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 carbohydrates would actually be better spent on protein and fat, says Roussell.

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. No Hot Water From Delta Tub Faucet"There's nothing in the literature to support this," says Apovia. English Bulldog Puppies For Sale In Oc CaAny time you cut back on calories so drastically, you make yourself more prone to yo-yo dieting, says Roussell. Teacup Persian Kittens For Sale In Washington State

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

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 DownAll it takes is a 21 day diet meal plan to kick-start your weight loss goals. Will you lose all the weight you want in this time? If you want to lose 10 – 20 pounds, it’s possible, but it will take a lot of effort. You won’t lose 30+ pounds in this timespan – it won’t happen. However, a 21 day diet plan can produce massive results and will be the start of a healthy habit that your brain is already starting to grow accustomed to. The 21 day diet phenomenon only works so well, in the long-term, because it is the right amount of time to actually form a habit.

Science has proven that habits form in 21 to 30 days. Healthy habits, such as the way you eat, can be life changing. But, bad and good habits can be formed in the same amount of time. If you follow the 3-Week Diet, you’ll form these good habits and start blasting away fat. You are what you eat. This is more than just a saying; it is the truth. If you fill your body with fatty foods, sugar and empty calories, you’ll soon start noticing that you’re gaining weight. Before you know it, you’ll need to go on a diet to start losing those extra pounds that seemingly snuck up on you overnight. Before you actually start the healthy eating that you’ll want to maintain well after 21-days, you’ll need to follow the 21 day detox diet. Detoxes are growing in popularity because they work; Your body goes through undue stress because of the way you eat. All of these bad-for-you foods have to pass through the liver where fat is pumped through. This is why people call the liver the fat pumping organ.

If your liver isn’t working at 100%, you’ll start accumulating fat. The first part of the 3-Week Diet meal plan can help you lose up to 10 pounds in just a week by detoxing the liver and providing it with a healthy jolt. This is done through eating mainly proteins and vegetables. Some of the veggies you’ll eat include: Inside of the program, you’ll find a massive list of vegetables and proteins you’ll need to eat to prepare your body for a 21 day diet. This is an essential part of any diet because when your liver works optimally, it will pump out excess fat flawlessly. I skipped a lot of the 3-Week Diet recommendations so that I can show you what a sample diet menu may look like. The one thing that I really like about this program is that your diet menu is really flexible. Dieting is all about calories. A 21 day diet will consist of detoxing and fasting, but once that has come to an end, it’s time to go back to somewhat normal eating. This is done by calculating your BMR.

For women, you’ll follow this calculation: Add all of these calculations together to get your BMR. Men will want to use the following calculation to get their own BMR: Once your BMR is found, you’ll use this to create a diet that works for you. Your BMR is meant to demonstrate how many calories you need per day to maintain your current weight. So, you will want to reduce these calories to find a good starting point to lose weight. I recommend BMR * .85. By reducing your calories by 15%, you’re going to see sustained weight loss without causing your body to go into survival mode and lower your metabolism. From here, it is up to you to ensure you remain within your caloric range. You can eat anything, but this is where a person’s results will vary greatly. Fat, protein and carbohydrates need to be eaten in proper amounts to achieve the results you desire. Ensure that you don’t get more than 25% of all calories from fat. Eating .5 grams of protein per pound of weight is sufficient and can be upped accordingly.