Introduction of High-density lipoproteins
The cholesterol required by using peripheral tissues, along with vascular cells, is provided both by new synthesis in the cells and by a delivery of low-density lipoproteins (LDL). When the level of LDL is high, they accumulate in the artery wall, there oxidize and taken up by foam cells in a process that leads to the development and progression of atherosclerosis.
High-density lipoproteins (HDL) oppose atherosclerosis directly, by removing cholesterol from foam cells, by inhibiting the oxidation of LDL, and by limiting the inflammatory processes that underlie atherosclerosis. HDL also has antithrombotic properties. As a result, HDL-cholesterol interrupts the process of atherogenesis at several key stages.
The cholesterol, transported in high-density lipoprotein (HDL) is called ‘good’ cholesterol. One of the important functions of HDL is to transport cholesterol from the cells and tissue back to the liver. High HDL-cholesterol is good as it takes cholesterol out of cells and the blood and helps to prevent excess cholesterol. Also removes cholesterol deposited in the walls of blood vessels.
HDL (High-density lipoproteins) cholesterol?
High-density lipoproteins are the smallest and densest of the lipoproteins, containing the highest proportion of protein to cholesterol. HDL stands for high-density lipoprotein and it is a kind of cholesterol in combination with soluble, conjugated proteins in the bloodstream.
It makes up 20% of the cholesterol in the body. HDL cholesterol is the good cholesterol because it protects your heart from the effects of any excess LDL cholesterol you may have.In a normal healthy individual, HDL carries about 1 / 4 of the total amount of cholesterol inside the blood, whereas most of the remainder is carried in LDL (low-density lipoprotein) “bad cholesterol” particles.
The framework of LDL is a single protein, as apolipoprotein B (Apo B), whereas HDL contains many proteins, the most important being apolipoprotein A1 (Apo A1). HDL is important for the synthesis of steroid hormones but it is better, for its protective role against cardiovascular disease.
A low HDL is thought to speed up the rate at which arteries fur up. HDL has 3 main benefits:-
- HDL, a positive anti-oxidant effect, helps protect blood cells and important chemical messengers in the blood from the break down.
- It removes excess cholesterol from the tissues and arteries and returns it back to the liver for recycling and removal from the body. The specialist also refers it as “reverse cholesterol transport”.
- Also helps protects the artery walls against LDL cholesterol (an anti-inflammatory effect).
Together these effects prevent arteries from furring up. Atherosclerosis is the term used to describe this process. It happens over many years and is caused by initial damage to the artery wall followed by fatty substances getting into the artery wall and causing fatty deposits, known as plaques.
Over time these fatty deposits can cause severe enough narrowing to reduce the flow of blood, or they can break up, causing sudden clots and blockages, resulting in heart attacks and strokes.
What is the ideal level?
HDL, “good” cholesterol because it carries cholesterol to the liver and body then eliminates it. High levels of HDL (especially over 60), believed to protect against heart disease, while low levels (less than 40) increase the risk of heart disease and possibly the risk of stroke.
Lipoproteins are complex chemicals that contain a core globule of fat surrounded by proteins – called apoproteins – that make them soluble in your body. Each class of lipoprotein contains different amounts and proportions of fats in the core, as well as specific apoproteins on their surface.
HDL, which stands for high-density lipoprotein, is the smallest and densest of the lipoproteins. The density of a lipoprotein depends on the relative amounts of lipid and protein in the particles. HDL is the densest lipoprotein because it contains a relatively low amount of fat compared to its protein content. HDL particles contain primarily cholesterol in their core and apoprotein A1 and A2 at their surface.
Desirable, good and very good levels:-
- Desirable — below 40 mg/dl or Below 1 mmol/L
- Good — 40 to 59 mg/dl or 1 to 1.5 mmol/L
- Very good — 60 mg/dl and above or above 1.5 mmol/L
Desirable, low levels by gender:-
- Optimal – more than 60 mg/dl – lower risk of heart disease.
- Low in men — less than 40 mg/dl – a higher risk factor for heart disease
- Low in women – less than 50 mg/dl – a higher risk factor for heart
What are the structure and functions of HDL?
HDL is a complex lipoprotein with a number of functions. One of its most essential function is known as reverse cholesterol transport. As cells die and replace, they release cholesterol into your blood. HDL binds the excess cholesterol and transfers it to other lipoproteins, such as LDL. It also gets rid of cholesterol in the walls of your arteries and takes it to the liver to remove from your body through the intestines.
HDL – “good” cholesterol because of its protective effect against atherosclerosis – the build-up of fatty plaques in your arteries. Higher HDL concentrations correlate with lower rates of atherosclerosis and a lower risk for heart disease and stroke. People who have naturally higher levels of HDL cholesterol – at lower risk of heart attacks and stroke.
However, it’s less clear whether that same benefit holds true for people who increase their HDL levels with medications. Interventions known to increase HDL have shown to lower the risk of heart attacks, like exercise, quitting smoking or improving the diet. However, medications that specifically increase HDL have failed to reduce the rate of heart attacks.
HDL levels – typically lower in people who have metabolic syndrome — a cluster of conditions that include obesity, increased blood pressure and high blood sugar levels:
- Stop smoking
- Limit animal fats (e.g. butter, cream, cheese, fried foods, Red meat)
- Eat more fiber (e.g. fruit, vegetables, cereals, baked beans), follow the DASH diet.
- Eat more fish
- Drink less alcohol
- Maintain a healthy weight
- Increase physical activity – aim for 30 minutes or more of moderate-intensity physical activity every day of the week
- Take medicine every day as directed by your doctor – medication can help reduce your cholesterol if it’s too high
HDL cholesterol is the lipoprotein, which carries the LDL cholesterol and the phospholipids from the blood to the liver and is known as the good cholesterol for all the right reasons. This is so because HDL promotes the uptake of the LDL cholesterol from the tissues, consisting of the vascular wall, and the Atheroma (cholesterol plaques deposited on the arterial walls) to the liver for excretion into the small intestines via the bile either directly as cholesterol or as bile acids.
Doctors call it as a reverse cholesterol transport and are indicative of the beneficial effect of HDL. HDL along with its protein and the cholesterol content has an inhibitive action on the oxidation of LDL cholesterol. Oxidized LDL is the main culprit that induces the formation of atherosclerosis. HDL prevents the LDL from getting oxidize, thereby preventing atherosclerosis.
Activation of the endothelium. The endothelium is the innermost lining of all of the blood vessels of the circulatory system. HDL helps in maintaining a healthy endothelium, which performs important functions including maintaining blood pressure, forming new blood vessels, preventing atherosclerosis and coagulation of blood.