Understanding Metabolic Adaptation and Weight Loss

We've all heard that energy 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 leads to plateaus and can make it harder to lose or maintain a healthy weight long term.

How does this all work, and is there anything you can do? Let's take a closer look.

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Metabolism Explained

Metabolism refers to the chemical processes within your cells to transform nutrients from the food you eat into energy to store or use for body functions. 

Digestion jumpstarts the process by breaking down protein, fat, and carbohydrates into smaller compounds like amino acids, fatty acids, and simple sugars. These smaller compounds are then absorbed into the bloodstream, where they travel to your cells.

With the help of enzymes, nutrients are used for energy to control all the functions of the body. Your body needs the right energy balance daily to function correctly, known as your basal metabolic rate (BMR).  

BMR measures the calories your body needs to perform essential functions like breathing and thinking when at rest. Around 60 to 75% of your total daily energy expenditure is used for these functions. For weight loss, the goal is to burn more calories (kcals) than you take in (negative energy balance), so the number of calories your body burns at rest plays a significant role.

What Is Metabolic Adaptation?

Metabolic adaptation (sometimes called adaptive thermogenesis) 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 maintain homeostasis and protect itself from starvation if calorie intake, body mass index (BMI), 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 diet-induced weight loss. So, it responds the same way to both threats by slowing your metabolism to burn fewer calories and prioritizing the available energy intake for essential functions like breathing and thinking.²

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 severe calorie restriction or rapid body mass loss. For some people, metabolic adaptation can mean it takes longer to reach weight loss goals, and it can occur in as little as two weeks, according to some research.³

A study from the University of Alabama at Birmingham examining metabolic adaptation levels and weight loss found that participants lost weight rapidly at first. But the weight loss significantly slowed after five months and beyond, even though they continued to follow the same diet pattern.

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. ⁴ But others suggest the adaptations can last for years, even if most of the body 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.   

Why do some people experience metabolic adaptation and some don’t? 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.  In this way, metabolic adaptation could contribute to the link between yo-yo dieting and obesity.

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.

<p class="pro-tip"><strong>Learn More: </strong><a href="metabolic-syndrome-diet">A Metabolic Syndrome Guide: Diet, Foods to Eat/Avoid + Tips</a>.</p>

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 with weight loss maintenance. 

As you lose weight and body composition shifts (including muscle mass), your body requires less energy to function, so your BMR declines, and you burn fewer calories.²

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 and Metabolic Adaptation

Hormones play a crucial role in controlling our metabolism. Hormones are chemical messengers that facilitate metabolic processes, regulate our appetite, and even influence how many calories we burn.

Weight loss can cause hormone changes that influence your metabolism, making it more challenging to lose weight:

• 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, making you more hungry, so you’ll eat more (and protect against going too low calorie)
• 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.  It's another way your body protects itself against starvation‍
• Thyroid: Your thyroid hormone controls metabolism. 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: 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 body fat storage. Insulin's job is to keep glucose levels in check, but 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 body's desire to maintain its current weight but also against some fundamental hormonal changes.

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).

4 Signs of Metabolic Adaptation You Should Know About

The primary sign of metabolic adaptation is that weight loss stalls despite eating less and exercising more, but there are several other signs to look out for, including:

• Feeling hungrier than usual
• Constant food cravings
• Weight gain despite stable food intake
  

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, make sure you’re eating the right amount of calories for your body, and create a plan to address metabolic adaptation.

Tips to Fix 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 to consider:

• Don't Over-Restrict: 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 eat to nutrient-dense foods like fiber-rich vegetables, lean protein, and healthy fats‍
• Consider Reverse Dieting: Reverse dieting is another way of saying "slowly increasing calories." Slowly increasing calories or taking diet breaks can refeed the body to help RMR normalize over time, and some research suggests those who alternate between regular balanced meals and energy restriction lose more weight than those who continuously follow a low-calorie diet. The tricky part is that you identify exactly how many calories you need to eat to maintain weight, so working with a coach or dietitian may help‍
• Play the Long Game: 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 that may help you move over the hill of metabolic adaptation‍
• 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 fat-free mass, and help preserve muscle mass. Protein-rich foods include chicken, meat, fish, eggs, legumes, and tofu‍
• Check Your Workout Routine: The added stress of too much physical activity 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 supports metabolic health and weight loss‍
• Manage Stress and Prioritize Sleep: Sleep and stress can be an overlooked determinant for weight management and hormone balance. Aim for 7 to 9 hours of sleep and find healthy ways to manage stress regularly

Metabolic Adaptation and Your Health

Slow and steady weight loss as a result of healthy habits is always a goal over a short-term crash diet, but metabolic adaptation can still happen. You may be able to avoid or minimize metabolic adaptation with slower weight loss, healthy eating habits (like not over-restricting calories), and exercise.

The key is finding an eating and living approach 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) with the Signos app. Learn more about nutrition and healthy habits that benefit your metabolic health on the Signos blog, or find out if Signos is a good fit for you by taking a quick quiz.

<p class="pro-tip"><strong>Also Read: </strong><a href="metabolic-health-101">Metabolic Health 101</a>.</p>