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

What Is the F-Factor Diet , Health food concept for a high fiber diet with fruit, vegetables, cereals, whole wheat pasta, grains, legumes and herbs. Foods high in anthocyanins, antioxidants, smart carbohydrates and vitamins on marble background top view.

It’s not news that fad diets aren't the best in regards to long-term weight loss. There’s plenty of evidence that any diet or cleanse that calls for making extreme changes like drastically cutting calories or avoiding entire food groups is unsustainable, and can even lead to disordered eating habits and bingeing.

The latest fad to get called out for being ineffective and potentially harmful is the F-Factor Diet, created by celebrity dietitian Tanya Zuckerbrot. High-profile clients reportedly pay $15,000 for the plan and personal sessions with Zuckerbrot, but the diet is also available to the masses in book form, with The F-factor Diet: Discover the Secret to Permanent Weight Loss.

The diet isn't necessarily new—Zuckerbrot published her book in 2006—but this summer, wellness influencer Emily Gellis Lande began posting on her Instagram stories other women’s complaints and bad experiences on the F-Factor diet, particularly after eating F-Factor’s branded bars and supplements. The purported side effect included hair loss, amenorrhea, rashes, lost periods, disordered eating habits, and GI distress. And although The New York Times reports that some of the more extreme claims may have been fabricated, nutrition experts have long had serious concerns about the diet and how restrictive it is. Here's what registered dietitians want you to know about the eating plan.

What is the F-Factor Diet?

Essentially, F-Factor is a very high-fiber diet (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 urges followers to track five nutrients and macronutrients—fiber, protein, fat, carb, and net carbs—and to alter their diet depending on meeting the recommendations for those types of foods. It's also important to note that, while the diet doesn't limit the number of a dieter's calories per day, the recommendations for other foods ultimately limits overall calorie intake.

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

  • Step one: This step typically lasts two weeks, and requires dieters to eat at least 35 grams of fiber per day. That's 10 grams more than what’s recommended by the Academy of Nutrition and Dietetics 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).
  • Step two: Dieters follow this phase until they’ve lost as much weight as they want to lose. Recommendations include sticking with the high fiber count, but net carbs increase to 75 grams per day.
  • Step three: In this phase, known as the maintenance phase, dieters eat 125 grams of net carbs per day but stick 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.

In all three phases, dieters are also encouraged to get 30% of their calories from fat, which the F-Factor website says is around 33 grams per day. Again, though the diet doesn't directly count calories, it helps to know that fat has 9 calories per gram, so 33 grams equal 297 calories. If 33 grams of fat make up 30 percent of your total calories, you’re eating 990 calories per day. For most people, that’s dangerously low. “[This is] not enough calories for most grown women,” Spence says.

The diet also encourages high protein intake—between 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).

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.

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

The diet's literally named after fiber, so the nutrient is a person's best friend on the diet: That means fruits, non-starchy vegetables, legumes, and select whole grains like high-fiber cereal and bulgur, per the website. You're also required to eat lean proteins and some fats, as stated earlier.

Conversely, F-Factor cautions against eating high-fat meat and dairy, 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.

Will the F-Factor Diet lead to weight loss?

In the short term? Yes, it likely will. In the long term? It’s doubtful.

“[F-Factor] looks like a diet my grandmother was on decades ago,” says Leslie P. Schilling, RDN, CSCS, CEDRD-S, a dietitian in the Las Vegas area, 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.”

If the word “starvation” alone isn’t enough to freak you out, know this: The Minnesota Starvation Experiment, one of the landmark starvation studies Schilling is referencing, found that after six months on a low-calorie diet, 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 plenty of 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.

Is the F-Factor Diet ever a good idea?

Probably not. While 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.

But there’s such a thing as eating too much fiber. 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. The F-Factor Diet also recommends getting fiber through its branded fiber-and-protein bars and powders, as well as GG crispbread crackers. The DGAs, on the other hand, recommend getting it from whole food sources like fruits, vegetables, whole grains, nuts, seeds, and legumes, which each provide several important nutrients.

Plus, no one nutrient—like fiber in this case—is no more important in a diet than another. “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.

The bottom line: The F-Factor Diet is restrictive and provides too few calories to be healthy for most people. While its focus on fiber isn’t necessarily bad, you’re better off eating plenty of fiber-rich foods—fruits, vegetables, whole grains, nuts, seeds, and legumes—without severely restricting other parts of your diet.

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