To lose fat, you have to consistently use more calories than you take in. Typically, the best way to do this is to cut calories from your diet instead of relying on exercise alone to do the trick. But losing fat via exercise alone is possible, too; it just requires a lot more work and time.
New research published in Medicine & Science in Sports & Exercise suggests you can lose fat through exercise alone. The catch is you have to exercise a lot. In terms of numbers, we’re talking 300 minutes of exercise every week, which equals roughly 40 minutes a day. That’s twice the minimum amount of the 150 minutes of physical activity recommended by the CDC.
Here, we explain where that number came from and what it means for your fat-loss goals.
If you want to understand why it takes so much exercise to lose fat, you first have to understand your body.
Any time you reach a caloric deficit (needed for fat loss), your body kicks in with various compensatory mechanisms to balance the scales. One of the ways your body achieves this is by slowing your metabolism. Essentially, you become more efficient at using calories, both during exercise and at rest. This compensatory response is essential for surviving famines and food shortages and for running faster marathons. It can be a roadblock if you’re trying to lose fat, according to Kyle Flack, PhD, lead researcher of the study published in Medicine & Science, and assistant professor of dietetics and human nutrition at the University of Kentucky.
Exercise also affects the hormones that regulate hunger and satiety. One 2018 analysis of 72 randomized controlled trials found that engaging in an exercise program for two weeks or more is associated with a decrease in leptin, a hormone that helps prevent over- and under-eating. Exercise has also been associated with increases in acylated ghrelin (a hormone that regulates appetite) and decreases in peptide YY and insulin (hormones that suppress appetite), which may encourage you to eat more, according to the Medicine & Science study.
Exercise can also prompt reward-driven eating behaviors, Flack says. For example, if you’ve just finished a tough workout, you may feel you’ve earned that treat you’ve been craving. Before you know it, you’ve eaten back the 300 calories you just burned — and likely missed out on helpful nutrients you might have gotten if you’d picked a healthier meal or snack.
Depending on how many calories you burn through exercise, the cumulative effects of these compensatory mechanisms — metabolism, hormones and eating behaviors — can bring your total calorie expenditure back to zero. In fact, previous research by Flack and colleagues found that these mechanisms typically compensate for 1,000 of the total calories sedentary adults burn through exercise every week, regardless of whether they burn 1,500 calories a week, or 3,000 calories a week. The most recent study confirmed these findings.
So, if you burn 1,000 calories through exercise in a single week, these compensatory mechanisms could add those 1,000 calories right back in.
Three hundred minutes is roughly how much sedentary adults need to exercise to out-run the compensatory mechanisms.
To find that 300-minute number, Flack and fellow researchers gathered 44 sedentary, overweight adults, and tracked how many calories they burned through exercise over a 12-week period. One group was instructed to exercise for 40–60 minutes six times per week (equal to approximately 300 minutes), a second group to exercise for 90–120 minutes two times per week, and a third group (the control) to perform no exercise.
The exercise groups did only aerobic activities (like walking or jogging), and were allowed to work at an intensity of their choosing, provided they achieved a minimum intensity of 50–59% of heart rate reserve (HRR). (To find your HRR, subtract your resting heart rate from your maximum heart rate.)
The six-day group burned roughly 500 calories per workout for a weekly total of 3,000. Meanwhile, the two-day group burned about 750 calories per workout, which equals 1,500 each week.
Researchers also found both groups compensated with 1,000 calories. This means the six-day group burned 3,000 calories, compensated with 1,000, and still ended up with a weekly deficit of 2,000 calories. The two-day group, on the other hand, burned 1,500 calories per week, compensated with 1,000, and achieved a 500-calorie deficit every week.
As a result, only the six-day (300 minutes) group had lost a significant amount of body fat (4 pounds) by the end of 12 weeks.
The researchers also kept tabs on how exercise might affect the levels of certain hormones that impact hunger and satiety. They found that exercise made people more sensitive to leptin, a hormone that controls appetite.
Some overweight and obese people may experience a condition known as leptin resistance. Similar to insulin resistance, people with leptin resistance are less sensitive to its effects, which can make it harder for them to feel satisfied after a meal. “What I’ve shown is if I can get people to exercise, they will improve their leptin sensitivity, and allow leptin to do its job better at a lower level,” Flack says.
These effects were especially noticeable in the six-day group and was likely a key contributor to their weight-loss results.
In an ideal fat-loss scenario, you’ll pair exercise with a healthy diet.
While you can lose fat through exercise alone, be warned: It takes a lot to out-run that compensatory response. “Basically, if you don’t want to go on a diet, you need to exercise to expend at least 3,000 calories a week, or 300 minutes, to see real weight loss,” Flack says. You can always do less, but your progress will be slower, he adds.
For now, it’s tough to say whether you’ll see the same fat-loss results if you incorporate resistance training into those 300 minutes, but be sure to score at least two full-body strength sessions per week, as recommended by the CDC.
Check out “Workout Routines” in the MyFitnessPal app to discover and log workouts or build your own with exercises that fit your goals.