When it comes to dieting, research shows that the majority of people will regain some – if not most – of the weight they lost. While there are many reasons this recovery can occur, some popular claims on the internet are that dieting permanently wrecks your metabolism. But while it’s true that diet slows down your metabolism, it also improves your metabolism in many positive ways.
When we talk about metabolism we are usually referring to your metabolic rate. This is the number of calories your body burns at rest. The more activity we do, the more calories we naturally burn. In order to lose weight through diet, you need to consume fewer calories than you consume. This forces the body to use its energy stores – such as fat – to remedy the deficiency. As a result, your metabolic rate also changes.
An example of the effect of adaptive thermogenesis was seen in a 2016 widespread study examining former candidates on US reality TV show The Biggest Loser. It found that the participants had a significant drop in their metabolic rate even several years after the initial weight loss. Participants had to eat up to 500 calories less than expected every day.
Other studies have also shown that metabolism slows with weight loss, but with much smaller decreases (about 100 fewer calories per day to keep the weight off). However, there is less certainty that this slowdown will continue when people are weight stable.
Research appears to show that most of the adaptive thermogenesis occurs in the actual diet phase as a transient response to the amount of weight lost. Overall, there is no conclusive evidence to suggest that metabolic rates will remain slowed over the long term (over a year after dieting).
It’s worth noting that many factors can affect metabolic rate, so post-diet changes can vary between people. For example, a study of fasting diets showed that metabolic rate actually decreased – but those who had the greatest decrease in metabolic rate already had higher metabolic rates. Overestimating metabolic rates at the start of a study or errors in predicting metabolic rate after weight loss can also affect study results.
When we lose weight, we mainly see a decrease in body fat. This decrease causes our fat cells to shrink – they don’t actually go away
There is consensus that due to weight loss, the metabolic rate slows down, both due to the reduction in height and the maintenance of essential tissues and fuel reserves. However, there is currently no consensus on how much it will slow down. We are currently investigating quantifying and predicting this slowdown at the University of Surrey.
However, a decrease in metabolic rate is just a change that occurs while losing weight.
When we lose weight, we mainly see a decrease in body fat. This decrease causes our fat cells to shrink – they don’t actually go away. This shrinking of fat cells signals that the body’s fuel stores are emptying and the hormone leptin is falling. Normally, leptin inhibits appetite and increases metabolic rate – but when leptin levels drop, metabolic rate slows and hunger increases.
The gut also releases fewer incretins (hormones that regulate appetite) when we lose weight, which can be beyond diet. Less leptin and fewer incretins can make us feel hungrier and lead to overeating.
As fat cells shrink, they can absorb glucose and store fat more efficiently to restore the lost fuel. Your body is also creating more fat cells, so you can store more fat in the future to better handle that calorie crisis next time.
But as contradicting as it sounds, all of these changes actually result in a more efficient and ultimately healthier metabolism. For example, smaller fat cells are better for our health, since excessive “diseased” fat cells do not do so well in getting rid of excess sugar and fat. This can lead to high levels of sugar and fat in the blood and increase the risk of insulin resistance, diabetes, and cardiovascular disease.
Adam Collins is a Senior Nutrition Fellow at the University of Surrey. Aoife Egan is a PhD student in mathematical modeling of weight loss at the University of Surrey. This article first appeared on The Conversation.