Blood lipids

Blood lipids (or blood fats) are lipids in the blood, either free or bound to other molecules. They are mostly transported in a protein capsule, and the density of the lipids and type of protein determines the fate of the particle and its influence on metabolism. The concentration of blood lipids depends on intake and excretion from the intestine, and uptake and secretion from cells. Blood lipids are mainly fatty acids and cholesterol. Hyperlipidemia is the presence of elevated or abnormal levels of lipids and/or lipoproteins in the blood, and is a major risk factor for cardiovascular disease.

Hyperlipidemia
Hyperlipidemia is the presence of elevated or abnormal levels of lipids and/or lipoproteins in the blood.

Lipid and lipoprotein abnormalities are extremely common in the general population, and are regarded as a highly modifiable risk factor for cardiovascular disease. In addition, some forms may predispose to acute pancreatitis. One of the most clinically relevant lipid substances is cholesterol, especially on atherosclerosis and cardiovascular disease. The presence of high levels of cholesterol in the blood is called hypercholesterolemia.

Hyperlipoproteinemia is elevated levels of lipoproteins.

Fatty acids
Blood fatty acids are in different forms in different stages in the circulation. They are taken in through the intestine in chylomicrons, but also exist in very low density lipoproteins (VLDL) after processing in the liver. In addition, when released from adipocytes, it fatty acids exist in the blood as free fatty acids

Intestine intake
Short- and medium chain fatty acids are absorbed directly into the blood via intestine capillaries and travel through the portal vein. Long-chain fatty acids, on the other hand, are too large to be directly released into the tiny intestine capillaries. Instead they are coated with cholesterol and protein (protein coat of lipoproteins) into a compound called a chylomicron. The chylomicron enters a lymphatic capillary and enters into the bloodstream first at the left subclavian vein.

In any case, the concentration of blood fatty acids increase temporarily after a meal.

Cell uptake
After a meal, when the blood concentration of fatty acids rises, there is an increase uptake of fatty acids in different cells of the body, mainly liver cells, adipocytes and muscle cells. This uptake is stimulated by insulin from the pancreas. As a result, the blood concentration of fatty acid stabilizes again after a meal.

Cell secretion
After a meal, some of the fatty acids taken up by the liver is converted into very low density lipoproteins (VLDL) and again secreted into the blood.

In addition, when long time has passed since the last meal, the concentration of fatty acids in the blood decreases, which triggers adipocytes to release stored fatty acids into the blood as free fatty acids, in order to complement e.g. muscle cells with energy.

In any case, also the fatty acids secreted from cells are anew taken up by other cells in the body, until entering fatty acid metabolism.

Cholesterol
The fate of cholesterol in the blood is highly determined by its constitution of lipoproteins, where some types favour transport towards body tissues and others towards the liver for excretion into the intestines.

The 1987 report of National Cholesterol Education Program, Adult Treatment Panels suggest the total blood cholesterol level should be: <200 mg/dl normal blood cholesterol, 200-239 mg/dl borderline-high, >240 mg/dl high cholesterol.

The average amount of blood cholesterol varies with age, typically rising gradually until one is about 60 years old. There appear to be seasonal variations in cholesterol levels in humans, more, on average, in winter.

Intestine intake
In lipid digestion, cholesterol is packed into low-density lipoproteins in the intestine, which are delivered to the bloodstream.

In lipoproteins
Cholesterol is minimally soluble in water; it cannot dissolve and travel in the water-based bloodstream. Instead, it is transported in the bloodstream by lipoproteins - protein "molecular-suitcases" that are water-soluble and carry cholesterol and triglycerides internally. The apolipoproteins forming the surface of the given lipoprotein particle determine from what cells cholesterol will be removed and to where it will be supplied.

The largest lipoproteins, which primarily transport fats from the intestinal mucosa to the liver, are called chylomicrons. They carry mostly fats in the form of triglycerides and cholesterol. In the liver, chylomicron particles release triglycerides and some cholesterol. The liver converts unburned food metabolites into very low density lipoproteins (VLDL) and secretes them into plasma where they are converted to low-density lipoprotein (LDL) particles and non-esterified fatty acids, which can affect other body cells. In healthy individuals, the relatively few LDL particles are large. In contrast, large numbers of small dense LDL (sdLDL) particles are strongly associated with the presence of atheromatous disease within the arteries. For this reason, LDL is referred to as "bad cholesterol".

High-density lipoprotein (HDL) particles transport cholesterol back to the liver for excretion, but vary considerably in their effectiveness for doing this. Having large numbers of large HDL particles correlates with better health outcomes, and hence it is commonly called "good cholesterol". In contrast, having small amounts of large HDL particles is independently associated with atheromatous disease progression within the arteries.

Intestine excretion
After being transported to the liver by HDL, cholesterol is delivered to the intestines via bile production. However, 92-97% is reabsorbed in the intestines and recycled via enterohepatic circulation.

Cell uptake
Cholesterol circulates in the blood in low-density lipoproteins and these are taken into the cell by LDL receptor-mediated endocytosis in clathrin-coated pits, and then hydrolysed in lysosomes.

Cell secretion
In response to low blood cholesterol, different cells of the body, mainly in the liver and intestines, start to synthesize cholesterol from acetyl-CoA by the enzyme HMG-CoA reductase. This is then released into the blood.

Hypercholesterolemia
Hypercholesterolemia is the presence of high levels of cholesterol in the blood. It is not a disease but a metabolic derangement that can be secondary to many diseases and can contribute to many forms of disease, most notably cardiovascular disease. It is closely related to the terms. Familial hypercholesterolemia is a rare genetic disorder that can occur in families, where sufferers cannot properly metabolise cholesterol.

Hypocholesterolemia
Abnormally low levels of cholesterol are termed hypocholesterolemia. Research into the causes of this state is relatively limited, and while some studies suggest a link with depression, cancer and cerebral hemorrhage it is unclear whether the low cholesterol levels are a cause for these conditions or an epiphenomenon.