I've been interested in the biology of digestion since I took grade 13 biology class for a second time - I'd failed the first time around. So I thought I'd chat a little about getting energy from food.
Different food is made up of different things. Most of those things contain what I call biologically available calories -BACs (the ash contained in some foods, small pieces of grass, insoluble fibre, and small chunks of bone are not available to us - if we chewed our cud or if we were
Osedax frankpressi we'd characterize some of this input as biologically available). We put BACs into the largest opening in our faces with our limbs, it travels into our gut void and with a little help from our smooth muscles is digested with more help from a bit of detergent we call bile acids (thanks in part to our gall bladder - more below), some regular old-fashioned gastric acid that helps break down connections between the chunks, good old fashioned mastication (which generally happens when the BACs are first introduced to the oral cavity), and the concerted and mindless mechanical attention of the (and here comes one of my favourite words)
rugae, and other general biological encouragement (including non-human life forms like our cute little gut flora), and passes out through our anus and in urine out through our genital regions (our naughty bits) after much of the caloric energy, nutrients and water is separated out and pulled into our greater biological system. And if our greater biological system is already sufficiently primed with the different stuff in the different biologically appropriate regions in our greater biological system, we either store it, purge it, or force a bit more of it into freely available and ready stores (less stable, than say, glycogen hanging around in our liver - lipids swimming around our blood stream). I've always been intrigued by bile. Our gall bladders make bile - it's like sunlight detergent, fussing about with the fat from a Big Mac as we digest it. Keep in mind this stuff needs cholesterol and luckily our livers help out by creating a bunch of it.
Bile acids are facial amphipathic, that is, they contain both hydrophobic and polar faces - one face is nervous around lipids (fats) and tends to turn the other cheek, and this other check really isn't sure how it feels about lipids, so it kind of clings and grabs onto lipids in sheer terror. Like holding on tight to your keys after they fall through a gap in the fence - you have to let go in order to get them out, but that goes against your better judgement - hence you freeze. Their amphipathic nature enables bile acids to carry out two important functions:
1. Emulsification of lipid aggregates: Bile acids have detergent action on particles of dietary fat which causes fat globules to break down or be emulsified into minute, microscopic droplets, as I suggested above re: Big Macs. Emulsification is not digestion per se, but is of importance because it greatly increases the surface area of fat (remember your physics classes everyone), making it available for digestion by lipases, which cannot access the inside of lipid droplets. Lipases are enzymes, silly. Enzymes catalyze chemical reactions without breaking down themselves. Cute little buggers really. The chemical reactions are the things that liberate the food energy i.e. calories.
2. Solubilization and transport of lipids in an aqueous environment: Bile acids are lipid carriers and are able to solubilize many lipids by forming micelles - aggregates of lipids such as fatty acids, cholesterol and monoglycerides - that remain suspended in water. Bile acids are also critical for transport and absorption of the fat-soluble vitamins. We're talking about Vitamin A, and I think D, oh and E!
(On a somewhat unrelated note: If you consume a polar bear liver, the vitamin A in it will cause your skin to slough off and you will die a most unpleasant death. So don't do that. Women do this in a less drastic fashion - they apply Vitamin A acid - retinol - creams to their faces so their wrinkly skin sloughs off revealing younger looking more vulnerable skin. But the whole polar bear thing, you actually slough off all your skin, in great slabs. Now that's disturbing.)
Anyway, back to the point I'd started with. It's quite simple. Our bodies maintain a dynamic equilibrium - if it needs more stuff stored, it helps to give it lots daily. If it doesn't need more stuff stored, then try not to give it lots daily. If it needs more stuff freed up for chemical energy to be converted into kinetic energy in our muscles, for example, then it will free it up, and that means there's more room for more stuff.
I weigh 205 lbs. I believe I should weigh 185 lbs. If I think carefully about what I've said above, keep my gall bladder in good working order, keep away from polar bear livers, and be mindful of the dynamic equilibrium expressed by my body, I should lose weight.
And if I ignore what I've said, well, I'll likely stay fat.
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