Who knew that when Procter and Gamble accidentally created Olestra by attaching up to eight fatty acids to regular table sugar, they had also created a monster? While the molecule tastes and looks like fat to the human eye, in the body its freakishly spiderlike structure can’t be broken down at all, thus, the intestine cannot absorb it for caloric energy.
After a fatty meal, the body doesn’t even really start digesting the triglycerides until they reach the small intestine (although there is a bit done by saliva and stomach fluid) .
As fats reach the duodenum, they form into large globules that stimulate secretion of bile from the liver that emulsifies them into tiny globules or droplets [1p716]. A protein called colipase binds to the droplets, which allows various enzymes called lipases secreted by the pancreas to access them more easily [1p779]. Lipases hydrolyze triglycerides freeing a couple of the fatty acids from their glycerol backbone so that both the monoglyceride and fatty acids can be transported by intestinal cells [1p716].
It’s not a perfect system. The hydrolysis is absolutely necessary for absorption, but then those fatty acids and monoglycerides are just made into triglycerides again [1p716]. But in the bloodstream they can be hydrolyzed again to be degraded by beta-oxidation into acetyl CoA for production of ATP or, as witnessed by my love handles, can be stored in adipose tissue .
Unlike triglycerides, Olestra would never be broken down or absorbed at all because it is unrecognizable to colipase and lipases. The colipases are particular in binding to droplets, and lipases, like all enzymes, are specific in the way of binding to substrates [1p639].
The “fake fat”, thus, is pooped out as slimy diarrhea and, along with it, fat-soluble nutrients. Yes, it’s true, those P&G snacks are fortified with extra vitamin A and D to cancel out the leaching.
1. Denniston KJ, Topping JJ, Caret RL. General, Organic, And Biochemistry, 5th ed. New York: McGraw Hill; 2007.
2. Cutting edge. “Fat Blockers”. Interactive Concepts of Biochemistry. Available at http://www.wiley.com/legacy/college/boyer/0470003790/cutting_edge/fat_blockers/fat_blockers.htm. Accessed on October 4, 2008.