If you’ve ever been to a group fitness class where everyone’s heart rates and estimated calorie burns are displayed on a screen, you know that these statistics vary greatly from person to person. You’ve probably also noticed that, generally, men tend to burn more calories than women. But have you ever wondered why different people burn calories at such different rates, even during the same workout?
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The truth is that metabolism — an umbrella term for all the processes in your body that break down nutrients for energy, fuel growth, and more — is far from simple. “There is a constant ebb and flow of reactions that build or repair our body (anabolism) and reactions that break down food and energy stores for fuel (catabolism),” says Anya Rosen, RD, a virtual functional medicine practitioner based in New York City. “It is an extremely complex topic that is very challenging to research,” she adds. Various factors play into how fast or slow you’re burning calories at any given time. Here are the six that experts say have the greatest impact on how many calories you burn while working out.
1. Body Weight
“Generally, the more you weigh, the more calories you’ll burn per session,” says Kyle Gonzalez, a San Francisco–based certified strength and conditioning specialist and performance coach at Future. “Calories are just a measure of energy, so the more you weigh, the more energy it takes to move your body.” Put differently, of two people with different weights, the one who weighs more will burn more calories, because they have a greater energy expenditure when moving.
People with larger bodies also tend to have larger internal organs (such as the heart, liver, kidneys, and lungs), which is a significant factor in how many calories are burned during exercise and at rest, because these organs and their processes require energy. One study found that up to 43 percent of the variation in total calorie burn between people could be explained by differences in the size of their internal organs.
This is one of many reasons that weight loss is so complicated — your body burns fewer calories as your weight decreases, which can lead to a weight loss plateau or even regaining weight. That said, it’s not the only reason. A previous review explains that weight loss can trigger other physiological adaptations as well, including the body’s tendency to burn stored fat for energy, a process called fat oxidation; greater hunger, due to higher levels of the hormone ghrelin; and less satiety, as levels of the hormone leptin dip.
If you’re looking to lose weight and have hit a plateau, consider working with a registered dietitian who specializes in weight loss and can help you meet your goal in a healthy and sustainable way. Find one at the Academy of Nutrition and Dietetics.
Also, keep in mind that exercise is a boon for overall health regardless of whether you lose weight. A review published in October 2021 in iScience suggests that while increased exercise doesn’t typically lead to long-term weight loss, improved cardiorespiratory fitness is associated with better health outcomes and a lower risk of premature death, regardless of weight.
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2. Muscle Mass
Here’s where things get a little bit trickier. Someone with more muscle mass will burn more calories than someone else who weighs the same but has less muscle. “Muscle tissue burns more calories than fat tissue,” says Jenaed Brodell, RD, a London-based private practice sports nutritionist. Yet claims about how many calories a pound of muscle will burn are often wildly overstated. In reality, evidence suggests that a pound of muscle burns about five calories per day, whereas a pound of fat burns about two calories per day.
During exercise, having more muscle mass will increase your total calorie burn, because your body needs to produce more energy to support the increased rate at which your muscles are contracting. Long story short, if you want to enhance your calorie burn, consider stepping up your strength-training game. “Evidence shows that lifting weights burns more fat [than cardio exercise] and has more promising long-term results,” says Brodell, adding the caveat that everyone’s goals and capabilities are different, and ultimately it’s up to you to choose how you exercise.
3. Birth Sex
“Generally, men burn more calories at rest and during exercise than women,” Gonzalez says. But there’s nothing magic about why this is — it’s because men tend to be larger than women, and they have more muscle mass than women of the same age and weight. “Males generally burn 5 to 10 percent more calories than females at rest, and this percentage usually increases with exercise,” Gonzalez says.
And while women can certainly add muscle mass through strength training, physiological differences mean that, in general, women can’t be as lean as men. “Women are genetically predisposed to lay down more fat to support hormone production and childbearing,” Brodell explains.
The National Institute of General Medical Sciences explains that body fat is also essential for functions such as storing energy, protecting internal organs, and supporting key functions like growth, immunity, hormone production, reproduction, and metabolism.
The American Council on Exercise (ACE) states that men need at least 2 to 5 percent body fat to support health, while women need a minimum of 10 to 13 percent. But these minimum numbers may not be sufficient. While there’s no official recommendation for optimal body fat percentage, the most cited study on the topic states that a healthy range for adults younger than age 40 is 8 to 20 percent for men and 21 to 33 percent for women. That said, the relationship between health and body fat is complex and not perfectly understood.
Instead of fretting about how your birth sex affects your calorie burn, focus on the things you can control. “The bottom line is that men and women should focus on building muscle and improving cardiovascular health with a well-balanced cardio and strength-training program,” Gonzalez says.
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“As we age, we tend to lose muscle mass,” Brodell says. “After age 30, you begin to lose as much as 3 to 5 percent of your muscle mass per decade.” The reasons for this aren’t perfectly understood, but a review published in July 2017 in Ageing Research Reviews explains that it’s likely because your body becomes more resistant to hormones that promote the protein synthesis that’s key to muscle maintenance. This loss of muscle mass lowers your metabolic rate — the speed at which you burn calories — at rest and during exercise.
A study on human metabolism, published in the August 2021 issue of Science, made headlines for its findings that metabolic rate may not decline throughout adulthood, but rather that it plateaus between the ages of 20 and 60 then begins its decline. In the study, authors measured the energy expenditure of 6,421 men and women between 8 days old and 95 years old using the doubly labeled water technique, the gold standard for this kind of measurement.
But this doesn’t necessarily mean that everyone’s calorie burn stays constant through adulthood. “This study on daily energy expenditure through the human life course is interesting, but it is not necessarily definitive proof that our understanding of metabolism is wrong,” Rosen says, adding that it’s impossible to perfectly quantify metabolism. Also, aging is accompanied by multiple changes in human physiology — not all of which may have been adjusted for, even by expert eyes.”
While you can’t stop your body from aging, you can preserve or even increase your muscle mass with regular strength training, Gonzalez says. “Strength training can help you increase your resting metabolic rate, which helps you burn more calories at rest over time.”
5. Fitness Level
The more you do a certain type of workout, the easier it seems. That’s not in your head — your body actually does adapt to do things more easily over time, Gonzalez explains. Overall, this is a good thing. It means that you can run faster or for longer with practice, and that your muscles will be able to lift heavier weights with proper training.
But it also affects your calorie burn. “As your body adapts to training, you will burn less calories with the same workouts,” Gonzalez says. “From your lungs to your muscles to your heart to your brain, your body becomes more efficient as you become more fit.” That’s why a newbie might burn significantly more calories than someone who’s been doing the same workout for years — and it’s why changing your workout routine can increase your fitness level and potentially enhance your calorie burn.
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6. Training Intensity
It’s also possible that two people doing the same workout are burning a different number of calories because they’re not actually doing the same workout. Brodell explains that someone exercising at a high intensity, meaning you’re breathing heavily and can’t carry on a conversation, can burn twice as many calories in the same amount of time as someone exercising at a low intensity. And just because you’re covering the same distance as someone else, or going through the same motions, doesn’t mean that the two of you are working out at the same intensity.
While the Department of Health and Human Services (DHHS) explains that walking and running deliver a lot of the same benefits when it comes to lowering blood pressure and reducing your risk of chronic conditions like heart disease and type 2 diabetes, a previous study found that adults who walked one mile burned roughly 89 calories, whereas adults who ran that same mile burned around 113 calories.
A target of 150 minutes of low-intensity per week is enough to yield many health benefits, including reduced anxiety, better sleep, lower blood pressure, increased cardiovascular fitness and reduced risk or slowed progression of certain chronic conditions, according to the DHHS. Incorporating higher-intensity exercise into your routine will boost your calorie burn and magnify these benefits even further. To increase the intensity of your workouts, ACE recommends increasing either your speed, range of motion, or the amount of weight you’re using for strength-training exercises.
The bottom line that you should try not to fret too much about things that are out of your control. Exercise has countless benefits beyond of just burning calories, so the most important thing is to find types of movement that are enjoyable and feel sustainable. “The type of exercise that is better for a person ultimately depends on that person’s goals, physical fitness, and capabilities,” Brodell says.