If you’ve got three weeks, you have enough time to get your body in show-off shape—at least as far as Biggest Loser trainer Bob Harper is concerned. Earlier this week, Harper’s new book,  Jumpstart to Skinny, hit shelves—and it promises amazingly fast results. We’re talking 20 pounds in 21 days type results.

Of course, you have to follow some pretty stringent rules to drop so much weight so quickly. All of your meals must be 40 percent carbohydrates, 40 percent protein, and 20 percent fat—and you can only take in 800 calories a day. What’s more, complex carbohydrates are only allowed at breakfast, and you also have to do 15-20 minutes of the “jumpstart” exercises listed in the book five days a week, in addition 45 minutes of daily cardio.

The good news is that Harper allows for unlimited vegetables, and they don’t count toward the daily calorie allotment. When Harper appeared on the TODAY Show on Monday morning to promote his book, he emphasized that this is a short-term strategy only meant for when you need to drop pounds fast.

So will Harper’s plan get you slim in time for summer? And is it even safe?

“Eating 800 calories per day for three weeks isn’t going to ‘hurt’ you,” says Mike Roussell, PhD, founder of Naked Nutrition and head of nutritional services at Peak Performance gym in New York City. “You won’t develop any vitamin or mineral deficiencies in that time.” Caroline Apovian, MD, Director of Nutrition and Weight Management Clinic at Boston Medical Center and author of The Overnight Diet agrees.

But while this plan is safe, it may not be as effective as possible. The calorie deficit tells your body to lose weight, but having such a high percentage of your calories come from carbs causes you to produce insulin, which makes your body hold onto fat. “It is like driving with your foot on the gas and the brake at the same time,” says Roussell, who recommends limiting carbs to 20 percent of your calories. Some of the remaining 20 percent that Harper suggested should go to carbyohydrates would actually be better spent on protein and fat, says Roussell.

Yes, fat. A woman following Harper’s recommendations would be eating just 17 grams of fat per day. To put that in perspective, one egg alone has six grams of fat. Research conducted by the Women’s Health Initiative indicates that it’s incredibly difficult to stick with such low-fat diets. “You’ve got to be more liberal with fat to make a diet doable without being hungry,” says Apovian.

In his book, Harper says that his 40/40/20 plan helps prevent the cycle of weight loss and gain—but experts disagree. “There’s nothing in the literature to support this,” says Apovia. Any time you cut back on calories so drastically, you make yourself more prone to yo-yo dieting, says Roussell. “Despite eating lots of vegetables on this plan, you’ll still be hungry,” he says. As a result, you’ll likely head straight for high-fat foods once you get off of Harper’s diet—and regain any weight you lose.

Jumpstart to Skinny may not be perfect, but it does have some useful takeaways:

Veg out
You might not want to OD on vegetables (they still contain calories after all), but Roussell recommends consuming veggies at every meal—preferably green ones. “You’re not going to get fat eating broccoli or apples,” he says, “but you might fill yourself up on that broccoli.”

Power up with protein
Making sure that 40 percent of your calories come from protein will help keep your body fueled and set you up to lose weight. A high-protein diet, plus exercise, helps build body mass—which is the best way to boost metabolism, says Apovian.

Change your carbs
Swap refined carbohydrates for whole grains, fruits, and vegetables, says Apovian. They have more nutrients and fewer calories—win-win!

Don’t forget fitness
Harper’s plan has dieters engaging in daily exercise throughout the entire three-week period—a smart move considering that focusing on diet and exercise simultaneously yields greater weight loss results than adding fitness to a preestablished diet routine, according to recent research from Stanford University School of Medicine.

More From Women’s Health:
Protein-Packed Recipes for Weight Loss
Weight Loss Success Stories
13 Ways to Stay Motivated to Slim Down

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