What Is the F-Factor Diet, and Is it Safe?

The diet, created by celebrity dietitian Tanya Zuckerbrot, has caused quite a bit of controversy.

The F-Factor diet hit the diet scenes around 2006 and the fad diet focused on filling up with fiber-rich foods. In 2020, influencer Emily Gellis Lande set out to bring attention to the potential dangers of the diet popular among influencers and celebrities.

Gellis Lande, via her Instagram, shared the complaints from anonymous people who had terrible experiences on the F-Factor Diet, particularly after eating the diet's branded bars and supplements.

Gellis Lande had never completed the diet herself. But the stories she shared from anonymous people reported side effects like hair loss, amenorrhea (lack of menstrual bleeding), rashes, disordered eating habits, and gastrointestinal distress.

Ultimately, in an article published in 2020 in The New York Times, F-Factor creator Tanya Zuckerbrot, RD, denied those accusations. The article even reported that some more extreme claims might have been fabricated.

But nutrition experts have raised serious concerns about the diet and its restrictions. Here, two registered dietitians and I—a dual MPH/RD candidate at the University of North Carolina's Gillings School of Public Health in Chapel Hill—weigh in on what we think you should know about the F-Factor Diet.

What is the F-Factor Diet?

The F-Factor Diet isn't new: Zuckerbrot published her book, The F-factor Diet: Discover the Secret to Permanent Weight Loss, in 2006.

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

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, told 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, explained Spence. 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 its followers to track fiber and net carbs. You should note that the term “net carbs” currently doesn’t have a legal definition and is not used by the FDA. The only carbohydrate information regulated by the FDA is provided in the Nutrition Facts label, which lists total carbohydrates and breaks them down into dietary fiber and sugars.

The F-Factor Diet also has recommendations for additional grams of fats each day, ounces of protein you should have at each meal, and a snack.

The protein recommendation equates to about 10 to 14 ounces of lean protein per day for women and 20 to 27 ounces for men. That amount is significantly above the Department of Agriculture's recommended amount of five to six ounces daily for most adult women and six to seven for most adult men.

It's also important to note that while the diet says it doesn't limit your daily calories, the recommendations for other foods ultimately limit overall calorie intake. That provides a built-in "calorie cap," according to Zuckerbrot. Each step, outlined below, also has its 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 seven grams more than what the Dietary Guidelines for Americans (DGA) recommends. It also entails eating fewer than 35 grams of net carbs (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. Still, net carbs increase to 75 grams daily, along with 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 body mass index, or BMI) and 125 grams of net carbs per day while sticking with the high fiber count. As a reference, the DGAs recommend getting 45% to 65% of your total calories from carbs—so if you eat 2000 calories per day, that's 225-325 grams per day.

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

What Can You Eat on the F-Factor Diet, and What’s Off-Limits?

Before we dive into this, it's essential to know that even when a food contains carbohydrates or fats, it may not be classified as a "carb" or "fat" on the F-Factor Diet. I'm not going to beat around the bush here. It's pretty confusing. 

According to the diet, non-starchy vegetables count as "zero carbs." The fat you find in lean protein, like beef, poultry, and dairy, isn't included in the "additional fats" allowed on the diet.

The diet emphasizes high-fiber foods, like fruits, non-starchy vegetables, and legumes like chickpeas. It selects whole grains like high-fiber cereal and bulgur. 

The F-Factor Diet also recommends getting extra fiber through its branded fiber-and-protein bars, powders, and 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, eating any of these things is fine 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 two net carbs to your daily carb count. The F-Factor blog explains that, while no one should start drinking alcohol if they don't already, drinking in moderation can be part of the diet plan.

According to the F-Factor, not eliminating alcohol allows you not to sacrifice your social life. Although, you should avoid high-sugar drinks and recommends low-calorie options like dry wine and liquor mixed with club soda.

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 nutritionist specializing in sports nutrition, weight concerns, and family wellness in Las Vegas, reviewed the F-Factor Diet: "[F-Factor] looks like a diet my grandmother was on decades ago," Schilling told 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," continued Schilling. "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 essential to recognize that Schilling's "starvation" reference doesn't refer to the literal definition of starvation.

Instead, Schilling referenced 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 daily.

The subjects (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 constantly talking and reading about it.

"First and foremost, someone's energy needs have to be met for the body to maintain basic processes," said Schilling. "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." 

Additionally, Schilling warned that the diet is far more likely to cause long-term harm than short-term good.

There's some evidence to back this up. First, some evidence suggests that most people can 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 a lack of willpower or follow-through. Research suggests that rapid weight loss can trigger physiological changes. Those changes—including fewer calories burned, less fat oxidation, and increased or decreased production of hunger hormones—make it hard to lose weight and promote weight gain.

However, there are some instances in which long-term weight-loss maintenance is possible. Research has found that approximately 30% of overweight individuals who pursue a weight-loss program are successful at long-term weight loss (losing 10% of their initial body weight).

On that note, the F-Factor Diet does have some proponents. 

"Bravo for the F-Factor Diet," Lisa Sasson, MS, RDN, clinical professor of nutrition and food studies at the NYU Steinhardt School of Culture, Education, and Human Development in New York, 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," continued Sasson. "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 W. Galluccio, MD, an adjunct assistant professor at the NYU Grossman School of Medicine in New York, called Zuckerbrot's approach to nutrition "sensible and based on science rather than fads," adding "under her guidance, my patients have significantly lowered their cholesterol, lost weight, and increased their chances of longevity.

Is It a Good Idea To Try the F-factor Diet?

The dietitians I interviewed and I recommended 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," said Schilling. "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 carbs.

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 the United States promotes a diet based on fiber-rich nutrition, including the Department of Agriculture, 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 fiber, isn't always good. Fiber is good for you, and eating enough (about 28 grams daily for women, although Schilling warned that not everyone wants or can tolerate that amount) has health benefits.

But 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 [gastrointestinal] disruptions such as bloating, abdominal pain, and constipation, among other symptoms," said Spence.

Indeed, studies have noted that dietary fiber is partially or completely fermented in the distal small bowel and colon, which produces gases like carbon dioxide, hydrogen, and methane. Switching from a low-fiber diet to a high-fiber diet may increase the occurrence of bloating.

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."

A Quick Review

As with all diets, your decision to try the F-Factor diet should be based on your individual diet goals and health condition. While diets work for some people, many nutritionists say that diet plans that are too restrictive fail. Or, worse: It can be harmful in the short or long term. 

If you have questions or concerns about a diet plan, speak with your healthcare provider.

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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