Having a “slow metabolism” means your body burns fewer calories at rest than average for someone your size, age, and sex. The average male burns about 1,696 calories per day just to keep basic body functions running, while the average female burns about 1,410. If your body falls well below those numbers relative to your body composition, your metabolism is on the slower end. But the phrase gets tossed around loosely, and the reality is more nuanced than most people think.
What Metabolism Actually Controls
Your metabolism isn’t one single process. It’s the sum of every chemical reaction keeping you alive: your heart beating, lungs breathing, cells dividing, brain firing, and organs filtering blood. The energy your body needs for all of this while you’re doing absolutely nothing is called your basal metabolic rate, or BMR. It accounts for roughly 60 to 70 percent of the calories you burn each day.
On top of that, your body burns calories digesting food and through physical activity. Digestion alone uses a meaningful amount of energy, and the type of food matters. Protein costs your body the most to process, burning 15 to 30 percent of its calories during digestion. Carbohydrates use 5 to 10 percent, and fats use almost nothing at 0 to 3 percent. So when people talk about a “slow metabolism,” they’re usually talking about a low BMR, but diet composition plays a role too.
Why Some People Burn Fewer Calories
There’s no single threshold that defines a slow metabolism. Everyone’s BMR is shaped by a mix of factors, and some of the biggest ones are things you can’t control.
- Body composition: Muscle tissue burns roughly 4.5 to 7 calories per pound per day at rest. Fat tissue burns far less. Two people who weigh the same can have very different metabolic rates depending on how much of their weight is muscle versus fat.
- Thyroid function: Low levels of thyroid hormone (hypothyroidism) directly reduce your BMR. This is one of the most common medical causes of a genuinely slow metabolism.
- Hormonal conditions: Women with polycystic ovary syndrome (PCOS), especially those with insulin resistance, can have significantly lower metabolic rates. One study found that women with PCOS and insulin resistance burned only about 1,116 calories per day at rest, compared to 1,868 in women without the condition, even after adjusting for age and body size.
- Menopause: Hormonal shifts during menopause lead to a decrease in lean muscle mass, which in turn lowers BMR.
- Undereating: Skipping meals, fasting, or eating very few calories signals your body to conserve energy. Your BMR drops in response.
Age Matters Less Than You Think
Most people assume metabolism tanks in their 30s or 40s. A large 2021 study published in Science found something different. Metabolism stays remarkably stable between ages 20 and 60 when you account for changes in body size and composition. The real decline doesn’t begin until around age 60, and even then it’s gradual: about 0.7 percent per year. By age 90, total energy expenditure is roughly 26 percent lower than in middle age.
What does change earlier is muscle mass. People tend to lose muscle and gain fat starting in their 30s and 40s, and that shift in body composition is what makes metabolism feel slower. The metabolic machinery itself isn’t breaking down. It’s just working with less calorie-hungry tissue.
What a Slow Metabolism Feels Like
A slow metabolism doesn’t announce itself with one obvious symptom. Instead, it shows up as a collection of subtle patterns. You might notice that you gain weight more easily than other people despite eating similar amounts. You might feel cold more often, since your body generates less heat when it burns fewer calories. Persistent fatigue is common, not the kind from a bad night’s sleep but a baseline low energy that doesn’t fully lift with rest.
If hypothyroidism is the cause, you may also notice dry skin, thinning hair, constipation, or brain fog. If PCOS or insulin resistance is involved, irregular periods, difficulty losing weight around the midsection, and blood sugar swings are typical. These patterns overlap, which is why identifying the underlying cause matters more than just labeling your metabolism as “slow.”
Your Body Fights Back Against Dieting
One of the most frustrating aspects of a slow metabolism is that dieting can make it slower. When you cut calories significantly, your body doesn’t just burn through fat reserves and carry on. It actively fights back through a process called metabolic adaptation. Hormones that regulate hunger and energy (like leptin and thyroid hormones) drop, and your nervous system dials down its activity. The result is that your resting metabolic rate decreases beyond what you’d expect from the weight you’ve lost.
This is why many people hit a plateau after weeks of dieting, or regain weight quickly after returning to normal eating. Their body is now burning fewer calories than it did before the diet, even at the same weight. It’s not a willpower failure. It’s a built-in survival mechanism that evolved to protect against starvation.
How to Measure Your Metabolic Rate
Online calculators that estimate your BMR based on height, weight, age, and sex give a rough starting point, but they can be significantly off. The most common formula (the Harris-Benedict equation) has been shown to overestimate actual metabolic rate by over 200 calories per day in some populations, with poor agreement when compared to clinical measurements.
The gold standard is a test called indirect calorimetry, where you breathe into a device that measures the oxygen you consume and the carbon dioxide you produce. From that, clinicians calculate exactly how many calories your body burns at rest. Some dietitian offices, sports medicine clinics, and hospitals offer this test. If you suspect a genuinely slow metabolism is interfering with your health or weight management, this measurement gives you a real number to work with instead of a guess.
What You Can Do About It
You can’t completely override your genetics or hormonal profile, but you can influence your metabolic rate more than most people realize. The most effective lever is building and maintaining muscle. Since muscle tissue burns more calories at rest than fat, increasing your lean mass raises your baseline energy expenditure around the clock. A single session of high-intensity resistance training can temporarily boost your resting metabolic rate for up to 48 hours afterward. Over time, consistent strength training builds the kind of tissue that keeps your metabolism higher even on rest days.
Protein intake matters too. Because protein has a much higher thermic effect than carbs or fat, eating more of it means your body uses more energy just to digest your meals. This doesn’t mean you need to follow a high-protein diet, but making sure protein is present at each meal can make a measurable difference over time.
Avoiding prolonged severe calorie restriction helps prevent metabolic adaptation. Moderate, sustainable calorie deficits preserve more muscle and provoke less of a metabolic slowdown than crash diets. If you’ve been undereating for a long time, gradually increasing calories while adding resistance training can help restore some of the metabolic rate you’ve lost.
If you suspect a medical cause, thyroid testing and hormone panels can identify treatable conditions. Hypothyroidism, for example, is manageable with medication, and treating it can bring your metabolic rate back toward where it should be. PCOS-related metabolic changes often improve with strategies that address insulin resistance, including exercise, dietary changes, and sometimes medication.