The F-Factor Diet Is Getting Major Backlash from Dietitians

In light of the F-Factor diet controversy, registered dietitians are working to de-bunk fiber myths.

Woman balancing loaf of bread on finger
Photo: Francesco Carta fotografo/Getty

More than a decade before the keto diet and its emphasis on healthy fats took a stronghold in the nutrition scene, another diet with laser-focus on a single nutrient was stealing the hearts (and stomachs) of those looking to lose weight: the F-Factor diet.

Created by Tanya Zuckerbrot, M.S., R.D. in 2007 alongside the publication of her groundbreaking book of the same name, the F-Factor Diet is rooted in the idea that fiber (a bioavailable type of carbohydrate) is the ~magic key~ to losing weight without ever having to feel hungry. The diet focuses on consuming high-fiber carbs (think: lots of beans and legumes; veggies like beets, broccoli, and cauliflower; fruits like apples, berries, and oranges; whole-wheat bread instead of white) and lean protein — a combo meant to help you feel full more quickly while consuming fewer calories, according to the program’s website.

The diet has three "steps," each with different purposes and nutrition recommendations. The first step of the plan is all about jumpstarting weight loss by swapping the refined carbs in your diet with high-fiber carbs and adding more of those high-fiber carbs. During the first step, women are advised to eat at least 35 grams of fiber per day (FYI, that’s 7grams more than what’s currently suggested for women by the USDA), eat less than 35 grams of net carbs (your total carbs minus fiber) per day, and consume roughly 33 grams of fats (or 30 percent of your daily caloric intake), and 10 to 14 ounces of protein daily (approximately the same amount of fat and nearly two to three times more protein than what's recommended by the USDA for women). Those on the diet are recommended to stick to Step 1 practices for about two weeks, and during that time period, you're incorporating protein, fats, and carbs into your breakfasts, lunches, snacks, and dinners, Zuckerbrot tells Shape.

When carbohydrates — which are converted to glucose and stored in the muscles and liver as glycogen (aka energy) — are eaten in excess of what your body can physically store, the glycogen is converted into fat, explains Zuckerbrot. "[But] in the absence of glucose, the body burns fat for fuel," she says. So, the first step of the F-Factor diet "is more restrictive for carbohydrates because we want that jumpstart. We want to empty out the glycogen stores, and typically people begin the program with their glycogen stores relatively full," explains Zuckerbrot.

In Step 2, F-Factor diet followers are advised to eat three additional servings of carbohydrates, preferably high-fiber options, throughout the day, with the recommended maximum net carb intake being 75 grams. To help folks plan their meals without all the guesswork, the F-Factor book provides the Food Exchange Lists created by the American Diabetes Association and Academy of Nutrition and Dietetics, which detail the serving sizes of certain foods and the number of carbohydrates, protein, fat, and calories in one serving, says Zuckerbrot. Followers can eat everything from pasta, quinoa, and sweet potatoes, to fruits, legumes, and dairy products throughout this step, which continues until they reach their desired weight or see their desired health benefits, she says.

"Not everyone comes to F-Factor for just weight loss," notes Zuckerbrot. "As a clinical private practice, we are dealing with GI disorders, we are dealing with cardiovascular disease, diabetes... So as a clinical dietitian, I have the ability to tailor F-Factor to meet their needs, and it's not always about weight loss. If you don't want to lose weight, you go right to maintenance."

Maintenance, or Step 3, typically involves adding an extra three or more servings of carbohydrates to your daily diet, capping the net carb intake at 125 grams, all with the goal of managing the weight or health benefits the diet follower has achieved, according to F-Factor's website. For context, the USDA recommends 130 grams of carbs per day for women.

Despite the seemingly simple recommendations online, Zuckerbrot stresses that this last step is not one-size-fits-all and should be adjusted to fit each person's unique needs. "[Maintenance] really varies depending on someone's height, weight, gender, or their activity factor," says Zuckerbrot. For instance, "athletes have to carb-o-load — they need much more carbohydrates [to help with] weight management." (FWIW, though, not all athletes need to carb-o-load for weight management.)

The Backlash Against the F-Factor Diet

To help you reach your fiber goals, F-Factor sells its own fiber and protein bars and powders, which boast 20 grams of each nutrient, helping you supplement when you're in a pinch. Sounds like a no-stress way to get your protein *and* fiber fix after your strength training session, right?

Not entirely. Some people are now anonymously calling out the company on Instagram, alleging that they’ve experienced severe health problems after eating F-Factor products — from full-body rashes, to severe gastric distress, and even amenorrhea. Some also claim the products may contain lead and asked F-Factor to release a Certificate of Analysis (COA), which would provide lab results of their products' quality control tests. On Thursday, the company shared a COA of its chocolate fiber/protein powder, showing that the product received a "pass," meaning the number of trace metals in the products was below the legal limit. Based on the COA shared on Instagram, it's unclear who issued the "pass" result, as the post blocks the name of who, specifically, conducted the analysis. That said, the COA showed that the product contained the exact amount of dietary ingredients listed on its label, and contained 0.014 parts per million (ppm) of lead, which is below the 0.015 ppm allowed by the U.S. Environmental Protection Agency for lead in public drinking water. (FWIW, F-Factor's products do have the Prop 65 warning mandated by California law, which indicates that products contain certain chemicals that could "cause cancer, birth defects or other reproductive harm.")

Along with the backlash against F-Factor products, others are saying that F-Factor can foster disordered eating habits by encouraging followers to meticulously count carbs and fiber and "approach food in a very regimented, controlled way," according to the New York Times. The environment within the F-Factor office is also being called into question, with former staff members alleging that "the culture was a pursuit of thinness at any cost,” the Times reports.

While the allegations are highlighting the potential dangers of following this diet without the personalized guidance of a nutritionist (not to mention the safety of supplements, which aren't regulated by the FDA, in general), they’re also bringing into question the validity of the F-Factor’s program in the first place — and Lisa Hayim M.S., R.D. is one nutrition expert leading the charge. In response to all of the controversy, the New York City-based registered dietitian recently posted a Fiber 101 lesson on her Instagram, detailing the exact pros and cons of fiber — and why it's not the end-all, be-all nutrient.

Hayim’s first issue with high-fiber diets (particularly fiber-packed supplements): Fiber on its own — totally separated from the good-for-you micronutrients and carbs you’d find in whole foods like fruits, nuts, and whole grains — is going to make you feel full, but it’s not going to give you enough energy to actually power through your day, she says in the video. “You may call this blood sugar control, but I call this faux fullness,” she explains. “Remember, you need energy and nutrients to be a functioning human, to have a regular period, to be able to carry children, or just feel good. If you eat fiber from real foods, you’re still going to get all the benefits from fiber, but also the nutrients and energy to make you feel like you.”

For that same reason, the Mayo Clinic recommends opting for fiber-rich whole foods, rather than fiber supplements, to get your daily dose. Basically, fiber-packed whole foods fuel your body with a variety of vitamins, minerals, and nutrients that pills and powders just can’t provide.

And if you’re slurping down a fiber- and protein-heavy shake (such as F-Factor’s) immediately after a workout, you might not see the biggest #gains. During a sweat sesh, your body uses stored glycogen (aka energy from carbs) to help you keep moving (unless your body uses fat instead, as mentioned previously). In order for your exhausted muscles to rebuild and repair and for your energy to pick back up, you need to replenish those glycogen stores by consuming both carbs and protein, according to the Academy of Nutrition and Dietetics. But there’s a hitch: “If all of your carbohydrates are coming from fiber — [as] it does in this [F-Factor] protein powder — it looks like you’re getting carbs, but you’re not because your body can’t break that down and use it,” Hayim says in her video.

So, is the controversy surrounding the F-Factor diet legit?

Fiber isn't so simple that all of its benefits can be broken down so simply. ICYMI, there are two different types of fiber, and each reacts differently in your body. Soluble fiber, found in foods like oats, beans, apples, and blueberries, dissolves in water to form a gel-like material and helps lower blood cholesterol and glucose levels. Insoluble fiber, on the other hand, is found in foods like whole-wheat bread, brown rice, cauliflower, and potatoes and doesn't dissolve in water. It helps increase stool bulk and move everything through your digestive system, helping to prevent both constipation and irregular trips to the bathroom, according to the Mayo Clinic.

When soluble fibers are digested and fermented by the bacteria in your gut, they produce short-chain fatty acids that can be absorbed and metabolized to produce energy, and this process actually contributes to up to 10 percent of daily energy intake, according to the Linus Pauling Institute at Oregon State University (OSU). Insoluble fibers, however, aren't digested, so they go straight through the gastrointestinal tract and don't give your body any energy, according to OSU. F-Factor's chocolate brownie bar, for example, contains soluble corn fiber and oats, as well as almond and peanut butter, which provide insoluble fiber, according to the Mayo Clinic. As for the company's powders, the vanilla protein powder is made with guar gum, which is a soluble fiber, according to OSU. Translation: F-Factor's products *do* give you some limited energy to bounce back from your workout.

Confused by all of this fiber talk and how you can possibly get all the micronutrients you need? Instead of trying to zero in on specific types of fiber or exactly how much you're getting, Keri Gans, M.S., R.D.N., C.D.N, a Shape Brain Trust member, encourages you to take a different approach. "The truth is, I actually hate to decipher between the two [types of fiber]," she explains. "I feel like people should be focused on getting all types of fiber because it's hard enough trying to get fiber in your diet, and then having to figure out which is soluble, which is insoluble, and which does what for us [makes it more challenging]. But if you focus on, overall, including foods that have fiber in them, you're going to eat more foods that have more variety of fiber in them."

So, is fiber a good-for-you nutrient to incorporate into your post-workout snack or not? In addition to giving you a boost of energy, fiber can help balance your gut when you eat *too much* protein, Wendy Dahl, Ph.D., an associate professor in food science and human nutrition at the University of Florida, previously told Shape. When you overload on protein, some of that macronutrient might not get digested, in which case it ends up being broken down by gut bacteria, creating inflammation-causing compounds in the process, explained Dahl. Introduce fiber into the equation, and the bacteria will break that down instead of the protein, meaning you won’t have to deal with inflammation, and protein can do its job of helping you feel satiated and maintaining muscle mass, said Dahl.

A far as F-Factor, specifically, not all experts in the nutrition field 100-percent agree with the backlash. For starters, F-Factor bars aren't just fiber bombs. They also support your body with protein from brown rice protein, pea protein, almond and peanut butter, and oats, as well as bone-strengthening calcium from the calcium concentrate — ingredients you'd find in other popular protein bars on the market, like Vega and Tone It Up. "These are ingredients, as a registered dietitian, that I don't object to at all," says Gans. "There's not one that calls out to me like, 'Oh, avoid this ingredient.' I think all these ingredients that are included in the products are safe."

Not to mention, F-Factor products aren't touted as full-on meal replacements, but rather as supplements that, "if needed, can help you meet your daily fiber needs in a more convenient and enjoyable way," according to the F-Factor website. Gans notes that this suggestion isn't all that unusual. "As registered dietitians, we always suggest food first, and then if you have problems meeting your nutritional needs for whatever reason, then perhaps a supplement is needed after discussing it with your registered dietitian or doctor," she says.

And even though the F-Factor diet praises fiber as a "miracle" nutrient and encourages people to amp up their intake, to Gans, the premise behind the diet is less worrisome than others with cult followings. "I'm warier of a diet that eliminates nutrients or food groups," she explains. "If they were encouraging one nutrient in place of another healthy nutrient, I would be concerned, and that's exactly what a lot of fad diets do. A diet that's encouraging a nutrient like fiber, that's known to have health benefits, and at the same time not focusing on eliminating any food groups, I can't really say I have a problem with that."

As for the poor side effects allegedly caused by F-Factor products, Gans stresses the idea you learned way back in kindergarten: Everyone is different — and so is their microbiome and digestive system. "There might be one ingredient in one of these products that doesn't agree with you," she says. "It doesn't mean it's unsafe. It doesn't mean it's harmful. It just means that it doesn't agree with you."

What about the F-Factor diet's seemingly restrictive practices?

Products and fiber supplementation aside, Hayim says in a second Instagram video that F-Factor's foundation appears to be rooted solely in weight loss, with little to no regard for other health factors, such as exercise, which makes it inherently troublesome. Case in point: One of F-Factor's core principles is to "work out less," according to the company's website.

Despite the website's wording, Zuckerbrot tells Shape that the idea that F-Factor has an anti-exercise message couldn't be farther from the truth. "In fact, there’s an entire chapter [in the F-Factor book] dedicated to exercise," says Zuckerbrot. "The messages that you've been reading are so contradictory to a book that’s been around for 14 years. We don’t have an anti-exercise message. It’s about working out smarter."

By "smarter," Zuckerbrot means reserving cardio just for the times you need a boost of endorphins, want to clear your head, or any reason besides weight loss. Instead, F-Factor encourages clients to focus on building muscle, which increases the capacity of the body's glycogen stores so they can eat more carbs and still manage their weight, she says. (More here: 11 Major Health and Fitness Benefits of Lifting Weights)

As for the diet's focus on losing weight, Gans believes that there's nothing fundamentally wrong with the goal of weight loss — if it's for the right reasons. "For some individuals, losing weight may be recommended," she explains. "I think people are being shunned today if they desire to lose weight, and I think that is not how we should look at it." If a person wishes to lose weight in a healthy way, with the intention of being a healthier person, and they have a healthy relationship with food *and* their body, then there's no reason they shouldn't be able to act on that desire, says Gans.

"It's hard to keep lumping us all, especially women, together," she adds. "We come in different shapes and sizes, we exercise differently, we have different genetic backgrounds, and as long as we're going about weight loss in a healthy manner that comes from a place of compassion versus a place of hatred of yourself, then I don't see why, as a registered dietitian, I can't help somebody reach their goals."

In response to claims that F-Factor's program may feel restrictive, Zuckerbrot says that notion is "antithetical to all of [her] messaging." She points to the fact that F-Factor does not encourage people to count calories or restrict them. She does, however, acknowledge that "each step of the F-Factor Diet has a calorie cap that guarantees you will lose weight. You will feel satiated and satisfied, and you need to receive the proper nourishment in order to keep up your energy level and remain in good health," Zuckerbrot writes in her book.

On the flip side, Zuckerbrot sees the recommendation to count carbs and fiber as necessary education to help clients meet their goals. "What is undisputed is that if you want to burn fat for fuel, your body has to rely on fat and not glucose," says Zuckerbrot. "Of course our clients need to understand what a carb is and how many carbs they’re eating; otherwise, how will they know whether they’re doing the program correctly? The whole idea about counting fiber is to make sure that they’re eating at least 35 grams of fiber because, in the absence of fiber, they’re going to be hungry and they’re not going to be receiving fiber’s numerous health benefits."

That's not to say that there aren't some potential issues with this specific diet. According to the F-Factor book, “the F-Factor Diet will teach you that magic number and show you which carbs you should choose to ensure that you do not exceed your stores, yet still feel satisfied and energized throughout the day." Yet, the amount of calories a woman ends up eating, due to the net carb limits and other macronutrient suggestions, during Step 1 is between 1,000 to 1,200 calories, says Zuckerbrot, who referenced the book's guidance.

If that sounds like a scarily low number, you're not completely wrong. "A diet consisting of 1,000 to 1,200 per day is rather low for the average person; however, for some people, it may help kickstart their weight loss," says Gans. "For a healthy individual, I wouldn’t expect any long-term adverse effects from the two-week recommendation; however, short-term they may experience some minor side effects, such as fatigue and irritability. Adverse health effects could arise if a person chooses to stay on Step 1 longer than recommended." According to the USDA, eating patterns that contain 1,200 to 1,500 calories each day can help most women lose weight safely.(FWIW, Zuckerbrot also notes that a person's caloric intake on the diet can be tailored to meet their needs through her private practice.)

Plus, for some folks, the F-Factor diet's limitations on daily net carb intake could become an issue. "Many diets, in general, can lead to under-consuming calories, especially if someone is prone to disordered eating and gets too strict with themselves," says Gans. "But I really think there is no straight 'yes or no' answer to this [question of under-consumption on the F-Factor diet] because there really isn’t a one-size-fits-all when it comes to losing weight. Depending on the height of a person, age, sex, and daily activity level, some may actually need more carbs, others may not. That is why it is important to speak with a registered dietitian to help customize a diet to meet your personal needs." (More here: Should You See a Dietitian?)

At the end of the day, Zuckerbrot says she stands behind her physician- and dietitian-endorsed program, her book, and her products. She notes that the way in which her program has been talked about online may have been the spark that ignited the flames. "I will make sure moving forward that the sound science that’s in the book is better communicated on social media," says Zuckerbrot. "Because I have a feeling that’s what happened here: People are taking snippets from social media and they may not be reading the book, so they’re misinterpreting some of my messaging. And for that, of course, I take full responsibility and I will do a better job moving forward."

The key takeaways from this whole mess? Number one: A well-balanced diet is essential — and fiber can and should hold a spot within it. But don't expect fiber — or any singular nutrient — to be a silver bullet.

Secondly, as Gans noted, if you have a history of disordered eating, a diet such as F-Factor that requires hitting specific numbers probably isn't the best route to take.

Lastly, if you want to lose weight, do you. Just talk with your doc or R.D. first to make sure you follow an overall healthy living plan that works best for your unique self. (Translation: Don't follow in Regina George's footsteps and only consume nutritional supplement bars — or the messages touted on the internet.)

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