The F-Factor Diet Has Been a Favorite Among Celebs and Influencers for Weight Loss—But Is it Safe?


Earlier this summer, influencer Emily Gellis Lande set out to bring attention to the potential dangers of a certain diet, popular among influencers and celebrities alike: the F-Factor Diet.

Gellis Lande, via her Instagram Stories, began sharing the complains from anonymous women, who had bad experiences on the F-Factor Diet, particularly after eating F-Factor's branded bars and supplements. Though Gellis Lande had never been on the diet herself, the stories she shared from anonymous women reported side effects that included hair loss, amenorrhea, rashes, lost periods, disordered eating habits, and GI distress.

Ultimately, via The New York Times, F-Factor creator Tanya Zuckerbrot, RD, vehemently denied those accusations—and the newspaper even reported that some of the more extreme claims may have been fabricated. But nutrition experts have long had serious concerns about the diet and how restrictive it is. Here, two registered dietitians, and myself—a dual MPH/RD candidate at the University of North Carolina's Gillings School of Public Health—weigh in on what we think you should know about the F-Factor Diet.

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What is the F-Factor Diet?

The F-Factor Diet isn't new: Zuckerbrot, who has worked high-profile clients like Megyn Kelly and Katie Couric, and is the official dietitian to the Miss Universe Organization, per the Times, published her book, The F-factor Diet: Discover the Secret to Permanent Weight Loss, in 2006.

The F-Factor Diet, in the broadest of terms, is a high-fiber diet for men and women (the "F" stands for fiber, according to the diet's website). The reasoning here, is that "fiber has zero calories—so you get to fill up, without filling out," the website says.

First, a quick reminder about what fiber is: “There are two types of fiber,” Shana Minei Spence, MS, RDN, a dietitian based in New York City, tells Health. “There’s soluble fiber, [which] dissolves in water to form a gel-like material and is said to help lower blood cholesterol and glucose levels.” Soluble fiber is found in oats, peas, beans, apples, citrus fruits, carrots, barley and psyllium. Then there’s insoluble fiber, which promotes bowel movements and can bulk up your stool, Spence says. It’s great for constipation or irregular stools, and can be found in whole-wheat flour, wheat bran, nuts, beans, and fibrous vegetables. Since your digestive system can’t break down fiber into energy, it also helps make you feel full.

The F-Factor Diet encourages followers to track fiber and net carbs, but also has recommendations for additional grams of fats you can have each day, as well as ounces of protein you should have at each meal, plus a snack. That equates to about 10 and 14 ounces of lean protein per day for women, about 90-126 grams—which is significantly above the amount recommended by the World Health Organization, which suggests 0.8 grams per kilogram of bodyweight (for a 150-pound woman, that's 55 grams a day).

It's also important to note that, while the diet says it doesn't limit the number of a dieter's calories per day, the recommendations for other foods ultimately limits overall calorie intake, providing a built-in "calorie cap," according to the F-Factor book. Each step, outlined below, also has its own cap on calories.

To help break things down a bit more, the F-Factor Diet consists of three phases or steps:

  • Step 1: This step typically lasts two weeks, and asks dieters to eat 1,000-1,2000 calories and 35 grams of fiber per day. That's 10 grams more than what’s recommended by the Academy of Nutrition and Dietetics for women, and 7 grams more than what the Dietary Guidelines for Americans (DGA) recommends. It also entails eating fewer than 35 grams of net carbs (that’s total carbs minus fiber), and six servings of fat.
  • Step 2: Dieters follow this phase, known as "Continued Weight Loss" until they’ve lost as much weight as they want to lose. Recommendations include 1267-1467 calories per day, while sticking with the high fiber count, but net carbs increase to 75 grams per day, along with an additional three servings of carbs and three servings of fat.
  • Step 3: In this phase, known as the maintenance phase, dieters are allowed to eat 1,600-2,000 calories per day (depending on BMI) and 125 grams of net carbs per day while sticking with the high fiber count. As a reference, the DGAs recommend getting 45-65% of your total calories from carbs—so if you eat 2000 calories per day, that’s 225-325 grams of carbs per day.

Lastly, water's a huge part of the F-Factor Diet, because "fiber needs water to work its magic," the website says, suggesting dieters drink three liters, or more than 12 cups of water a day. That's not too far off from the the Institute of Medicine of the National Academies of Sciences, which actually recommends 2.7 liters (11 cups) for women and 3.7 liters (almost 16 cups) for men, per day of total water intake.

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What can you eat on the F-Factor Diet, and what’s off-limits?

Before we dive into this, it's important to know that even when a food contains carbohydrates or fats it may not necessarily be classified as a "carb" or "fat" on the F-Factor Diet (I'm not going to beat around the bush here, it's quite confusing). According to the diet, non-starchy vegetables count as "zero carbs," and the fat you find in lean protein, like beef, poultry, and dairy, aren't included in your "additional fats" allowed on the diet.

On the whole, the diet puts the most emphasis on high-fiber foods, like fruits, non-starchy vegetables, legumes, and select whole grains like high-fiber cereal and bulgur, per the website. The F-Factor Diet also recommends getting extra fiber through its branded fiber-and-protein bars and powders, as well as GG crispbread crackers.

Conversely, F-Factor cautions against eating high-fat meat, and encourages dieters to avoid saturated fats and added sugar. But still, the diet repeatedly advertises that you can lose weight “without losing everything you love.” Presumably, it’s fine to eat any of these things, as long as you hit the diet's fiber, net carb, protein, and fat goals.

One thing F-Factor makes sure to note is that alcohol is not off limits, and that a 4-ounce glass of wine contributes 2 net carbs to your daily carb count. The F-Factor blog explains that, while no one should start drinking alcohol if they don’t do so already, drinking in moderation can be part of the diet plan from the outset. “Without including some alcohol into your diet from the outset, you make it much more difficult to continue with your normal social life, go out after work with friends or even just socialize on a Saturday evening,” one blog post states. The post goes on to advise against high-sugar drinks and recommend low-calorie options like dry wine and liquor mixed with club soda.

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Will the F-Factor Diet lead to weight loss?

In the short term? Yes, it likely will. But it's harder to come to that conclusion in the long-term.

Leslie P. Schilling, RDN, CSCS, CEDRD-S, a dietitian in the Las Vegas area, reviewed the F-Factor Diet and has some thoughts: “[F-Factor] looks like a diet my grandmother was on decades ago,” she tells Health. “It's just another low-calorie diet with a catchy name. It is not nutritionally-sound because the total energy looks to fall below what most adults would need to eat in a day. To put it in perspective, this calorie level, which appears to be a "one-size-fits-all' approach, looks to be lower than the calorie levels in the landmark starvation studies.”

It's important to recognize that Schilling's "starvation" reference doesn't refer to the literal definition of starvation, but rather she's referencing the Minnesota Starvation Experiment, a landmark study conducted in 1944. The study followed 36 men who spent six months on a low-calorie diet of 1,570 calories a day. The subjects (who were all healthy men) experienced significant drops in strength, body temperature, heart rate and sex drive, as well as increased levels of depression, irritability, and fatigue. Worse, they became obsessed with food, dreaming about it often, and talking and reading about it constantly.

“First and foremost, someone's energy needs have to be met for the body to maintain basic processes,” Schilling says. “The body doesn't know the difference between another fad diet and a famine. This [diet] is a recipe for syndromes of low-energy availability and starting a restrict-binge cycle.” She warns that the diet is far more likely to cause long-term harm than do short-term good.

There’s some evidence to back this up. First, an April 2020 meta-analysis published in The BMJ looked at 121 previously conducted weight loss studies and found that the vast majority are able to lose weight during the first six months of a restrictive diet, but gain that weight back within a year.

And it’s not because of lack of willpower or follow-through. A 2015 review study published in the International Journal of Obesity explains that rapid weight loss can actually trigger physiological changes that make it really hard to lose more weight and actually promote weight gain, including fewer calories burned, less fat oxidation, increased production of hunger hormones, and decreased production in hunger hormones.

It should be noted, however, that there are some instances in which long-term weight-loss maintenance is possible: A 2005 article published in The American Journal of Clinical Nutrition stated that about 20% of overweight individuals who pursued a weight-loss program were successful at long-term weight loss—when that weight loss was defined a losing 10% of their initial body weight—and maintained it for at least one year.

On that note, the F-Factor Diet does have some proponents. "Bravo for the F-Factor Diet," Lisa Sasson, MS, RD Clinical Associate Professor, New York University Department of Nutrition, Food Studies and Public Health, said in an endorsement provided to Health by Lanny J. Davis, attorney and spokesperson for F-Factor. "It is not a weight-loss diet but a lifestyle approach to eating healthy, satisfying, and delicious foods. It is not based on deprivation, but instead focuses on an expansive list of foods to create well-balanced meals and snacks that come together to comprise a healthy diet.”

In another endorsement, Ronald Galluccio, MD, a cardiologist at Eastside Medical and Cardiovascular Associates, called Zuckerbrot's approach to nutrition "sensible and based on science rather than fads," adding that, "under her guidance, my patients have significantly lowered their cholesterol, lost weight, and increased their chances of longevity.

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Is it a good idea to try the F-Factor Diet?

Myself and the dietitians I interviewed recommend against trying the F-Factor Diet, essentially because of the lower-than-suggested calorie counts and the emphasis on one nutrient. “When I have a client that is hyperfocused on one particular nutrient, I will typically find out that they are lacking in other areas,” Schilling says. “This seems particularly true when eating too much fiber. I like to tell my clients that they may feel full, but they are [not adequately] fed.” In other words, getting too full from fiber-rich foods means you might not have room for adequate protein, fat, and starchy carbohydrates.

And while, yes, eating plenty of fiber is a healthy choice, the F-Factor didn’t invent this recommendation. It claims to be "the only dietitian-created program for weight-loss and optimal health that is based on fiber-rich nutrition,” but that couldn’t be further from the truth. Every legitimate healthy organization in America promotes a diet based on fiber-rich nutrition, including the USDA, the American Diabetes Association, the American Heart Association, the Institute of Medicine, and the Academy of Nutrition and Dietetics.

It's also worth noting that too much of anything—even something like fiber—isn't always a good thing. While fiber is good for you and eating enough—about 28 grams per day for women, although Schilling warns that not everyone wants or can tolerate that amount—has proven health benefits, anyone who’s ever eaten an entire sheet pan of roasted cauliflower in one sitting knows that too much fiber can lead to some nasty side effects. “Too much can definitely cause GI disruptions such as bloating, abdominal pain, and constipation, among other symptoms,” Spence says.

Still, in a statement provided to Health by Davis, on behalf of Zuckerbrot, she defends her diet:

"I am a registered dietitian. I've been eating the F-Factor way for twenty years, and I will be eating this way for the rest of my life to manage my weight without hunger while improving my health. For me, the F-Factor Diet is about food freedom. It is not a calorie counting diet and it is not restrictive. I developed the F-Factor diet to provide ample calories, which is why I ask my customers not to try to customize or alter the program that is prescribed in the book or take it to any extremes. F-Factor is about learning how to eat to honor your health and your waistline. Our goal is to educate people on how to improve their health through their relationship with food. With education comes empowerment, and we are committed to empowering every one of our customers with the tools they need to live happier, healthier lives."

Editor's Note: This post was updated to reflect the views of the supporters and founder of the F Factor Diet.

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