Every few years, someone repackages a ~2,500-year-old garment and sells it as a weight loss aid. In the 1800s, they were called corsets. Throughout the 20th century, they were called girdles. By the 2000s, they were called Spanx. In the 2010s, they were called waist trainers. And now, on the eve of the 2020s, they’re making a resurgence as sauna or sweat belts, or hot body shapers. All have been intended to help users look slimmer, and in recent decades and iterations, their purpose has been expanded to help users lose weight, too. Unfortunately, these sauna belts don’t reduce belly fat.

The theory of these sweat belts is that if you heat up your body you’ll burn more fat, especially if you’re exercising while wearing them, as most of these products advise. They are, essentially, corsets made of rubber, neoprene, or other non-breathable materials, that increase sweating while compressing the abdomen to give the appearance of slimness — a personal, portable, spot sauna, if you will.

The only problem is, the premise is faulty. Just as in a real sauna, heating up your body, or a part of it will only help you lose water weight, not fat. This might initially give the impression of less fat or a slimmer frame — which is probably why one sauna slimming belt available on Amazon.in advises “For Best Results: Drink 1-2 glasses of lukewarm water before use” — but it doesn’t last as long as you rehydrate.

In the meantime, these sweat belts — advertised for use for a couple of hours at a time, while being active during “GYM exercise, Outdoors exercise or activities, Home Workouts, Jogging, and Chores [sic],” according to one — put users at risk of dehydration and even heat stroke. “If you want to be able to exercise strenuously and get the most out of it, you have to have a way to dissipate heat,” Gary Hunter, a professor in the school of nutrition at the University of Alabama at Birmingham, in the U.S., told the Los Angela Times. “The [belly] is a big source of heat loss, so this could be dangerous.”

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Aside from the health risks, sauna belts might actually make it harder to lose belly fat; you can’t work out as hard or as effectively when you’re overheated. Nor can you work out effectively when breathing is inhibited. According to the American Board of Cosmetic Surgery, waist trainers can lessen lung capacity by 30% to 60%. “At best, this can result in low energy and discomfort, but scarier consequences — passing out, fluid buildup in the lungs, inflammation, — are also a real possibility.”

On top of this, weight loss waist trainers, if overused, could hamper proper digestion, reduce appetite, cause blockages in the digestive tract, and prompt acidity — all of which work against healthy weight loss and exercise efforts.

Ultimately, there is no proof these sweat belts actually contribute to weight loss generally, much less belly fat loss specifically (spot reduction is a myth); there’s a reason these products are advertised for use during exercise — it’s the exercise and/or the right diet that leads to weight loss. One small-scale study in 2010 did try to probe whether waist compression belts (corsets or waist trainers) aided weight loss above and beyond a low-calorie diet among obese people, but ultimately was unable to reach any conclusions; while participants stuck to the low-calorie diet, most abandoned their corsets due to discomfort before the study completed.

Sauna belts for belly fat also fail in their other claims. They tout toned abs, but do the opposite of what it takes to tone abs. Abdominal muscles are toned through exercises that activate them; belly compression belts isolate these muscles instead.

“Girdles just temporarily compress and redistribute fat and skin around the abdomen. When it comes to a flat stomach, diet and exercise — not undergarments — are what count,” writes Dr. Edward R. Laskowski for the Mayo Clinic. With too much use, these waist belts may actually weaken core muscle strength, leading to poor posture and balance problems. Laskowski recommends abdominal crunches, leg lifts, planks, bridges, as well as Pilates or fitness ball exercises, as successful ways to tone abs.

Still, for some, the appearance of slimness might be enough, even with weight loss claims debunked.

If sauna belts to trim belly fat arise out of the myth that more sweat means more weight loss, they also arise from the myth that fat is automatically unhealthy. With no proof that weight loss belts work, it’s time to lay these specious products — as well as these myths — to rest.