Metabolism of body fat

  1. Hi,

    Does anybody know or have a link on the chemical/metabolic pathways for human body fat? I've been trying to look this up and can't seem to find an answer.

    I'm curious about this concept. Human body fat is a form of animal fat, so metabolizing it should raise your LDL cholesterol level? What this would mean is that if you're losing weight, you'd expect higher LDL cholesterol. Is that right?


    Thanks.
     
  2. jcsd
  3. Body fat consists of a glycerol ester-linked to three fatty acid chains (called triglycerides). The fatty acid chains undergo beta oxidation which removes two carbons (an acetyl group) from the chain at a time. These acetyl groups are carried by coenyzme A (CoA) to the mitochondrial matrix, where they are bound to oxaloacetate to form citric acid (the first step of the Krebs or citric acid cycle). From this point on, chemical energy is extracted via the citric acid cycle and electron transport chain, just like acetyl-CoA from glycolysis during sugar-substrate respiration.

    The opposite metabolic process, fatty acid synthesis, is discussed here. Cholesterol metabolism is discussed here. For the big picture, take a look at the Roche pathway chart and search for 'fatty'.

    I don't know off the top of my head how metabolism of body fat (as opposed to ingested fat) affects LDL levels.
     
    Last edited: Sep 8, 2009
  4. Thanks for the info and the links. I think in a very vague sense any saturated fat will promote high LDL, so assuming human body fat is mostly saturated, metabolizing it will promote high LDL. Definitely don't have all the details on the reactions, though, and don't even know if human fat is mostly saturated.

    Here's a pretty good and simple link on fat metabolism.

    http://www.unisanet.unisa.edu.au/08366/h&p2fat.htm
     
  5. Here's a couple of other points I forgot to mention.

    1. You can make an educated guess that human fat is hydrogen saturated. It needs to be solid at room temperatures. Plants can get away with unsaturated fat because they have cell walls and exoskeleton like superstructures to store the fat. I would guess that insects have unsaturated fats for that reason, but am not sure. Cold water fish also have unsaturated fat because they live in the cold and have the added suspension of water to keep their fat from oozing out. I would also guess that the fat stored in a camel's hump is predominantly unsaturated.

    2. This is actually an important concept in a clinical sense. What normally happens is that over weight people are told they have high cholesterol. They start dieting and getting more exercise, and losing weight. Their cholesterol is still elevated, so they get on statins. There wouldn't be much research into this phenomenon because most of it is paid for by the statin manufacturers. If this concept is correct, which I admittedly don't know, then you really need to check a person's ldl levels when they've stopped losing weight.

    3. Still don't know what the biochemical pathway between saturated fat and ldl is. If anybody has some info, I would be greatful.

    Thanks.
     
  6. I remember learning something like this, but I don't think fatty acid diffusion across the membrane is as important as you make it out to be. Recall that saturated fatty acids have more energy-rich oxidizable hydrogen-carbon bonds than unsaturated fatty acids of the same length. As an energy storage medium, they are therefore more valuable. However, their tendency to solidify at room temperature is a problem. Fatty acids need to be in aqueous solution to interact with enzymes and to be metabolized; their hydrophobic character makes their solubility low, and this becomes even worse when they are solid. Humans and other endotherms can utilize saturated fats for energy storage because we maintain our bodies at temperatures where the fats are at least semi-fluid. (Our fat is not solid, but is liquid in vesicles within adipocytes.) Ectothermic organisms cannot depend on ambient temperatures remaining above the melting point of saturated fats, so they compromise and utilize more unsaturated fats for energy storage. This is why fish and other ectotherms (including plants and insects) have a much higher percentage of unsaturated fatty acids relative to endotherms.
    I just found the following on wiki's (completely unsourced) article on fatty acid metabolism. Emphasis mine.
    It sounds like fat released/catabolized from adipocytes during weight loss and excercise is carried by serum albumin in the form of individual fatty acids and not by lipoproteins (which transport intact triglycerides). VLDL/LDL turnover sounds more closely tied to levels of ingested and newly synthesized lipids.
     
    Last edited: Sep 9, 2009
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