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On, somebody asked a question about the fat in chicken stock sometimes being liquid and sometimes solid once the stock has cooled. The only answer until now suggests that the difference is due to the percentage of saturated fat in the chicken lard, which will vary depending on the chicken's food.

I am not entirely happy with this explanation. I am aware that the proportion of saturated to unsaturated fat in an organism's cell walls will vary with diet, and could imagine that the fat stored within its adipose tissue will be different too. The person who provided the answer also posted references which confirm that part. But I doubt that the effect will be strong enough that, if an animal on low-saturated-fat diet is processed to lard, the resulting lard will be a viscous liquid, as opposed to the usual soft solid. If my memory doesn't fail me, my grandma used to keep her animals on a mostly starch-based diet, and still was able to process them for (solid) lard.

Can somebody confirm or disprove this theory?

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What is your reason for thinking that "cell walls" (I assume that you mean cell membranes) composition will vary with diet? My recollection is that the fluidity of the phospholipid bilayer is tightly regulated, and since organisms can saturate or desaturate acyl chains this would be easy to achieve. As far as stored lipid is concerned my feeling is that fluidity might be somewhat less critical so could vary. – Alan Boyd Sep 28 '13 at 14:23
@AlanBoyd I have read it as the explanation to why saturated fats are unhealthy: the cell walls membranes of the eater become stiffer and less flexible by incorporating molecules made with saturated instead of unsaturated fatty acids. I don't remember my sources, they were some kind of popular science, not peer-reviewed literature, so it is possible that they were wrong, or that they were oversimplified enough to give me a wrong understanding of what is actually happening. – rumtscho Sep 28 '13 at 14:46

Without going into the biochemical details of fatty acid digestion, mobilization, and synthesis, the brief answer is that the animal regulates its own fatty acid composition to within a specified range regardless of its food. Lipid metabolism does not differ from the metabolism of other compounds, in that it is a highly regulated, enzyme-mediated process, which ultimately is genetically determined.

It must be stressed that a heterotroph assimilates its food, not as complete macromolecules, but as their base components. For example, the starch-fed chickens you described would digest the starch down completely to glucose, which would then be converted in the liver to fatty acids. These fatty acids would be, of course, those that the chicken had the enzymes to synthesize, which are consequently mostly saturated fatty acids of a certain length that happen to solidify when cooled.

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