Anything insoluble in water or blood, such as cholesterol, has to be delivered in a lipoprotein (a lipid/protein delivery vehicle).
The mighty Chylomicron - the largest lipoprotein – is sent out from the intestines (gut) to deliver dietary fat – exogenous fat.
The 2nd largest lipoprotein – VLDL – Very Low Density Lipoprotein – is sent out from the liver to deliver liver made fat – principally from excess dietary carbohydrates.
|Chylomicron||gut wall||apoB-48||dietary fat|
|LDL||offspring of VLDL||apoB-100||cholesterol to body|
|HDL||Liver||various||reverse cholesterol transport|
To understand the difference between chylomicrons and VLDL, let’s compare a high fat breakfast with a high carb breakfast.
High Fat Breakfast (Two or three eggs fried in lard plus two lamb chops)
When you eat fat and protein for breakfast, the fats (lipids) become separated from the protein in the intestine, are broken down into molecules, absorbed into the gut wall, and are reassembled into triglycerides and placed into the chylomicron. (Cholesterol and other fat soluble nutrients like vitamin E share the ride.)
If the chylomicron is the size of an 18-wheeler, liver-made VLDL is a large delivery truck, LDL is a van, and HDL is a motorcycle. Though smallest, HDL is the most numerous particle and you want as many of these motorcycles as possible bringing cholesterol back to the liver for recycling.
Released in the lymph from the gut wall, chylomicrons travel in the blood until they have delivered their fatty bounty, shrink and disappear. (Chylomicron remnants are picked up by the liver.) Because chylomicrons have a specific apoB-48 protein attached, the cells in the body recognize the vehicle and send enzymes out to snatch up this fat – of dietary origin.
Immediately after a high fat meal, you will have elevated chylomicron levels (apoB-48) that will fall relatively quickly – in 2 or 3 hours – with no direct effect on VLDL, LDL, or triglyceride levels. A high carbohydrate diet does the opposite.
High Carbohydrate Breakfast (Cheerio’s, skim milk, banana, and orange juice)
Unlike fats, carbohydrates are absorbed as sugar or glucose directly in the blood – except fructose which is sent to the liver and turned into fat. (Sugars are soluble in water so they are not carried in a lipoprotein.) After our high carb breakfast, there is no immediate effect on lipid levels; instead you get an immediate rise in blood sugar and insulin.
A certain amount of glucose is absorbed by muscle and liver cells and stored as glycogen. When the glycogen storage is exceeded, the excess sugar is absorbed by the liver, turned into liver-made fat (triglycerides) and is loaded onto VLDL and sent out into the blood.
(The word “triglyceride” has two meanings: (1) describes how fats are packaged in the body and (2) a term used in blood work to measure fat levels in your blood. Triglycerides (TG) over 100 represent increased risk of heart disease.)
VLDL production can go on for hours after a carbohydrate or mixed meal
Excess carbs lead to ongoing triglyceride production which calls for additional VLDL to haul away the liver-made fat. Also, as VLDL is assembled in the liver, HDL must give up its proteins and disappear. In a teeter-totter manner, Triglycerides go up; HDL goes down.
And the greater your triglyceride and VLDL production, the more likely LDL – the offspring of VLDL – will be the small, dense particle associated with increased risk of heart disease. In other words, if your triglycerides are elevated – say over 150 – your HDL will be depressed and your LDL will become a smaller, denser particle more susceptible to oxidation.
VLDL and LDL are a continuum. As VLDL gives up its fat, it morphs into LDL delivering mostly cholesterol. LDL metabolism, regulated by the liver, is not related to dietary fat – which is metabolized and delivered via gut-made chylomicrons.
As you can see, Dr. Atkins was right. Dietary fat and cholesterol – delivered in the chylomicron system (B-48) – have little or no influence on LDL and VLDL (both B-100). While it takes 8-10 hours for the liver to process out B-100s, it takes just a few hours for B-48s to be snatched up in the bloodstream and disappear.