Each individual’s weight management journey is unique. Some people have much more difficulty losing weight than others. Some people who are able to lose weight seem to lose pounds fairly quickly, whereas for others, it can take a very long time to achieve significant weight loss. One of the reasons why weight loss can be so difficult to achieve and maintain is metabolic adaptation (also known as ‘adaptive thermogenesis’), which refers to the fact that energy burn decreases with weight loss – even more than what would be expected by the weight loss alone.
In other words: If a person loses 20lb (9kg), think of taking off a 20lb backpack. With 20lb less weight to carry around, it makes sense that daily energy burn goes down. However, in addition , there is a further reduction in energy burn, due to a slowing in resting metabolism. I picture the energy producing mitochondria in each of our billions of cells as little furnaces, all turned down with weight loss.
In a recent study, they investigated whether this additional reduction in energy burn with weight loss (metabolic adaptation) had an impact on the time it took for women with elevated body weight to achieve a weight loss goal.
In the study, they enrolled 65 women (average age 36) with elevated body weight (average BMI 28.6), and supported them to follow an 800 calorie diet until their BMI reached 25, for a 16% weight loss. (Note, we would not recommend such a low calorie diet as a permanent life change – this was for research purposes. We also do not recommend a particular BMI or particular weight goal in clinical practice – it’s about improving health, and not the numbers on the scale!).
They measured resting metabolic rate at the start of the study, and 4 weeks after target weight loss was achieved.
The average time taken to achieve an average of 16% weight loss was 155 days. Adherence to the diet was on average 64%.
They found that there was indeed a significant reduction in metabolism after weight loss, and that the greater this reduction in metabolic rate, the longer it took to reach the weight loss goal. After adjusting for target weight loss, energy deficit, and dietary adherence, metabolic adaptation explained 63% of the variation in the time required to achieve the weight loss goal.
The average reduction in resting metabolism was quite small in the study at -46 kcal/day, measured after 4 weeks of weight stability. The authors note that based on prior research, this number is probably quite a bit higher during the active weight loss phase, estimated at about -110kcal/day in this group of people. Also, importantly, there was enormous variation in the change in resting metabolism from person to person. Based on their data, they calculated that those with the largest amount of metabolic adaptation would take an additional 70 days to reach the same weight loss goal, compared with a person with no metabolic adaptation. This helps to explain some of the variation in time needed to reach the weight loss goal in the study, which ranged from 66 to 252 days.
BOTTOM LINE: Metabolism decreases during and after weight loss, which is part of the body’s natural response to defend body weight. There is a huge variation in this change in metabolism from person to person – an important reason why each individual’s weight management journey and responses to treatment are unique. Increasing physical activity can help to offset this reduction in metabolism!
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