Some of you have probably seen this before in other fora, and I've mean to post it up here for a while, but hadn't summoned enough round tuits. A reasonable chunk of my audience here are quite aware of this stuff already, but there's also plenty who aren't. If you're one of the former, feel free to drop corrections in if any are spotted. And if you're one of the latter, feel free to ask for clarifications on anything that seems complicated or confusing or wrong. The idea is that even if you know absolutely nothing about this, you should hopefully be able to understand a bit more about it after this post.
"Calorie Counting" Is Bogus (ETA: in the sense of not useful on its own).
"Calorie counting" doesn't take into account 90% of what's going on in the body when food is processed and used. This is biochemistry. It's not that simple. Calorie counts don't work on their own. It's the wrong thing to be measuring. It's like comparing different brands of computer by measuring their CPU speeds in MHz and claiming that's all that matters. It's not the only factor that matters, and there are plenty that matter more.
What Are We Made Of?
Humans (indeed most animals) are composed basically (leaving out micronutrients) of water, proteins (composed of amino acids joined together into twisted up chains), fats/lipids (fatty acids - long, flat chainlike molecules), and carbohydrates/polysaccharides (simple sugars/monosaccharides stuck together like globs of multi-angled lego).
All cells are composed of proteins, lipids, and carbohydrates. What varies is the types of stuff and the proportions based on what the cell's job is. Muscle cells have more proteins than any other types, because they build long protein chains that allow them to contract when triggered. Fat cells have more lipids, because their job is to store lipids for times of hunger.
What's Going On Under The Hood?
The primary biochemical cycle that our cells use to do any sort of work is known as the "Krebs" cycle, named after the main discoverer of it. Mostly known as the "Citric Acid" cycle, these days, since that's a molecule that features heavily in the cycle.
Basically, the Krebs cycle is a long chain of sequential chemical reactions which break down stuff, resulting in the conversion of ADP (Adenosine Di-Phosphate) to ATP (Adenosine Tri-Phosphate) at several points along the chain. The conversion of ATP to ADP is then used to fuel most other reactions in the cell (eg, muscular contraction, etc). Think of ATP as a little charged battery, and the Krebs cycle as a process that churns out these little charged batteries, which then float around and get used up elsewhere inside the cell.
The Krebs cycle when fully used is designed for breaking down glucose (the simple sugar molecule/monosaccharide that we use) by adding Oxygen, and turning it into Carbon Dioxide plus water. However, because it's a multi-step process, there are plenty of extra points where things can get inserted.
Fatty acid metabolism happens quite close to the end of the Krebs cycle. Basically small bits get chopped off the end of the fatty acid, and they get inserted into the Krebs cycle, and generate a small amount of ATP each.
Amino acid metabolism is more complicated than I want to know about, but basically it also results in a shortcut of the Krebs cycle, and generates ATP. The insertion point is usually earlier than that for fatty acids, and is easier.
So, there are three basic sources of energy for cells. Amino acids, glucose, and fatty acids. Glucose is floating around in the
bloodstream, and same for amino acids and fatty acids, although in much lower proportion. Muscles have amino acids on site, which they are made of.
Food is digested (at various points for different substances, ranging from starting at the mouth for carbohydrates through the various intestines) into it's basic components before being absorbed into the body. The more "complex" the thingy, the longer it takes to digest into its absorptible bits (ie, amino acids for proteins, fatty acids for fats, monosaccharides for carbohydrates).
Once those bits are absorbed, they travel from the intestines and stomach, via the only vein in the body not to go directly back to the heart (the hepatic portal vein), straight to the liver.
The liver usually then grabs as much of the monosaccharides as it can, and turns them into glycogen, which is the body's main store of carbohydrate. Glycogen is stored mostly in liver cells, but also it's stored in muscle cells, which pull glucose out of the bloodstream, and convert it to glycogen.
If blood sugar is low, body cells convert glycogen to glucose, and vice versa for high.
Fatty acids are collected by fat cells, and bundled together, if there is a lot of fatty acids in the bloodstream, and vice versa... but this process is slower than the process for glycogen/glucose conversions.
Amino acids are used everywhere, but the biggest store is in the muscles. Again, muscle cells build up proteins more when there is high amino acid blood content, but a bigger factor in this process is whether the muscles have been used recently.
I'm Running I'm Running I'm Running!
Now, when one exercises, the muscles start to burn glucose, run out of glucose, and start converting glycogen to glucose (well, continue to - this is a dynamic equilibrium - since they always do some conversion, the rate just goes way up). Also the liver starts dumping glycogen into glucose also, to maintain blood-sugar levels.
Eventually, the entire glycogen store runs out, at which point the only energy sources are fatty acids and amino acids.
Muscles, if they run out of glucose during exercise, don't have fatty acids to burn. Or not much, anyway. So, they generally eat amino acids, plus whatever fatty acids are in the bloodstream at the time.
In fact, exercise burns fatty acids, amino acids and glucose, all at the same time, sucking them out of the bloodstream as you exercise.
As they come out of the bloodstream, they are replenished into the bloodstream from storage sources (the liver, for glucose, fat cells for fatty acids, muscles for amino acids).
So - longer, slower exercise has more time to suck fat out of the fat cells than short fast exercise, since you spend longer with "low" levels of fatty acids in the bloodstream.
It's got nothing to do with the calorie count. You can burn more "calories" by running fast for 10 minutes instead of walking slow for an hour, but most of those calories will come from glycogen storage and amino acids, and hardly any of them will be from fatty acids.
Your brain can detect the levels of various substances in your bloodstream, and if you practice, you can learn to tell what state your body is in, to distinguish between false hunger and real hunger, and what your body is actually hungry for.
Aerobic exercise is exercise which burns sugars "properly", meaning that your bloodstream is carrying enough oxygen to actually complete the Krebs cycle fully. Anaerobic exercise is exercise which occurs too fast for full burning to occur, resulting in the byproduct "lactic acid" being produced. Fast and heavy exercise (e.g. sprinting) is anaerobic, gentle exercise (e.g. walking) is aerobic.
The way to tell the difference? Listen to your muscles... if they're feeling "crunchy" during exercise, that's bad. "Crunchy" means that you have lactic acid buildup, and lactic acid is the direct byproduct of anaerobic metabolism. (ETA updated info)
Doing fifteen minutes of aerobic exercise will raise your metabolic rate for quite some hours afterwards. This overall raise in metabolic rate is likely to burn more energy than doing fifteen minutes of fast running, after which your body collapses and reduces your metabolic rate for hours afterwards.
Basically, fat metabolism occurs last. Your body burns protein (i.e., muscle tissue) in preference to fats! Thus, over-exercising whilst hungry does nothing for reducing fat. All it does is burn off muscle tissue. You should eat a balanced diet, doing so by learning to tell whether your body is hungry or not, and you should do 15 to 30 minutes of gentle aerobic exercise daily. That will reshape your body much better than counting every calorie.