Understanding Metabolic Adaptation

The term metabolic damage has gained lots of traction over the years. Researchers initially observed a reduced metabolic rate in subjects who had lost a substantial amount of weight. This is far from shocking, since reducing an individual’s body weight will simultaneously reduce their energy demands. What was unique in this case however was the metabolic rates of some individuals were far lower than what the researchers projected.

These findings became popularized within various fitness circles and were quickly labeled as metabolic damage. However, at the moment there isn’t any convincing evidence to support the existence of metabolic damage within this context. What researchers were observing is more accurately defined as metabolic adaptation and adaptive thermogenesis (1).

During a period of caloric restriction accompanied by a reduction in body weight, your body undergoes several physiological changes to adapt to the changing environment – both internal and external. Leptin is a hormone whose primary function is regulating energy balance and maintaining body weight. Often called the satiety hormone, it helps regulate an individual’s drive to consume food. Because it’s synthesized in adipocytes leptin is sensitive to body fat stores (2). At some point when body fat is lost during a period of caloric restriction, serum leptin concentrations decrease. This reduction in leptin concentration is accompanied by a cascade of neurochemical alterations that can significantly increase hunger and reward seeking behaviour (3).

Various other hormones including thyroid are also impacted. Thyroid hormone has been demonstrated to be an important variable in determining energy expenditure and basal metabolic rate (4). It’s been observed that fat loss during a sustained caloric deficit can reduce thyroid values, thereby decreasing basal metabolic rate (5).

Additionally, Adenosine Triphosphate (ATP) synthesis becomes more efficient. Typically ATP synthesis is roughly 40% efficient which means approximately 60% of energy is lost via thermogenesis (6). However in the presence of low energy availability and reduced body fat, mitochondrial efficiency increases. Proton leak, a process regulated by uncoupling proteins causes energy to be lost as heat. But increased mitochondrial efficiency reduces proton leak and increases ATP synthesis as an adaptive response (7). We also see other aspects of our physiology such as muscular work efficiency increase as calories are restricted and weight is reduced (8).

As these adaptations occur we also see a reduction in NEAT (non exercise activity thermogenesis). This is associated with spontaneous, non-exercise related physical activity, and accounts for the majority of energy expenditure (9). Researchers have observed that caloric restriction and loss of body weight can reduce an individual’s NEAT significantly. Unfortunately this is largely unconscious so there’s not much that can be done. Adopting a daily step count is a common practice to keep account for and regulate energy expenditure. However, because this is for the explicit purpose of expending calories it’s not technically NEAT. It’s Exercise Activity Thermogenesis. But I digress.

Researchers have found that our bodies like consistency. Enter settling point theory. As one paper described it “The set point model is rooted in physiology, genetics and molecular biology, and suggests that there is an active feedback mechanism linking adipose tissue (stored energy) to intake and expenditure via a set point, presumably encoded in the brain” (10). Although this does not account for all relevant variables it does explain to some degree the body’s desire to preserve homeostasis from a body weight and energy availability standpoint. Essentially as energy availability from external (ie. food) and internal (ie. body fat stores) sources decrease, our body tries to resist this change via several physiological and neurochemical changes. As mentioned previously, changes in thyroid, leptin and even increased hedonic dive for food are just some of the numerous adaptive responses.

As you reduce your bodyweight, the energy requirement for locomotion decreases accordingly (11). NEAT has been shown to vary between individuals of the same size by as much as 2000kcal per day (12). In a previous article I wrote for Kabuki Power I mentioned “a paper by Rosenbaum and colleagues cited a reduction in total energy expenditure (TEE) of 10-15% which was unexplained by changes in body composition. Of this 10-15% reduction, roughly 85% could be explained by reductions in non resting energy expenditure of which NEAT is the largest contributor” (13)(14). Once we account for these changes the vast majority of discrepancies between estimated BMR and actual BMR are accounted for.

So, is metabolic adaptation an issue? Absolutely. But does it suggest some form of damage? Well, at the moment there doesn’t seem to be strong supporting evidence of this. But what can be done in order to manage some of these adaptive responses so you can successfully maintain your new body weight/composition? One potential approach is utilizing a high energy flux approach (15). Researchers have consistently found that regular physical activity is strongly associated with successful weight management. By increasing energy intake in proportion to energy expenditure, we can offset some of the adaptive responses of dieting, and increase energy intake, all while staying within a predetermined bodyweight range.

Increasing calories can reduce hunger, increase the thermic effect of food, and help decay psychological fatigue that accumulated over the course of your diet. Adopting a more gradual approach to weight loss (ie. 1% of your body weight lost per week) may delay some of these adaptive responses since the acute change in energy availability is not dramatic. Additionally it’s important to establish clear timelines and end dates for your diet periods. Dieting more than three months is typically not recommended since you often see diminishing returns beyond that point. Utilizing maintenance phases to slowly increase your energy intake while remaining weight stable will set you at a higher caloric starting point at the onset of the next diet phase.

To summarize, metabolic damage doesn’t appear to have strong supporting evidence at this time. What we typically observe instead is metabolic adaptation. These adaptations are entirely reversible in the vast majority of cases. When done correctly, dieting can be an important aspect of healthy eating and optimizing body composition.

Thanks for reading along, and I hope you guys got some value out of this!