Can metabolic damage arrest weight loss?

Author- Dr Akshay Alawani ANutr, Faculty Head, INFS

Evolution has taught humankind that ‘the one who adapts - survives’. Humans became the dominant species not because they are the fastest, strongest, or biggest but because they are the smartest. Physiological processes in the human body have also adapted dramatically to acclimatize to the stressors of the environment and safeguard the survival of the species.

Just a few thousand years back when farming was not discovered, food scarcity was a significant issue. Long-term starvation was a harsh reality. Even after the discovery of farming, occasional famines kept this threat alive.

To avoid tissue mass (muscles and organs) loss and eventual death, the human body developed adaptation strategies to deal with caloric insufficiency. The concept of these processes and their effect is collectively termed ‘Metabolic Adaptation’. This is a vast area, but this article will focus on real-life understanding of the concept that is important from dieters’ (targeting weight loss) perspective.

The term metabolism is often used to signify daily overall energy expenditure. Interestingly, energy expenditure is proportional to energy intake. If you eat more, you spend more; if you eat less, you will spend less. To put it simply, the human body has a very strong ‘energy-saver’ mode. Like your cellphone’s saver mode reduces activities to minimal to prevent faster shutdown, physiological processes in a dieter can adapt dramatically to prevent loss of tissue.

Metabolic adaptation consists of several processes including the lowering of basal metabolic rate and activity expenditure. This means that lying down in your bed or during an activity, you will spend lesser calories during the energy deficit and weight loss phase compared to when you are eating more or maintaining weight. It is important to add that as anyone loses weight, due to tissue loss, the metabolic rate is bound to decrease. This is natural – you get smaller and so do your energy needs. However, individuals that lose weight, show an additional decline in overall energy expenditure which cannot be explained by loss of tissue mass. This additional drop is ‘metabolic adaptation’.

An old experiment (Minnesota Starvation) done with soldiers, involved starvation protocol and then recovering from the same by refeeding. In this study, dramatic descent in the rate of energy expenditure after starvation was reported. Loss of tissue could not alone explain the whole decline.

A more recent example would be an experiment conducted with American TV show (The Biggest Loser) participants. They had lost a dramatic amount of weight in a short span. After several months, when energy expenditure was measured in the participants, it was found to be less than expected. Metabolic adaptation was reported here as well. The probable reason - they were still eating less than their maintenance and to prevent further tissue loss, their body was resistant to using these adaptations.

Though these participants had lost a lot of tissue mass, they were still overweight when these energy measurements were taken. Still, their body wasn’t promoting or responding to energy deficit with weight loss like in the initial stage. As you may have already sensed, they are not alone in this pickle. Every dieter faces slow weight loss after a certain point, and there is considerable data showcasing that lack of adherence is not the only thing stopping their progress.

Metabolic adaptation was once necessary, and it has helped the survival of humans. However, after globalization, food distribution channels have shown radical change, and food availability is no more an issue in most parts of the world. Questions arise - if we were able to evolve to survive famine, why hasn’t our body adapted to burn fuel faster to avoid obesity? Moreover, why does it try to prevent weight loss with such adaptations? The answers are not very clear; most likely, physiological processes haven’t had enough time to catch up to such an intense surge in food availability.

Extent of Adaptation

In a study by Martins, after a weight loss intervention that led to participants losing 7-17 kg weight, metabolic adaptation was on average 50 kcal/day (at maintenance diet). Even during a 50% energy deficit, Heinitz reported ~200 kcal adaptation.

To conclude, the existence of metabolic adaptation (the additional drop in energy expenditure) is beyond doubt. However, when someone thinks that they are adapted to 800 kcal/day and are not losing weight further, another look into adherence and close monitoring of energy intake should be done. In a controlled setting with continual guidance, the deficit-inducing weight loss studies have shown considerable weight loss in participants. So, don’t lose hope – re-evaluate the situation, trust the process, and ‘adapt’.

References

• Fothergill, E. et al. (2016) ‘Persistent metabolic adaptation 6 years after The Biggest Loser competition’, Obesity (Silver Spring, Md.), 24(8), p. 1612. doi: 10.1002/OBY.21538.

• Heinitz, S., Hollstein, T., Ando, T., Walter, M., et al. (2020) Early adaptive thermogenesis is a determinant of weight loss after six weeks of caloric restriction in overweight subjects. Metabolism. [Online] 110, 154303. doi:10.1016/j.metabol.2020.154303

• Keys, A. et al. (1950) ‘The Biology of Human Starvation, Vols. 1 & 2’, The Biology of Human Starvation, Vols. 1 and 2, 1,2.

• Martins, C. et al. (2020) ‘Metabolic adaptation is not a major barrier to weight-loss maintenance’, The American Journal of Clinical Nutrition, 112(3), pp. 558–565. doi: 10.1093/ajcn/nqaa086.

• Müller, M. J. et al. (2015) ‘Metabolic adaptation to caloric restriction and subsequent refeeding: the Minnesota Starvation Experiment revisited’, The American Journal of Clinical Nutrition, 102(4), pp. 807–819. doi: 10.3945/AJCN.115.109173.