For my regular readers, this article will be a bit off-topic – it’s written for a couple of friends.
Eating fat does not make you fat. Well, not directly. All the food you put into your face must eventually be turned into glucose – sugar, basically. That’s the only fuel your body can burn.
When you eat simple carbohydrates – that is, sugars – your body absorbs the sugar directly in to the bloodstream from your digestive tract. Sugar doesn’t require any digestion to become sugar, although certain types of sugar go through some extra processing before they turn into the kind of sugar your body runs on. This is why sugar is such a quick hit of energy – it gets into the blood fast. A gram of simple carbs turns into 4 calories of energy. (Technically, 4 kilocalories, but in the US we refer to ’em as calories).
Complex carbohydrates, like starches, take longer to break down. Your body uses enzymes in the digestive tract, mainly the mouth (saliva), stomach and small intestine, to break them down into sugars, which are absorbed into the blood through he small intestine. Many complex carbs are accompanied by dietary fibre, which slows down the enzymes’ reaction, thereby slowing the release of sugar into your blood. A gram of carbs gets you 4 calories of energy.
Protein is harder. Most of the digestive work happens in the stomach and small intestine, primarily by hydrochloric acid and pepsinogen, two substances in the stomach. Protein can take a long time to break down, compared to complex carbs, and proteins don’t break down into sugar. Instead, they break down into amino acids. It’s those amino acids that get absorbed into your blood, and your body can use them to repair and strengthen existing proteins – muscles – in your body. This is important, because your body loses a minimum of 250 grams of protein per day, and needs to rebuild that 250 grams. Some of the broken-down protein is actually recycled, though, so it’s not like you need to eat 250 grams a day! Protein is kind of a pain to digest. In fact, although every gram of protein yields 4 calories, it takes about 1 calorie to actually digest the stuff. In terms of energy, protein just isn’t efficient source of energy.
Then there’s fat, which is broken down into sugars, and which yields an amazing 9 calories per gram. But remember: It’s broken down into sugar. Fat doesn’t automatically go to your gut. It doesn’t matter (from a weight perspective) what kind of calories you eat – they all turn into the same energy at the end. And all energy can become fat, if you have too much of it.
Finally, there’s alcohol. This is basically a super-carbohydrate, turning into 7 calories per gram. And, best of all, when you’re burning energy from alcohol, you won’t burn energy from any other source. That means your liver is a lot more likely to turn any other free calories into fat for later use.
So where does all this energy go? Your bloodstream can store about 600 free calories, and your muscles store about 2000. The trick with muscular calories is that they aren’t shared; they can only be used to power the muscle they’re stored in. That’s why a full-body workout can burn so many calories – you’re depleting hundreds of tiny calorie storage units, which then have to be refilled. Building more muscle increases calorie storage, too.
Once your body is “full” of calories, the extra gets turned into fat by your liver. That’s why, if you are fairly sedentary, you shouldn’t eat more than 400-600 calories at once. Think about it: Your muscles are full of energy because you’re not doing anything. Your blood only holds about 400-600 calories, and you only burn about 80 calories an hour. You can easily go 4 hours without eating, just on what’s in your bloodstream. But if you then chow down a 1,200-calorie hamburger, less than half of that will go toward replenishing your blood’s energy supply. The rest is immediately turned into fat. Do that every day, and in a week you’ll have earned a pound of fat.
And it does no good to starve yourself later to “make up for it.” Your body won’t turn to those fat stores unless it’s pretty desperate – it’d much rather eat its own muscle. That’s basically a survival technique, especially in women, whose body chemistry demands more fat be present than is needed in men. Working out correctly – think high-intensity interval training – can force your body into a “survival panic” where it’ll start tapping into those fat stores.
(Apologies for the massive oversimplifications)