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 2 hours daily, you could expect to drop 7 to 9 pounds a week, says obesity expert Yoni Freedhoff, M.D., author of the forthcoming book Why Diets Fail and How to Make Yours Work. (Transform your own body in only 82 days with Speed Shred, the new 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 has 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:

Trying to shed pounds?
Ditch the soda.
Check out
the Men's Health report on
The Strange Reason Diet Soda
Makes You Fat
.

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