excerpt from M Kendrick article in Media forum
"Malcolm Kendrick article-
"In order to understand why a high fat diet should, and does, raise HDL levels and lower VLDL levels (and may also lower LDL levels), you need to understand a bit about fat and sugar metabolism and the role of lipoproteins in your blood. Starting here.
When you eat fat it is absorbed by the gut and stuffed into very large lipoprotein known as a chylomicron. The fat in a chylomicron is almost all stored in the form of three fat molecules attached to a glycerol molecule, a structure known as a triglyceride. Three fats and a glycerol = tri-glyceride. By the way, cholesterol also sits in chylomicrons as a co-passenger. (Anything insoluble in water/blood, such as cholesterol, has to be carried around in a lipoprotein)
Chylomicrons are then released into the bloodstream and travel through the body losing chunks of triglyceride all the while as they pass fat cells. (Fat cells attack chylomicrons with a ?lipase? enzyme, chopping bits off). As this happens chylomicrons shrink, turning into Very Low Density Lipoproteins (VLDLs), which are otherwise known as? ?triglycerides.? How confusing is that?
In fact, the nomenclature in this area must be the most confusing in all of medicine.
* LDL is known as ?bad? cholesterol
* HDL is called ?good? cholesterol
* VLDLs are named triglycerides?
It?s little wonder that most people haven?t the faintest idea what anyone is talking about in lipid metabolism. Chylomicrons, VLDL, HDL and LDL are all lipoproteins. I wish that people would stop calling them things like ?cholesterol? and ?triglycerides?, and ?good? cholesterol and ?bad? cholesterol. It really doesn?t aid understanding.
Anyway, moving on. Apart from chylomicrons, the gut also sends out VLDLs de-novo, and the VLDLs do pretty much the same thing as chylomicrons, dropping off triglycerides here and there (mainly into fat cells) and shrinking. Quite what the difference is between a shrunk down chylomicron and a VLDL is, I don?t know. (By the way, just in case you?re wondering, VLDLs also contain cholesterol as a co-passenger. All lipoproteins have cholesterol in them)
Not all chylomicrons and VLDLs travel round dropping off triglycerides. Some go straight to the liver where they are absorbed, broken down, and unpacked. And their contents are used to make other things the body needs.
However, wherever they go, all of the ?fat containing? chylomicrons and VLDLs produced by the gut drop off their fat load, shrink, are then absorbed and completely disappear. So a few hours after a meal they are gone. And if you were to measure VLDL levels a few hours after a high fat meal they would have returned to ?normal?. Whatever normal may be.
Thus, if you eat a high fat meal, almost all sign of it will have disappeared in a relatively short space of time. And there will be no change in any lipid level. Or at least not any lipid level that anyone can be bothered measuring.
However, if you eat a high carbohydrate meal, the metabolism acts in a very different way. Carbohydrates are absorbed and transformed into sugars in the gut, from whence they go straight into the bloodstream, same as fat. But because sugars are soluble in water they don?t need to be carried in a lipoprotein, so there is no immediate effect on lipid levels from a high carb meal. You just get a sharp rise in blood sugar level.
A certain amount of the sugar will be absorbed into fat and muscle cells, and then stored as glycogen. But if you eat a big carbohydrate meal, the fat and muscle storage cannot cope, and the excess sugar has to be absorbed by the liver to prevent the sugar level getting too high.
However, the liver cannot store that much sugar, so it starts to convert it into fats, in the form of triglyceride. At which point, the liver then packs this excess triglyceride into a VLDL and sends it out into the bloodstream - along with some cholesterol. (Unlike with sharks, the liver in humans is not an energy storage organ)
So you get a kind of delayed VLDL rise after eating carbohydrates. But there is a key difference between the VLDL made by the guts, and the VLDL made by the Liver. The VLDL made by the liver, unlike that made in the gut, shrinks into a low density lipoprotein (LDL). The dreaded heart disease causing lipoprotein ? the one they call co-lest-erol.
Why does this happen to ?liver manufactured VLDL?, when it doesn?t happen to the VLDL made in the gut? Well, as liver manufactured VLDL leaves the liver, it interacts with an HDL molecule which transfers it?s proteins to the VLDL molecule. One of the proteins transferred is apolipoprotein B-100. And the apo B-100 molecule is the unique LDL ?identifier.?
On the other hand, VLDL made in the gut has apolipoprotein B-48 attached to it and this VLDL doesn?t become an LDL molecule as it shrinks."
This may be why the doctor recommends Atkins.
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