While writing up that eight-toxic-foods rebuttal the other day, I started reading up on Olestra, the "fake fat" that made the list. While it has to be considered a failure for its developers, I found the chemistry behind it interesting, and it got me to thinking.
First off, for those outside the chemical/biochemical field, a brief introduction to fat. We (and most other organisms) all store it in pretty much the same way: a backbone of glycerol (three carbons in a row, each with an alcohol), and each alcohol turned into an ester with a fatty acid. Those fatty acids are long carbon-chain compounds with a carboxylic acid group on the end, and when you've combined three of them onto a glycerol (fully loaded, as it were), you have a triglyceride. When the body wants to break that down for use as energy, it cleaves off the fatty acids one at a time (leading to diglycerides and monoglycerides), and the fatty acids are then chewed up two carbons at a time. (They're made two carbons at a time, too, so the ones found in living creatures are very heavily biased towards even-numbered carbon counts).
That intro is enough to make sense of some of the things you'll see in a blood test, like the tryglyceride levels and the free fatty acids. But there are a lot of details hidden in there. For one thing, there's a whole suite of different enzymes that do the work of assembling and breaking down the glycerides, and they're under all sorts of control mechanisms. And while glycerol is glycerol, the fatty acids themselves come in a huge variety - different lengths, presence of single or multiple double bonds up and down the chain (and keep in mind that double bonds in the middle of such a chain come in both cis and trans varieties), etc. So with this long list, glycerides get produced in all kinds of combinations, depending on diet, the tissue involved, and other factors. And beyond that, most all these components, up and down the list, are involved as signaling molecules on various proteins, substrates for other enzymes, starting materials for whole other chemical sequences, etc. Lipidology gets very complicated very quickly, and you may have noticed (via the changing dietary advice over the years) that we don't quite have it figured out yet. Nowhere near.
So what's Olestra? It's nothing more than table sugar (sucrose) with its alcohol groups given the fatty-acid-ester treatment. What you end up with is a molecule that acts very much like normal fats - both of them are polyhydroxy compounds decorated with fatty acids, after all. But the enzymes that cleave the various fatty ester groups don't recognize an esterified sucrose as anything they've seen before, and thus Olestra goes on its way uncleaved and unmetabolized. That, actually, is one of the things that seems to have sunk it in the market. A good-sized dose of Olestra has to go somewhere, namely, right through your digestive tract. The reports of the side effects this could bring on were not a good selling point, although there's a debate about how often they were observed in the real world.
Otherwise, though, it seems to have been a reasonably convincing substitute for actual fats. I've never had any Olestra myself - it would be interesting to see if I could distinguish potato chips made with it from the conventional ones. Procter and Gamble were of the opinion that there was no discernable difference in taste or texture, but I've heard from people who say that they can tell under blinded conditions
Another side effect is that the stuff would tend to dissolve greasier substances and carry them along. Thus the problems with fat-soluble vitamin absorption with Olestra, which was compensated for by adding more of these vitamins (such as A, D, and K) the to potato chips made with it. It should be remembered that potato chips are not a major source of vitamins - well, not for most consumers - but the concern was that a steady diet of Olestra-containing foods could interfere with nutrient absorption from the other foods eaten at the same time. This is purely a greasiness/water solubility issue (logP being the medicinal chemist's measuring scale), and Olestra has also shown an ability to sequester and remove things like ingested PCBs, for the same reasons. It doesn't know a vitamin from anything else; it just knows what it can dissolve.
Olestra spent a lot of time in human testing. Since lipid molecules (as mentioned above) are involved in a lot of different processes, these studies were done to see if there were any signs of Olestra participating in other pathways. Nothing was ever found; the stuff was too odd-looking to the body's enzymes to be digested, and too odd-looking to work its way into these other mechanisms as well. It just sort of made its way through.
But such cross-bred biomolecule hybrids are an interesting class. Just as you don't see fully-esterified sugar molecules in cells, there are many other things like this that don't show up - at least, as far as I know. The carboxylic acids at the C-terminals of amino acids, oligopeptides, and proteins don't get handled in living systems as esters much (or if they do, I've missed it). Imagine glycerol with peptides esterified off the OH groups, for example, in sort of a protein-fat hybrid. Now try it with glucose - I've never seen that, either. In the same way, the OH groups on amino acids like serine are available to be esterified, but that's another class of compounds I don't know much about. Phosphorylation, yes, but not plain esters. It's not like esters are somehow alien to biochemistry - you have the glycerides, for one, and esters of cholesterol are a well-known class of compound. Biochemistry as we know has just never gotten around to using these things.
It's easy to imagine a slightly alien life form using fatty acid esters of the higher sugars as its energy storage class rather than stopping at glycerol. These creatures would have enzymes that would take Olestra apart like a wooden puzzle, and might be baffled at our own molecules. Somewhere, some unusual-looking alien is perhaps proposing glycerol esters as an indigestible substitute for the diet - worried, perhaps, about the way everyone's tentacles are getting so swollen these days, what with the overabundance of cheap food and all, and sensing a market opportunity. Perhaps Zarkon & Yipslarg will succeed where Procter and Gamble failed.