Fad diets and crash diets: why they don’t work

It seems there is a new diet every week, each promising to melt away pounds with one special food, one forbidden food group or one clever trick. Many contain a grain of truth, but most are much better at marketing than at keeping weight off. Here is how to recognize a fad or crash diet, and why a slower approach works better.

What makes a diet a fad

Fad diets come and go, but they tend to share a few features:

• One food, or one food group, is the answer. Grapefruit, cabbage soup, juice, or fruit only for a few days are classic examples.
• Whole food groups are forbidden without a medical reason, such as all carbohydrates or all fat.
• Rigid rules about food combinations or eating times, with no evidence behind them.
• A product to buy: a pill, powder, tea or plan that is said to do the work for you.

Some popular eating patterns, such as lower-carbohydrate diets, can help some people lose weight when they lead to eating fewer calories overall. The problem comes when a plan is so restrictive that it is hard to follow for long, cuts out foods that supply important nutrients, or is sold as a shortcut.

Crash diets: why fast loss doesn't last

A crash diet cuts calories drastically, often to a few hundred a day, for example by living on lemon water with maple syrup and cayenne pepper, or on juices alone. You will lose weight quickly, but much of the early loss is water and stored carbohydrate, and some of it is muscle.

Very low-calorie diets can also cause fatigue, dizziness, constipation and nutrient shortfalls, and losing weight very fast raises the risk of gallstones. When normal eating resumes, the water returns at once, and most people regain the rest over time. Eating far too little also makes the body use somewhat less energy, which makes regain easier. Very low-calorie diets do have a place in medicine, but only under a doctor's supervision.

Red flags in diet ads

The US Federal Trade Commission warns that some weight loss claims are simply false. Be wary of any product or plan that:

• promises you will lose weight without changing what you eat or how active you are
• says it works for everyone, or that the weight loss will be permanent
• promises dramatic losses in a short time
• relies on before-and-after photos, celebrity endorsements or "news" reports that are really ads

What works instead

There is no shortcut, but the reliable approach is not complicated. The US Centers for Disease Control and Prevention (CDC) suggests losing weight at a gradual, steady pace of about 1 to 2 pounds a week; people who lose weight that way are more likely to keep it off.

• Eat a little less than you burn, rather than a lot less.
• Build meals around vegetables, fruit, whole grains, beans, and lean protein, with modest amounts of healthy fats.
• Cut back on sugary drinks, sweets and highly processed snacks rather than banning whole food groups.
• Move more, in ways you enjoy enough to keep doing.
• Choose changes you can live with for years, not weeks.

For more, see our pages on weight loss and eating to lose weight. If you have a health condition, talk to your doctor before starting any diet.

Sources

• Steps for losing weight (US Centers for Disease Control and Prevention)
• Claims in weight loss ads might be false (US Federal Trade Commission)

Last reviewed November 2026. This article is general information, not medical advice.