The Mighty Chylomicron - or Why Dr. Robert C. Atkins was right!
The Mighty Chylomicron - the largest lipoprotein - is sent out from the intestines (gut) to deliver dietary fat and cholesterol. Lipoproteins are specially designed vehicles used to transport cholesterol and triglycerides around the bloodstream. (A triglyceride is the way fat is assembled in the body - three fatty acids attached to glycerol.) Anything insoluble in water or blood, such as cholesterol, has to be carried/delivered in a lipoprotein; that is, a lipid/protein vehicle.)
There are two distinct lipoprotein systems, chylomicrons to deliver dietary fat, and VLDL - Very Low Density Lipoprotein - to deliver fat made in the liver - principally from excess carbohydrates. As VLDL circulates delivering its fat, it morphs into LDL, the lipoprotein that delivers mostly cholesterol.
That's right - LDL is the metabolic offspring of VLDL - intimately related to carbohydrate metabolism.
Made in a separate pathway in the liver, HDL is reverse transport - scavenging loose cholesterol and returning it to the liver for recycling.
Of course, it would be a good idea if the medical profession stopped using terms like 'good' cholesterol and 'bad' cholesterol. LDL is not 'bad' cholesterol - LDL is a lipoprotein that delivers cholesterol. This sloppy terminology doesn't aid our understanding of heart disease, and evolved primarily to defend the failed hypothesis that cholesterol and fat cause heart disease.
Diet Heart News 1 and 2 discussed the liver-made carbohydrate-related VLDL system and the importance of the Triglyceride/HDL ratio. This report discusses dietary fat or chylomicron metabolism. To understand the difference between these systems, let's compare a high fat breakfast with a high carb breakfast.
High Fat (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, are absorbed within the gut wall, and then are reassembled into triglycerides and placed into the very large lipoprotein known as a chylomicron. (Cholesterol and other fat soluble nutrients like vitamin E share the ride.)
If the chylomicron - the largest lipoprotein - 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 from the gut wall through the lymph, 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. So a few hours after our fatty breakfast, the chylomicrons are gone. If you were to measure apoB-48 levels a few hours after a high fat meal, they would have returned to a 'normal' fasting state.
Eat a high fat breakfast, almost all sign of the fat - apoB-48 - will have disappeared in a relatively short space of time. And there will be no change in any lipid level - your VLDL, LDL, and triglyceride numbers are not directly affected. As you can see, dietary fat is readily and almost completely absorbed by healthy people.
High Carbohydrate (Cheerio's, skim milk, banana, and orange juice)
Unlike dietary fat, carbohydrates (except fructose) are absorbed as sugar or glucose directly in the blood. 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 a rise in blood sugar and an insulin response.
A certain amount of glucose can be absorbed into muscle and liver cells and stored as glycogen. But if you eat a big carbohydrate breakfast - like the one described above - the glycogen storage is exceeded and the excess sugar is absorbed by the liver and turned into liver-made fat - also called triglycerides - three fatty acids linked to glycerol. (The word "triglyceride" describes how fats are assembled in the body and also is a term used in blood work to measure residual blood fats in your blood. Triglycerides (TG) over 100 represent increased risk of heart disease.)
Unlike chylomicron metabolism, VLDL production can go on for hours after a carbohydrate or mixed meal. The liver stays busy making and packing triglycerides into VLDL which is sent out into the bloodstream - along with some cholesterol. If you are also consuming excess fructose, which goes directly to the liver, you are at risk of developing fatty liver disease, essentially a metabolic traffic jam in your liver.
Excess carbs > elevated blood sugar > chronic triglyceride production > more VLDL trucks needed to haul away the liver-made fat. Elevated liver-made triglycerides result in chronic high levels of VLDL. So, as you can see, LDL - the lipoprotein the experts call 'bad' cholesterol - is not related to dietary fat consumption. Dietary fat consumption is related to chylomicron metabolism and chylomicrons do not morph into LDL - they simply disappear.
As we learned in Diet Heart News, Vol. 1, No. 2, the greater your triglyceride production and VLDL level, the more likely the LDL offspring will be the small, dense particle associated with increased risk of heart disease. Also, as VLDL is assembled in the liver, HDL must give up its proteins and is reduced in a teeter totter manner: Triglycerides go up; HDL goes down.
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 fat restricted carbohydrate diet reduces VLDL, raises HDL, and may even lower LDL. A high carbohydrate diet does the opposite.
In short, the metabolism does exactly what you would expect it to. So you see, Atkins was right - excess carbohydrates - not dietary fat - make you fat and predispose you to metabolic diseases like diabetes and heart disease.
"Atkins was right - excess carbohydrates - not dietary fat - make you fat and predispose you to metabolic diseases like diabetes and heart disease"
"If the chylomicron - the largest lipoprotein - 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."
The Mighty Chylomicron - the largest lipoprotein - is sent out from the intestines (gut) to deliver dietary fat and cholesterol. Lipoproteins are specially designed vehicles used to transport cholesterol and triglycerides around the bloodstream. (A triglyceride is the way fat is assembled in the body - three fatty acids attached to glycerol.) Anything insoluble in water or blood, such as cholesterol, has to be carried/delivered in a lipoprotein; that is, a lipid/protein vehicle.)
There are two distinct lipoprotein systems, chylomicrons to deliver dietary fat, and VLDL - Very Low Density Lipoprotein - to deliver fat made in the liver - principally from excess carbohydrates. As VLDL circulates delivering its fat, it morphs into LDL, the lipoprotein that delivers mostly cholesterol.
That's right - LDL is the metabolic offspring of VLDL - intimately related to carbohydrate metabolism.
Made in a separate pathway in the liver, HDL is reverse transport - scavenging loose cholesterol and returning it to the liver for recycling.
| VEHICLE | ORIGINATES | PROTEIN | DELIVERING |
| Chylomicron | gut wall | apoB-48 | dietary fat |
| VLDL | liver | apoB-100 | liver-made fat |
| LDL | offspring of VLDL | apoB-100 | cholesterol to body |
| HDL | liver | various | reverse cholesterol transport |
Of course, it would be a good idea if the medical profession stopped using terms like 'good' cholesterol and 'bad' cholesterol. LDL is not 'bad' cholesterol - LDL is a lipoprotein that delivers cholesterol. This sloppy terminology doesn't aid our understanding of heart disease, and evolved primarily to defend the failed hypothesis that cholesterol and fat cause heart disease.
Diet Heart News 1 and 2 discussed the liver-made carbohydrate-related VLDL system and the importance of the Triglyceride/HDL ratio. This report discusses dietary fat or chylomicron metabolism. To understand the difference between these systems, let's compare a high fat breakfast with a high carb breakfast.
High Fat (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, are absorbed within the gut wall, and then are reassembled into triglycerides and placed into the very large lipoprotein known as a chylomicron. (Cholesterol and other fat soluble nutrients like vitamin E share the ride.)
If the chylomicron - the largest lipoprotein - 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 from the gut wall through the lymph, 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. So a few hours after our fatty breakfast, the chylomicrons are gone. If you were to measure apoB-48 levels a few hours after a high fat meal, they would have returned to a 'normal' fasting state.
Eat a high fat breakfast, almost all sign of the fat - apoB-48 - will have disappeared in a relatively short space of time. And there will be no change in any lipid level - your VLDL, LDL, and triglyceride numbers are not directly affected. As you can see, dietary fat is readily and almost completely absorbed by healthy people.
High Carbohydrate (Cheerio's, skim milk, banana, and orange juice)
Unlike dietary fat, carbohydrates (except fructose) are absorbed as sugar or glucose directly in the blood. 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 a rise in blood sugar and an insulin response.
A certain amount of glucose can be absorbed into muscle and liver cells and stored as glycogen. But if you eat a big carbohydrate breakfast - like the one described above - the glycogen storage is exceeded and the excess sugar is absorbed by the liver and turned into liver-made fat - also called triglycerides - three fatty acids linked to glycerol. (The word "triglyceride" describes how fats are assembled in the body and also is a term used in blood work to measure residual blood fats in your blood. Triglycerides (TG) over 100 represent increased risk of heart disease.)
Unlike chylomicron metabolism, VLDL production can go on for hours after a carbohydrate or mixed meal. The liver stays busy making and packing triglycerides into VLDL which is sent out into the bloodstream - along with some cholesterol. If you are also consuming excess fructose, which goes directly to the liver, you are at risk of developing fatty liver disease, essentially a metabolic traffic jam in your liver.
Excess carbs > elevated blood sugar > chronic triglyceride production > more VLDL trucks needed to haul away the liver-made fat. Elevated liver-made triglycerides result in chronic high levels of VLDL. So, as you can see, LDL - the lipoprotein the experts call 'bad' cholesterol - is not related to dietary fat consumption. Dietary fat consumption is related to chylomicron metabolism and chylomicrons do not morph into LDL - they simply disappear.
As we learned in Diet Heart News, Vol. 1, No. 2, the greater your triglyceride production and VLDL level, the more likely the LDL offspring will be the small, dense particle associated with increased risk of heart disease. Also, as VLDL is assembled in the liver, HDL must give up its proteins and is reduced in a teeter totter manner: Triglycerides go up; HDL goes down.
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 fat restricted carbohydrate diet reduces VLDL, raises HDL, and may even lower LDL. A high carbohydrate diet does the opposite.
In short, the metabolism does exactly what you would expect it to. So you see, Atkins was right - excess carbohydrates - not dietary fat - make you fat and predispose you to metabolic diseases like diabetes and heart disease.
