Many people believe that the more you sweat, the more weight you lose, and that if you sweat a lot during a workout, it means you’re working out harder than someone who may not be sweating as much. Both of these beliefs are incorrect.
The purpose of sweating is to cool your body down, therefore helping to regulate your core body temperature. Sweat is excreted from sweat glands in the layer beneath your skin. We are each born with between 2 million and 4 million sweat glands, and how much you sweat depends on the number of sweat glands you have (more glands equals more sweat). Men's glands are more active than women's, so they tend to sweat more.
On a superficial level, when you sweat, you do lose some water weight in the form of sweat. But all you're losing is water and electrolytes, and that will be gained right back as soon as you eat again or drink water. It's true that some athletes who are trying to make a certain weight class work out in sweatsuits or garbage bags right before they weigh in — and it can have an effect — but this is just to achieve a temporary number on the scale and does not reflect true weight loss.
This also applies to beliefs about sweating unaccompanied by the physical exertion of a workout. For example, sitting in a sauna or steam room, or participating in a workout class where the temperature of the room is raised before you even start to move (e.g., hot yoga or certain spin classes where they pack the room with people).
In these instances, you aren't necessarily burning extra calories by generating extra sweat.
That's not to say that sweating doesn't have its benefits.
Actually, sweating is great for detoxing, improving acne, and boosting your mood.
The one caveat I would throw in is that if you're sweating, unless you're in an artificially hot environment, you're likely exerting yourself in your workout, and that exertion is what's burning calories. Physically fit people tend to start sweating at a lower core body temperature, which then allows them to perform for longer, therefore boosting their workout.
Some people think that this raised body temperature increases your body's metabolic rate, resulting in an overall acceleration of the number of calories your body burns. While there are contradicting beliefs about this, these claims are not supported by scientific evidence.
Excessive sweating is definitely dangerous.
If you're sweating in an attempt to lose weight, stop! Dehydration is the biggest risk, which can lead to suboptimal performance and in extreme cases lightheadedness and loss of consciousness. Other risks include overheating (heatstroke), extreme loss of electrolytes (kidney damage/death), and other cardiovascular-related emergencies.
So, the simple answer to the question of whether sweating in itself makes you lose weight is a big old N-O.
However, if you're working out and that workout is causing you to sweat, that probably means you're burning calories. Combined with a healthy diet, this can lead to weight loss, improved health, and an overall sense of well-being.