In an effort to increase weight loss, or possibly lose weight at a more rapid pace, many people seek to increase the amount they sweat. Wearing a sweater will most likely accomplish that in certain conditions, but it won't translate into more calories being burned. In addition, increasing the amount you sweat can also increase the risk of dehydration. For this reason, consult a doctor before you exercise with a sweater or any other clothing that makes you sweat more.
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Your body needs a certain amount of calories to function, even when at rest — what's referred to as your basal metabolic rate. Your BMR calories go toward bodily functions such as breathing, cell growth and repair, and hormone replacement. In addition to your BMR, you need calories to provide energy when you are involved in any kind of activity. The majority of the calories are provided by your consumption of food and liquids. If you don't consume enough calories, your body turns to itself for the energy, mostly using fat as fuel. It takes an excess expenditure of 3,500 calories to lose 1 lb. of fat. The only way to do that is by decreasing your caloric intake or expending the calories through exercise, or preferably, a reasonable combination of the two.
When the body's internal temperature increases, sweat glands are triggered, and the body releases fluids on the surface of the skin to cool the body. Wearing a sweater will increase the temperature of the body when exercising in most cases, which increases the amount of sweat produced. However, the increased temperature won't increase the number of calories burned, it will only make you lose fluids.
Increased sweating will lead to a temporary weight loss depending on how much you sweat, but the weight lost is in the form of fluids and electrolytes. You will gain it back once you start to eat and drink again. This is why many athletes who need to be at a certain weight to compete use saunas and sauna suits the day before they need to be weighed; they can lose the weight the night before and then gain it back after successfully weighing in.
Wearing a sweater while working out is not as dangerous as wearing a rubber suit while exercising, but it does have its risks. If you are exercising on a hot day or in a hot room while wearing a sweater, your body temperature can increase quickly. Dehydration is the biggest concern, in addition to heat stroke, fatigue and exhaustion. The best way to manage calories is by exercising regularly and eating a healthy, low-calorie diet, and avoiding extreme exercise regimens.