What Is a Fad Diet? Plus, the Best and Worst Plans, per RDs

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Fad diets are alluring — especially if one of your goals is weight loss — and they make big promises about your future. But what are fad diets, and why do registered dietitians tend to advise against them?

“Fad diets are more of a quick fix for weight loss or an acute issue,” says Alyssa Smolen, RDN, who is based in Essex County, New Jersey. “These differ from therapeutic diets, such as the Mediterranean diet or DASH, which are intended to be a long-term lifestyle change,” she says.

It can be tough to tell a fad diet from a sustainable lifestyle diet, especially because in many cases, both have “diet” in their name. But that doesn’t make them the same thing. Here’s how to spot a fad diet, according to Leah Reitmayer, RD, a sports dietitian in Fayetteville, North Carolina.

• Promises rapid weight loss
• Designed as a short-term solution
• Relies on specific foods as a “magic cure” for weight loss
• Excludes certain food groups
• Makes bold claims without scientific support
• An influencer or celebrity claims it worked for them
• May be expensive to follow

Some fad diets are bizarre at best and dangerous at worst. (But some fad diets have been around for years and may seem to have credibility because of that longevity.) As one study pointed out, these types of diets may be harmful for your health if you were to follow them for years. Maybe it's because, as the authors point out, fad diets often “defy the basic principles of biochemistry and nutritional adequacy.” In other words: They don’t make scientific sense and don’t supply the nutrients your body needs. Other fad diets may not be inherently dangerous, but they have significant pitfalls.

Here’s what you need to know about some of the best and worst options out there.

Best: Keto

The keto diet — one of the most famous diets of the recent decade — is a high-fat, moderate-protein, and very low-carbohydrate diet. Because it so strictly reduces carbs, the diet tends to produce quick weight loss, notes a meta-analysis. “People tend to lose weight on a keto diet, but it’s not normally something that can last,” says Reitmayer. For example: One study included in the meta-analysis cited above found that four to six weeks of a keto diet led to about a 20 pound weight loss, though the effect leveled out after six months and long-term results were similar to a calorie-restricted eating plan. There are real limitations to going keto. “There’s a small number of people who have the discipline to do that long term. Our bodies run on glucose, and so we need carbohydrates,” Reitmayer says.

Worst: Carnivore Diet

Do you want to consume only red meat, poultry, fish, butter, and eggs? (And water, too!) That’s the small list of foods on the carnivore diet. “I would never recommend this diet to anyone,” says Smolen. “You are eating only one food group, and restricting so many foods is detrimental for health. I can’t emphasize enough that variety is needed for optimal health,” she says. There’s emerging evidence that this diet dangerously raises “bad” LDL cholesterol levels. What’s more, without any plant foods, like fruits, vegetables, or legumes, you’re not getting fiber, which can lead to GI problems, among many other issues.

Best: Vegan

A vegan diet eliminates all animal products, including meat and dairy. There are many reasons people try a vegan diet, including ethical ones, where the lifestyle aligns with their values concerning animals and the environment. But there’s also a subset of people who use it as a quick weight loss diet — and that’s where it veers into fad territory for some folks. Because of this and the fact that the vegan diet can be more expensive and difficult to maintain, it’s often not done correctly, Reitmayer points out. Although this diet focuses more on what to avoid, rather than what to eat, it is a plant-based diet that protects your heart health and has been found to support a healthy weight, as research notes. But that's only if you’re following it properly; many highly processed junk foods technically qualify as vegan, and those should be cut back in any diet, not increased.

Worst: HCG Diet

The HCG diet limits you to 500 calories a day. People on the diet also take hormone injections, pellets, sprays, or pills that mimic a hormone produced by the placenta during pregnancy, says Melissa Rifkin, RD, who is based in Mount Kisco, New York. Proponents claim the hormone supports weight loss by changing your metabolism. For its part, the U.S. Food and Drug Administration (FDA) says there’s no evidence behind it — and any weight loss comes from lack of food, not the HCG. A huge warning is that by eating so few calories, you are likely not getting the vitamins, minerals, and protein your body needs for optimal health. If you possess human chorionic gonadotropin (HCG) in any form, the FDA has this advice: “Quit using it, throw it out, and stop following the dieting instructions.”

Best: Intermittent Fasting

Intermittent fasting, as it’s dubbed, means that you restrict your eating window. There are several types of intermittent fasting, but many people do 16:8 (a version of time-restricted eating), in which they fast for 16 hours and eat during the other eight; you can also fast every other day or a couple of days per week. Relative to other fad diets, there’s strong research on intermittent fasting; a meta-analysis shows that these diets are just about as effective as calorie-restricted eating. People in trials tend to be able to stick with intermittent fasting, and so some find that this is an easier way to cut overall calorie intake than counting calories to stay in a daily deficit.

Worst: Cabbage Soup Diet Plan

The idea with the cabbage soup diet is to guzzle cabbage soup for seven days straight. If you can stand to eat cabbage soup for that long, the promise is that you’ll lose 10 to 15 pounds. You’re also given the green light to add in some foods throughout the week, such as apples, berries, brown rice, and beef. “The hype with these single-food diets burns out quickly,” says Rifkin. Although it’s designed for just seven days, who knows if you can even last that long: “The lack of variety tends to bore people,” she adds. Though cabbage itself is a healthy food, you lack fat and protein on this diet — two of the three macronutrients needed for health.

Best: SlimFast Diet

Ranked as number five in “Best Fast Weight-Loss Diets” in 2024 by U.S. News & World Report, SlimFast won honors for its efficacy for short-term weight loss. That said, it’s still a fad diet — meal replacement shakes or snacks teach you nothing about the often more sustainable approach of a diet rich in fresh, healthy whole foods — so it ranks near the bottom of the “Best Diets Overall” category. “Is it convenient? Sure. But I do think it lacks adequacy as a meal replacement,” says Rifkin.

Worst: Sleeping Beauty Diet

The premise of this diet is that you sleep so you don’t eat, which will make you lose weight, says Reitmayer. It’s incredibly dangerous, especially since you’re encouraged to take sedatives to make long sleeps more possible. Natural sleep is incredibly healthy — and is necessary for balanced appetite hormones, according to research — but this approach counts as disordered eating. Skip it.

Best: Atkins Diet

Atkins and keto may both be low-carb diets, but Atkins is distinct in that it’s high in protein. This diet also leads to rapid weight loss, per research. U.S. News & World Report agrees that it’s effective, ranking it No. 2 in “Best Fast Weight Loss Diets.” But with four phases of the diet, there is a real learning curve to getting started. Eventually, the diet is designed to put you into maintenance mode where you stay on it for life — making it one of the better choices. That said, keep in mind your body needs carbs to thrive, and long term, sticking to a low-carb eating plan when you don't have a medical reason is tough. Another thing to keep in mind is that a lot of the initial weight loss is water weight because of the carb restriction, says Smolen.

Worst: Cotton Ball Diet

Another dangerous fad diet, this involves consuming cotton balls to fill up your stomach; some versions involve dipping the cotton into liquids like juice before swallowing them. “I almost thought this was a joke, but it’s real, and it is dangerous,” says Rifkin. A "diet" like this not only robs you of the nutrients your body needs, but it can also cause intestinal blockages, according to OSF Healthcare. No fad is worth risking your health for.