By Richard N. Fogoros, MD 

Updated on May 25, 2023

 Medically reviewed by Richard N. Fogoros, MD

Everywhere you turn, you are admonished to pay attention to your cholesterol levels, and to a lesser extent, your triglyceride levels. Cholesterol and triglycerides are two forms of lipid, or fat, that circulate in your bloodstream. They are both necessary for life itself.

Cholesterol is critical for building and maintaining key parts of your cells, such as your cell membranes, and for making several essential hormones — including the estrogens, progesterone, vitamin D, and steroids. Triglycerides, which are chains of high-energy fatty acids, provide much of the energy needed for your tissues to function. So you can't live without either of these types of lipids.¹

But when blood levels of cholesterol or triglycerides become too high, your risk of developing heart attack, stroke, and peripheral vascular disease is significantly increased. And this is why you need to be concerned about your lipid levels.²

John E. Kelly / Photolibrary / Getty Images

Overview

There are two sources for cholesterol and triglycerides — dietary sources and "endogenous" sources (manufactured within the body). Dietary cholesterol and triglycerides mainly come from eating meats and dairy products. These dietary lipids are absorbed through your gut and then are delivered through the bloodstream to your liver, where they are processed.³

One of the main jobs of the liver is to make sure all the tissues of your body receive all the cholesterol and triglycerides they need to function. Generally, for about eight hours after a meal, your liver takes up dietary cholesterol and triglycerides from the bloodstream. During times when dietary lipids are not available, your liver itself produces cholesterol and triglycerides. In fact, about 75% of the cholesterol in your body is manufactured by the liver.⁴

Your liver then places the cholesterol and triglycerides, along with special proteins, into tiny sphere-shaped packages called lipoproteins, which are released into the circulation. Cholesterol and triglycerides are removed from the lipoproteins and delivered to your body's cells, wherever they are needed.

Excess triglycerides — those that are not needed immediately for fuel — are stored in fat cells for later usage. It is important to know that many of the fatty acids stored in our bodies originated as dietary carbs. Because there is a limit to how many carbohydrates we can store in our bodies, any “extra” carbs we eat are converted to fatty acids, which are then packaged as triglycerides and stored as fat. (This explains why it is easy to become obese even on a low-fat diet.) The stored fatty acids are split from the triglycerides and burned as fuel during periods of fasting.⁵

Good and Bad Cholesterol

You will often hear doctors and dietitians talk about two different “types” of cholesterol — low-density lipoprotein (LDL) cholesterol (so-called “bad” cholesterol), and high-density lipoprotein (HDL) cholesterol (or “good” cholesterol). This way of talking about cholesterol is a convenient shorthand, but strictly speaking, it is not really correct.

Strictly speaking, as any good chemist will tell you, cholesterol is just cholesterol. One molecule of cholesterol is pretty much the same as another. So why do doctors talk about good and bad cholesterol? 

The answer has to do with lipoproteins.

Lipoproteins. Cholesterol (and triglycerides) are lipids, and therefore do not dissolve in a water medium like blood. In order for lipids to be transported in the bloodstream without clumping together, they need to be packaged into small particles called lipoproteins. Lipoproteins are soluble in blood, and allow cholesterol and triglycerides to be moved with ease through the bloodstream.⁶

The “behavior” of the various lipoproteins is determined by the specific kinds of proteins (called apolipoproteins) that appear on their surface. Lipoprotein metabolism is quite complex, and scientists are still working out all the details. However, most doctors concern themselves with two major types of lipoproteins: LDL and HDL. 

LDL Cholesterol — “Bad” Cholesterol. In most people, the majority of the cholesterol in the blood is packaged in LDL particles. LDL cholesterol is often called “bad” cholesterol. 

Elevated levels of LDL cholesterol have been strongly associated with an increased risk of heart attack and stroke. It is thought by many experts that when LDL cholesterol levels are too high, the LDL lipoprotein tends to stick to the lining of the blood vessels, which helps to stimulate atherosclerosis. So, an elevated LDL cholesterol level is a major risk factor for heart disease and stroke.⁷ 

While there is no question that elevated LDL cholesterol levels contribute strongly to cardiac risk, in recent years, experts have begun to question whether reducing LDL cholesterol levels itself necessarily reduces the risk. In particular, while lowering LDL cholesterol levels with statin drugs significantly reduces cardiac risk, reducing LDL cholesterol levels with most other kinds of drugs has not been shown definitely to do so. Current guidelines on treating cholesterol rely so strongly on the use of statins because they don't just lower cholesterol, but contribute to plaque stabilization and have possible anti-inflammatory effects.

"HDL Cholesterol — Good" Cholesterol. Higher blood levels of HDL cholesterol levels are associated with a lower risk of heart disease, and conversely, low HDL cholesterol levels are associated with an increased risk. For this reason, HDL cholesterol is commonly called "good" cholesterol.⁸

It appears that the HDL lipoprotein "scours" the walls of blood vessels and removes excess cholesterol. So the cholesterol present in HDL is, to a large extent, excess cholesterol that has just been removed from cells and blood vessel walls and is being transported back to the liver for recycling. The higher the HDL cholesterol levels, presumably, the more cholesterol is being removed from where it might otherwise cause damage.

In recent years, the notion that HDL cholesterol is always "good" has come under fire, and indeed, it now appears that the truth is a bit more complicated than simply "HDL = good cholesterol." Drug companies working hard to devise drugs for increasing HDL levels, for instance, so far have run into a brick wall. Several drugs that successfully raise HDL levels have failed to improve cardiac outcomes. Results like these are forcing experts to revise their thinking about HDL cholesterol. 

Causes of High Cholesterol

Elevated LDL cholesterol levels can be caused by several factors, including heredity conditions such as familial hypercholesterolemia. More commonly, elevated cholesterol levels are related to poor diet, obesity, sedentary lifestyle, age, smoking, and gender (pre-menopausal women have lower cholesterol levels than men).

Several medical conditions, including diabetes, hypothyroidism, liver disease, and chronic kidney failure can also increase cholesterol levels. Some drugs, especially steroids and progesterone, can do the same.¹

Triglycerides and Cardiac Risk

Many clinical studies have shown that having a high triglyceride blood level — a condition called hypertriglyceridemia — is also associated with a substantially elevated cardiovascular risk.² While this association is generally accepted by experts, it is not yet agreed that elevated triglyceride levels are a direct cause of atherosclerosis, as LDL cholesterol is thought to be. There is no generally accepted “triglyceride hypothesis.”

Still, there is no question that hypertriglyceridemia is strongly associated with elevated cardiovascular risk. Furthermore, high triglyceride levels are a prominent feature of several other conditions known to increase cardiac risk. These include obesity, sedentary lifestyle, smoking, hypothyroidism — and especially metabolic syndrome and type 2 diabetes. 

This latter relationship is particularly important. The insulin resistance that characterizes metabolic syndrome and type 2 diabetes produces an overall metabolic profile that tremendously increases cardiac risk. This unfavorable metabolic profile includes, in addition to hypertriglyceridemia, elevated CRP levels, high LDL cholesterol levels, and low HDL cholesterol levels. (In fact, there is usually a “see-saw” relationship between triglyceride and HDL cholesterol levels — the higher the one, the lower the other.) People with insulin resistance also tend to have hypertension and obesity. Their overall risk of heart disease and stroke is very high.²

Given the plethora of risk factors that usually accompany high triglyceride levels, it is understandable that researchers so far have been unable to tease out just how much of the elevated risk is directly caused by the hypertriglyceridemia itself.

Testing

Beginning at age 20, testing for cholesterol and triglycerides is recommended every five years. And if your lipid levels are found to be elevated, repeat testing should be done yearly.⁹

When to Seek Treatment

Deciding on whether you ought to be treated for high cholesterol or high triglyceride levels, whether that treatment ought to include drug therapy, and which drugs ought to be used is not always entirely straightforward. Still, if your cardiovascular risk is elevated, the right treatment aimed at your lipid levels can substantially reduce your chances of having a heart attack, or even of dying prematurely. So when it comes to treating cholesterol and triglycerides, it is important to get it right. You can read about current thinking on when and how treatment for blood lipids should be chosen.