Adaptive thermogenesis is defined as “the decrease in energy expenditure beyond what could be predicted from the changes in fat mass or fat free mass under conditions of standardised physical activity in response to a decreased energy intake”(1). This is the technical definition a common term used in the nutrition and fitness industry, “starvation mode”. This is effectively a plateau in weight loss despite being in a theoretical calorie deficit. Adaptive thermogenesis is also an important factor regarding maintenance of weight after a period of dieting.
Adaptive thermogenesis is a very complex issue that is still not fully understood by the scientific community. It involves a series of changes in hormones like thyroid stimulating hormone, leutinizing hormone, follicle stimulating hormone, growth hormone, cortisol, leptin, ghrelin, peptide YY etc. All of these changes are basically your body sending signals to the brain to say that there is a change in body fat percentage and the metabolism needs to slow down to prevent excessive weight loss leading to starvation. Your body doesn’t want to get lean and it doesn’t mind being fat. This is because, during our evolution, being fat was never a risk but being lean was and it is because of this that it will do all it can to fight your weight loss efforts.
The main contributing factors to adaptive thermogenesis are a prolonged hypocaloric state (calorie deficit) and body fat percentage. One possible way to reduce the impacts regarding the former are to incorporate periods of overfeeding or eating around maintenance calories. By doing this, it can up-regulate the hormones that were previously telling your brain that you could be starving. Leptin, insulin and peptide YY will increase while ghrelin (the appetite hormone) will decrease. This signals the hypothalamus that you are eating again. Studies have shown that the severity of adaptive thermogenesis is greater in lean individuals than obese. So as an individual diets down to lower body fat percentages, these refeeds are required more frequently.
Jeremy has dieted down from 100kg to 85kg. His friend Muzz has always been 85kg. Quite coincidentally, Jeremy and Muzz have very similar body compositions. Muzz’s maintenance calories are 2800. Jeremy assumes that his maintenance calories are the same as Muzz’s because of their similar statures. He eats 2800 calories also and is surprised that he starts to gain weight. As Jeremy has some “starvation mode” response going on his BMR is about 15% lower than Muzz’s and as they have similar activity levels his maintenance calories are actually 2400. (Note that the amount of energy expended is also slightly lower after a dieting phase) Jeremy is now eating at a 400 calorie surplus. Jeremy would need to either drop his calorie intake a little or increase energy output to maintain weight.
The decrease in resting metabolic rate by adaptive thermogenesis is to prevent starvation but the amount of this reduction has been shown in studies to be approximately 250-500 calories depending on the size of the person. This is the equivalent of a small to medium sized meal like 200g of chicken and a few servings of fibrous veggies or a McDonald’s cheeseburger. So there are a few possible options.
- Decrease calories further to create a hypocaloric state once again (may or may not be possible)
- Increase exercise (add cardio or increase intensity) to increase the total energy output
- Implement a diet break or refeed to slow the adaptive thermogenesis
Which option is best will depend on the individual situation. It’s important to remember that despite slower weight loss being better for preservation of fat free mass, you should not be in a hypocaloric state for ever and a return to maintenance calories will be required at some stage. How this can be done in the most effective way will be explained in the next topic regarding reverse dieting.