People who want to lose weight are often tempted to try one of the latest fad diets that are popular for a brief time and claim to help people shed pounds fast. But what is a fad diet? And do they actually help people lose weight long term?

Kara Burnstine, a registered dietitian at the Pritikin Longevity Center in Florida, told Live Science that a fad diet is defined as a "quick-fix way to lose weight that's usually popular for a short time and then fades away."

Another tell-tale sign of a fad diet, she said, is that claims of its success in helping people lose weight are mainly based on anecdotal reports.

"These diets don't usually have a lot of scientific evidence to back them up — they're based more on testimonials," she said. "You may see influencers touting benefits that appear too good to be true, which is often how these diets gain traction."

Weight loss can be tough, and with promises of rapid transformation, it's easy to see why these diets are so popular. They offer an easy solution to a complex problem, often promising quick results.

But fad diets don't work in the long term. Here, we'll delve into the science behind them to explore why. If you are thinking of making significant changes to your diet, however, make sure to consult a dietitian or a doctor first.

Why don't fad diets work?

However, the results are often short-lived because the diet is unsustainable as a lifestyle, Burnstine said. This can explain why most people regain the weight they've lost when they go back to their usual way of eating.

Fad diets also limit food groups that may be needed for essential nutrients, Burnstine said. Commonly restricted food groups include dairy and carbohydrates, according to the British Dietetic Association. Dairy is a good source of protein and calcium, while carbohydrates are needed for energy, as well as B-vitamins and magnesium.

Fad diets may also promote eating mainly one type of food — for example, the cabbage soup diet — or avoiding all cooked foods.

Wesley McWhorter, a registered dietitian in Texas and a national spokesperson for the Academy of Nutrition and Dietetics, told Live Science that “most fad diets remove entire food groups (fruits, vegetables, grains, etc.) or are extremely restrictive, reinforcing a negative relationship with food.”

"The biggest problem [with fad diets] is the shame-based rhetoric that blames the failure [to lose weight] on a lack of willpower — which we know is simply not true," McWhorter said. "There are many factors involved in why, what, and how we choose to eat. It is important to consider those issues to help us make sustainable changes."