We've all heard that metabolism is the key to weight loss. But what is metabolism, really? Metabolism is how our bodies turn food into energy and how we use that energy to power everything we do—from thinking and growing to moving and digesting.
Our metabolic rate—the speed at which we burn calories—can decrease if the body senses a need to conserve energy. This is known as metabolic adaptation, and while it's a normal part of your physiology, it can make it harder to lose or maintain a healthy weight long term.
So how does this all work, and is there anything you can do about it? Let's take a closer look.
What Is Metabolic Adaptation?
Metabolic adaptation is when your body becomes more efficient at using energy, burning fewer calories in the process. It's a built-in physiological safety button your body uses to protect itself from starvation if calories or weight drops too low.
Sounds a little dramatic, but when your body is presented with a calorie deficit (eating fewer calories than you burn), it can't tell the difference between a famine and intentionally eating less. So, it responds the same way to both threats by slowing your metabolism to burn fewer calories and prioritizing the available energy for essential functions like breathing and thinking.
When Does Metabolic Adaptation Occur?
The truth is that metabolic adaptation can occur even when weight loss is achieved healthily through exercise and a nutritious diet. But it's more likely to happen with over-restriction or rapid weight loss, and for some people, metabolic adaptation can mean it takes longer to reach weight loss goals.
For example, a study examining metabolic adaptation levels and weight loss found that participants lost rapidly at first. But the weight loss significantly slowed after five months and beyond, even though they continued to follow the same diet pattern. 3
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So does this mean that once you hit a certain weight, you can never lose any more? Not necessarily, but it could make it more challenging.
Some studies suggest that over time, metabolism can return to normal. But others suggest the adaptations can last for years, even if most of the weight is regained (which theoretically should help metabolism return to baseline).
A study on the contestants from the television show "The Biggest Loser" found that metabolic adaptation occurred six months after weight loss, but they continued to burn significantly fewer calories for six years after the initial weight loss, even after gaining the weight back.
So why do some people experience metabolic adaptation and some may not? It likely depends on how the weight was lost in the first place. Studies suggest that certain genetic dispositions could differentiate those more susceptible to metabolic adaptation.
Whether you experience metabolic adaptation and to what degree really varies from person to person—a perfect example of bio-individuality. What works for one person may not work for you.
It has nothing to do with willpower or motivation. It's simply how you're made. But the more you understand your body, the more you can work with it instead of against it.
Why Does Metabolic Adaptation Happen?
When you lose weight, your total energy expenditure drops significantly. This helps explain why some people have such a hard time maintaining weight loss.
A big piece of your total energy expenditure is your resting metabolic rate (RMR). RMR measures the calories your body needs to perform essential functions like breathing and thinking when at rest.
RMR accounts for 60-75% of the calories you burn in a day, so when it slows down, your total energy expenditure does too.2
As you lose weight (including muscle mass), your body requires fewer calories to function, and your RMR declines.
You may move less or burn fewer calories while exercising because you now have less body mass to move around, you become more efficient at the exercise you're doing, and your body is smart about conserving energy.2
Metabolic adaptation helps explain why telling people to eat less and exercise more for weight loss often doesn't work in the long run. You're fighting against your natural physiology, including hormone changes, resting metabolic rate, and nervous system activity. 7
Even NEAT (the number of calories you burn from everyday activities like talking and fidgeting) can drop.
Hormones Play a Big Role in Metabolic Adaptation
Let's take a look at some of the hormonal adaptations that can occur with weight loss.
- Ghrelin. Think of ghrelin as the hormone that makes your stomach growl (although it does much more than that). It's often called the hunger hormone, signaling your body that it's time to eat.
Ghrelin levels go up before meals and drop afterward. But when you lose weight or your caloric intake decreases, ghrelin production increases to get you to eat more.
- Leptin. Leptin is a hormone stored in the fat cells that send satiety signals to your brain. It helps you recognize when you've eaten enough to feel full.
The problem is that leptin drops with weight loss, so the signal becomes less clear, and you can feel hungrier. 12It's another way your body protects itself against starvation.
- Thyroid hormone. Your thyroid hormone controls metabolism, and weight loss does appear to adversely affect thyroid hormones, especially when a significant amount of weight is lost. Even more moderate weight loss can affect the thyroid somewhat, although more research is needed.
- Cortisol and insulin. While we usually think of stress as emotional or mental, physical stress like weight loss or caloric restriction, can also increase cortisol levels. This stress hormone raises blood sugar and insulin levels, increasing fat storage.
Insulin's job is to keep blood sugar levels in check. Still, when constantly elevated, insulin resistance makes it harder for your body to process blood sugar and use it for energy (and puts you at risk for metabolic disorders).
Even too much exercise can increase cortisol levels. Women’s bodies are extra sensitive to nutrient scarcity, so much so that excess stress on the body can halt ovulation and menstruation.
The takeaway is that when you're trying to lose weight, you're not only up against your own body's desire to maintain its current weight but also against some fundamental hormonal changes.
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But that doesn't mean you can't meet your goals. In many cases, you can work with your body, not against it, by making strategic changes to how you eat and exercise (more on this later).
How Long Does it Take for Metabolic Adaptation to Happen?
While the answer to how long it takes for metabolic adaptation to occur can vary, research suggests that it can occur in as little as two weeks.1
Some studies also suggest changes in RMR can happen in the first six months of diet changes or weight loss. But once again, it likely depends on your body, what you’re eating, how you exercise, and all the different factors that make you unique (like your genetics and health history).
Signs of Metabolic Adaptation
The primary sign of metabolic adaptation is that weight loss stalls despite eating less and exercising more. You may start to gain weight back even if you haven't changed any habits.
Other signs can include feeling hungrier than usual or constant cravings. If you're experiencing any of these signs, you may want to talk to your doctor or a registered dietitian to rule out any other possible causes and create a plan to address metabolic adaptation.
Reversing Metabolic Adaptation
If you're wondering whether metabolic adaptation can be reversed or fixed, the answer is yes. Your metabolism isn’t broken, despite what you’ve read.
The key to reversing metabolic adaptation is to find the right balance of calories and exercise for your body long term. This can be different for everyone, and it may take trial and error to find what works for you.
Before making any changes, it's crucial to ensure that your nutrition or fitness goals are realistic and safe for your body. Remember, metabolic adaptation is a safety adaptation. You don't want to go too low calorie or too hard with exercise if it's adversely impacting your health.
Once you've determined that your goals are appropriate, you can examine your nutrition and exercise routine. Here are some tips:
- Don't over-restrict total calories. It may sound counterintuitive, but you may need to eat more to keep your metabolism humming. Instead of cutting even more calories, focus on the quality of food you’re eating to nutrient-dense foods like fiber-rich vegetables, lean protein, and healthy fats.
- Consider reverse dieting or cycling. Reverse dieting is another way of saying "slowly increasing calories." This approach can be helpful if you've been severely restricting calories and your metabolism has slowed. Slowly increasing calories can refeed the body to help RMR normalize over time.
Reverse dieting is often used by people who have reached their goal body weight and are ready to find a way to eat that supports maintenance. The tricky part is that you identify exactly how many calories you need to eat to maintain weight. Many people who use this method hire a coach or dietitian to help them; some just find it burdensome to track long-term calories.
Interestingly, a study called MATADOR (Minimising Adaptive Thermogenesis And Deactivating Obesity Rebound) found that men who alternated two weeks of energy restriction with two weeks of regular, balanced intake lost more body weight and fat mass after 16 weeks than those who ate a low-calorie diet continuously.
In other words —giving the body a break from dieting led to better results. The goal for both of these is to find a way to eat that supports your body and metabolism and doesn't feel like a diet based on deprivation.
- Play the long game with weight loss. If you lose weight too quickly, your body may think it's starving and start to hold onto calories and store fat. Losing weight at a slow and steady pace of no more than two pounds per week is a smart approach.
- Eat more protein. Protein is your blood sugar-balancing, satiety-stimulating, muscle-building macronutrient. When it comes to metabolic health, protein is key.
A high-protein diet has been shown to help with weight loss, increase RMR, and help preserve muscle mass.20Protein-rich foods include chicken, meat, fish, eggs, legumes, and tofu.
- Take a second look at your fitness routine. The added stress of exercise can sometimes be a trigger for metabolic adaptation. You may need to step back and reassess your current fitness routine. Combining strength training, cardio, and rest all support metabolic health and weight loss.
- Don't forget other pillars of wellness like sleep and stress! Both sleep and stress can play a role in weight management and hormones like cortisol. Aim for 7 to 9 hours of sleep and find healthy ways to manage stress regularly..
Metabolic Adaptation and Your Health
Metabolic adaptation is a normal and necessary process that helps to protect your body. You may be able to avoid or minimize metabolic adaptation with slower weight loss, healthy eating habits (like not over-restricting calories), and exercise.
But if it happens, that's OK too. The goal is to focus on your overall health and wellness. What you do want to avoid is jumping on and off the dieting train because it can make it more challenging to maintain your healthy body weight as your body becomes more and more protective of its calorie stores.
The key is to find an approach to eating and living that feels good for you and meets your individual needs. You can learn more about how your body responds to food and exercise by using a continuous glucose monitor (CGM). This can help you figure out what foods work best for you and how to exercise in a way that supports your metabolic health.
If you have questions about your fitness routine or diet, don't hesitate to reach out to a certified trainer, coach, or registered dietitian who can help you create a plan that's right for you.
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Topics discussed in this article:
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