No topic may be more popular in the fitness industry than metabolic adaptation and reverse dieting. At the same time, no topic may have more misconceptions.
Can a metabolism get damaged to a point where an individual cannot lose weight?
Is a reverse diet a good way to lose weight?
The purpose of these articles are to address these and other misconceptions about metabolic adaptation and reverse dieting through a combination of science and experience.
Components of Metabolism
Before we get into the nuts and bolts of metabolic adaptation research, let’s first define several terms related to metabolic rate:
- Basal Metabolic Rate (BMR) – The number of calories burned at rest to keep an individual alive.
- Exercise Activity Thermogenesis (EAT) – The number of calories expended during exercise.
- Thermic Effect of Food (TEF) – The number of calories required to digest and absorb the food we eat (~10% of calories consumed).
- Non-Exercise Activity Thermogenesis (NEAT) – The number of calories burned through movement during daily life (e.g. doing housework, fidgeting while sitting, shopping, etc.)
- Total Daily Energy Expenditure (TDEE) – The total number of calories an individual burns daily. This is the sum of BMR, EAT, TEF and NEAT.
One other thing that is important to address prior to getting into the discussion of metabolic adaptation is the idea that the metabolism can get damaged to the point that an individual cannot lose weight, no matter how little food they eat or how much activity perform.
This notion is simply not true and evidence from starvation studies clearly shows that everyone will lose weight if caloric intake is low enough and activity is high enough to create a caloric deficit.
One study commonly cited is the Minnesota Starvation Study. This study was performed shortly after World War II to investigate how the human body adapted to caloric intake and activity levels similar to deployed soldiers.
Researchers recruited 36 men and placed them on a carbohydrate-rich, protein-poor diet that contained roughly half of their caloric intake for 6 months. Moreover, participants were given physical jobs that required them to walk 22 miles walking weekly on average. To us bodybuilders, this sounds like a recipe for muscle loss!
After 6 months the average body weight went from 152.6lbs to 115.6lbs and all participants lost a large amount of body weight.
What we can take from this study is that a very a low caloric intake combined with a large amount of cardio does not cause a metabolism to get “damaged” to the point an individual cannot lose weight. Anyone will lose weight if activity levels are high enough and caloric intake low enough to create a caloric deficit.
Although metabolic damage is not a real physiological phenomenon, metabolic adaptation does occur during dieting.
Evidence of metabolic adaptation after massive weight loss has been observed in several studies.
Johannsen et al.  recruited 16 obese participants with mean BF 49% and placed them on a 30 week rapid weight loss program that resulted in loss of 38 percent of body weight. As a result of this loss, BMR decreased further than expected based upon body weight and lean mass loss suggesting metabolic adaptation had occurred.
Similarly, Knuth et al.  followed 13 contests from the Biggest Loser TV show and also observed metabolic adaptation beyond what was predicted. In addition, researchers observed that the larger the caloric deficit, the more adaptation that occurred.
Metabolic adaptation has also been observed in a natural bodybuilder during contest preparation. In a case study of a professional natural bodybuilder preparing for a contest in which he placed high enough to qualify for the IFPA Yorton Cup Pro World Championships, a reduction in BMR of nearly 50% from baseline was observed despite the competitor only losing 6lbs of lean mass during contest prep .
Taken together these studies clearly show that metabolic adaptation does occur during weight loss in both obese individuals and natural bodybuilders.
Why does metabolic adaptation happen?
There are a number of reasons why metabolic adaptation occurs while dieting:
- Decreased lean mass – Muscle mass is a major determinant of BMR. As muscle mass is lost during weight loss (which even occurs in high level natural bodybuilders ) BMR is reduced.
- Decreased food intake – Roughly 10 percent of calories consumed are expended through digestion and absorption. As caloric intake is reduced, TEF is reduced.
- Decreased NEAT – As an individual diets, it is not uncommon to feel a more sluggish resulting in less movement throughout the day. Moreover, sub-conscious movement, such as fidgeting while sitting, may be reduced during a caloric deficit in an attempt to preserve energy.
- Decreased leptin, insulin, thyroid, estrogen, and testosterone – A reduction in a number of hormones that affect several aspects of TDEE occurs during an energy deficit.
- Increased mitochondrial efficiency – Mitochondria are the organelle in a cell where the majority of energy (in the form of ATP) is produced. However, this process is not perfectly efficient and some potential energy is lost in the form of heat. During dieting, the mitochondria become more efficient at converting substrate into energy and the amount of energy lost as heat is reduced. This means a greater percentage of nutrients consumed are being converted to energy.
- Increased gut microbe nutrient extraction – Undigested food matter enters the large intestine where a portion is fermented by bacteria into a number of products including short chain fatty acids which are up-taken by the human body and can be used for energy. During dieting bacteria become more efficient at extracting nutrients from food consumed resulting in less energy loss through feces.
- Adaptive component beyond what is expected – After accounting for all of the aforementioned factors, there may be an additional adaptive component of metabolism that is responsible for adaptation that occurs beyond what is expected. This is a controversial area of research, but something that may or may not exist and contribute to metabolic adaptation with weight loss.
Those interested in reading more about the mechanisms by which metabolic adaptation occurs during weight loss are encouraged to read .
Body Fat Overshooting
Metabolic adaptation during dieting combined with increased hunger can set individuals up for rapid re-gain. This increased hunger is not normalized until lost lean mass is re-gained ; however, the majority of initial weight gain coming out of a caloric deficit is body fat . This sets individuals up for weight gain beyond the point at which they started their cut and is likely the reason why it is common for individuals who yo-yo diet to end up heavier over time.
Overshooting has been observed in a number of different populations. In an animal study  rats were divided into 3 groups: high calorie, low calorie, or calorie cycling (alternating between periods of high calorie and low calorie). After 6 months body weight and body fat did not differ in high calorie or calorie cycling groups despite cycling group consuming periods of low calorie intake. Moreover, rats in calorie cycling group got more efficient at storing body fat each time they came off of a diet.
Body fat overshooting has also been observed in humans. Weyer et al.  studied 5 individuals that lived in a biosphere for 2 years during which time they ate in a slight deficit. Metabolic rate was measured at the end of the 2 year study and also 6 months later. After returning to their normal lives for 6 months, bodyweight was normalized back to where it had be prior to the experiment; however, metabolic rate had not normalized compared to an age, gender, height, weight, and body fat percentage-matched reference group.
In bodybuilders, overshooting is commonly observed after competition. In a case study from the 1990’s, an amateur bodybuilder lost 23lbs during contest preparation and gained back 30lbs in 3 weeks after competition .
Similar findings have been observed in more recent case studies. Metabolic rate remained lower than at start of prep at 3  and 5 (Pardue unpublished data) months post-contest after weight was regained. This suggests that hormone levels and metabolic rate lag behind body weight gain post-diet and simply re-gaining weight quickly is not enough to normalize levels.
Take Home Point
Metabolic damage does not exist; however, metabolic adaptation does occur during weight loss and sets an individual up for body fat overshooting during the post-diet period. Although this sounds like a story of doom and gloom, in part 2 of this series we will discuss techniques commonly utilized to reduce adaptation during dieting and to help combat re-gain post-diet.
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