To lose weight, aim for 300 minutes of exercise
Adults who worked out 6 days a week benefited in study
Can exercise help us shed pounds? Aninteresting newstudy involving overweightmenand womenfound thatworking out can help us loseweight, in part by remodeling appetite hormones. But to benefit, the study suggests, we most likely have to exercise a lot— burning at least 3,000 calories aweek. In the study, thatmeantworking out six days aweek for up to an hour, or around 300minutes aweek.
The relationship betweenworking out and ourwaistlines is famously snarled. The process seems as if it should straightforward: We exercise, expend calories and, if life and metabolismswere just, develop an energy deficit. At that point, wewould start to use stored fat to fuel our bodies’ continuing operations, leaving us leaner.
But our bodies are not always cooperative. Primed by evolution tomaintain energy stores in case of famine, our bodies tend to undermine ourattempts to drop pounds. Startworking out and your appetite rises, so you consume more calories, compensating for those lost.
The result, according tomany past studies of exercise andweight loss, is that most peoplewhostart anewexercise program without also strictlymonitoringwhat they eat do not lose asmuchweight as they expect— and some pack on pounds.
ButKyle Flack, an assistant professor of nutrition at theUniversity of Kentucky, began towonder if this outcomewas inevitable. Maybe, he speculated, therewas a ceiling to people’s caloric compensations after exercise, meaning that if theyupped their exercise hours, theywould compensate for fewer of the lost calories and lose weight.
For a study published in 2018, he and his colleagues explored that idea, asking overweight, sedentary menandwomento start exercising enough that they burned either 1,500 or 3,000 calories aweek during theirworkouts. After three months, the researchers checked everyone’sweight loss, if any, and used metabolic calculations to determine howmany calories the volunteers had consumed in compensation for their exertions.
The total, it turned out, wasanaverage of about 1,000caloriesaweek of compensatory eating, no matter howmuch people hadworked out. By that math, themenandwomen whohad burned 1,500 calories aweek with exercise had clawed back all but about 500 calories aweek of their expenditures, while those burning through 3,000 calories with exercise ended up with a netweekly deficit of about 2,000 calories. (No one’s overall metabolic rate changed much.)
Unsurprisingly, the group exercising the most lostweight; the others did not.
But that study leftmany questions unanswered, Flack felt. The participants hadperformedsimilar, supervisedworkouts, walkingmoderately for30or60 minutes, five times aweek. Would varying lengths or frequencies ofworkoutsmatter to people’s caloric compensation? And whatwas driving people’s eating? Did the differing amounts of exercise affect people’s appetite hormones differently?
To find out, he and his colleagues decided to repeatmuchof the earlier experiment, but with novel exercise schedules this time. For the newstudy, whichwas published inNovember inMedicine& Science in Sports& Exercise, they gathered another group of 44 sedentary, overweightmenand women, checkedtheirbody compositions, and asked half of them to start exercising twice aweek, for at least 90 minutes, until they had burned about 750 calories a session, or 1,500 for theweek. They couldwork out however theywished — many chose towalk, but some chose other activities — and theywore a heart rate monitor to track their efforts.
The rest of the volunteers began exercising six times aweek for about
40 to 60 minutes, burning close to 500 calories a session, for aweekly total of about 3,000 aweek.
The researchers also drew blood, to check on the levels of certain hormones that can affect people’s appetites.
After 12weeks, everyone returned to the lab, where the researchers recomputed body compositions, repeated the blood draws and began calculating compensations.
And again, they found a compensatory threshold of about 1,000 calories. As a consequence, only the menandwomenin the group that had exercised the most— six days aweek, for a total of 3,000 calories — had shedmuchweight, dropping about four pounds of body fat.
Interestingly, the researchers did uncover one unexpected difference between the groups. Those burning about 3,000 calories aweek showed changes nowin their bodies’ levels of leptin, an appetite hormone that can reduce appetite. These alterations suggested that exercise had increased the exercisers’ sensitivity to the hormone, enabling them to better regulate their desire to eat. Therewere no comparable hormonal changes in themenand womenworking out less.
Flack says thenew experiment “reinforces the earlier finding” that most ofuswill eat more if weexercise, but onlyupto about the 1,000-caloriesa-week inflection point. If we somehowcan manage to burnmore than that amountwith exercise, we probably can dropweight.
But, of course, burning thousands of calories aweek with exercise is daunting, Flack says. Plus, this study lasted only a few months, and cannot tell us whether later changes to our appetites ormetabolismswould augment or undercut any subsequent fat declines.
Still, for those of us hoping that exercise might help us trim ourwaistlines, the morewe canmove, it seems, the better.