivary testosterone concentrations of only about 50% compared to men in the U.S. In many studies, physical activity is also associated with later age at menarche, suggesting that the earlier onset of menarche in developed countries may partially be a consequence of the lack of physical activity that many children experience in today’s world.
The research by Pontzer and colleagues has important implications for achieving weight loss and health. As discussed above, increased physical activity would lead to an initial weight loss in people who are mostly sedentary and decide to start exercising. However, as the exercise regimen becomes more intense, there will be a neglibible effect on TEE and weight loss. Instead, individuals will lower their non-physical activity related energy expenditure. This may be bad news for those who are trying to rely on exercise to control their weight or lose extra kilograms and suggests that a healthy diet is the best way to lose weight. On the other hand, the constrained TEE energy model highlights important mechanisms through which exercise can improve our health. I already mentioned above that exercise likely reduces energy expenditure associated with mental stress, partly due to lower production of stress related hormones. Non-physical activity related energy expenditure also includes the inflammatory response, which is well-known to promote the development of various age-related diseases (cardiovascular diseases, cancer, Alzheimer’s disease).
The constrained TEE model also suggests why excessive exercise can have negative consequences, by lowering non-physical activity related energy expenditure below a critical level. The consequences include immunosuppression and loss in reproductive function, as often seen in elite athletes.
One interesting question is what actually determines our normal Total Energy Expenditure level, or TEE level. According to Pontzer, we all have a certain TEE set point. Under homeostatic conditions, our energy (food) intake matches the TEE. If we take in more energy than our TEE set point, then the extra energy will be converted into fat. If we take in less energy, we will lose weight, which highlights the primary importance of diet in controlling our weight.
Not everyone has the same TEE set point. Our TEE set point seems to be determined by the ability of our body to digest food and turn this into usable energy. Hence, the amount of energy we can spend is fixed and not dependent on how much food we take in. If we want to utilize more energy for physical activity, we can only do so by downregulating non-physical activity related energy expenditure.
Not surprisingly, athletes tend to have a high TEE set point and can therefore allocate a higher proportion of their total energy expenditure to physical activity. This means that athletes seem to have a better ability to turn food into usable energy. Nonetheless, there is a limit, which based on Pontzer’s calculations is about 4650 calories for a man weighing 85kg.
According to Pontzer, the reason for the higher TEE in athletes could be that they develop a higher set point through training during their development. Alternatively, the TEE set point may primarily be determined genetically and individuals may become successful athletes in part because they happen to have a high TEE set point.
Athletes can temporarily be so physically active that they exceed their TEE set point. However, this increased energy demand can usually not be maintained through eating more. Instead, excessive exercise in elite athletes is associated with extensive exhaustion and weight loss. Hence, an increase in our energy expenditure beyond the TEE set point is not sustainable. What is more, it will lead to severe downregulation of non-physical related activites, such as immune defense and tissue repair, leading to illness and injuries.
In summary, Pontzer’s research suggests that we all have a Total Energy Expenditure set point. Increasing exercise will not have a major effect on our overall energy expenditure, but instead lead to the downregulation of energy spent on non-physical activities such as stress, reproductive, immune and inflammatory responses. Thus, exercise is a great way to reduce stress levels. It is, however, not ideal for weight loss, where diet still remains the best approach.