When Jessica Simpson took to Instagram to announce that she lost 100 pounds in the six months after giving birth to her daughter Birdie Mae, her followers flocked to the comments section to ask exactly what she did to drop the pounds. In a recent interview, Simpson’s trainer, Harley Pasternak, gave fans the answer they were looking for. 

Pasternak told People the 39-year-old former pop star made diet changes based on his book The Body Reset Diet. “It’s three meals and two snacks a day, and each meal has protein, fiber and fat, and snacks are protein and fiber or protein and fat,” he said, explaining the gist of the diet. “It’s about balancing in a way that doesn’t make it painful or too much of a departure from your life before that.”

But, when you dig a little deeper into Pasternak’s Body Reset Diet, what exactly does it entail—and can it really help you lose weight like it helped Simpson? Here’s the lowdown on how it’s structured, and what to be aware of before you give it a try.

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What is the Body Reset Diet?

The diet is broken up into three phases. The first lasts five days, and you’re directed to eat five times a day and consume only smoothies and snacks—specifically three smoothies and two snacks. 

The smoothies are made from a combo of a liquid base (dairy or non-dairy milk, or water), protein (from a powder or plain non-fat Greek yogurt), healthy fat (from nuts, seeds, or avocado), and high fiber carbs (from fruit and/or veggies). There are plenty of smoothie recipes in the book.

As for snacks, there are several to pick from, like low-fat popcorn; celery sticks with almond butter, or a pear with sliced turkey. The snacks have guidelines around calorie, fiber, and protein content, which are outlined in the book. All of the snacks should be about 150 calories and contain at least 5 grams each of fiber and protein and less than 10 grams of sugar.

Phase one also involves light exercise—walking only (no boot camp classes or other hard core workouts), at a minimum of 10,000 steps daily.

During phase two, on days six through 10, you drink two smoothies, and eat one solid meal plus two snacks per day. Meals include salads, sandwiches, soups, stir fries, and other simple dishes. Meal prep is encouraged, and recipes are provided. You also add a five minute at-home, no-equipment workout three days a week, in addition to the 10,000 steps.

In phase three, days 11 through 15, smoothies are limited to one per day, plus two meals and two snacks. The workouts also ramp up a bit. 

Beyond day 15, the book includes “rest of your life” advice (which sounds similar to the diet Simpson followed). In this advice, Pasternak advocates continuing to eat five times a day—specifically one smoothie, two snacks, and two solid meals—with two “free” splurge meals per week, which may include some alcohol. The 10,000 steps a day is advised seven days a week for life, along with brief resistance training sessions five days a week.

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Can the Body Reset Diet help you lose weight?

We unfortunately can't say for certain if the Body Reset Diet will help you lose weight. Pasternak doesn't have data on the results, either short term or long term, and it ultimately depends on someone's starting weight, previous diet, age, and other factors. In the long term, you’re encouraged to do many of the things you’ve probably heard about from other weight loss plans, including planning ahead, eating more slowly and mindfully, and sticking to a schedule, all of which is solid weight-loss advice. 

The Body Reset Diet isn’t a drastic or fad diet. It encourages making healthy choices and developing lifestyle habits. In a nutshell, it takes many solid eating and exercise recommendations and puts them into a concrete, easy to understand structure. The book is straightforward, the strategy is easy to follow, and the advice is reasonable and actionable long term.

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What are the downsides of the Body Reset Diet?

The book was first published in 2013, and some of the recommendations seem dated compared to newer nutrition science and trends. For example, artificial sweeteners are allowed, and the plan encourages low-fat and fat-free foods, eggs without the yolk, and some more processed products, like deli turkey, crackers, and pita. Current nutrition thinking is focused on whole rather than processed foods and quality fat over total fat.

And while there are options for vegans, vegetarians, and those who follow a gluten-free or dairy-free diet, they aren’t readily tagged. You’ll have to look through the recipes, and you may need to make substitutions based on the diet you personally follow. For example, if you’re vegan you could make some of chicken broth soups with vegetable broth instead.

The plan also requires calorie counting or tracking. Many of my clients find this to be cumbersome or even stressful. But because the Body Reset Diet involves eating five times a day, it would be important to monitor calories to prevent over-consuming if you decide to follow this plan. In my experience, the more meals, the higher the chances are that you may overdo it on the calories, even just a bit, which can add up to enough of a surplus to stall weight loss.

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Should you try the Body Reset Diet?

Nutrition is always evolving, which makes following a book like this from a specific date a challenge. While the structure of this plan, a balanced diet and regularly times meals, holds up, if you decide to try it, go for updated versions of foods within certain categories, like grass-fed if you eat dairy, pasture-raised whole eggs for egg eaters, no artificial sweeteners, and simple, clean ingredient packaged foods. Also, consider ditching the beef, upping seafood if you eat it, and cranking up the number of plant based meals. 

Finally, it’s unclear how much weight you might lose in 15 days on this plan—or even how much weight you’ll lose in six months, like Simpson. But one thing is certain: losing weight and keeping it off does require finding a strategy you can really stick with, and one that makes you feel well both physically and emotionally. Whether that’s this exact plan, or your own modification, in the long run, healthy, sane, and sustainable always wins out over fast and furious.

Cynthia Sass, MPH, RD, is Health's contributing nutrition editor, a New York Times best-selling author, and a nutrition consultant for the New York Yankees.

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