When it comes to dieting, research shows that most people get back some – if not most – of the weight they lost.
While there are many reasons for this weight recovery to occur, there are some popular claims online that it is because diet permanently destroys your metabolism. But while it is true that diet slows down your metabolism, it also improves your metabolism in many positive ways.
When we talk about metabolism, we usually refer to your metabolic rate. This is the number of calories your body burns at rest. In fact, the more activity we do, the more calories we burn.
To lose weight through dying, you need to eat fewer calories than you consume. This causes the body to use its energy sources – such as fat – to meet the shortage. Your metabolic rate changes as a result.
Loss of lean (muscle) tension when you eat – which burns around 15-25 calories per kilogram each day – reduces the resting metabolic rate, meaning you need fewer calories than you did before. But the body also slows metabolism deliberately to conserve energy sources and reduce weight loss.
When the body senses depleted fat stores it stimulates altered thermogenesis, a process that reduces the resting metabolic rate – and potentially weight loss despite a strict diet.
Proper thermogenesis can begin within three days of starting a diet, and it is recommended that it last well beyond a diet – even inhibiting weight maintenance and favoring weight regain.
One example of the impact of adaptive thermogenesis was seen in a widely published 2016 study that looked at US reality TV show contestants The Biggest Man.
It showed that participants had a significant reduction in their metabolic rate, even several years after initial weight loss. Participants had to eat up to less than 500 calories per day.
Other studies have also shown that metabolic slowdown is accompanied by weight loss, but with a much smaller reduction (around 100 fewer calories per day to maintain weight). However, there is less certainty that this slowdown will continue once people are stable in terms of weight.
Research seems to show that most variable thermogenesis occurs in the actual diet as a temporary response to the amount of weight lost. Overall, we do not have conclusive evidence that supports the notion that metabolic rate remains reduced over the long term (over a year after diet).
It is worth noting that many factors affect metabolic rate, so changes to it after a diet can vary between people. For example, one study of fasting diet showed that metabolic rate actually decreases as a result – but those who already had the greatest decrease in metabolic rate already had a higher metabolic rate in the first place.
Above metabolic levels at the beginning of a study or errors in predicting metabolic rate after weight loss may also affect study results.
It has been agreed that metabolic rate slows down due to weight loss, both as a result of both a reduction in body size, and as a means of conserving key tissues and fuel sources. But at the moment there is no consensus on what it is slowing down.
We measure and predict this slowdown is something we are currently studying at the University of Surrey.
Metabolic changes
A decrease in metabolic rate is just one change that occurs with weight loss, however.
When we lose weight, the main change we see is a reduction in body fat. This reduction is actually our fat cells shrinking in size – they don’t disappear. This depletion of fat cells indicates that the body’s fuel stores are depleted, causing a drop in the hormone leptin.
Leptin usually suppresses cravings and increases metabolic rate – but when leptin levels fall, metabolic rate slows down and hunger increases.
The gut also releases fewer incretins (hormones that regulate appetite) when we lose weight, which can lead to death. Less leptin and fewer incretins can make us feel leaner and can lead to overeating.
When fat cells are reduced, they will be able to take in glucose and store fat more efficiently to help lose fuel. Your body also creates more fat cells so you can store more fat in the future to better deal with this “crisis” of calorie the next time it happens.
But as different as it is, all of these changes lead to a more efficient and ultimately healthier metabolism.
For example, smaller fat cells are better for our health, as “sick” fat cells with too much water do not work as well at getting rid of excess sugar and fat. This can lead to high blood sugar and fat levels, increasing the risk of insulin resistance, diabetes, and cardiovascular disease.
So diet does not technically damage your metabolism but improves it by helping it work better. But without care, this metabolic development can work together against you to regain the weight, and even exceed your original weight.
Studies show that exercise (or just physical activity) can be one way to prevent weight regain, by improving our ability to maintain our weight and potentially reduce weight gain. ‘reduce metabolic slowdown. Exercise can also help regulate appetite and fuel burning in the short term, and may make weight loss more sustainable in the long run.
Adam Collins, Principal Teaching Fellow, Nutrition, University of Surrey and Aoife Egan, PhD Researcher, Mathematical Modeling of Weight Loss, University of Surrey.
This article is republished from The Conversation under a Creative Commons license. Read the original article.