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Explained: What new study reveals about effects of physical activity on body weight, metabolic rate

The observations are a re-interpretation of an earlier study for which researchers had observed 14 contestants of a reality TV show in which people competed to lose weight through rigorous exercise and dieting.

By: Explained Desk | New Delhi |
Updated: December 27, 2021 9:40:43 pm
The Biggest LoserA contestant who featured in the reality show 'The Biggest Loser' (Youtube/The Biggest Loser)

A study performed on the contestants of ‘The Biggest Loser’, a reality TV show, found that while these contestants lost a huge amount of weight over a short period of time (typically over a 30-week period), they also had slow metabolism rates and low levels of a hormone that affects hunger.

‘The Biggest Loser’ is a reality TV show in which obese contestants compete to lose weight through rigorous exercise and dieting.

The study was published in the journal Obesity in 2016 after researchers observed 14 contestants, both during a season and after filming stopped. The study said that six years after the contestants took part in the show, most of them had regained much of the weight they had lost and that their metabolism rates had slowed down. In other words, after six years most contestants were heavier and were burning fewer calories.

Now, Kevin Hall, a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases and one of the co-authors of the 2016 research, has provided a new interpretation of this study. His perspective through which he tries to explain why the metabolic rate of the participants was low even after six years was published in Obesity last month.

The 2016 study and its findings 

In a detailed thread on Twitter, Hall said that during the competition, obese contestants performed three hours of vigorous exercise and cut their dietary calories by 65 per cent. As a result of the exercise and dietary changes, contestants experienced extremely rapid loss of weight and body fat.

Now, because of the extreme exercise, the participants’ total energy expenditure increased, but their resting metabolic rate (RMR) decreased “likely due to the decrease in diet calories and active weight loss,” Hall said on Twitter.

RMR determines the number of calories that are burnt while the body is at rest. These are the calories that are essential for maintaining functions such as breathing, blood circulation and basic neurological functions.

Hall explains that these findings (weight loss and low RMR) were not unexpected. The RMR typically goes down during weight loss, because of the body’s reaction to it. The body will react to a person’s weight loss as if it were starving. Therefore, when a person starts to lose weight, the RMR goes down (slower metabolism) in order to compensate for weight loss.

What was surprising were the findings that came six years after the competition. The participants had regained weight and while researchers expected that their RMRs would have gone back to original levels, it still remained low despite regaining 2/3rd of the weight they had lost. “We had no explanation at the time, but simply concluded that there was persistent metabolic adaptation,” Hall said.

Reinterpreting the 2016 results 

During that time (2016), Herman Pontzer, an Associate Professor of Evolutionary Anthropology at Duke University, was studying physically active hunter-gatherers and found that their RMR was not much different from those of sedentary Westerners. Pontzer proposed a model based on his observations called the ‘constrained energy expenditure model’ which says that increase in physical activity expenditure can result in compensatory reductions in other components of the energy budget.

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Hall notes that the participants they had observed had remained very physically active with a median 80 per cent increase in physical activity than what they did before the competition. What researchers observed in the participants they studied is that the participants with the most metabolic adaptation, which resulted in decreased RMRs, were also the ones who had the least weight regain. Significantly, those participants with the greatest sustained increase in physical activity were most successful at avoiding weight regain.

“So, my reinterpretation of the Biggest Loser study is that the persistent metabolic adaptation we observed after 6 years to decrease RMR may have been due to energy compensation as predicted by the constrained energy expenditure model. They didn’t “destroy their metabolism”!”Hall said.

In other words, the RMRs probably remained low even after six years because of continued physical activity, which also helped them regain weight.

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