Your metabolism can be slowed through changes in diet, activity level, muscle mass, and hormonal balance, but deliberately doing so comes with real health trade-offs. Most of the factors that lower your metabolic rate also increase your risk of weight gain, insulin resistance, and metabolic disease over time. Understanding what actually drives metabolic rate down can help you make informed choices, whether you’re trying to gain weight, managing a condition like hyperthyroidism, or simply curious about how your body regulates energy.
What Controls Your Metabolic Rate
Your basal metabolic rate (BMR) is the energy your body burns just to keep you alive: breathing, circulating blood, maintaining body temperature, repairing cells. It accounts for roughly 60 to 75 percent of your total daily calorie burn. The rest comes from physical activity and digesting food.
Several factors set your baseline. Muscle tissue burns more calories at rest than fat tissue, so your body composition matters significantly. Thyroid hormones act as a throttle on nearly every cell’s energy use. Your age, sex, and even the temperature of your environment all shift the number up or down. The key insight is that slowing your metabolism means changing one or more of these levers, and each lever has consequences beyond just calorie burn.
Eating Less Triggers a Survival Response
The most direct way to slow your metabolism is to eat significantly fewer calories than your body needs. Your body reads a large caloric deficit as a threat and responds by conserving energy. This is called metabolic adaptation, or adaptive thermogenesis, and it’s a well-documented survival mechanism.
In the Minnesota Starvation Experiment, one of the most detailed studies of prolonged caloric restriction, participants experienced a drop in resting energy expenditure of about 266 calories per day. Their heart rate fell by 14 percent, blood pressure by 7 percent, and the energy cost of simply walking decreased by 22 percent. Sympathetic nervous system activity, the “fight or flight” system that keeps your body revved up, dropped by 38 percent. Thyroid hormone (T3) fell by 39 percent, and testosterone dropped by 11 percent. The body essentially turned down every dial it could to conserve fuel.
About half of the metabolic slowdown in that study was explained by losing muscle and organ tissue. The other half was true adaptive thermogenesis, meaning the body was burning fewer calories than its remaining tissue should have required. Your body becomes more efficient at using less energy, which sounds appealing until you realize it also means fatigue, feeling cold, and losing the capacity for physical and mental performance.
The good news, if you’ve already been through this: metabolic adaptation isn’t permanent. Research from the University of Alabama at Birmingham found that the slowdown significantly reduces or even disappears after a couple of weeks of stable eating at maintenance calories.
Losing Muscle Lowers Your Baseline
Muscle is metabolically expensive tissue. Pound for pound, it burns more calories at rest than fat. This is why losing muscle mass is one of the most reliable ways your metabolism drops, and it happens naturally with age, inactivity, and caloric restriction.
During prolonged bed rest, substantial muscle atrophy sets in within just one week, along with whole-body insulin resistance. Sedentary behavior over longer periods causes a shift in muscle fiber type from oxidative (slow-twitch, calorie-hungry) fibers to glycolytic (fast-twitch, less metabolically active) fibers. This shift lowers the energy your muscles demand around the clock.
Menopause accelerates this process in women due to hormonal changes that reduce lean muscle mass, which is a primary reason BMR tends to decline during and after menopause. The same pattern happens gradually in men as testosterone levels fall with age.
Thyroid Hormones Are the Master Switch
No single factor influences metabolic rate more dramatically than thyroid hormone levels. Your thyroid produces hormones that tell cells how much energy to use, and shifts in those levels can move your metabolic rate by 15 percent or more in either direction.
People with an underactive thyroid (hypothyroidism) have a measurably lower BMR. In clinical studies, adjusting thyroid hormone levels from slightly elevated to slightly suppressed produced about a 15 percent swing in resting energy expenditure. On the opposite end, people with overactive thyroids can burn 40 percent more energy than predicted, which is why unintentional weight loss is a hallmark symptom of hyperthyroidism.
If you suspect a thyroid issue is behind an unusually fast metabolism, that’s worth investigating with a healthcare provider, because hyperthyroidism carries its own serious risks including heart problems and bone loss. Treatment to normalize thyroid levels will naturally bring metabolic rate closer to baseline.
Staying in a Warm Environment
Your body spends energy maintaining a core temperature of about 98.6°F. When you’re in a cool environment, it burns extra calories generating heat. When you’re in a warm environment within or above your thermoneutral zone, that extra calorie burn disappears and your metabolism stays at its baseline minimum.
The thermoneutral zone, where your body doesn’t need to work to stay warm, starts at roughly 72 to 73°F (22 to 23°C) for most people. Women tend to have a slightly lower threshold (around 71.4°F) compared to men (around 73.2°F). Keeping your home and workspace warm, wearing layers, and avoiding cold exposure will minimize the calories your body spends on temperature regulation. The effect is modest compared to diet and muscle mass, but it’s a real and measurable factor.
Sleep, Stress, and Metabolic Rate
Counterintuitively, poor sleep and high stress actually increase your resting metabolic rate rather than lowering it. Total sleep deprivation raises energy expenditure by about 7 percent, driven by elevated cortisol and increased sympathetic nervous system activation. A rise of just 10 micrograms per deciliter in cortisol corresponds to roughly 130 to 200 extra calories burned per day.
So if your goal is to slow your metabolism, getting consistent, high-quality sleep in the 7 to 9 hour range and managing stress will help. A calm, well-rested body runs at a lower, more efficient metabolic rate. This is one of the rare cases where the metabolically “slower” state is also the healthier one.
Why Slowing Your Metabolism Has Risks
Most of the methods that lower metabolic rate come with significant downsides. Research published in BMJ Open Diabetes Research & Care found that individuals with genetically low basal metabolic rates are more prone to weight gain, poor blood sugar control, and unfavorable cholesterol profiles. The combination of low energy expenditure and a sedentary lifestyle reduces the activity of enzymes that clear fat from the blood, impairs glucose uptake, and increases the risk of developing insulin resistance and type 2 diabetes.
In the Minnesota Starvation Experiment, the hormonal cascade from severe caloric restriction included a 54 percent drop in insulin levels, a 44 percent drop in leptin (the hormone that signals fullness), and a 49 percent drop in adiponectin (a hormone that helps regulate blood sugar). These aren’t just numbers on a lab report. They translate to constant hunger, difficulty concentrating, feeling cold all the time, and a body that aggressively stores fat the moment you eat normally again.
Prolonged inactivity, another effective way to lower metabolic rate, causes its own problems: loss of bone density, vascular dysfunction, increased inflammation, and reduced cardiorespiratory fitness. Even the metabolic flexibility of your cells suffers, meaning your body becomes worse at switching between burning fat and carbohydrates for fuel.
A Healthier Approach to Gaining Weight
If you’re searching for ways to slow your metabolism because you’re underweight or struggling to gain weight, the more effective strategy is to increase calorie intake rather than reduce calorie burn. Eating in a modest caloric surplus (300 to 500 calories above your maintenance needs) while doing resistance training encourages weight gain as muscle rather than fat. This approach actually raises your BMR over time, but it also helps you reach and maintain a healthy weight without the hormonal disruption, fatigue, and health risks that come with a genuinely suppressed metabolism.
For people with medical conditions causing an abnormally fast metabolism, such as hyperthyroidism, the solution is treating the underlying condition. Bringing thyroid hormone levels into the normal range can reduce resting energy expenditure by 20 percent or more, resolving the excess calorie burn without the need for behavioral changes that compromise your health.