How Much Weight Can You Lose in a Week?

This summer, Matthew McConaughey dropped nearly 30 pounds to go from a ripped stripper in Magic Mike to a drug-dealing HIV patient for an upcoming role. And contestants on The Biggest Loser often shed double digits’ worth of weight in a week.

But when it comes to your own weight loss, experts recommend you aim for a measly one to two pounds a week. It’s fair to wonder: Why?

The above examples and a little math confirm you certainly can lose a lot more. If you start at a weight of 250, cut back to 500 to 800 calories, and work out for two hours daily, you could expect to drop seven to nine pounds a week, says obesity expert Yoni Freedhoff, M.D., author of the forthcoming book Why Diets Fail and How to Make Yours Work. (Build muscle while you torch fat with Speed Shred, the new follow-along DVD series from Men's Health.)

But besides making your life a living hell, dropping weight this quickly has other downsides: Muscle loss, nutritional deficiencies, and loose skin, just to name a few. And don’t forget gallstones and even potentially fatal cardiac arrhythmias, Dr. Freedhoff warns.

OK, fine, so losing 9 pounds a week isn’t reasonable or healthy. But can you aim for three or four while still preserving your hard-earned muscles—and your health?

Trainer Marc Perry, C.S.C.S., C.P.T., founder of BuiltLean, says though it’s not typical, he’s seen clients healthfully lose up to 1.5 percent of their body weight from fat in a week. Start at 250 big ones? That’s four pounds of fat.

Could you become a similar success story? Research does tell us that your fat-burning potential depends on a few factors, some you can control and some you can’t. Namely:

Your starting point. The flabbier you are at first, the larger the percentage of lost weight will come from fat, says Kevin Hall, Ph.D., a researcher at the National Institute of Diabetes and Digestive and Kidney Diseases. Say you start at 300 pounds—a modest goal of 1 percent fat loss per week means you’ll shed three pounds in a week, Perry says. But if you’re just looking to drop 10 pounds from a relatively lean frame, you’ll probably have a harder time hanging on to all your muscle.

Your workout. You’ve heard us say it before: Resistance training is key to keeping muscle while burning fat. In one Columbia University study, participants cut calories and were assigned either to strength-train or do cardio three times a week. After eight weeks, everyone lost more than 9 percent of their body weight. But in the aerobic group, 20 percent of that came from lean tissue (mostly muscle), while the resistance group limited lean-tissue loss to 8 percent.

Your protein intake. Protein provides essential amino acids that your body uses to make muscle. Skimp and you’ll lose more muscle. In one study, athletes who cut calories by 60 percent were told to eat either 35 percent or 15 percent of their calories from protein; the high-protein group lost 20 percent less muscle over two weeks.

Your sleep habits. Not getting enough shut-eye throws hunger and metabolism hormones like leptin and ghrelin out of whack, Perry says. In a small study published last year in Annals of Internal Medicine, volunteers on a reduced-calorie diet slept either 5.5 or 8.5 hours a night. In two weeks, they both lost a little more than 6.5 pounds—but those who slept more lost twice as much of that from fat.

How much you’ve already lost. The smaller you are, the fewer calories you’ll burn, Dr. Freedhoff says. But there are even more complex hormonal and metabolic shifts at work, making it harder to burn fat the longer you’re losing. Scientists are still working to understand the mechanisms, but a paper Hall published last year in The Lancet outlines the results: Say you start at 220 pounds and cut 480 calories at day. You’d eventually hit 165, but it’d take you a year to get halfway there, and another two to lose the rest. Things get even wonkier the faster you lose. Hall’s group studied contestants on The Biggest Loser season 8, who after 30 weeks had lost more than one-third of their body weight, more than three-fourths of that from fat. However, their resting metabolisms dropped so much that they were torching 500 fewer calories daily than would be expected given their new weight.

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