Is the Warrior Diet Dangerous? Here's What a Nutritionist Says About This Intermittent Fasting Plan


There’s no shortage of options when it comes to weight loss, but one of the latest to bubble up (although it’s not actually new) is the Warrior Diet, which involves a rather extreme form of intermittent fasting. Here’s a summary of how the diet works, the potential pros and cons, and my bottom-line advice as a registered dietitian who has helped my many clients shed pounds over the past two decades.

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The Warrior Diet rules

This weight-loss plan doesn't require fasting completely. You eat very little for 20 hours a day, and then eat as much food as you’d like during a four-hour evening window, with no specific calorie targets or limits.

In the initial “detox” week, or phase one stage, foods like broth, hard boiled eggs, raw veggies and fruits, yogurt, cottage cheese, and vegetable juices are allowed in small portions during the 20-hour period, along with coffee, tea, and water. During the four-hour window, dieters are advised to eat a salad dressed with oil and vinegar, and unprocessed, primarily plant-based foods are encouraged, like veggies, beans, and whole grains.

During week two or phase two, the same foods are consumed in the 20-hour period, but during the four-hour window, more fat is encouraged after the salad—from nuts and animal protein, along with cooked veggies. Grains are excluded.

In the third week, dieters cycle between one or two higher-carb days within the four-hour eating window and one or two lower-carb days over the course of a week, with the foods allowed during the 20-hour period remaining the same.

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Once the three phases are completed, they can be repeated, or a dieter can stick with the 20:4 timing and focus on a higher -protein, lower-carb pattern. Throughout the entire diet, processed foods are discouraged, including candy, chips, baked goods, sugary drinks, artificial sweeteners, fried food, and fast food.

You may have heard about a simplified version of the Warrior Diet, in which people fully fast for 20 hours a day and eat anything they want within the four-hour window. This version, sometimes referred to as 20:4, is more extreme and risky than the original. Calories are further restricted, and some people binge on unhealthy foods during the eating window, without regards to nutrition.

Intermittent fasting concept with empty colorful plates. Time to lose weight , eating control or time to diet concept. warrior-diet-intermittent-fasting , warrior-diet , intermittent-fasting Intermittent fasting concept with empty colorful plates. Time to lose weight , eating control or time to diet concept. warrior-diet-intermittent-fasting , warrior-diet , intermittent-fasting as well as speed training. He also suggests taking supplements, including probiotics and a multivitamin. The more simplified 20:4 approach doesn’t have concrete rules around workouts and supplements, but exercising during strict fasting hours can potentially lead to dizziness, or even passing out, which can up injury risk.

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Potential pros

Several recent studies on intermittent fasting in humans have shown benefits that include not only weight loss, but also improvements in blood sugar and insulin levels, cholesterol, and inflammatory markers. However, it’s important to note that there is no one intermittent fasting or time restricted eating protocol. Numerous variations have been studied, and benefits have been seen in approaches that allow a much larger eating window, including up to 12 hours.

Few studies have been conducted to test a four-hour eating window. In one study only water and no food was allowed during the 20-hour period, and the 20:4 pattern was followed every other day for two weeks, rather than day after day.

Another older study involved two eight-week periods during which adults consumed all of the calories needed for weight maintenance in either three meals per day (breakfast, lunch, and dinner) or one meal per day within four hours in the early evening. There was an 11-week washout period between the two methods.

During the one meal per day experiment, subjects had a small reduction in body weight and body fat, which wasn’t seen when they consumed three daily meals. However, the volunteers didn’t become accustomed to the one meal per day diet. Some reported extreme fullness after the meal and difficulty finishing the food within the allotted time. Over time, the one meal a day dieters experienced increased hunger and desire to eat, and feelings of fullness decreased. This group also had an increase in blood pressure and cholesterol.

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Potential cons

Even if weight loss is achieved on the Warrior Diet, there are several potential downsides. Nutritionally speaking, it can be difficult to consume enough overall nutrients, which can ultimately impact energy and even immunity, particularly for the newer, simplified 20:4 version. Studies also show that in order to best utilize protein, it should be consumed throughout the day in four evenly distributed meals.

Undereating during active hours can lead to low blood sugar, hunger, irritability, and constipation. The pattern is also difficult to stick with socially and emotionally and can potentially lead to or worsen disordered eating—particularly for those prone to cycles of restriction followed by binge eating.

Other research on intermittent calorie restriction has shown that it may trigger trouble concentrating, eating-related thoughts, anger, and depression. Other potential effects include a disruption of the menstrual cycle.

Extreme fasting is definitely inappropriate for kids and teens, pregnant and breastfeeding women, people with chronic medical conditions like type 2 diabetes, people who need to take medications with food, those with a history of disordered eating, athletes, and very active people.

Finally, such an extreme approach just isn’t necessary for weight loss. Some studies show no difference in outcomes between intermittent fasting and continuous calorie restriction. The latter means creating and sustaining a calorie deficit that allows for weight loss (not a starvation diet) that’s consistent from day to day and doesn’t require extended hours of fasting.

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Bottom line advice

When it comes to weight loss, it’s important to keep in mind that what may work for one person may be completely inappropriate or ineffective for another. There’s also a difference between what it takes to lose weight and what’s optimal for health, including mental well being, immunity, digestive health, sleep, and disease prevention.

As a health professional, I want people to lose weight in a way that maximizes wellness, not compromises it. And if there’s one thing I can attest to after years of counseling clients, it’s that any method used to shed pounds must be sustainable long-term in order to keep the weight off.

While the original Warrior Diet emphasizes whole, unprocessed foods, which is great as an overarching strategy, it’s not realistic or necessary to go through the rest of your life never eating dessert. The 20:4 pattern is also not research-backed or nutritionally optional, and it’s nearly impossible to stick with. Find an approach that allows you to safely, sanely, and sustainably lose weight, based on lifestyle habits that nourish your body and enhance overall wellness.

Cynthia Sass, MPH, RD, is Health's contributing nutrition editor, a New York Times best-selling author, and a private practice performance nutritionist who has consulted for five professional sports teams.

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