There is a lot of talk in the fitness industry about reverse dieting, metabolic damage, very low calorie diets and types of dieting. Strong bases for these claims come from empirical evidence observed in physique athletes. Studies are currently being done to better understand this issue but there is still much that is not yet understood in relation to this topic.
What is the claim of “metabolic damage”?
The idea of metabolic damage is that very low calorie diets actually suppress overall metabolism in the body. The general school of thought is that when calories are restricted the body is forced to become more economical in the use of calorie expenditure. This means that if a certain activity used 250kcal before a harsh diet, that the same activity may only use 220kcal after a long or harsh period of caloric restriction. The “metabolic damage” would actually be an increased efficiency in calorie use, meaning that you could go back to a prior maintenance level and gain body fat after a lower calorie diet.
Metabolic damage, perhaps made popular by Layne Norton, is now better explained as metabolic adaptation. As in many other aspects the human body adapts to outside influences (training is another example). So when a person is continually in a caloric deficit, the body must become more efficient as calorie use in order to maintain energy balance. There is evidence pertaining to obese individuals concerning metabolic adaptations and changes in RMR that explain the difference between subjects concerning weight loss at an extreme caloric deficit.
But what about the evidence?
Studies have shown that low calorie diets can impact RMR, shown by differences in gas exchange (indirect calorimetry). The study showed that, at least in women, diet alone resulted in a significantly lower RMR at the end of the 12 week study. The diet and exercise group also showed a decrease in basal metabolic rate at 12 weeks but also had an increase in resting metabolic rate. So even though both groups had a decrease in BMR (normally taken fasted and immediately upon waking in a testing facility) the increase in RMR for the exercise group increased.
There is evidence that shows that how you diet may be as important as to how severely you diet. It has been shown that high protein low carb vs high carb low protein diets have significantly different outcomes in a 12 week study. The study resulted in a lower TEE (total energy expenditure) for the high carb, as well an increased energy balance by the 12 week point. There were also fullness ratings, which only differed in the beginning and showed the high protein to be more satiating. Protein has a higher thermic effect than carbs which also can play a role in determining how TEE is affected during caloric restriction and food choice.
There also is a lot of empirical evidence as can be seen in many physique athletes, women in particular, after coming off of low caloric intakes after a physique competition. Studies are currently being carried out on physique athletes because they are in a unique situation, coupling both high exercise and low caloric intakes given their energy output. There may be evidence that your body actually becomes extremely efficient at calorie use, thus allowing an individual to fuel the same activities with overall lower energy expenditure. This is likely linked to hormones such as T3, T4 as well as others like ghrelin and leptin that influence hunger and are not yet completely understood.
There are also protein markers from adipose tissue (fat cells) that correlate with differences in total energy expenditure. As these proteins increase, total energy expenditure also increases, and when one decreases so does the other. Now this does not tell us exactly why metabolic adaptations occur, but does give us some insight as to where to possibly look in the future.
How do we avoid weight gain following caloric deficits?
When coming off of a diet wherein an individual was in a caloric deficit the increasing belief is that it takes a similar amount of time for the metabolism to adapt back to a less efficient caloric use. This means that simply jumping caloric intake back to pre-diet levels will result in weight gain (fat being the main concern). It is recommended than when ending a diet, calories are slowly and periodically added back into the diet to minimize fat gain. The speed at which calories are reintroduced into the diet depends on the gender, activity level, and needs of the athlete.
Just as slow and steady is the healthy way to garner weight loss, the same principle should be applied to increasing caloric intake. For those who demand to stay leaner longer a slower approach is recommended. If an athlete’s physique is not an issue however, one can be a bit more liberal with the addition of calories post diet.
Due to the small amount of research into the subject it most certainly leaves a lot of room for interpretation. It is recommended that you slowly increase caloric intake to minimize excess weight gain and “rebounding” after a harsh diet. This should facilitate metabolic adaptations without allowing for excessive weight gain.