6.6.1: Understanding Blood Cholesterol

Understanding Blood Cholesterol

You may have heard of the abbreviations LDL and HDL with respect to heart health. These abbreviations refer to low-density lipoprotein (LDL) and high-density lipoprotein (HDL), respectively. Lipoproteins are characterized by size, density, and composition. As the size of the lipoprotein increases, the density decreases. Why are they referred to as “good” and “bad” cholesterol? What should you know about these lipoproteins?

Major Lipoproteins

Recall that chylomicrons are transporters of fats throughout the watery environment within the body. After about ten hours of circulating throughout the body, chylomicrons gradually release their triacylglycerols until all that is left of their composition is cholesterol-rich remnants. The liver uses these remnants as raw materials to formulate specific lipoproteins. Following is a list of the various lipoproteins and their functions:

• VLDLs. Very low-density lipoproteins are made in the liver from remnants of chylomicrons and transport triacylglycerols from the liver to various tissues in the body. As the VLDLs travel through the circulatory system, the lipoprotein lipase strips the VLDL of triacylglycerols. As triacylglycerol removal persists, the VLDLs become intermediate-density lipoproteins.
• IDLs. Intermediate-density lipoproteins transport a variety of fats and cholesterol in the bloodstream and are a little under half triacylglycerol in composition. While traveling in the bloodstream, cholesterol is gained from other lipoproteins while circulating enzymes strip its phospholipid component. When IDLs return to the liver, they are transformed into low-density lipoproteins.
• LDLs. As low-density lipoproteins are commonly known as the “bad cholesterol," it is imperative that we understand their function in the body so we can make healthy dietary and lifestyle choices. LDLs, which carry cholesterol and other lipids from the liver to tissue throughout the body, consist of very small amounts of triacylglycerols and house over 50 percent cholesterol and cholesterol esters. How does the body receive the lipids contained therein? As the LDLs deliver cholesterol and other lipids to the cells, each cell’s surface has receptor systems specifically designed to bind with LDLs. Circulating LDLs in the bloodstream bind to these LDL receptors and are consumed. Once inside the cell, the LDL is removed, and cholesterol is released. In liver cells, these receptor systems aid in controlling blood cholesterol levels as they bind the LDLs. A deficiency of these LDL binding mechanisms will leave a high quantity of cholesterol traveling in the bloodstream, which can lead to heart disease or atherosclerosis. Thus, LDL receptors are critical for regulating cholesterol levels. An elevated LDL cholesterol level is a primary cause of atherosclerosis.¹ Lipoprotein a (Lp(a)) is an LDL-cholesterol-like lipoprotein that increases heart disease risk. It is considered to have a genetic component. Research is ongoing to consider possible therapies for elevated Lp(a) levels.²
• HDLs. High-density lipoproteins carry cholesterol out of the bloodstream and into the liver, where it is either reused or removed from the body with bile. HDLs have a very large protein composition and low cholesterol content (20-30%) compared with other lipoproteins. Thus, these high-density lipoproteins are commonly called “good cholesterol.”

Contrasting LDL and HDL

Heart attack and atherosclerosis are conditions often caused by cholesterol that has accumulated and thickened in the walls of arteries. HDLs and LDLs are directly connected to these life-threatening ailments. By comparing and contrasting the roles each of these lipoproteins serves in the health of heart and blood vessels, you will be able to construct and evaluate a plan of action for your personal health. Consider the following lipoprotein facts:

• LDL/HDL composition. LDL is approximately 25% protein and 75% cholesterol and other fats. LDL is bigger (yet lighter) and richer in cholesterol than HDL. HDL is 50% protein and 50% cholesterol and other fats. HDL is smaller, more dense, and richer in protein.
• LDL/HDL function. LDLs carry cholesterol into cells for normal usage, but LDLs can also deposit cholesterol into the walls of blood vessels, which can lead to harmful disease. HDLs scavenge excess cholesterol from the cells, tissues, and blood vessels and deliver these back to the liver, where they are either reused or excreted.
• LDL/HDL and inflammation. LDLs carry pro-inflammatory lipids that may contribute to heart disease. HDLs transport anti-inflammatory lipids that may reduce the occurrence of heart disease.
• LDL/HDL warnings. High LDL values warn of increased health risks for heart disease, while high HDL values indicate a reduced risk for heart disease.
• Oxidized LDL. LDLs become more dangerous when oxidized. Oxidation is defined as the loss of electrons between two substances via a chemical reaction. If an LDL oxidation occurs, the oxidized LDL is left unstable. Oxidized LDL can speed up the process of plaque formation in the arteries. It is believed to hasten the deposition of cholesterol into the arterial walls and to induce a chronic inflammatory effect throughout the body’s vast network of vessels. This activity promotes atherosclerosis and significantly increases the risk of heart attack or stroke.

Testing Your Lipid Profile

Testing your lipid levels can provide the foreknowledge necessary to take action to help prevent or treat cardiovascular disease. Talk to your healthcare provider about these tests. Current guidelines recommend testing as follows:³

• Age 19 or younger: Screen at ages 9 to 11 and repeat every 5 years. If there is a family history of high blood cholesterol, heart attack, or stroke, screening may be as early as age 2.
• Age 20 to 65: Screen every 5 years. Men aged 45 to 65 and women ages 55 to 65 should be screened every 1 to 2 years.
• Older than 65: Screen yearly.
• Testing requires a blood sample to be drawn after 9 to 12 hours of fasting for an accurate reading. (By this time, most of the fats ingested from the previous meal have circulated through the body, and the concentration of lipoproteins in the blood will have stabilized.)

Blood Cholesterol Recommendations

According to the National Institutes of Health (NIH), the following total cholesterol values are used to target treatment.⁴

Desirable cholesterol numbers for adults

• Total cholesterol less than 200 mg/dL
• LDL cholesterol less than 100 mg/dL
• HDL cholesterol greater than or equal to 60 mg/dL
• Triglycerides less than 150 mg/dL

Note: Cholesterol levels are measured in milligrams (mg) of cholesterol per deciliter (dL) of blood.

For healthy total blood cholesterol, the desired range you would want to maintain is under 200 mg/dL. More specifically, for individual lipid profiles, a low amount of LDL and a high amount of HDL prevents excess buildup of cholesterol in the arteries and wards off potential health hazards. An LDL level of less than 100 milligrams per deciliter is ideal, whereas an LDL level above 160 milligrams per deciliter would be considered high. In contrast, a low value of HDL is a sign that a person is not benefiting from the protection the HDL can offer. Values of less than 40 milligrams per deciliter for men and 50 milligrams per deciliter for women mark a risk factor for developing heart disease. In short, elevated LDL blood lipid profiles indicate an increased risk of heart attack, whereas elevated HDL blood lipid profiles indicate a reduced risk.

Figure \(\PageIndex{1}\): A 4-ml sample of hyperlipidemic blood with lipids separated into the top fraction (sample is in an EDTA collection tube.). This is a standard EDTA (cell count) tube, as may be used to obtain a full blood count or iron studies. The tube was set upright for four hours and was not centrifuged. This sample was not analyzed; however, a similar sample showed a serum triglyceride of over 130mmol/L (over 5000mg/dl) and an LDL over 40mmol/L (over 1540mg/dl). "Hyperlipidaemia - lipid in EDTA tube" by Mark-shea is in the Public Domain.

Attributions:

Zimmerman, "An Introduction to Nutrition (Zimmerman)", CC BY-NC-SA 3.0. Text was updated. References added and updated.

References

1. Virani S, Newby LK, Arnold SV, et al. 2023 AHA/ACC/ACCP/ASPC/NLA/PCNA Guideline for the management of patients With chronic coronary disease: a report of the American Heart Association/American College of Cardiology Joint Committee on Clinical Practice Guidelines. J Am Coll Cardiol. 2023;82(9):833-955. doi:10.1016/j.jacc.2023.04.003.
2. About Lipoprotein (a). Centers for Disease Control and Prevention. Updated May 15, 2024. Accessed August 4, 2023. https://www.cdc.gov/heart-disease-family-history/about/about-lipoprotein-a.html?CDC_AAref_Val=https://www.cdc.gov/genomics/disease/lipoprotein_a.htm.
3. Blood Cholesterol Diagnosis. NIH National Heart, Lung and Blood Institute. Updated March 24, 2022. Accessed August 4, 2023. https://www.nhlbi.nih.gov/health/blo...erol/diagnosis.
4. Cholesterol & Your Heart: What You Need to Know Fact Sheet. National Heart, Lung, and Blood Institute, National Institutes of Health. Published September 2022. Accessed August 5, 2023. https://www.nhlbi.nih.gov/resources/...now-fact-sheet.