Saturday 14 May 2005

Pic of the day: Screenshot from the anime Sensei no Ojikan. What is this feeling?

Ketogenesis?

OK, probably not. Or at least not to any large extent. And since most of us never learned about ketogenesis in school, let me share my newfound fascination. ^^

The body lives by burning food, much like a candle flame lives by burning stearin or wax. But our bodies are doing a lot more advanced stuff with the energy, and we are able to burn a lot of different stuff at room temperature, which is impressive. But also complicated. You probably learned in school that the 4 types of fuel are CARBOHYDRATES, FAT, PROTEIN and ALCOHOL. OK, you perhaps didn't learn the fourth in class. Not all of these fuels are created equal. Not even the first three.

When given the choice, the body prefers carbohydrates. Basically this is sugar, since starches are broken down into sugar pretty quickly (it starts while you are still chewing them). All cells in the body can use sugar. It doesn't give all that much energy, but it is quick and easy to use. The brain is particularly fond of sugar.

When the level of sugar in the blood falls because you are exercising, the muscles thoughtfully switch to burning fat instead, leaving the sugar for the brain. The brain cannot really burn fat, though it needs fat for other purposes, mostly in childhood, so don't starve your babies. After the exercise (or manual work, in those parts of the world where this still happens) the body will tank up on carbs again from the digestive tract. Food typically uses a couple days on passing through, so there is usually some available at any one time. The body will now store some of the sugar as "animal starch" (glucogen, which roughly means sugar-maker) for the next occasion. So far, so good.

But what if there's not enough carbs in the food? Either because you are out of food (in which case you should probably sell your computer and buy food instead) or because you are too sick to eat, or perhaps even starving yourself to impress your god or your friends? Well, then there's always ketogenesis.

In this small marvel of forethought, the liver converts fat to a carbs substitute, called "ketone bodies" (there are three of them). These small pseudo-sugars can be used instead of sugar by any part of the body except the liver itself, even the brain. The cells will greedily accept them whenever there is a shortage of sugar. They burn completely into CO2 and water, leaving no dangerous waste products. It seems like a perfect plan. In fact, it is, except sometimes in people with type I (youth) diabetes the body will produce ketone bodies while there is still sugar. In this case they accumulate and poison the blood.

So, now you can talk with confidence about the nature and causes of ketogenesis. The day was not wasted after all!

***

As for my humble self, however, I burned over 600 kcal going to the shop and back. When I came home and attempted to replenish my carbs reserves, however, my stomach acted up. I've had worse, but feeding was not on the agenda for some hours. So I suppose some ketogenesis may have taken place. But probably not much. It is not like I have completely exhausted my body's reserves or anything. I have lost about one and a half kg (3 pounds) since Tuesday, and I doubt I've burned that much fat, so it probably means there is less stuff in my digestive tract than before. I feel mostly like drinking, naturally, so the carb intake is a bit below par. But I'm not fasting or anything. Except briefly when the stomach is on strike. Since ketogenesis is associated with the onset of starvation, I think it may still be a bit away... Let's hope so.