The 90-Day Challenge Diet

Although you'll find many variations on 90-day challenge diets, one of the most prominent is meant to accompany the 90-Day Fitness Challenge. The 90-Day Fitness Challenge is a book — and accompanying eating program — from The Biggest Loser alums Amy and Phil Parham.

The Parhams' book, The 90-Day Fitness Challenge, is inspired by what they learned during their time on The Biggest Loser. Although they didn't win the show, they lost a combined total of 256 pounds and made an enormous change to their lifestyle.

The book starts with a look at the Parhams' personal story and then, as the publisher explains, teaches the nuts and bolts of the fitness and diet principles that helped them make such a big life change. Their Christian faith features very prominently in the book, and as the Parhams said in an interview with the Christian Broadcasting Network (CBN), it's actually intended for church congregations and other community groups to work through together.

With that said, aside from the daily devotional readings and the Parhams' accounting of how their faith influenced their journey, the book is also grounded in simple, healthy and manageable habits that anybody can use to help get their diet and fitness back on track.

In their interview on the CBN, the Parhams explain several key principles of their diet: combining protein, carbs and fats in every meal or snack, and eating every three to four hours. Don't skip meals, they warn, because that can cause your metabolism to shut down. "You eat to lose. I eat more now than I ever have," Amy Parham says in the interview.

The Parhams also encourage readers to buy in bulk and prep their meals once a week, then parcel them out into pre-measured portions. The idea is that always being prepared, with healthy food at hand, makes it easier to stay on the diet plan — and the weekly prep sessions make these foods easier to squeeze into a typical American life.

The meals the Parhams recommend focus on natural, less processed foods instead of the ultra-processed foods that make up a shockingly high percentage of the food supply. You can expect to eat lots of vegetables, along with healthy proteins such as poultry, eggs, tofu and beans; healthy whole grains like oatmeal and brown rice; fruit; and healthy fats from foods like nuts, avocados and oils rich in unsaturated fats.

The Parhams also encourage you to drink water: Actually, they say you should "guzzle" it and never be without a water bottle. To estimate how much you should drink, they recommend dividing your weight in half and converting that number into ounces. So if you weigh 200 pounds, you should drink 100 ounces of water every day.

Other key tenets of the diet include increasing your fiber intake and limiting your intake of added sugar, fat and sodium. All of these recommendations echo key advice from numerous healthy authorities, although the point about eating frequently has yet to be conclusively proven in clinical trials.

Read more: 6 Health Risks of Eating Too Many Processed Foods

The Parhams' 90-day diet and fitness challenge is just the tip of the iceberg when it comes to 90-day weight-loss plans. You'll find 90-day diets and fitness challenges promoted in books, on websites and even in mobile apps.

But why 90 days? Doesn't it take 21 days to form a new habit? Not necessarily, say the authors of an article in the December 2012 issue of the British Journal of General Practice. They speculate that the myth of 21 days to create a new habit might come from anecdotal evidence of plastic surgery patients who needed 21 days to adjust psychologically to their new appearance.

They also highlight evidence that shows a habit's tendency to become second nature plateauing, on average, around 66 days after you first make it daily practice. That's a little over two months, so a three-month diet or fitness challenge is actually the perfect length to give you a little wiggle room if you backslide or leave you a little extra time to figure out exactly what habits work best for you.

If you want to lose weight and keep it off, you should be building sustainable habits that will last a lifetime — not just 90 days. But that 90-day period feels imminently more manageable than "forever," and by the time those 90 days are over, you'll probably feel much more comfortable and confident in your new habits.

Read more: 10 Ways to Stay Fit and Healthy

As explained by the Harvard T.H. Chan School of Public Health, having a meal plan that you can easily stick with over time is one of the most important components of any diet. In the same article, Dr. Eric Rimm, then an associate professor at the school, suggests key points for choosing the right diet for you. Some of the points most salient to this discussion are:

• Steer away from processed foods; choose unprocessed foods instead.
• Look for a diet that offers a range of food choices, including fruits, vegetables, whole grains and nuts.
• A diet that's tailored to your personal and cultural preferences increases your odds of success.

But perhaps one of Rimm's most important takeaway points is that there is no single magic bullet "best" diet out there. The key is finding a healthy lifestyle that you can keep up over the long term, which is why finding that easy-to-stick-with meal plan is so important.

Read more: Pros and Cons of the 10 Most Popular Diets

Not sure how to tell if a particular 90-day diet reset plan is really good for you? Try asking yourself these quick questions or see how well they match up with Dr. Rimm's points already mentioned. On balance, the Parhams' 90-day weight loss plan comes out looking very good.

You can also compare the diet plan to landmark references such as Harvard T.H. Chan School of Public Health's Healthy Eating Plate, the USDA's ChooseMyPlate, or the U.S. Department of Health and Human Services' (DHHS) key recommendations in its Dietary Guidelines for Americans, 2015-2020.

In fact, those key recommendations are a good thing to look for in any sustainable diet plan. They emphasize eating a wide variety of vegetables and fruits, whole grains, high-quality protein sources, fat-free or low-fat dairy, and oils that are rich in healthy, unsaturated fats.

Not sure if the diet you're looking at provides a healthy balance of fat, protein, carbohydrates, fiber and the like? The DHHS offers a helpful table that shows how these nutrients should be balanced according to your gender and age.

The DHHS dietary guidelines also place limits on several key components, which should sound familiar from the review of the Parhams' 90-day diet reset: Fewer than 10 percent of your daily calories should come from saturated fat; ditto for added sugar. Meanwhile, you should consume less than 2,300 milligrams of sodium per day, and if you drink, do so in moderation. The DHHS defines moderate drinking as up to one drink per day for women or up to two drinks per day for men.

You can even create your own 90-day diet challenge as long as you focus on setting yourself up for success. According to findings from the National Weight Control Registry, you'll get the best odds of success by combining a healthy diet with exercise.

Using the principles already described, set your fitness and diet goals to be specific, measurable, attainable, realistic and time limited; UMass Dartmouth offers more detail about this S.M.A.R.T. goal-setting methodology, which is useful in not only fitness but also every aspect of your life.

And remember: You're in this for a lifetime. So even if your DIY 90-day diet plan doesn't go quite as planned, or perhaps you try the Parhams' or anyone else's 90-day fitness challenge and find it's not a fit, you've got another 90 days coming up to try something else.