Type 2 diabetes raises blood sugar levels and, over time, damages blood vessels, nerves, and organs throughout the body. It starts with a problem called insulin resistance, where your cells stop responding normally to insulin, the hormone that moves sugar from your blood into your tissues for energy. As the disease progresses, your pancreas struggles to keep up with the extra demand for insulin, and blood sugar climbs higher. That sustained high blood sugar is what drives the complications most people worry about: damage to the eyes, kidneys, heart, and nerves.
How Insulin Resistance Works
Insulin acts like a key that unlocks your cells so glucose can enter. In healthy muscle cells, insulin triggers a chain of signals that sends glucose transporter proteins (called GLUT4) to the cell surface, where they pull sugar inside. In type 2 diabetes, that signaling chain gets disrupted. Fatty molecules that build up inside cells, particularly one called diacylglycerol and another called ceramide, interfere with the signal at multiple points. The result: fewer transporters reach the cell surface, and less glucose gets in.
This doesn’t just affect muscle. Insulin normally tells your liver to stop making new glucose when blood sugar is already adequate. In people with type 2 diabetes, the liver ignores that signal. Research published in the Journal of Clinical Investigation found that glucose production from the liver was about 25% higher in people with type 2 diabetes compared to controls, and that the liver’s glucose-manufacturing process accounted for 88% of total glucose output versus 70% in people without diabetes. So your muscles are taking in less sugar while your liver is pumping out more of it. Blood sugar rises from both directions.
What Happens to Your Pancreas
Your pancreas doesn’t fail all at once. In the early stages of insulin resistance, often years before a diabetes diagnosis, the insulin-producing beta cells compensate by working harder and churning out extra insulin. For a while, this keeps blood sugar in a normal range. But that overproduction takes a toll.
The constant demand to manufacture large amounts of insulin strains the cell’s internal machinery. Proteins start to misfold, triggering stress responses inside the cell. The tiny energy factories within each beta cell (mitochondria) also become overworked, producing harmful byproducts called reactive oxygen species that further damage the cell. Over time, beta cells begin to burn out. Some die outright, but many actually revert to a more primitive state, losing their ability to produce insulin without technically dying. Others get reprogrammed into different cell types that produce the wrong hormones entirely. The net effect is the same: your pancreas makes less and less insulin, and blood sugar control deteriorates.
Why High Blood Sugar Causes Symptoms
The classic symptoms of type 2 diabetes, frequent urination, intense thirst, and fatigue, all trace back to excess glucose in the blood. When blood sugar gets high enough, your kidneys can’t reabsorb all of it. The extra glucose spills into your urine, and through a process called osmotic diuresis, it pulls water along with it. You urinate more, lose fluid, and get dehydrated. That dehydration triggers thirst. Meanwhile, because glucose isn’t getting into your cells efficiently, your tissues are starved for fuel even though your blood is saturated with sugar. That’s the fatigue.
Many people with type 2 diabetes, though, have no obvious symptoms for years. Blood sugar can be elevated enough to cause slow, silent damage long before it’s high enough to make you feel noticeably different.
Damage to Blood Vessels
Persistently high blood sugar damages the lining of blood vessels, both the tiny capillaries that feed your eyes and kidneys and the larger arteries that supply your heart and brain. Chronic hyperglycemia is an independent risk factor for coronary heart disease, separate from other factors like cholesterol or blood pressure.
The damage happens partly because high glucose impairs the blood vessel lining’s ability to produce nitric oxide, a molecule that keeps vessels relaxed and open. At the same time, insulin resistance shifts the balance toward producing more of a compound that constricts blood vessels. With high insulin levels (common in the earlier stages of type 2 diabetes), that constricting signal gets amplified further. The combination of less relaxation and more constriction raises blood pressure and accelerates the buildup of plaque in artery walls. This is why type 2 diabetes roughly doubles the risk of heart attack and stroke.
What It Does to Your Eyes
The retina, the light-sensitive tissue at the back of your eye, is fed by an intricate network of tiny blood vessels. High blood sugar damages these vessels over time, causing them to leak blood and fluid into the retina. This early stage can blur your vision or cause no symptoms at all.
As the damage progresses, the retina becomes starved for blood flow. In response, the body grows new blood vessels on the retina’s surface to try to restore supply. These replacement vessels are fragile and poorly formed. They leak blood and fluid into the vitreous, the gel-like substance that fills the eye, which can cause sudden vision changes, floaters, or in severe cases, retinal detachment and blindness. Diabetic retinopathy is the leading cause of new blindness in working-age adults, but it’s largely preventable with good blood sugar control and regular eye exams.
Kidney Damage Over Time
Each kidney contains about one million tiny filtering units called glomeruli. Each glomerulus is a bundle of microscopic blood vessels that sift waste from the blood while keeping useful proteins and cells in circulation. High blood sugar and high blood pressure (which often accompany diabetes) damage these delicate capillaries, causing scarring that gradually reduces the kidney’s filtering ability.
Early on, the damage shows up as small amounts of protein leaking into urine, something you wouldn’t notice but that a lab test can detect. As more filtering units scar over, kidney function declines. Without intervention, this can progress to kidney failure requiring dialysis or transplant. About one in three people with type 2 diabetes develops some degree of kidney disease.
Slow Wound Healing
If you’ve heard that people with diabetes heal slowly from cuts and injuries, there’s a specific biological reason. Normal wound healing moves through phases: first inflammation to clear debris and fight infection, then a rebuilding phase where new skin and blood vessels form. In type 2 diabetes, wounds get stuck in the inflammatory phase and can’t transition to repair.
High blood sugar reprograms immune cells called macrophages so they stay locked in an aggressive, inflammation-promoting state. These overactive macrophages keep pumping out inflammatory signals long after they should have switched to a cleanup-and-rebuild mode. At the same time, the skin cells responsible for closing a wound (keratinocytes) migrate more slowly in a high-glucose environment because of changes in how certain genes are expressed. New blood vessel growth into the wound is also reduced, starving the healing tissue of oxygen and nutrients. This is why diabetic foot ulcers are so common and so dangerous, affecting roughly 15 to 25% of people with diabetes at some point in their lives.
How Type 2 Diabetes Is Diagnosed
Diagnosis relies on blood tests that measure how much sugar is in your blood. The most common is the A1C test, which reflects your average blood sugar over the past two to three months. An A1C below 5.7% is normal. Between 5.7% and 6.4% is considered prediabetes, a warning stage where insulin resistance is present but hasn’t fully progressed. An A1C of 6.5% or higher on two separate tests confirms diabetes.
A fasting blood sugar test (taken after at least eight hours without eating) offers a snapshot rather than an average. Normal is below 100 mg/dL, prediabetes falls between 100 and 125 mg/dL, and 126 mg/dL or above indicates diabetes. Many people are diagnosed during routine bloodwork before they ever experience symptoms, which is why regular screening matters, especially after age 35 or with risk factors like obesity, family history, or a sedentary lifestyle.
What Makes It Progressive
Type 2 diabetes tends to worsen over time if nothing changes, but “progressive” doesn’t mean “inevitable.” The disease advances because each element feeds the others. Insulin resistance forces the pancreas to overproduce insulin, which eventually exhausts beta cells, which raises blood sugar further, which damages blood vessels, which worsens insulin delivery to tissues. It’s a cycle.
Breaking the cycle at any point helps. Weight loss of even 5 to 10% of body weight can meaningfully improve insulin sensitivity. Exercise has a direct effect on muscle cells’ ability to take up glucose, partly by activating glucose transporters through pathways that don’t even require insulin. For many people, these changes can lower A1C enough to reduce or delay complications. Some people with early type 2 diabetes who make substantial lifestyle changes achieve blood sugar levels in the normal range and stay there, a state sometimes called remission. The disease hasn’t disappeared, but its destructive effects are paused.