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There is a question I encountered.

Why is fat harder to digest ? A) It has ester bonds. B) It is not easily soluble in water.

Which is the most appropriate answer?

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Not everything in biology is black or white (in fact, almost everything is gray). A combination of these factors and others play a role in the observation. –  LanceLafontaine Jun 12 '13 at 19:51
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1 Answer 1

up vote 3 down vote accepted

I think that the main reason (but probably not the only one) is because fats are hydrophobic, while the environment of the digestive tract is aqueous. Most of the fats in the diet are triglyceride and in hydrophilic environment they form globules. Triglycerides must be hydrolysed to be absorbed by the duodenum and this work is done by pancreatic lipase, that is a water soluble enzyme and can only hydrolyse the esteric bonds at the surface of fat globules.

Therefore, during the digestion amphipathic molecules such as bile salts and phospholipids emulsify those droplets in smaller ones, increasing the surface area where lipases, together with colipase can digest.

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Fats are not metabolized in the digestive tract. The metabolic pathways that break down and utilize fat turn fats into glycerol which is much easier to obtain from carbohydrates. –  shigeta Jun 12 '13 at 23:33
    
My Lehninger principles of biochemistry says that "products of lipase action [(monoacylglycerols, diacylglycerols, free fatty acids and glycerol)] diffuse into the epithelial cells lining the intestinal surface". Is this incorrect or maybe I didn't explain myself well? I omitted this last part of the process because I thought that it was superfluous. –  Mattia Rovetta Jun 13 '13 at 6:48
    
You might be right. I'd thought that the question is about metabolizing for energy. –  shigeta Jun 13 '13 at 13:11
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