Low density lipoprotein (LDL) is a particle in your blood that carries cholesterol from the liver to cells throughout your body. It’s often called “bad cholesterol,” but that label is misleading. LDL itself serves a necessary biological function. The problems start when too much of it circulates in your bloodstream, where it can build up inside artery walls and raise your risk of heart disease and stroke.
What LDL Actually Is
LDL isn’t pure cholesterol. It’s a tiny sphere made up of fats (including cholesterol and triglycerides) wrapped in a shell of proteins and other lipids. The key protein on its surface is called apolipoprotein B-100, or apoB for short. Each LDL particle carries exactly one apoB molecule, which acts like an address label, allowing the particle to dock with receptors on your cells.
When an LDL particle reaches a cell that needs cholesterol, apoB binds to a receptor on the cell’s surface. The cell then pulls the entire particle inside, strips out the cholesterol, and uses it. Cholesterol is essential for building cell membranes, producing hormones, and making vitamin D. Every cell in your body needs it, and LDL is one of the main delivery vehicles.
How Your Body Clears LDL
Your liver is responsible for pulling most LDL out of the bloodstream. It does this through LDL receptors on its surface. When an LDL particle binds to a receptor, both are drawn inside the cell. In the acidic environment inside the cell, the receptor changes shape, releases the LDL particle for breakdown, and then recycles back to the surface to grab another one. This recycling process is what keeps your LDL levels in check.
A protein called PCSK9 interferes with this recycling. When PCSK9 binds to an LDL receptor, it locks the receptor in place so it can’t release and recycle. Instead, both the receptor and the LDL particle are destroyed. The result: fewer receptors on the liver’s surface and more LDL left circulating in the blood. People who naturally produce more PCSK9 tend to have higher LDL levels, while people with less PCSK9 activity tend to have lower levels. This mechanism is the basis for a newer class of cholesterol-lowering medications.
When LDL Becomes Dangerous
LDL particles are small enough to slip through the lining of your arteries, especially in areas where blood flow creates turbulence, like at branch points. Once inside the artery wall, LDL can become oxidized, a chemical change triggered by free radicals and enzymes in the tissue. Oxidized LDL is what really drives damage.
Your immune system treats oxidized LDL as a threat. White blood cells called macrophages rush in and swallow the modified particles. But macrophages aren’t designed to handle large amounts of oxidized cholesterol. They become engorged, turning into what researchers call foam cells. These foam cells accumulate in the artery wall, forming the fatty streaks that are the earliest stage of plaque.
Over time, foam cells release inflammatory signals, enzymes that weaken the surrounding tissue, and growth factors that cause the plaque to expand. Some plaques develop a thick, stable cap and narrow the artery gradually. Others become unstable, with a thin cap that can rupture suddenly, triggering a blood clot that blocks the artery. This is how heart attacks and many strokes happen. Research published in the American Heart Association’s journals has shown that plaques rich in macrophages and oxidized LDL are more prone to this dangerous instability.
Not All LDL Particles Are the Same
LDL particles come in a range of sizes, roughly 22 to 28.5 nanometers in diameter. Researchers have grouped them into two broad patterns: Pattern A (larger, more buoyant particles above 25.5 nm) and Pattern B (smaller, denser particles at 25.5 nm or below).
Small, dense LDL particles have several properties that make them particularly concerning. They stay in the bloodstream longer, are more easily oxidized, bind more readily to artery walls, and slip through the artery lining more efficiently. That said, large LDL particles are not harmless. People with familial hypercholesterolemia, a genetic condition that causes very high LDL, typically have large, cholesterol-rich particles, and they develop premature heart disease at high rates. The overall conclusion from research is that LDL concentration matters more than particle size when assessing risk. Measuring particle size doesn’t add much predictive value beyond knowing your standard LDL number and other risk factors.
Healthy LDL Levels
LDL cholesterol is measured in milligrams per deciliter (mg/dL) of blood. General guidelines from MedlinePlus list the following healthy targets:
- Adults age 20 and older: less than 100 mg/dL
- Children and teens (age 19 or younger): less than 110 mg/dL
These are population-level guidelines. Your personal target may be lower if you have existing heart disease, diabetes, or other risk factors. Some people are treated to targets well below 70 mg/dL.
LDL is typically not measured directly in routine blood work. Instead, it’s calculated using a formula that takes your total cholesterol, HDL cholesterol, and triglycerides and works backward. This calculation, known as the Friedewald equation, loses accuracy in two common situations: when triglycerides are elevated (150 mg/dL or higher) and when LDL is already low (below 100 mg/dL). In one analysis, among patients with high triglycerides whose calculated LDL appeared to be below 70 mg/dL, up to 41% actually had levels at or above 70 when measured with a more precise method. If your triglycerides tend to run high, a direct LDL measurement may give a more reliable number.
What Raises LDL Levels
Several factors push LDL higher, some within your control and some not.
Diet plays a direct role. Eating large amounts of saturated fat increases LDL cholesterol, total cholesterol, and HDL cholesterol. Research from the American Heart Association found that overfeeding saturated fat not only raised LDL particle counts across multiple size categories but also changed the composition of those particles, enriching them with compounds like sphingomyelins and ceramides that make LDL more prone to clumping together. This increased aggregation susceptibility may make the particles more harmful once they enter the artery wall.
Genetics are equally important. Your genes determine how many LDL receptors your liver produces, how efficiently those receptors work, and how much PCSK9 your body makes. Some people eat carefully and exercise regularly but still have high LDL because their genetic makeup limits clearance. Familial hypercholesterolemia, the most well-known genetic cause, affects roughly 1 in 250 people and can push LDL well above 190 mg/dL from a young age.
Other factors that raise LDL include carrying excess body weight (particularly around the midsection), physical inactivity, smoking, and certain medical conditions like hypothyroidism and kidney disease. Age also matters: LDL tends to rise as you get older, partly because LDL receptor activity on liver cells declines over time.
How LDL Differs From Other Lipoproteins
Your cholesterol panel includes several related particles, and understanding how they differ helps make sense of your results. HDL (high density lipoprotein) works in the opposite direction from LDL, picking up excess cholesterol from tissues and returning it to the liver for disposal. Higher HDL levels are associated with lower cardiovascular risk. VLDL (very low density lipoprotein) is a larger, triglyceride-rich particle produced by the liver. As VLDL loses triglycerides in the bloodstream, it gradually shrinks and becomes LDL. This is why high triglyceride levels and high LDL levels often travel together.
Total cholesterol on your blood test is the sum of cholesterol carried by all these particles. It’s a useful screening number, but your LDL level is the more specific measure that drives most treatment decisions. When your doctor focuses on one number from your lipid panel, it’s almost always LDL.