If you’ve ever been to a group fitness class where everyone’s heart rate and estimated calorie burn are displayed on a screen, you know these statistics vary wildly from person to person. You’ve probably also noticed that, in general, men tend to burn more calories than women. But have you ever wondered why each person burns calories at a different rate, even during the same workout?
The truth is, metabolism—a catch-all term for all the body’s processes that break down nutrients for energy, fuel for growth, and more—is anything but simple. “There are constant fluctuations between the reactions that build or repair our bodies (anabolism) and the reactions that break down food and stored energy for fuel (catabolism),” says Anya Rosen, a registered dietitian and functional medicine practitioner based in New York City. “It’s a very complex and difficult topic to research,” she adds. A variety of factors affect how quickly or slowly you burn calories at any given time. Here are six that, according to experts, have the greatest impact on the amount of calories you burn during exercise.
1. Body Weight
“Usually speaking, the more you weigh, the more calories you burn per session,” says Kyle Gonzalez, CSCS, chief content officer of Momentus, a Los Angeles-based supplement company. “Calories are simply a measure of energy, so it takes more energy to move your body.” In other words, of two people of different weights, the one who weighs more will burn more calories because they use more energy to move.
People with bigger bodies also tend to have larger interior organs (such as the heart, liver, kidneys, and lungs), which is a key factor in the number of calories burned during exercise and at rest, since these organs and their processes require energy. One study found that up to 43% of the variation in total calorie burn between people can be clarified by differences in the size of their internal organs.
This is one of the many reasons why losing weight can be so tricky: as you lose weight, your body burns fewer calories, which can lead to a plateau or even further weight gain. However, this isn’t the only reason. One study explained that weight loss can also cause other changes in how the body functions. For example, when you lose weight, hormones make you feel hungrier and less full.
If you want to lose weight and have hit a plateau, consider consulting a registered dietitian specialising in weight loss to help you reach your goal in a healthy, sustainable way. Find one through the Academy of Nutrition and Dietetics.
Also, remember that exercise is beneficial for overall health, whether or not you lose weight. A 2021 review suggested that while augmented exercise doesn’t typically lead to long-term weight loss, better cardiorespiratory fitness is related with better health and a lower risk of premature death, irrespective of weight.
2. Muscle Mass
This is where things get complicated. A being with more muscle mass will burn more calories than someone of the same weight but with less muscle. “Muscle flesh burns more calories than fat tissue,” says Jenaed Brodell, a registered dietitian and sports nutritionist with a private practice in London.
During exercise, consuming more muscle mass will increase your overall calorie burn, as your body has to produce more energy to support the higher rate of muscle contraction. In short, if you want to boost your calorie burn, consider increasing your strength training.
3. Sex at Birth
“In general, men burn more calories at rest and during exercise than women,” says González. But there’s nothing magical about it: it’s because men tend to be larger than women and have more muscle mass than women of the same age and weight. “Men typically burn between 5% and 10% more calories than women at rest, and this percentage usually increases during exercise,” explains González.
And while women can gain muscle mass finished forte training, physiological differences mean that, in general, they can’t lose as much weight as men. “Women have a genetic predisposition to store more fat to support hormone production and childbirth,” explains Brodell.
The American Heart Association states that body fat also plays a role in storing energy, absorbing nutrients, supporting cellular function, protecting internal organs, and producing hormones.
The American Council on Exercise (ACE) states that men need 2% to 5% body fat to survive, while women need 10% to 13%. However, these figures are only the minimums for the body to function and should not be considered a weight-loss goal. While there is no official recommendation for an optimal body fat percentage, an estimate from Human Kinetics suggests a range of “good” to “acceptable” for men (11%-20%) and women (16%-30%). However, the relationship between health and body fat is complex and not fully understood.
4. Age
“As we get older, we incline to lose muscle mass,” says Brodell. “After 30, you start losing about 3 to 5 per cent of your muscle mass per decade.” The reasons aren’t fully understood, but one study suggests it’s likely because the body becomes more resistant to the hormones that promote protein synthesis, which is key to maintaining muscle mass. Losing muscle mass reduces your metabolic rate (the rate at which you burn calories), both at rest and during exercise.
A 2021 study made headlines with its results: your metabolic rate may not decline throughout adulthood; rather, it peaks between the ages of 20 and 60 and then begins to decrease. However, that doesn’t mean everyone’s calorie burn remains the same throughout adulthood. “This study on daily energy expenditure over a person’s lifetime is interesting, but it’s not necessarily definitive proof that our sympathetic of metabolism is wrong,” Rosen said, adding that it’s impossible to measure metabolism perfectly. Furthermore, ageing brings about many changes in human physiology, and not all of them are irreversible, even for experts, she said.
While you can’t stop the body’s ageing process, you can maintain or even increase muscle mass with regular strength training, González says. “Strength exercise can help increase your resting metabolic rate, which in turn container help you burn more calories at rest over time.”
“The important thing is for men and women to focus on building muscle and improving cardiovascular health through a balanced cardio and strength-training program,” says González.
5. Fitness Level
The more often you do a particular type of workout, the easier it will seem. It’s not just a theory; your body adapts to do things more easily over time, explains González. Generally, this is a good thing. It means you can run faster or for longer with practice, and your muscles can lift heavier weights with the right training.
But it also affects your calorie expenditure. “As your body adapts to the workout, you’ll burn fewer calories during the same workouts,” says González. “From your lungs to your muscles, heart, and brain, your body improves as you get fitter.” That’s why a beginner can burn more calories than someone who has been doing the same workout for years. This is also why changing your workout routine—such as the time of day, or the type or order of exercises—can increase your fitness level and potentially boost your calorie expenditure.
6. Exercise Intensity
It’s also possible that two people doing the same workout can burn different amounts of calories because they aren’t actually doing the same exercise. According to Brodell, someone exercising at high intensity—that is, breathing heavily and barely able to speak—can burn twice as many calories in the same amount of time as somebody exercising at low intensity. And just because they run the same distance as someone else, or perform the same movements, doesn’t mean they are both exercising at the same intensity.
The Section of Health and Human Services (DHHS) states that walking and running offer many of the same benefits for lowering blood weight and plummeting the risk of chronic diseases such as heart disease and type 2 diabetes. One study found that adults who walked 1.6 km burned approximately 89 calories, while adults who ran the same mile burned approximately 113 calories.
According to the DHHS, 150 minutes of moderate-intensity or 75 min of vigorous-intensity workout per week is enough to reap numerous health benefits, including reduced anxiety, improved sleep, lower blood pressure, better cardiovascular fitness, and a lower risk or slower progression of various chronic diseases. Incorporating higher-intensity exercise into your routine will increase calorie burn and further enhance these benefits. To increase the intensity of your workouts, the ACE recommends increasing your speed, range of motion, or the weight you use for strength training. High-intensity intermission training (HIIT) can also be an real and efficient way to increase exercise intensity and calorie expenditure.
Ultimately, exercise has countless benefits beyond calorie burning, so the most important thing is to find movements you enjoy and that are sustainable for you. “The right type of exercise for each person depends on their goals, fitness level, and abilities,” says Brodell.