Weight loss and calorie management are interesting topics. Since a good portion of us live in a consumer society, managing our calorie consumption and weight is a difficult task. We then look for advice on how to better manage this aspect of our lives via doctors, health professionals, coaches, and other sources. One common recommendation, especially when losing fat is the primary goal, is to exercise more. The more one exercises, the more one loses weight, right? Yes, but to some degree. However, recent studies show that this methodology is not as fail-proof as we might have thought. In fact, there is critical evidence that indicates that this way of thinking is incomplete. It explains why active people can still gain weight, even when sticking to the recommended calorie intake for their size and activity level.
Three Energy Expenditure Models
Before we can understand how our bodies utilize and manage energy, we must first understand how energy is expended. Once we understand how energy is used in our body, we can then theorize different models and understand how it divvies up said energy. Energy is expended in three different ways: to maintain body functions, to digest, absorb, and store food, and to create physical movement.
Maintaining body functions includes those that occur at rest, such as breathing, circulating blood, supporting organ functions, and maintaining stable body temperature. There are two types of energy consumption rates with regards to maintaining body functions: basal metabolic rate (BMR) and resting metabolic rate (RMR). BMR measures the minimum number of calories required to keep our bodies functioning without dying. RMR measures the number of calories used for pumping blood throughout your body, breathing, keeping appropriate body temperature, maintaining levels of different chemicals, and other functions that are required to keep you alive at a bare minimum. RMR is the amount of energy the body requires at rest to support ongoing physiological processes beyond bare survival. RMR encompasses eating, walking for short periods, using the bathroom, sweating or shivering, boosting the immune system, and other non-essential but ongoing physiological processes the body performs at rest.
The thermic effect of food (TEF), also known as diet-induced thermogenesis (DIT), refers to the energy the body expends to digest, absorb, and store nutrients from food. TEF expenditure varies based on the type of food we consume. It is highest for protein, moderate for carbohydrates, and lowest for fat. The body uses different amounts of energy to digest, absorb, and metabolize each one. For example, protein requires a significant amount of energy for the body to process. If you consume 100 calories of protein, roughly 20–30 of those calories are used during digestion and metabolism. By comparison, carbohydrates require only about 15% of their energy to be processed, while fat requires roughly 5%.
Lastly, there is the activity energy expenditure (AEE), which is the energy spent on physical movement, including planned exercise and non-exercise activity. As the name suggests, it refers to the calories expended on activities beyond basic bodily maintenance, such as running, lifting weights, going for a walk, standing, fidgeting, and other movements that exceed resting metabolic needs.
The Additive Total Energy Expenditure Model
The Additive Total Energy Expenditure (ATEE) model is the intuitive framework we are accustomed to using. This model suggests that our total energy expenditure is the sum of all forms of expenditure, in which BMR, RMR, and TEF remain relatively constant, while AEE fluctuates. Thus, increasing AEE, primarily through physical activity, leads to an increase in total energy expenditure. If one wants to lose weight, for example, it is intuitive to increase physical activity, allowing the body to burn not only the calories consumed that day, but stored calories as well.
The ATEE model is a common understanding on how our body regulates calorie consumption. There is a strong correlation between increased physical activity and total energy expenditure. However, this understanding is not complete. Although this theory is accurate over short periods of time, it fails to account for the cybernetic adjustments the body undergoes as it adapts to recurring habits.
Evidence ATEE Model is Incomplete
Firstly, when physical activity dramatically changes for a short period of time, the ATEE model accurately predicts calorie consumption. This strategy is effective for those that have a sedentary lifestyle and change to a more active one within a short period of time. The evidence of fat loss alongside increased physical activity is staggering. However, once the body adapts to the new norm, it is shown that fat loss and calorie consumption overall diminish. This effect is due to the body adapting and modifying consumption to compensate for the new source of expenditure.
A study demonstrated this phenomenon with people involved in long-term exercising. The study showed that these people had a reduction in BMR, such as suppressed ovarian activity and lower estrogen production in response to the moderate exercise. Contrary to traditional belief, BMR doesn’t stay constant but rather adapts, modifying our understanding and calculation for total calorie expenditure. Other species also exhibit this behavior, showing that an increase in long-term physical activity reduces energy expenditure on growth, somatic repair, and BMR.
Studies also show that more active populations of people do not have higher total energy expenditure as one might think. It was found that highly active populations burn the same amount of calories per day as sedentary populations of the same body size. Thus, an individual who expends only 100 calories per day through physical activity due to a sedentary lifestyle may ultimately expend a similar total amount of energy as someone who expends 600 calories per day through an active lifestyle. This new understanding contradicts the ATEE model entirely.
It is shown that the amount of calories expended is capped off at a threshold, and doesn’t behave linearly. If it was linear, using the previous example, the active person would have to eat 500 more calories a day than the sedentary lifestyle person. However, if they did, we would observe that the active person would in fact gain weight over time, due to over-consuming. Thus, we must consider a new theory that captures this phenomenon.
New Potential Understanding: Constrained Total Energy Expenditure Model
It is evident with current research that the body has a limited total energy budget. Energy expenditure increases with physical activity when the body is accustomed to lower activity levels. However, this increase in energy expenditure plateaus when the body has adapted to higher physical activities. In the paper “Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans”, Herman Pontzer and colleagues express the following:
… energy allocation among physiological tasks responds dynamically to long-term shifts in physical activity, adapting to maintain total energy expenditure within some relatively narrow range.
Contrary to the previous belief that energy expenditure increases linearly with activity, the body in fact compensates for long-term physical exertion by reducing the energy it spends on other systems, such as reproduction and metabolism. First, let’s try to understand this phenomenon by taking a glimpse of our ancestors.
Looking at our evolutionary history helps explain why this theory makes sense. Our bodies evolved in a much different environment, where resources were much more limited and it was physically demanding to survive and collect food. Our bodies adapted to endure over a long period of time, so when physical activity increased, our energy expenditure was constrained to not drain ourselves completely. If the energy spent obtaining food simply cancelled out the energy gained from eating it, our species would most likely have died out. Thus, to keep the Total Daily Energy Expenditure (TDEE) constrained, our body developed a compensatory mechanism.
Evidence of this new model is apparent in farmers. A study found that subsistence farmers and traditional hunter-gatherers total energy expenditures are similar to those of more developed population where their daily tasks involve much less physical activity. Two similar body types require relatively the same amount of calories, even though one spends much more daily energy than the other.
The research suggests that only around 72% of extra calories burned through additional activity translate into a net increase in total daily energy expenditure. In other words, 28% of calories of said activities is compensated or offset by other components of energy expenditure. These other components of energy expenditure are BMR or other non-exercise activity thermogenesis (NEAT). NEAT activities are ways we expend energy for activities we do that is not sleeping, eating, or sports-like exercise. It includes walking to work, fidgeting, standing, typing, doing household chores, and shivering.
How Does the Body Use Excess Energy?
This new understanding of calorie expenditure suggests that increased activity may yield diminishing returns due to compensatory responses in non-physical energy expenditure. Thus, if we spend more energy on physical activity, less energy will be used for non-physical activities. However, the reverse is also true. Less energy used on physical activities will result in more energy used for non-physical activities. If there is energy remaining due to a sedentary lifestyle, the body reallocates the energy toward internal physiological processes. This reallocation can cause these processes to overwork and cause harm to the body.
For example, the immune system is one of the non-physical processes that will be used to compensate with excess energy. One may think “Wouldn’t this be a boost in my immune system?” Unfortunately, the immune system can be harmful if in a state of overcompensation. One such side effect is inflammation. When a person is more active, their body reserves energy, forcing the immune system to be efficient. However, if the immune system has excess energy, the overworking can lead to chronic low-grade inflammation. Instead of being useful, like healing a cut, an immune system may overwork and damage healthy tissues. Over time, this over-inflammation can cause diseases such as heart disease, Alzheimer’s, and type 2 diabetes.
Excess energy can also trigger stress responses, as these non-physical processes tend to ramp up during periods of sedentary behavior. Similarly to the immune system, if your stress response activities have excess energy, your body will use it. Sedentary lifestyle may lead to higher resting cortisol levels and other stress hormones. These excess levels, over time, can wear down your cardiovascular system, contribute to high blood pressure, and negatively affect mental health, such as an increase in anxiety and depression.
All in all, an active lifestyle will help you live a healthier life due to the balance of energy expenditure in the body. Non-physical systems will not over work and optimize the energy to maintain the body efficiently. Thus, if calories are going to be used and stored regardless, how does one lose weight?
Weight Loss
Modern techniques don’t work for weight loss because they fail to incorporate the Constrained Total Energy Expenditure model. With techniques that focus solely on physical activity, participants lose weight, then quickly regain it back. Losing weight solely on physical activity is difficult because of the body’s compensation and metabolic adaption. A drop in TDEE closes the energy deficit, making continued weight loss harder and setting the stage for weight regain once dietary restrictions are relaxed or intense exercise is reduced.
A weight loss program cannot be universal, but must be tailored to the individual since everyone requires a different assessment. For example, studies show that people with higher fat mass exhibit a higher degree of energy compensation. Contrary to popular belief, it is theorized that people with higher fat mass are more inclined to compensate for increased activity by conserving energy elsewhere. As people get fatter, their bodies may compensate more for calories we burn that day, making fat loss progressively harder. People with higher energy compensation tend to have higher fat mass. Thus, weight loss programs need to consider not only the individual’s fat mass, but their body type, age, and other factors that affect our energy compensation mechanism.
So, to lose weight, this new theory suggests overcoming this constrained energy budget by achieving true caloric deficit. Caloric deficit is key because it sustains an energy imbalance. If your body consistently lacks the appropriate amount of calories to perform its daily functions, it will need to retrieve said energy from storage. Exercise also accelerates fat loss by complementing the caloric deficit. This is obviously a generic understanding of the phenomenon and will help you think differently about our bodies and energy expenditure. However, you should always refer to a professional before making any drastic changes to your diet and lifestyle.
It is fascinating science is unveiling a different perspective on our body’s energy compensation. Although long-term physical activity does not help tremendously with weight loss, physical activity helps in other areas. It helps prevent certain diseases, helps our immune system perform efficiently, and helps with our mental state. It also uncovers efficient ways of understanding one self and learning how to properly lose weight. Dieting is a dominant factor in weight loss in which caloric restriction is a critical method to reliably create long-term energy deficiency. Increases in TDEE from exercise are often smaller than expected due to metabolic compensation.