You’ve probably heard the mantra: Weight loss is a matter of calories in vs. calories out. Most of us have a decent grasp of the “calories in” part—it’s all about what you eat! When it comes to “calories out,” however, many of us are downright mystified. How much energy do our bodies actually need and use in a given day? Is that what metabolism means? And if so, is it possible to make metabolism faster so the body burns more calories?
Whether or not you’re trying to lose weight at the moment, gaining a fuller understanding of metabolism can be helpful in your wellness journey. To that end, we asked experts to walk us through what metabolism actually is, how it plays out the body, and whether it’s really adjustable for weight management. Read on to find out everything you wanted to know about metabolism.
What is metabolism?
Many people think of metabolism as a simple measure of how quickly our bodies burn through calories. There’s a bit more to it than that, says Dr. Scott Summers, PhD, chair of the department of nutrition and integrative physiology at the University of Utah School of Medicine. Metabolism is actually a collective term for the many processes that convert food and nutrients into energy and building blocks the body needs, he explains.
Metabolic processes fall into two main categories. Those that contribute to growth are described as anabolic (think: the creation of new cells). On the flip side of the coin are catabolic processes, which break down complex substances for the body to use in other ways. Both types of processes require energy.
Though metabolic functions are a big determinant of your daily calorie needs, metabolism isn’t just about weight, Dr. Summers emphasizes. It’s really about how the body captures and utilizes energy to carry out the tasks that keep us alive.
Anabolism vs. catabolism
As noted, metabolism is actually made up of two “isms”: anabolism and catabolism. Anabolism is constructive, taking simple molecules and building them into more complex materials for the body’s use. Catabolism works in the opposite way: It breaks down substances into simpler parts. Catabolism fuels anabolism; both types of processes are important to human physiology.
Anabolism
Your body is pretty much a factory that’s always cranking. Day and night, it produces important raw materials, including cells, tissues, hormones, and enzymes, to name a few. Called anabolism, this family of processes is your body’s growth and building mode, says registered dietitian nutritionist Kathleen Zelman, MPH, founder of No Nonsense Nutrition. Anabolism also occurs with the muscle development that results from strength training, she notes.
Catabolism
“Catabolism is the opposite of anabolism,” Zelman says. “It’s the process of breaking down substances within the body.” An example of a catabolic process is glycolysis, the 10-step series of reactions in which enzymes extract energy from dietary glucose. Most catabolic processes in the body release energy.
Basal metabolic rate
Whether you’re biking on a park trail or reading calmly at a desk, you need fuel to get through your days. The minimum amount of energy your body needs—i.e., while you’re doing absolutely nothing—is known as your basal metabolic rate (BMR). “That’s the energy it takes to be alive,” says Dr. Douglas White, PhD, an associate professor in the department of nutrition, dietetics, and hospitality management at Auburn University. BMR, usually expressed as calories per day, covers the vital functions of the heart, lungs, digestive system, and more, he explains.
In general, BMR likely accounts for 60% to 80% of all calories you metabolize. Figuring out your exact BMR, however, would require some fairly involved measurement. The process generally requires an overnight stay in a lab setting under strict conditions—no eating for 12 hours beforehand, with a full night of sound sleep required. First thing in the morning, a lab specialist would have to measure your internal heat production by analyzing the gases you breathe in and out (a technique known as indirect calorimetry), then use those findings to arrive at the baseline number of calories your body needs per day. Most people do not know their actual BMR—nor do they need to in the absence of a metabolic issue or related health concern, Dr. White says.
You may have also heard of a similar measurement called resting metabolic rate (RMR), or resting energy expenditure. In clinical settings, specialists calculate RMR using the same calorimetry technique used for BMR but under less restrictive conditions—no fasting required, for example. An RMR value is generally a bit higher than a BMR value because it may include heat generated from, say, digesting the blueberry waffles you had for breakfast that day. Among metabolic specialists, BMR is considered the preferred standard over RMR, Dr. White says.
If you’ve ever done a Google search for metabolism, you may have come across quickie formulas aimed at calculating your BMR or RMR based on factors such as your age, weight, height, and sex. Just note that these tools, while seemingly convenient for estimating baseline calorie needs, vary in the results they produce and may not align with what a lab test would tell you.
What bodily functions does metabolism include?
Metabolism affects every process in the body that uses or converts energy, including:
- Muscle contraction
- Brain function
- Temperature regulation
- Breathing
- Blood circulation
- Digestion
- Waste elimination
As mentioned earlier, the functions that are essential for life, such as brain function and blood circulation, account for the lion’s share of calories you burn each day. Wondering about the rest of your energy expenditure? Here’s a rough breakdown:
- Physical activity: 15% to 30%. Metabolically speaking, physical activity isn’t limited to formal workouts. “Just walking up stairs—just standing up—that’s going to burn some calories,” Dr. White says. That said, this percentage goes up with increasing duration and intensity.
- Food digestion: about 10%. You need fuel to get fuel! The body requires a fair amount of energy just to digest, absorb, and release energy from protein, fat, and carbs, Dr. White says. This is known as diet-induced thermogenesis, or the thermic effect of food.
- Adaptive thermogenesis: rare instances. This is basically your metabolism’s emergency regulatory mode. For example, adaptive thermogenesis can up your calorie burn to keep you warm during exposure to cold conditions. It can also downshift your metabolic rate to conserve energy if food becomes scarce.
Metabolic rate and body weight
All other things being equal, a higher body weight generally means a higher metabolic rate. That may seem counterintuitive, but the simple fact is that more pounds require more energy for the body to carry and maintain, Dr. White explains. In this sense, a “faster” or “higher” metabolism doesn’t always translate to a smaller number on the scale.
Metabolic rate actually tends to dip in people who have lost weight, says Dr. Lawrence Cheskin, MD, chair of the department of nutrition and food studies at George Mason University. This underscores the importance of a healthy maintenance plan for those hoping to stay at a reduced weight. Because a smaller body requires fewer calories per day to function, returning to former dietary habits could result in quickly regaining weight that was lost, Dr. Cheskin cautions.
Taking that scenario to the extreme, crash diets that induce rapid weight loss can trip the body’s adaptive thermogenesis response mentioned above. “When you [severely restrict] calories, your body thinks it’s starving,” Zelman says. “As a result, your metabolism slows down to burn fewer calories.”
What else determines my metabolic rate?
Metabolism is essentially the body’s engine, Zelman says, and some people’s engines just run faster than others. Apart from weight—which is modifiable—much of a person’s metabolic rate is hardwired by the following factors.
- Heredity: “There’s a huge genetic component to how [our bodies utilize] the food we eat,” Dr. Summers says. Genes determine up to 80% of metabolism, he adds.
- Sex-based factors: The hormone testosterone supports muscle growth, and muscle mass is one factor that stokes BMR, White says. That may be one of several reasons cisgender men have been shown to have higher energy requirements than cisgender women.
- Age: Somewhere around age 30, our bodies naturally start to lose 3% to 8% of their muscle mass per decade, a loss that can ultimately ding our metabolic rate, Dr. White says. Strength training may be helpful for maintaining muscle mass and minimizing age-related declines in BMR, he says.
- Height: Similar to the way a higher weight increases the body’s daily energy needs, tall people tend to have higher BMRs than folks of smaller stature.
How does food and nutrition affect my metabolic rate?
Maybe you’ve heard that certain foods and drinks—such as coffee, spicy peppers, or green tea—can speed up your metabolism. While some studies have looked at whether such additions to your diet could impart a boost, there’s no evidence they make a meaningful difference in how quickly your body burns calories, according to the National Library of Medicine.
Some research has suggested that a high-protein diet could slightly raise metabolic rate by producing a greater thermic effect during digestion than carbs and dietary fat do. According to one formula put forth in a 2002 research review, someone following a 2,000 calorie-a-day diet who consumes 30% of their calories from protein would burn 23 more calories per day—roughly what a person could burn in a few minutes of walking—than someone getting 15% of their calories from protein. A 2020 study in the Journal of Nutrition found that a diet of 30% protein may help keep people from regaining weight after they lose it—again, in part by increasing RMR.
How does exercise affect my metabolism?
Regular physical activity that includes strength training could boost your daily energy expenditure beyond the calories you burn during the activity itself. Some of the credit goes to lean muscle mass. “Adding muscle can increase metabolism and help you burn calories at a higher rate,” Zelman says. That's not to say you need to be pumping iron every day. Several studies report that people who regularly engage in almost any form of exercise—whether swimming, running, or tennis—may burn more calories independent of those activities than people whose lifestyles are sedentary.
Are there tips for boosting your metabolism?
To date, research hasn’t turned up any quick “hacks” that can noticeably turn up the dial on your body’s rate of calorie burn. While a few small studies suggest that lifestyle measures such as getting better sleep, turning down the thermostat, or drinking more water may boost metabolism, the findings aren’t consistent and most effects observed are marginal. (Adequate sleep and hydration are good for other reasons, though!) Experts say metabolism is largely determined by heredity, as well as sex-based factors, body size, age, and physical activity. Becoming more physically active, increasing muscle mass through strength training, and eating more protein may be ways to increase the body’s rate of calorie burn.
The upshot: What does metabolism mean?
Though many people associate metabolism with just weight, metabolism is actually a collective term for the many processes that take place within our cells to provide energy for life processes (such as blood circulation) and for creating materials the body needs (such as bone tissue). Metabolism includes anabolic processes, which synthesize new materials from simpler parts, and catabolic processes, which break down complex substances to free up energy and more.
Much of our metabolic rate—the amount of energy we expend per day—goes toward supporting vital life functions. Metabolism is largely shaped by immutable factors such as heredity and age. In terms of weight management, there’s scant evidence that particular foods or drinks can “speed up” metabolism and increase the body’s overall rate of calorie burn. That said, physical activity that builds lean muscle mass may help in that regard. Whatever your personal metabolic rate, long-term weight management is possible with a lifestyle that includes a healthy pattern of eating and regular physical activity.
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Karen Ansel, MS, RDN is a journalist and author specializing in nutrition, health, and wellness. Her latest book is Healing Superfoods for Anti-Aging: Stay Younger, Live Longer.