A fad diet is a popular weight-loss plan that promises rapid results, eliminates whole food groups, leans on testimonials instead of clinical research, and usually comes with a product to buy. Mayo Clinic, Houston Methodist, and the Academy of Nutrition and Dietetics all converge on the same pattern. If a diet hits four or more of the red flags below, dietitians treat it as a fad regardless of how compelling the marketing looks.

• Definition pattern: rapid-loss promise + food-group elimination + weak evidence + product or supplement push
• The 10 dietitian-flagged red flags are at the top of the article
• Common examples covered: HCG, Master Cleanse, Werewolf, 500-calorie “shock” diet, 13-day, cayenne pepper, pineapple, strawberry
• Why they fail: most early loss is water and glycogen; metabolism downregulates; restriction breeds binge cycles
• What actually works: a modest calorie deficit, adequate protein, real food, sleep, and consistency over months, ideally under RD or physician guidance for any underlying condition

What Is a Fad Diet? Red Flags, Examples, and Why They Don’t Work Long-Term

A fad diet is a popular weight-loss plan, typically short-lived in the public eye, that promises rapid results, restricts whole food groups, and rests on testimonials rather than peer-reviewed research. Mayo Clinic and the Academy of Nutrition and Dietetics use slightly different language, but both converge on the same pattern: rapid loss + food restriction + weak evidence base + a marketing engine.

What is Fad Diet – 8 Types

The pattern matters more than any individual rule. If a diet hits four or five of the red flags in the next section, dietitians treat it as a fad regardless of who is selling it.

This article covers the working definition, the 10 dietitian-flagged red flags, eight specific named protocols (including HCG, the Master Cleanse, and the 500-calorie “shock” diet), why fad diets reliably fail in the long run, and what the long-term research actually supports instead.

What Counts as a Fad Diet

There is no single accepted definition. Mayo Clinic’s working description is “a popular eating plan that promises quick weight loss.” Houston Methodist adds “based on anecdotal evidence rather than peer-reviewed research.” Across clinical sources, the same elements show up:

• Promises faster weight loss than 1 to 2 pounds per week
• Eliminates whole macronutrient groups (all carbs, all fats, all dairy) without a medical reason
• Sold with a product, supplement, pill, or program fee
• Cites testimonials and before-and-after photos, not randomized trials
• Has a short half-life in popular search interest

Diets with long-term clinical data behind them, like the Mediterranean diet and DASH, are not fads even when they become popular. Both show sustained adherence and outcomes in decade-long studies. Fad diets do not.

10 Red Flags Dietitians Flag

This list pulls from Mayo Clinic, Houston Methodist, Baptist Health, the Human Performance Resource Center, and the Academy of Nutrition and Dietetics fad-diet guidance.

1. Promises more than 2 pounds per week of weight loss
2. Eliminates an entire food group with no medical reason
3. Requires you to buy a specific product, supplement, or pill
4. Uses before-and-after photos or personal testimonials as primary evidence
5. Cites a single study or no studies at all
6. Demonizes specific foods as “toxic” or “evil”
7. Promises results “without exercise” or “without changing what you eat” (the magic-ingredient framing)
8. Has a short, dramatic timeline (3-day, 7-day, 13-day)
9. Comes with a celebrity endorsement and a brand
10. Cannot be sustained for life without obvious harm

If you see four or more of these, the working assumption is fad diet, regardless of how compelling the marketing is.

8 Fad Diet Examples (the Most-Searched Ones)

These are presented in alphabetical order. Each section covers what the diet claims, what it actually does to the body, and what dietitians say.

Cayenne pepper diet

Claim: capsaicin in cayenne pepper boosts metabolism enough to burn fat. Often paired with the Master Cleanse.

What it actually does: cayenne can produce a mild, short-term thermogenic effect (a few extra calories burned), but not at a level that produces meaningful weight loss. Any actual weight change comes from the severe calorie restriction that accompanies the protocol.

Dietitian view: not a diet. A spice with mild thermogenic properties used as a marketing hook for restrictive plans.

HCG diet

Claim: human chorionic gonadotropin (HCG), taken alongside a 500-calorie-per-day diet, causes the body to burn fat from “stored deposits.”

What it actually does: the U.S. Food and Drug Administration has stated there is no substantial evidence that HCG increases weight loss beyond what would be expected from caloric restriction alone. In other words, the weight loss is from eating 500 calories a day, not the hormone. The FDA has issued public warnings against over-the-counter HCG weight-loss products, and several have been recalled.

Dietitian view: not recommended. A 500-calorie diet without medical supervision carries risk of gallstones, electrolyte imbalance, muscle loss, and rebound weight gain. The HCG component adds nothing beneficial. Speak to a physician or registered dietitian before considering any very-low-calorie protocol.

Master Cleanse (lemonade diet)

Claim: drinking only water mixed with lemon juice, cayenne pepper, and maple syrup for 4 to 10 days “cleanses toxins” and produces dramatic weight loss.

What it actually does: extreme calorie restriction (usually under 600 calories per day, almost entirely from sugar). Most weight loss is water, glycogen, and muscle. Reported gains return within weeks of resuming normal eating.

Dietitian view: not a cleanse. The liver and kidneys handle detoxification on their own. There is no clinical evidence the protocol provides any health benefit. Risks include dizziness, fainting, electrolyte imbalance, and disordered eating patterns in vulnerable people.

Pineapple diet

Claim: eating only pineapple (or mostly pineapple) for several days promotes rapid weight loss because the bromelain enzyme “burns fat.”

What it actually does: bromelain helps digest protein but does not burn fat. The weight loss comes from severe calorie restriction (typical pineapple-only plans run under 1,000 calories per day).

Dietitian view: a mono-diet. Mono-diets are nutritionally incomplete and reinforce restrictive eating patterns. Not recommended.

500-calorie diet (also called the “shock diet”)

Claim: a “shock” intervention to rapidly trigger weight loss in a short window.

What it actually does: 500 calories per day is below the World Health Organization’s safety threshold for unsupervised adults. The body responds with rapid initial weight loss (mostly water), followed by metabolic downregulation, hunger spikes, and rebound on resumption of normal eating.

Dietitian view: not safe without medical supervision. Very-low-calorie diets at 500 to 800 calories per day are sometimes used in clinical obesity medicine, but only under MD/RD oversight and only for specific patients. Self-prescribing at this calorie level is not appropriate.

Strawberry diet

Claim: a strawberry-heavy plan promotes rapid weight loss because of the fruit’s antioxidants and fiber.

What it actually does: strawberries are nutritious, but a strawberry-led plan is another mono-diet variant. Weight loss is from caloric restriction, not the fruit.

Dietitian view: same as pineapple. Single-food protocols are nutritionally incomplete.

13-day diet (Copenhagen / “Royal Danish Hospital” diet)

Claim: a strict 13-day low-calorie protocol, marketed with the false attribution to a Danish hospital, that claims to “reset” metabolism through specific food combinations.

What it actually does: provides roughly 600 to 800 calories per day. Weight loss is from caloric restriction. The “reset metabolism” claim is unsupported.

Dietitian view: a structured very-low-calorie diet without medical oversight. The Royal Danish Hospital has publicly disowned the diet — they did not create it, and the attribution is a marketing fiction.

Werewolf diet (lunar diet)

Claim: fasting and eating cycles tied to the lunar phases produce extra weight loss.

What it actually does: 24-hour fasting cycles. Any weight loss is calorie restriction, not lunar timing.

Dietitian view: there is no biological mechanism for the moon to affect body composition. The fasting protocol can be inappropriate for people with eating disorder histories, type 1 diabetes, pregnancy, and several other conditions.

Why Fad Diets Don’t Work Long-Term

Three findings keep showing up in research.

Most early “weight loss” is not fat. When calorie intake drops sharply, the body burns through stored glycogen first. Each gram of glycogen holds roughly 3 grams of water. So a person who loses 5 pounds in the first 4 days has lost mostly water and glycogen, not body fat. Resumption of normal eating restores glycogen stores and the water weight returns.

Metabolic adaptation. A 2016 follow-up study of The Biggest Loser contestants (Fothergill et al., Obesity) showed that 6 years after rapid weight loss, resting metabolic rate had dropped substantially below predicted levels. Severe restriction trains the body to operate on fewer calories, and that adaptation can persist long after the diet ends.

Restriction-binge cycles. Cognitive behavioral research on dieting (Polivy and Herman) shows that strict dietary restriction reliably predicts subsequent overeating, especially in response to perceived “diet breaks.” The cycle is the mechanism, not a personal failure.

The consistent finding across longitudinal studies: most people who lose weight on a fad diet regain it within 1 to 3 years, and a meaningful subset finish heavier than they started.

What Actually Works (the Boring Answer)

Long-term studies of the Mediterranean diet, DASH, the Diabetes Prevention Program, and Look AHEAD all converge on the same core practices.

• A modest calorie deficit (250 to 500 fewer per day, not thousands)
• Adequate protein (around 0.7 to 1.0 g per pound of lean body mass for active people)
• Real food, mostly plants, with room for what you actually like
• Sleep. Under-slept people eat more and lose less fat per pound of weight loss
• Consistency over months, not days
• Movement that you’ll actually do, not “the right” exercise
• For an underlying condition (diabetes, heart disease, kidney disease, thyroid): work with a registered dietitian or your physician

This list is boring on purpose. Long-term data only exists for boring approaches. If you want to know whether a source is credible, the boring-list test works: a credible source’s recommendations look roughly like the bullets above. A fad diet’s recommendations look nothing like them.

For help finding a credible practitioner, the difference between a registered dietitian and a nutritionist matters and is worth understanding before you book an appointment.

FAQ

Is the HCG diet safe?

The FDA has stated there is no substantial evidence that HCG produces weight loss beyond what 500 calories a day would do alone. The agency has issued warnings against over-the-counter HCG weight-loss products. Any rapid weight loss on the protocol comes from severe calorie restriction, which carries risk of gallstones, electrolyte imbalance, and muscle loss. Talk to a physician or registered dietitian before considering any very-low-calorie protocol.

Is the 500-calorie diet safe?

Five hundred calories per day is below the safety threshold for unsupervised adult weight loss. Very-low-calorie diets at this level are sometimes used in clinical obesity medicine, but only under medical supervision and only for specific patients. Self-directed 500-calorie diets are not recommended.

What about the Master Cleanse?

The Master Cleanse provides roughly 600 calories per day from sugar (maple syrup), with no protein, fat, fiber, or essential micronutrients. There is no clinical evidence it produces lasting weight loss or any “detox” benefit. Reported risks include dizziness, electrolyte imbalance, and rebound weight gain.

Are mono-diets like the pineapple or strawberry diet okay short-term?

Single-food diets are nutritionally incomplete and reinforce restrictive eating patterns. Even short-term, they can cause electrolyte imbalances and trigger problematic eating behaviors in vulnerable people. Not recommended.

Is intermittent fasting a fad diet?

Intermittent fasting has more research behind it than most fad diets, and major bodies including the American Heart Association have published cautious commentary on it. It is not classified as a fad in most clinical sources. IF can still be inappropriate for people with eating disorder histories, type 1 diabetes, pregnancy, or who are under 18. See our overview of intermittent fasting for more.

Is keto a fad diet?

Keto has decades of clinical use for refractory epilepsy and a meaningful research base for short-to-medium-term weight loss. Most clinical sources do not classify keto as a fad. The marketing claims around some consumer keto products (e.g., “burn fat without exercise”) often hit fad-diet red flags. The diet itself is not a fad. Some products around it are.

How can I tell if a new diet that just went viral is a fad?

Run it through the 10 red flags above. If it hits four or more, treat it as a fad. The single strongest signal is “promises more than 2 pounds per week of weight loss” combined with “sold alongside a supplement or program fee.”

What should I do if I’ve already started a fad diet?

If you are early in (under 7 days) and feeling fine, you can stop without consequence. If you’re feeling lightheaded, weak, or noticing rapid weight loss, stop and check in with your physician or a registered dietitian. If you have a history of eating disorders, stop and connect with your treatment team or the National Alliance for Eating Disorders helpline at 1-866-662-1235. Our guide for families on eating disorder support covers what to do next.

Editorial Note and Disclaimer

This article is informational and was written by the Diets Meal Plan editorial team, drawing on guidance from the Academy of Nutrition and Dietetics, Mayo Clinic, Houston Methodist, the U.S. Food and Drug Administration, and the peer-reviewed sources cited above. It is not medical advice. If you have a health condition or a history of disordered eating, talk to a registered dietitian or your physician before changing your diet.

See our editorial policy for how we research and update health content.