When it comes to diet, Research shows that most people will regain some – if not most – of the weight they lost. Although there are many reasons why this weight gain can happen, some popular online claims are that this is because the diet permanently destroys your metabolism. But while it is true that diet slows down metabolism, it also improves metabolism in many positive ways.
When we talk about metabolism, we usually refer to its metabolic rate. This is the number of calories your body burns at rest. Of course, the more activities we do, the more calories we burn. To lose weight through diet, you need to consume fewer calories than you are using. This forces the body to use its energy stores – such as fat – to supply the deficiency. As a result, your metabolic rate will also change.
The loss of lean tissue (muscle) when you go on a diet – which burns about 15-25 calories per kilogram per day – decreases your resting metabolic rate, which means you need fewer calories than before. But the body also deliberately slows down metabolism to preserve energy stores and minimize weight loss.
When the body realizes that fat stores are depleted, it triggers adaptive thermogenesis, a process that further reduces the resting metabolic rate – and can prevent weight loss, despite a strict diet. Adaptive thermogenesis can take effect three days after starting a diet and it is suggested that it persists far beyond the diet – even though it makes weight maintenance difficult and favors weight recovery.
An example of the effect of adaptive thermogenesis was seen in a widely publicized 2016 study that looked at former US reality show contestants The biggest loser. He showed that participants had a significant decrease in their metabolic rate, even several years after the initial weight loss. Participants needed to eat up to 500 calories less than expected daily.
Other studies have also shown metabolic slowdown with weight loss, but with much smaller decreases (about 100 fewer calories per day to maintain weight). However, there is less certainty whether this slowdown persists when people are stable.
Research seems to show that most adaptive thermogenesis takes place in the real diet phase as a temporary response to the amount of weight being lost. Overall, we have no conclusive evidence to support the notion that the metabolic rate remains low in the long run (more than a year after diet).
It is important to note that many factors can affect the metabolic rate, so changes after diet can vary between people. For example, a study on fasting diets showed that the metabolic rate did in fact decrease as a result – but those who had the biggest reduction in metabolic rate already had a higher metabolic rate to begin with. Overestimating metabolic rates at the start of a study or errors in predicting metabolic rate after weight loss can also affect the results of the study.
It is a consensus that the metabolic rate decreases due to weight loss, due to the decrease in body size and as a way to preserve essential tissues and fuel reserves. But there is currently no consensus on how much it decreases. Quantifying and predicting this slowdown is something we are researching 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 decrease in body fat. That decrease is actually our fat cells decreasing in size – they don’t actually go away. This shrinking of fat cells signals that the body’s fuel stores are running low, causing a drop in the hormone leptin. Normally, leptin inhibits appetite and increases the metabolic rate – but when leptin levels plummet, the metabolic rate decreases and hunger increases.
The intestine also releases less incretins (hormones that regulate appetite) when we lose weight, which can persist beyond diet. Less leptin and less incretins can make us feel more hungry and can lead to overeating.
When fat cells shrink, they are able to absorb glucose and store fat more efficiently to help restore lost fuel. Your body also creates more fat cells so that you can store more fat in the future to better deal with this calorie “crisis” the next time it happens.
But as contradictory as it may seem, all of these changes actually result in a more efficient and, ultimately, healthier metabolism. For example, smaller fat cells are better for our health, as over-inflated “sick” fat cells don’t work as well to get 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.
Therefore, the diet does not technically ruin your metabolism, but improves it, helping it to function better. But, without care, this metabolic improvement can conspire against you to regain weight and even exceed your original weight.
Studies show that exercise (or simply physical activity) can be a way to prevent weight recovery, by improving our ability to maintain our weight and can potentially minimize the metabolic slowdown. Exercise can also help regulate appetite and fuel burning in the short term and can make weight loss more sustainable in the long run.
This article was originally published at The Conversation in Adam Collins and Aoife Egan at UCL. Read the original article here.