Heart disease is caused by a combination of factors that damage your blood vessels over time, with the most common being high cholesterol, high blood pressure, smoking, diabetes, physical inactivity, and genetics. In 2023, cardiovascular disease killed over 919,000 people in the United States alone, making it the leading cause of death. Most of these deaths trace back to a slow, silent process happening inside your arteries that begins years or even decades before symptoms appear.
How Artery Damage Builds Over Time
The underlying cause of most heart disease is atherosclerosis, a gradual buildup of plaque in the walls of your arteries. Plaque is a sticky mix of fat, cholesterol, calcium, and other substances. As it accumulates, your artery walls grow thicker and harder, and the opening that blood flows through gets narrower. This process is sometimes called “hardening of the arteries,” and it’s usually completely silent in its early stages. You won’t feel it happening.
The trouble starts when the inner lining of an artery gets damaged. Once that lining is compromised, cholesterol particles can slip into the artery wall and trigger an inflammatory response. Your immune system sends white blood cells to clean up the cholesterol, but over time those cells become overwhelmed, die, and form a fatty core inside the vessel wall. A fibrous cap forms over this core, creating what we recognize as plaque. If that cap ruptures, a blood clot forms rapidly at the site, potentially blocking the artery entirely. That’s a heart attack.
Inflammation plays a role at every stage of this process, from the initial damage to the final rupture. Doctors can measure a blood marker called high-sensitivity CRP to gauge cardiovascular inflammation. Levels below 1 mg/L suggest low risk, 1 to 3 mg/L indicate moderate risk, and above 3 mg/L signals high risk for a future heart attack or stroke.
Cholesterol: The Central Player
LDL cholesterol is the primary material that builds up inside artery walls to form plaque. When you have too much LDL circulating in your blood, more of it penetrates the artery lining and fuels the process described above. HDL cholesterol works in the opposite direction. It picks up excess cholesterol from the blood and carries it back to the liver, where it’s flushed from the body. Higher HDL levels are protective.
Triglycerides, a type of fat your body uses for energy, add another layer of risk. High triglycerides combined with low HDL or high LDL create a particularly dangerous mix that significantly increases your chance of a heart attack. This combination is common in people who eat diets high in refined carbohydrates and sugar, drink heavily, or carry excess weight around the midsection.
High Blood Pressure
Blood pressure is the force of blood pushing against your artery walls with each heartbeat. When that pressure stays elevated over months and years, it physically damages the delicate inner lining of your arteries. Think of it like water running through a garden hose at too high a pressure: the walls take a beating. Those damaged spots become entry points for LDL cholesterol and inflammatory cells, accelerating plaque formation. High blood pressure also forces your heart to work harder to pump blood, which over time can cause the heart muscle itself to thicken and stiffen, eventually leading to heart failure.
Diabetes and Blood Sugar
People with type 2 diabetes face a dramatically higher risk of heart disease, and the reasons go beyond just blood sugar. Chronically elevated glucose triggers a cascade of damage inside blood vessels. It increases oxidative stress, meaning the body produces too many harmful molecules that directly damage proteins, fats, and DNA in vessel walls. This oxidative stress also triggers inflammation, creating a self-reinforcing cycle of damage.
High blood sugar also produces compounds called advanced glycation end products, which physically stiffen blood vessel walls and heart tissue, making them less flexible and harder for blood to flow through. Autopsy studies of diabetic patients show reduced numbers of small blood vessels in the heart, thickened vessel walls, and increased scarring of heart tissue. The heart muscle itself tends to enlarge in people with diabetes, a structural change that impairs its ability to pump efficiently. These changes explain why heart disease is the leading cause of death in people with diabetes, not the blood sugar itself.
Smoking
Cigarette smoke delivers a toxic mix of more than 7,000 chemicals into your bloodstream with every puff. These chemicals damage the inner lining of your arteries, trigger inflammation, and make your blood thicker and more prone to clotting. That combination is especially dangerous: damaged arteries accumulate plaque faster, and thicker blood is more likely to form a clot at a plaque site. When blood cells can’t move easily through narrowed, plaque-filled arteries, the result can be a heart attack or stroke. Smoking is one of the most preventable causes of heart disease, and the damage begins to reverse within weeks of quitting.
Physical Inactivity
Sitting for long stretches is an independent risk factor for heart disease, even if you’re otherwise healthy. A large study published in the Journal of the American College of Cardiology found that the risk of heart failure, heart attack, irregular heartbeat, and cardiovascular death all climb once you spend more than 10.6 hours per day sitting. That threshold matters: people who crossed it faced the steepest increases in risk.
The good news is that even small changes help. Replacing just 30 minutes of sitting (in people who sit more than 10.6 hours daily) with any kind of movement reduced the risk of heart failure by 7% and cardiovascular death by 8%. Swapping that time for moderate-to-vigorous exercise, like brisk walking or cycling, cut heart failure risk by 15% and cardiovascular death by 10%. Even light activity like casual walking or household chores reduced cardiovascular death risk by 9%.
One important caveat: people who sat for long periods still had increased cardiovascular risk even if they met the standard guideline of 150 minutes of exercise per week. Exercise helps, but it doesn’t fully cancel out prolonged sitting. Breaking up long periods of inactivity throughout the day matters on its own.
Genetics and Family History
Some people inherit a higher risk of heart disease regardless of their lifestyle. One of the clearest genetic risk factors is a blood particle called lipoprotein(a), or Lp(a). Unlike regular LDL cholesterol, Lp(a) levels are almost entirely determined by your genes and don’t change much with diet or exercise. Levels above 50 mg/dL are considered high, and they’re surprisingly common.
Lp(a) is dangerous for three reasons. Like LDL, it builds up in artery walls to form plaque. But it also promotes blood clotting, which can rapidly block a narrowed artery. And it drives inflammation, making existing plaques more likely to rupture. Chronic Lp(a)-driven inflammation can even cause calcium to build up on the aortic valve, stiffening it and restricting blood flow from the heart. If you have a strong family history of heart attacks, particularly at a young age, Lp(a) testing can reveal a hidden risk that standard cholesterol panels miss.
Sleep Apnea
Obstructive sleep apnea, a condition where your airway repeatedly collapses during sleep, is a significant and often overlooked cause of heart disease. Each time the airway closes, your blood oxygen level drops and your body floods with stress hormones to jolt you awake enough to breathe again. This cycle can repeat dozens or even hundreds of times per night.
The repeated drops in oxygen and spikes in stress hormones damage the cardiovascular system in multiple ways. They increase blood pressure, promote inflammation and oxidative stress in artery walls, and accelerate plaque buildup. The dramatic swings in pressure inside the chest during obstructed breathing can physically stretch and remodel the heart’s chambers over time, promoting irregular heart rhythms and eventually heart failure. Sleep apnea also increases the risk of pulmonary hypertension, where blood vessels in the lungs stiffen from chronic oxygen deprivation, forcing the right side of the heart to work harder until it begins to fail.
How These Causes Work Together
Heart disease rarely has a single cause. These risk factors amplify each other. Diabetes raises blood pressure. Inactivity worsens cholesterol levels and blood sugar control. Smoking accelerates the damage from high blood pressure. Obesity contributes to sleep apnea, which raises blood pressure further. A person with three moderate risk factors can face a higher total risk than someone with one severe one, because the biological pathways of damage overlap and compound.
This also means that addressing even one or two factors can have outsized benefits. Quitting smoking, moving more throughout the day, and managing blood pressure and blood sugar don’t just reduce their individual risks. They slow the shared underlying process of artery damage and inflammation that drives nearly all heart disease.