How are diabetes and cardiovascular disease connected?
Diabetes and cardiovascular disease are often treated as two separate health problems. In reality they are deeply connected — so much so that having diabetes is one of the strongest predictors of cardiovascular disease we know of. Cardiovascular disease means disease of the heart and blood vessels — including heart attack, stroke and poor circulation to the limbs, not just the heart itself. It's often called heart disease, though strictly that term refers to the heart alone. Understanding how diabetes drives cardiovascular disease reveals something important: managing blood glucose, while essential, is not on its own enough to protect against cardiovascular disease.
How big is the risk?
People with diabetes have roughly double the risk of cardiovascular disease compared with people of the same age who don't have diabetes. That includes heart attack, stroke, heart failure and reduced blood flow to the legs. The increase holds even after accounting for other factors such as blood pressure and cholesterol, which means diabetes is contributing something over and above those familiar risks.
Cardiovascular disease is a leading cause of death and disability in people with diabetes. And, for type 2 diabetes, the risk doesn't switch on at the moment of diagnosis — it begins to build during the years of insulin resistance that typically precede it, when blood glucose may still look close to normal. People with type 1 diabetes also carry an elevated cardiovascular risk, though the reasons are not identical.
It's not just high blood glucose
The most common assumption is that diabetes harms the heart and blood vessels simply because blood glucose is too high. High glucose does cause damage — but it is only part of the story.
Type 2 diabetes rarely travels alone. It usually sits within a cluster of metabolic disturbances, each of which independently places strain on the heart and blood vessels:
- Insulin resistance — in which the body's cells respond poorly to insulin. This has effects throughout the body well beyond glucose. In type 2 diabetes, this typically precedes the onset of diabetes by a number of years, while in type 1 diabetes, it often develops over time after diabetes develops.
- An unfavourable cholesterol profile — often raised triglycerides, low "good" HDL cholesterol, and LDL particles that are smaller and denser, making them more likely to lodge in artery walls.
- High blood pressure — which frequently accompanies insulin resistance and adds mechanical stress to the arteries.
- Chronic low-grade inflammation — a persistent, smouldering inflammatory state that accelerates the build-up of plaque.
- Endothelial dysfunction — the delicate lining of the blood vessels loses its ability to regulate blood flow and resist clot and plaque formation.
- A more clot-prone state — the blood itself becomes more likely to form clots.
Central weight gain and fatty liver disease often sit within this same cluster. The key point is that by the time glucose is high enough to diagnose type 2 diabetes, the cardiovascular system has usually been under pressure from these other factors for years.
Because so much of this is shared ground, the connection appears to run in both directions. Just as diabetes raises the risk of cardiovascular disease, some cardiovascular conditions appear to raise the risk of later developing type 2 diabetes — the clearest example being heart failure. This isn't necessarily one disease directly causing the other. More often it reflects the same underlying metabolic disturbance surfacing in different ways. It's a useful reminder that type 2 diabetes and cardiovascular disease are best thought of as two expressions of one shared process, rather than as separate problems that happen to occur together.
Two sizes of blood vessel, two different stories
Diabetes damages blood vessels of two very different sizes, and the story differs for each.
Small vessels (microvascular disease)
Diabetes affects small, microscopic blood vessels, with the eyes, kidneys and nerves being particularly susceptible to such damage, leading to the complications many people associate with diabetes — vision problems, kidney disease and nerve damage. This kind of damage is strongly driven by how much glucose the vessels are exposed to over time.
Large vessels (macrovascular disease)
These are the major arteries of the body. Damage here causes heart attack, stroke and poor circulation to the legs and feet. This is driven not by glucose alone, but by the whole metabolic cluster described above.
This distinction — small vessels shaped mainly by glucose, large vessels shaped by the wider picture — is the hinge for understanding why glucose control behaves the way it does.
Why controlling glucose alone doesn't fix cardiovascular risk
If high glucose causes the damage, it seems logical that lowering glucose should undo it. For the small vessels, that logic largely holds: good glucose control clearly protects the eyes, kidneys and nerves.
For the large vessels, the picture is more complicated. Major clinical trials that pushed blood glucose down intensively succeeded in reducing small-vessel complications — but they did not deliver the expected fall in heart attacks and strokes.
The reason follows from the biology. Large-vessel risk is driven by the entire metabolic cluster — blood pressure, cholesterol, inflammation and the rest — of which glucose is only one contributor. Lowering glucose while the other factors remain elevated leaves most of that risk untouched.
Two things are worth keeping in mind so this isn't misread. First, glucose control still matters enormously: it protects the small vessels, and getting control right early and maintaining it appears to carry long-term cardiovascular benefit, in both type 1 and type 2 diabetes. Second, the lesson is not that glucose is irrelevant to cardiovascular health — it's that lowering glucose on its own is not enough to protect the heart and blood vessels.
This is why modern diabetes care treats cardiovascular risk as something to be managed across several fronts at once — glucose, blood pressure, cholesterol, weight, kidney function and smoking — rather than through glucose alone. It's also why certain newer classes of glucose-lowering medication, notably the SGLT2 inhibitors and GLP-1 receptor agonists, have changed the field: they reduce cardiovascular events in ways that go beyond their effect on blood glucose. The point is not the specific drugs, but what they demonstrate — that protecting the heart and blood vessels in diabetes means addressing the whole cardiometabolic picture.