Losing weight can be a challenge and keeping it off can be even harder. Many people who successfully shed pounds find that they must eat even fewer calories to maintain that weight then they did before they gained and lost. For example, if someone starts at a weight of 180 lb, gains 50 lb over a few years then loses 50 lb over the same time period, when they get back to their starting weight of 180 lb, their resting metabolic rate (RMR) will be on average 20-30% (or more!) lower than it was when they started. This represents a significant number of calories for most people.
This reduction in RMR after weight loss is a major factor contributing to weight regain and to the overall difficulty in weight maintenance. But why does this happen? Let’s dive into some of the scientific theories behind it.
What is Resting Metabolic Rate (RMR)?
Resting Metabolic Rate refers to the number of calories your body needs to maintain basic functions like breathing, digestion, and cell repair while you're at rest—so if I put you at bedrest and you’re not moving at all but just lying there, then the number of calories your body needs to stay alive is your RMR. RMR accounts for the majority of your daily energy expenditure. Many people focus on calories in versus calories out when trying to lose weight, but RMR plays a significant role in how your body uses the calories you’re taking in and how efficient it is in that calorie use.
Why Does RMR Decrease After Weight Loss?
Adaptive Thermogenesis and Increased Efficiency
One of the potential reasons your metabolism slows after weight loss is adaptive thermogenesis. This refers to the body's ability to adjust its heat production in response to changes in diet (or environmental temperature), essentially meaning that the body can regulate its metabolism to burn more or less energy depending on the situation. When you lose weight, your body sees it as a potential threat and attempts to thwart weight loss by reducing the number of calories used at rest.
As you lose weight, your body becomes more efficient at using the energy it gets from food. This means that even if you continue to eat the same amount, your body might use fewer calories to perform the same daily activities, further slowing down your metabolism.
A thousand years ago, we did routinely face famine, and it was protective for our bodies to be able to flex our metabolism to protect us. I heard a metaphor that I thought beautifully described this phenomenon: we are cactus living in the abundance of a rain forest. Because our genes are built for living in a desert, they can sabotage our attempts to lose weight and maintain that weight loss.
Loss of Lean Muscle Mass
Muscle cells require more calories than fat cells to stay alive. As muscle mass decreases, your RMR drops because your body needs fewer calories to maintain less muscle. This is one of the reasons why people who lose weight without strength training often find that their metabolism may slow. Some loss of lean mass is normal, even unavoidable, with weight loss, but including resistance training and optimizing protein intake will mitigate this as much as possible. I know my patients get sick of hearing this, but protein intake and weight training are so important during weight loss (and after!).
Hormonal Changes
Hormones also play a crucial role in regulating metabolism, and weight loss can disrupt the balance of several key hormones:
Leptin: This hormone helps regulate hunger and energy balance. After weight loss, leptin levels drop, which increases hunger and reduces energy expenditure to protect against further weight loss. Leptin is produced by fat cells, so low leptin levels can signal to the brain that the body is in a state of energy deficit, leading to increased hunger and decreased energy expenditure.
Thyroid hormones: The thyroid gland produces hormones that control metabolism. When you lose weight, particularly with extreme calorie restriction, levels of thyroid hormones can decrease, slowing down metabolism.
Cortisol: This stress hormone can rise during weight loss, particularly with extreme dieting. Elevated cortisol levels can lead to increased fat storage and muscle breakdown, both of which contribute to a lower RMR.
Scientific Studies on Reduced RMR
Multiple studies have explored the connection between weight loss and reduced RMR. One well-known study on contestants from the Biggest Loser TV show found that after significant weight loss, the participants' measured RMRs were significantly lower than expected (calculated), even years after they regained nearly all their weight. Their measured (not calculated, which is an important distinction) RMR after regaining nearly all their weight lost was nearly the same as at the nadir of their weight loss. Certainly, when people lose weight, we expect their RMRs to decline because they have less body mass to keep alive. But this study showed that despite regaining nearly all their weight, the contestants did not see any increase in their RMR.
By the numbers, their average RMR before weight loss was 2600 kCal and that dropped to an average of 1990 kCal at their lowest weight, but upon regaining nearly all the weight (70-90%), their average RMR was essentially unchanged at 1900 kCal back at their starting weight. This is remarkable and shows how well the body can adapt when faced with what it interpreted as catastrophic weight loss.
A recent study in the European Journal of Clinical Nutrition confirms how much difference there is in measured vs calculated RMR and how we need to interpret RMR calculations cautiously, particularly in people who have gained and lost weight even one time. This study showed that after bariatric surgery there was significant discordance in people’s measured vs calculated RMR, particularly in those with BMIs>30. Their calculated RMRs were on average 30% higher than measured!
Conclusion: The Challenge of Maintaining Weight Loss
The body’s natural response to weight loss—slowing down the metabolism—is a key reason why many people struggle with long-term weight maintenance. Factors like hormonal changes, loss of muscle mass, and adaptive thermogenesis all contribute to a reduced RMR after weight loss. Understanding these mechanisms is crucial for anyone trying to lose or maintain weight, as it explains why simply cutting calories may not always be enough in the long run.
What to do?
Tune in next week to learn more about strategies for weight loss and weight maintenance!
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