This biological process could be hindering your ability to lose and maintain weight loss! To lose weight you need to be in a negative energy balance, meaning you consume less calories than the number of calories your body uses daily.
CALORIES IN < CALORIES OUT = WEIGHT LOSS
When we look at calories out, or total daily energy expenditure (TDEE) there are 4 main components
Basal metabolic rate (BMR): the amount of energy needed for basic body processes
Non-exercise adaptive thermogenesis (NEAT) and non-exercise physical activity (NEPA): these include non-exercise activities in our day to day lives like fidgeting, doing the dishes and walking from our bedrooms to the fridge and back again.
Exercise activity thermogenesis (EAT): this is the energy spent exercising.
Thermic Effect of food (TEF): the amount of energy that is required to digest and absorb nutrients in our food. Protein and fiber have a higher TEF than other nutrients like fats and simple carbohydrates.
Calories in is pretty self-explanatory - it’s the food and drink we consume.
SO WHAT IS METABOLIC ADAPTATION?
Metabolic adaptation is a physiological response to reduced calories in. As you eat less calories and lose weight, your TDEE decreases to reach a homeostatic point where your bodyweight and your caloric intake match your daily energy expenditure. This is an evolutionary advantage for us as humans and originates from the benefit it provides in preventing us from starving and dying during times of famine – for most of our history excess access to food wasn’t common and starvation was a threat.
Metabolic adaptation is one of the reasons people struggle to keep weight off after losing it in the first place. Starvation mode is a phrase often used to refer to this process as our bodies adapt to the stress of relative starvation – it’s our bodies way of managing its role in energy conservation.
The diagram below from Trexler et al., (2014) is a theoretical model for metabolic adaptation and shows the ways in which our bodies respond to a decrease in energy intake.
From Trexler et al., (2014) A theoretical model of metabolic adaptation and potential strategies to attenuate adaptations. A/A/T hormones = Anabolic, Anorexigenic, and Thermogenic hormones; O/C hormones = Orexigenic and Catabolic hormones. Dotted lines represent inhibition of metabolic adaptation.
SO WHY DOES THIS MATTER FOR AN ATHLETE MANIPULATING THEIR WEIGHT?
In order to manipulate an athletes weight (body mass), their energy intake is adjusted in relation to their energy expenditure. Energy expenditure is dynamic by nature and because of this, adjusting energy intake accordingly can seem less simple than a simple calories in vs calories out.
In response to weight loss, there are reductions in TDEE (BMR, NEAT, NEPA, EAT & TEF) – the magnitude of these adaptations is proportional to the size of the energy deficit created by the reduction of calories in. Besides the performance implications of large calorie deficits, when trying to drop weight as an athlete, it makes sense to instead make the most out of the smallest calorie deficit possible that provides appreciable weight loss. This will reduce the rate of weight loss, but works to limit adaptations that can make reducing fat mass a lot harder. Studies have shown that adaptive thermogenesis and decreased energy expenditure can persist after a weight loss period, even in those that have maintained the reduced bodyweight for several months to a year (Rosenbaum et al., 2008; Thom et al., 2020).
Athletes who aggressively diet for a competitive season and rapidly regain weight afterwards may find it more challenging to achieve optimal body composition in subsequent seasons. To avoid rapid weight and fat gain after dieting ‘reverse dieting’ has been used amongst physique athletes. Reverse dieting involves incrementally increasing calories – the theory behind this is that providing a small caloric surplus helps to restore hormone levels and energy expenditure to pre-diet levels whilst also closely matching an athletes energy intake with their recovering metabolic rate to limit fat gain. Although this is the case, there has been limited scientific research on reverse dieting so far and most evidence for success is anecdotal (de Jonge et al., 2018).
de Jonge, L., 2018. Could Slow Reintroduction of Calories after Weight Loss Prevent Metabolic Adaptation after Weight Loss?. JAMA, 295(13), pp.1549-1555.
Rosenbaum, M., Hirsch, J., Gallagher, D.A. and Leibel, R.L., 2008. Long-term persistence of adaptive thermogenesis in subjects who have maintained a reduced body weight. The American journal of clinical nutrition, 88(4), pp.906-912.
Thom, G., Dombrowski, S.U., Brosnahan, N., Algindan, Y.Y., Lopez-Gonzalez, M.R., Roditi, G., Lean, M.E. and Malkova, D., 2020. The role of appetite-related hormones, adaptive thermogenesis, perceived hunger and stress in long-term weight-loss maintenance: a mixed-methods study. European journal of clinical nutrition, 74(4), pp.622-632.
Trexler, E.T., Smith-Ryan, A.E. and Norton, L.E., 2014. Metabolic adaptation to weight loss: implications for the athlete. Journal of the International Society of Sports Nutrition, 11(1), pp.1-7.