Someone on this post once wrote that exercise helps you lose weight by increasing the breathing rate, which burns calories. Is this so? I always thought an increased heart rate is what burns calories, and the increased breathing rate is a by-product of fatigue. Or am I wrong? Can I burn calories by sitting at my computer and breathing heavily?
I don’t know who first said “everyone’s a critic,” but I think it’s a really stupid saying.
It depends on why you’re sitting at your comuter breathing heavily, Pete. 
Your Quadell
If you think about it, you are using muscles (exercising) to breathe, specifically your diaphragm and the intercostal muscles between your ribcage, not to mention the energy required to keep your heart pumping to circulate the blood through the lungs and rest of the body. Whether it’s good exercise is an entirely different question…
Your body is like a car; if it’s on, its using gas (calories/energy etc). 'Course I don’t 'spect you want it turned off! 
…it has never been my way to bother much about things which you can’t cure.
Sound like you have a kinna similar view voiced in the question “Can I get an aerobic workout from Drinking coffee?” that was answered on the straight dope show. God I miss that show.
The facts are that elevated heart rate and breathing are byproducts of highly increased metobolism, which is a fancy word for buning calories. The oxygen and sugars are required for running your muscles. If you start exerting yourself, your circulatory system has to step up the delivery by jacking up your breathing and heart rate.
So the obvious answer is, No you cant melt away the fat by sitting on you butt and just breathing hard. You actually have to exert your self and then you’ll start breathing hard.
However, EVERYTHING you do burns some calories. Breathing more, burns more calories, than breathing less. So if you really stuck with it and did your heavy breathing for 30-40 minute sessions 5 times a week, you might actually see some results. Frankly, Madonna is the first person I have ever hear of, that can keep herself fit that way. If your not madonna, you’ll probably need to diet and execise like everyone else.
Pete aksed: Someone on this post once wrote that exercise helps you lose weight by increasing the breathing rate, which burns calories. Is this so? I always thought an increased heart rate is what burns calories, and the increased breathing rate is a by-product of fatigue. Or am I wrong? Can I burn calories by sitting at my computer and breathing heavily?
Technically, yes, if you don’t eat anything either.
Increased breathing is an effect of fuel burning, not a cause (a significant cause, that is). Real exercise creates a demand for more fuel-burning, which uses up O[sub]2[/sub] and releases CO[sub]2[/sub]. The increased CO[sub]2[/sub] causes the diaphragm to work the lungs faster to eliminate the CO[sub]2[/sub] and get more O[sub]2[/sub].
Since living itself is an exothermic (heat-making) process, you must burn fuel to live. And forcing your diaphragm to move faster is a lame form of exercise and thus burns fuel slightly faster. But faster breathing is hard to maintain if your blood doesn’t need faster gas exchange.
The best solution: exercise for real. Walking just 30 minutes a day (constantly, not up & down office jaunts to the coffee machine) is a minimal but effective way to start an exercise program.