Is it true that i should wear thick clothes such as a hoody while doing cardio to sweat more? Or is it just water weight that gets put back on when you drink? I have heard both.
Currently there is not a lot of research in human studies on the effects of high ambient temperature and increased energy expenditure. As is pointed out in another answer, the artificial inhibition of the cooling mechanisms of the body by wearing either many layers or heat trapping clothing will negatively impact the length and quality of exercise sessions.
There is a study (With the caveat that it was funded by an energy drink company) that shows an unnamed commercially available thermogenic drink (Monster/Rockstar type) does have an effect on lipolysis, attributable to the already known effects of caffeine and EGCG (The active ingredient in green tea). That study can be found here. (Also of interest, near the end of the first paragraph in the discussion is a suggestion that EGCG also alters food digestability).
There are two studies that I found suggesting that high ambient temperatures actually slow down, rather than speed up metabolisms and subsequent energy expenditure. This study using Zucker rats (Lab rats specially bred for obesity research) shows in this graph that metabolism and energy expenditure markedly drop with increases in ambient temperature, for both obese and non obese rats.
This study on male weasels shows a similar curve. There is an associated decrease with both low and high ambient temperatures, with a peak in the "nominal" range, falling off even below the lower temperature curve as the temperature increases. There is also a suggestion found here that in endothermic animals, being unable to dissipate body heat is a limiting factor as well, which is a new theory called Head Dissipation Limit. This theory suggests that energy expenditure is limited by the ability of the subject to dissipate heat.
The HDL hypothesis has somewhat been validated, as there have been studies on the effects of cold on energy expenditure, and as this study shows, cold environments tend to increase lipolysis in humans.
The common misconception that wearing lots of clothing increases weight loss is a correlation error, in that people wear a ton of clothes, sweat a lot, and mistakenly think that the water weight they have lost is equivalent to fat weight lost. This is reinforced by the common practice of boxers, wrestlers and other athletes that need to make weight being shown exercising in many layers.
While there has not been a definitive study on high heat environments and the effects on lipolysis in humans, there is quite a bit of evidence that metabolism and energy expenditure both decrease in response to high heat environments, which is the opposite of what you want when trying to reduce fat.
If pure weight loss is what youre after ( and I mean just kg/pounds, not fat loss), then that extra hoodie could help you.
Amy Nutt makes a great point about heat and weightloss. Sweating helps body get rid of the salts. And as we know, salt, besides making our food taste different, has the side effect of lessening the water throughput of your body. In other words, it keeps the water inside the body, therefore increasing, or certainly not lessening, your bodyweight.
And if we assume, that you are on a good diet, there should be quite a small intake of salt, so there will be initial gains, but not in the long run. It may help you loose those first couple of kg/lbs that people are so ecstatic about, but will not help your weight loss in the long run.
I have seem bodybuilder wearing loads of cloth for their cardio workout just before a competition. They do it to minimize the water body which will in a lean and trained body cause the muscles to appear more defined.
This, however, is not healthy. Your body regulates the temperature by sweat that evaporates from your skin and cools the body down (You can cool down a drink in summer by wrapping it in a wet newspaper and put it in the sun - same effect).
Sweating more will do not do anything because your body will just be carving for more water after your workout. So wear whatever you are comfortable with.
For any given fitness level, you can burn more energy the better you can dump the waste heat into the environment. The amount of energy you can burn on the long run is going to by determined by the heat transfer coefficient.
Your maximum body temperature, the temperature of the environment, the thermal properties of your clothes, and your body area define a maximum sustainable rate at which you can dump heat into the environment (taking also in to account how much sweat can still evaporate from your clothes). The more clothes you are going to wear, the less heat per unit time you'll be able to dump into the environment. The amount of work you can perform will then also drop, so the total amount of energy you can burn per unit time will go down.
Sweating is one of many body responses to higher temperature, and begins when other responses are working to maximum efficiency, such as vasodilation (dilation of capillaries close to skin) and the increase in heart rate, both working to cool the body.
An increase in cellular respiration performed by the body's cells, is needed to fuel these systems, and the hotter the body is the more the body must work to maintain a satisfactory temperature. Obviously there are many more factors to consider when exercising such as water intake and the types of exercise being performed, but if applied appropriately, wearing thick clothes during cardio WILL provide better weight loss results.
Sweating helps detox sodium overload. I've been told by many athletic trainers that when eating too much sodium, which is common for a lot of people, it can not be released out of the body other than through sweat. If you do not go out on a sweaty workout after eating too much sodium, you will become bloated due to the fact that people believe drinking water will help detox away the sodium. water weight is developed due to over levels of sodium. That's why people wear thick clothing. To help with too much sodium intake.