Let’s get the skinny on sweat right at the top with a really quick lesson on how your body rids itself of toxins and fat.
Water-soluble toxins are flushed out through the blood and kidneys, but fat-soluble toxins press your liver into service before taking the blood/kidney root. And we’re talking fat-soluble toxins like processed and preserved foods.  So the junk you eat not only helps pack on the pounds in the form of fats rife with toxins, your liver must work overtime.
Now, back to sweat. Not only does perspiration, no matter how dripping wet, not rid your body of toxins, any weight lost as a result of profuse perspiration is simply water weight—you’re dehydrated, folks. And while this (hopefully) temporary condition might work help a wrestler make weight (or help you to fit into the too-tight dress for a bit) the minute you consume anything, the weight comes back. 
And let’s not play with dehydration. Symptoms run the spectrum from dry mouth, headache, and weakness to rapid heartbeat, low blood pressure, dizziness, even delirium—the latter less likely, but why risk it? 
All this said, there’s a lot of money being spent and made on sauna suits,  full-body jumpsuits made from some type of not-found-in-nature plastic—think polyvinylchloride—that people do when working out (or just hanging out) to induce perspiration to lose weight.
I think we’ve already established that this process is bunk through good old biology. Sweating itself doesn’t make you lose weight,  but if you’re working out at, say high-intensity aerobics or running or virtually any exercise where you sweat your ass off…well, then, perspiration is an indicator: visible proof that you’re busting your butt and likely burning calories.  But any weight you lose from the actual sweat is water; you’ll immediately regain it after replacing that fluid.
Meanwhile, the sauna suit business is hot.  People love their sauna suits, especially Kutting Weight suits which run $50 to $100.  It’s incredible, really, when one takes a look at the reviews—of nearly 1,750 reviews on Amazon,  the overwhelming majority of users have given Kutting Weight sauna suits a five-star rating with headlines like, “Buy This Now,” “Wow!” and “LOVE, LOVE, LOVE.” And the really negative, one- and two-star reviews, about 80 were almost entirely related to fit and comfort, with nothing mentioned about weight loss or lack thereof.
Kutting Weight says its sauna suit products have been “scientifically proven to contribute to a significantly increased metabolism (20.8%) and to facilitate greater weight loss (40.4%) when compared to exercise performed under similar conditions with no sauna suit clothing.”
An elastic neoprene with mesh “ventilation zones in the underarm and inner leg sections add comfort and breathability, without losing the body heat essential to maximize sweat and enhance calorie burning.”
I’m at a loss here to understand how it’s possible that a product science says will not help anyone sweat away pounds can otherwise have convinced thousands that it does work. What’s going on here?
Maybe it’s the neoprene; it’s comfy and soft. But a keyword search of Amazon reviews found that more than 170 people claiming they have “lost weight” wearing the suits. And on the Kutting Weight “Success Stories” page, scores of people sing the sauna suit’s praises. 
Scratching my head.
There are no shortage of sauna suit brands. In many cases, popular gym chains and big-box retailers label their own, like Gold’s Gym Sauna Suits, which one may purchase at Wal-Mart for about $20. The heavy-duty vinyl, two-piece sauna suits earned mixed reviews,  but like myriad others the complaints had virtually nothing to do with weight loss. What people either loved or hated was the way the suit felt and fit—and, in many cases, whether it even survived a workout.
Another company, GoFit—whose suits are made from “durable vinyl,” and described as a “sweat suit (which) promotes perspiration for weight loss”—has quite a few very negative reviews,  but again, it’s nothing to do with weight loss. Rather, customers complained about the fabrication and fit.
Hmm. Maybe we’re onto something here: is the reason one brand has such stellar reviews and the other crushingly poor ones because of how people felt when wearing the suit during a workout? I mean, they’re working out, right? Burning calories. Weight loss. Makes sense.
A few months ago, a hugely famous celebrity posted a video on social media of herself preparing for a workout wearing a sauna suit. There were surprisingly few comments—only ten—so the one calling her out for promoting dehydration stood out. 
A fitness blog, on a website devoted to the military and their families, cautions against electrolyte loss when working out in a sauna suit. In fact, it says sauna suits as a weight loss tool is a “myth,” that the suits are really only effective within world of competitive fitness, especially in gyms and in sports where an athlete needs to make a specific weight class.
“But sweating excessively has no useful purpose in healthy weight loss. The dangers of sweating to lose weight are severe—they include overheating (heatstroke), extreme loss of electrolytes (kidney damage/death), and other cardiovascular related emergencies.” 
The common wisdom appears to be that while high-end sauna suits may be really comfortable and people feel good in them while working out, there’s no evidence to demonstrate weight loss beyond that of a pound of dropped (and temporary) water weight.
But speaking of water, instead of taking the water out in the form of sweat while wearing a sauna suit, consider putting lots more water in.
There’s voluminous proven, peer-reviewed research on the health benefits of increasing your water intake generally, but especially when you’re looking to lose weight. And the matter has been definitively decided by researchers at Virginia Tech. In clinical trials, scientists found that people who drank a glass or two of water before meals ate fewer calories and lost weight. Scientists announced at the 240th National Meeting of the American Chemical Society that ordinary water is the weight loss “elixir.” Yes, plain old H2O. 
That’s not to say a sauna suit isn’t a good fit for you—if you feel better in one and don’t mind spending the money, don’t sweat it—go for it.