Some interesting tidbits while I study for my biochemistry final:

-When your body breaks down carbohydrates for energy, it converts glucose into fructose (and then other things, but that’s one of the first steps of glucose breakdown, aka glycolysis.) So it’s not really true that “your body doesn’t know what to do with fructose!” because when you eat fructose in the diet (most commonly as table sugar, which is 50% fructose) it feeds directly into glycolysis. Now, what it does on the way there might be relevant to health effects, but I’m not sure. I’ve heard something about it not triggering leptin, and leptin is supposed to suppress appetite, therefore not triggering it makes people TEH FAT — but I’ve also read that fat people tend to have plenty of circulating leptin anyway, so your guess is as good as mine.

-When your body breaks down fats for energy, one of the first steps is insert a double bond (if the fats are saturated), then to change the cis-double bonds to trans-double bonds (during ß-oxidation.) But this occurs within the mitochondria of cells, after the regular old cis fats have been carried through your bloodstream. I’m not sure exactly what happens when trans fats are attached to lipoproteins and carted around. Something bad! I don’t know the precise mechanism of why they’re said to cause atherosclerosis. Something about lowering HDL while they simultaneously raise LDL.

So, obviously, it’s not as straightforward as “well your body uses fructose and trans fats as natural intermediates in metabolism, therefore they can’t be bad for you,” but I always find it funny when I see stuff like this. These molecules, while they may cause unintended effects when ingested in the diet, do serve certain natural purposes, and are not 100% man-made. I mean, unless by “man-made” you mean “my body makes them when I eat food.”

Your body also makes stuff like ammonia during protein catabolism, but it’s pretty well-controlled or converted into non-toxic forms. You certainly wouldn’t want to drink it!