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Metabolism of body fat

by JakeA
Tags: body, metabolism
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JakeA
#1
Sep4-09, 05:16 PM
P: 42
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.
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grumpymrgruff
#2
Sep8-09, 01:43 PM
P: 11
Quote Quote by JakeA View Post
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.
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.
JakeA
#3
Sep9-09, 07:58 AM
P: 42
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

JakeA
#4
Sep9-09, 10:14 AM
P: 42
Metabolism of body fat

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.
grumpymrgruff
#5
Sep9-09, 11:03 AM
P: 11
Quote Quote by JakeA View Post
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.
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.
Quote Quote by JakeA View Post
3. Still don't know what the biochemical pathway between saturated fat and ldl is. If anybody has some info, I would be greatful.
I just found the following on wiki's (completely unsourced) article on fatty acid metabolism. Emphasis mine.
Digestion and transport

Fatty acids are usually ingested as triglycerides, which cannot be absorbed by the intestine. They are broken down into free fatty acids and monoglycerides by pancreatic lipase, which forms a 1:1 complex with a protein called colipase which is necessary for its activity. The activated complex can only work at a water-fat interface: it is therefore essential that fatty acids (FA) be emulsified by bile salts for optimal activity of these enzymes. People who have had their gallbladder removed due to gall stones consequently have great difficulty digesting fats. Most are absorbed as free fatty acids and 2-monoglycerides, but a small fraction is absorbed as free glycerol and as diglycerides. Once across the intestinal barrier, they are reformed into triglycerides and packaged into chylomicrons or liposomes, which are released into the lacteals, the capillaries of the lymph system and then into the blood. Eventually, they bind to the membranes of hepatocytes, adipocytes or muscle fibers, where they are either stored or oxidized for energy. The liver acts as a major organ for fatty acid treatment, processing chylomicron remnants and liposomes into the various lipoprotein forms, namely VLDL and LDL. Fatty acids synthesized by the liver are converted to triglyceride and transported to the blood as VLDL. In peripheral tissues, lipoprotein lipase digests part of the VLDL into LDL and free fatty acids, which are taken up for metabolism. This is done by the removal of the triglycerides contained in the VLDL. What is left of the VLDL absorbs cholesterol from other circulating lipoproteins, becoming LDLs. LDL is absorbed via LDL receptors. This provides a mechanism for absorption of LDL into the cell, and for its conversion into free fatty acids, cholesterol, and other components of LDL. The liver controls the concentration of cholesterol in the blood by removing LDL. Another type of lipoprotein known as high density lipoprotein, or HDL collects cholesterol, glycerol and fatty acids from the blood and transports them to the liver. In summary:

* Chylomicrons carry diet-derived lipids to body cells
* VLDL's carry lipids synthesized by the liver to body cells
* LDL's carry cholesterol around the body
* HDL's carry cholesterol from the body back to the liver for breakdown and excretion.

When blood sugar is low, glucagon signals the adipocytes to activate hormone sensitive lipase, and to convert triglycerides into free fatty acids. These have very low solubility in the blood, typically about 1 μM. However, the most abundant protein in blood, serum albumin, binds free fatty acids, increasing their effective solubility to ~ 1 mM. Thus, serum albumin transports fatty acids to organs such as muscle and liver for oxidation when blood sugar is low.
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.


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