Sweating itself is the way the body cools itself and gets rid of toxins.  You are sweating because you are hot and the body gets rid of the heat in form of energy which is measured in calories.  If you burn more calories than you take in for the day, you lose weight.

More info:

http://health.howstuffworks.com/sweat2.htm

http://health.howstuffworks.com/calorie.htm

http://btc.montana.edu/olympics/nutrition/eat02.html

An interesting subject is regarding how efficiently a human operates as if it were a machine! We know that automobiles of the 1970s generally operated at around 15% overall thermal efficiency (which has now been increased to around 21%). We know that electric power plants and the grid distribution system is around 13% overall thermal efficiency, regarding the energy that was in the coal or uranium and the electricity that actually gets to us. We know that photosynthesis in plants (such as corn and wheat crops) is only around 1% to 2% efficient regarding the energy received as sunlight. So what about us?
These "overall thermal efficiency" figures are a rigorous scientific evaluation of any machine, taking the total developed energy or power (such as the actual energy and power that an automobile creates and makes available to its wheels) divided by the total amount of energy that is available from the source fuel (such as the 126,000 Btu of energy available from a gallon of conventional gasoline) (times 100 to get percent). This number is really one of the best guides for an overall comparison of different types of machines.

We generally have a decent idea of the energy that we take in as food. A 2,000 Calorie daily diet regimen means that the person takes in food that includes 2,000 (kilo-) Calories of thermochemical energy in it. No problem there. By the way, 641.2 (kilo-) Calories equals one horsepower-hour of energy.

Early in the 20th century, factory owners did many studies regarding just how much actual work they could get out of their employees! It was generally found that a healthy 35-year-old (European) man could use up a total of 0.49 horsepower for an 8-hour shift. If you do the math, you can see that accounted for around 2,500 Calories used up during that work shift! (That worker's body would use up at least another 1,200 Calories of energy in the remaining 16 hours of that day for a total energy consumption of around 3,700 Calories per day.) People working in such factories HAD to eat enough food each day to account for that!

Unfortunately for those factory owners, only around 0.1 horsepower of that was actually available as useful work. That's about 64 Calories per hour or 75 watts. The other 0.39 horsepower (about 256 Calories per hour or 1,000 Btu/hr) was used up in metabolic activities and maintaining body temperature. In "staying alive!" Bummer! This data indicates that such human factory workers were capable of around 20% overall net thermal efficiency during their work shift. More recent research has suggested that a maximal efficiency for a human is probably around 25%, but that most existing mechanisms are not able to efficiently deal with the herky-jerky way we tend to create such work!

(Younger 20-year-old men could produce 15% more (or around a net thermal efficiency of 23%) and 60-year-old men could produce 20% less (or around 16% net thermal efficiency) than these values.) The metabolic requirements depend on health and environmental conditions, particularly the air temperature. In an extremely cold environment the body must expend even more energy in maintaining body temperature.

These results encouraged factory owners to move toward automation where steam engines and electric and gasoline motors did most of the work.

When a person is not fully exerting oneself like in those factories, the metabolic rate drops somewhat. A sedentary or desk-person needs far less than that 0.39 horsepower rate, actually around 0.16 hp (or according to the ASHRAE Handbook charts, 390 to 450 Btu/hr or around 100 Calories per hour). This is getting close to the minimum possible metabolic rate which ensures survival, the so-called basal metabolic rate (or BMR) Relatively few Americans are now in factory jobs that are as demanding as those harsh tests considered. Therefore, the necessary daily dietary intake does not need to be as high as it was a hundred years ago.

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The body is interesting in that it chooses to allocate that 0.1 hp of available work as it finds necessary. Digestion is a process that requires quite a bit of energy. If there is no immediate requirement for physical activity or for mental exertion, the body will allocate nearly all of the available work toward digesting a meal. This greatly explains why we are often in a mood to take a nap after a Thanksgiving feast, as the body recognizes that it has a lot of food to digest! Creative thinking also seems harder right after a giant meal!
If there is NOT a large amount of food to digest, the body will allocate most or all of that available work to either mental (thinking) activities or to physical activities or both.

IF you take in more Calories (actually kilo-calories or Kcal) in a day than you use up, the extra energy is turned into sugars and fats to be stored away for a possible future survival need, and you will gain weight. Equally, if you use up more Calories in a day than you take in, some of that existing fat and sugars is converted back into forms that can become energy, and some of the bodyfat is therefore converted into that work and weight is lost. The premise behind "working out" is closely related to this fact, and it would work to some extent, if it were not that all that exercise often creates a healthy appetite!

Where the Energy Goes
There are a LOT of (metabolic) processes that go on in the human body, nearly all of which use up energy! The heart pumps blood, the lungs pump air, and countless chemical reactions occur to accomplish various functions. Virtually all of them result in HEAT being created as a result. In some circumstances, such heat is considered waste. But in the body of any warm-blooded animal, that heat is very important, in maintaining a constant warm body temperature. Many of those chemical reactions cannot happen very well in cold environments, and so the heat that seems wasted is actually accomplishing a very important task. Cold-blooded animals have less efficient chemical processes and so they tend to have less efficient brains and muscles.
In any case, nearly all of that wasted heat eventually gets to the skin to be either radiated or convected away; or the heat goes into warm air inside the lungs to be exhaled. We can do some rough calculations regarding these things.

For heat radiated away, there is a standard equation that describes this so-called Black Body Radiation. The Stefan-Boltzmann Law is that the amount of radiation is equal to a constant (called the Stefan-Boltzmann constant, s) times the area of surface times the FOURTH power of the absolute temperature. For a situation where a radiating object is within a room which is at a lower temperature, it becomes = s * A * (T14 - T24). (We are leaving out here the constants regarding the emissivities of the surfaces involved, assuming them both to be 1.0).

If all of a human's skin were at the same temperature, this could be easy! We have around 20 square feet of surface area (A) and the constant is 0.1713 * 10-8 Btu/sf/hr/°R4. The head is maintained at a fairly high temperature, to ensure clear thinking in case of emergency! The arms and legs tend to be cooler. The way the body does this is by restricting or permitting blood to flow freely to different areas. (When someone falls in very cold water, the body attempts to nearly completely shut down blood flow to the limbs, to try to conserve body heat for the brain and torso where it is urgently needed.)

For our estimate, let's say that the body attempts to keep the AVERAGE skin temperature to be 5°F above the ambient room temperature. In that case, we would have 20 sf * 0.1713 * 10-8 Btu/sf/hr/°K4 * ((78+459)4 - (72+459)4). This is about 104 Btu/hr, which is around 26 Calories per hour of RADIATED heat.

For convective heat losses, we are going to simplify by assuming that there is no wind. Therefore we can use formulas for Natural Convection. A very simplified version gives h = 0.2 * (T1 - T2)1/3; for our situation above, h = 3.42. The convective heat loss is then that number times the surface area (of 20 sf) times the temp difference (5°F) or 340 Btu/hr. (around 85 Calories per hour) If a person were naked, this would apply, but the effect of clothing tends to insulate some parts of the body, particularly the very important torso, and we are going to suggest here that the effect of clothing will generally reduce this CONVECTIVE heat loss to around 70 Calories/hour.

The third method that the body discards heat is by exhaled breath. In normal breathing, we generally exhale about 0.5 liter of air about twelve times every minute. This is therefore around 6 liters of air per minute. This air is at our core body temperature, of 98.6°F temperature, air that had been inhaled a few seconds earlier at room temperature. If the room is at 68°F that means the room air had been raised in temperature (by the body) by around 30°F. The 6 liters of air/minute is 360 liters/hour or about 13 cubic feet per hour which has a weight of around 1 pound of air per hour. Air has a thermal capacity of around 0.24 Btu/lb/°F. In raising 1 pound of air by 30°F, that means that the amount of heat added to the air (from inside our body) is (30 * 1 *.24) or around 7 Btu/hr of EXHALED heat in the dry air.

There is also water vapor in the exhaled breath. As the air is inside the lungs, the relative humidity there quickly rises to 100%. Therefore, water along the walls of the lungs evaporates into the air to be exhaled. This is additional heat energy that gets carried away. Using standard analysis, the partial pressure of the saturated water vapor at 98.6°F is around 0.9 PSI. This defines the (weight) proportion of the water vapor and the dry air to be around one to 27. We will not go through all the math here but it is pretty simple to determine how much weight of water vapor is exhaled per hour. Most of the energy is involved in evaporating the water into water vapor, but then it also has to be warmed from the inhaled breath air temperature up to the 98.6°F that gets exhaled. Evaporating sufficient water at room temperature and then raising it to 98.6°F for the 6 liters of air represents around 1/25 pound of water per hour being evaporated and heated, or around 40 Btu/hr.

Between these two components of the exhaled breath, we have around 47 Btu/hr or 12 Calories/hr lost due to EXHALED BREATH. We must remember that this was based on a 68°F room, and standard (waking, sedentary) rates of breathing and depth of breathing. For example, during sleep, the respiration rate generally slows down and becomes more shallow, so those heat losses become less, while during heavy exercise or exertion, respiration becomes faster and often deeper, so that greater heat losses in the breath occur then.

This then gives a daytime resting total of around (26 + 70 + 12) or around a ballpark estimate of 110 Calories per hour of heat energy sent away from the body. During the night, the heat loss is generally slightly lessened. A ballpark number we could consider a day of losing 110 Calories for 16 hours, or 1760 Calories per day as being credible. During our eight hours of sleep, it has been scientifically confirmed that we normally lose around 80 Calories per hour, so we have a 24-hour day total of around 110 * 16 + 80 * 8 or 2400 Calories, a number that is is reasonable agreement with the accepted value of food energy consumption for a sedentary adult individual.

(The body also has the capability of dumping quite a lot of heat by sweating, where the evaporation of the water removes heat from the room air very close to the skin, and therefore increases the (local) temperature differentials we discussed above. (This is why your skin feels cooler when you sweat.) This then allows the body to dump substantial amounts of heat when the body is in danger of overheating. In a sedentary situation, the body creates a very small amount of sweat, and we are ignoring that energy loss here.) This then accounts for a ballpark of 2,200 or 2,400 Calories of input energy, in rough agreement with what dieticians say.

This analysis does not include the "productive" work output. When the body is more active, the metabolic activity increases, to power all the needed operations inside the body, while also producing productive work output which could be as much as an additional 1/4 of that (as noted above). In what is considered heavy work by ASHRAE, the amount of productive work done can be around 0.15 horsepower, or 96 Kcal/hr or 110 watts. The body has to increase its metabolic rate to accomplish everything necessary, with the net efficiency being around 20%. Therefore, the body is actually then using up around 480 Kcal/hr or 550 watts of total consumed energy. This must all be disposed of by radiation and convection from the skin (therefore bloodflow near the skin is increased so that the skin temperature rises to accomplish this) and the rate and depth of breathing is increased to also dump more heat, and finally, the body sweats to dispose of additional heat.

The numbers above are generally meant to apply to adult men of around 200 pounds weight. Women generally have smaller total surface area and therefore they need to use less energy to maintain their core body temperature, so they tend to need to eat less, and therefore have lower daily dietary intake. But the reasoning is still completely valid.

The analysis of this web-page is the basis for a unique potential weight-loss concept of another web-page, A VERY Unusual Weight Loss Approach. Modern people tend to use heavy covers when they sleep, or have the room quite warm, or have a heating pad or warm waterbed. That situation can reduce the body's heat loss down to around 20 Calories/hour during sleep instead of the normal 80 Calories/hour mentioned above. Over an eight-hour period of sleep, this difference (60 Calories/hour) would seem to represent a total of 480 Calories of heat that your body does NOT get rid of!

The premise would then be that over a week, maybe an additional 3500 Calories (7 * 480) might get used up by the body by sleeping with just a sheet instead of a heavy blanket. Since one pound of bodyfat has an energy content of about 3500 Calories, this might suggest that it may be possible to lose about one full pound of bodyfat EVERY WEEK just because of this blanket/sheet difference!

http://mb-soft.com/public2/humaneff.html