When you have diabetes, your body loses its ability to move sugar from your bloodstream into your cells efficiently. Sugar builds up in your blood instead, and over time that excess sugar damages blood vessels, nerves, and organs throughout your body. About 40 million people in the U.S. have diabetes, roughly 1 in 8, and more than 1 in 4 of them don’t know it yet.
What happens next depends on which type you have, how long it goes unmanaged, and how well blood sugar levels are controlled. Here’s what’s actually going on inside your body at each stage.
Why Blood Sugar Rises in the First Place
Normally, when you eat, your body breaks carbohydrates down into glucose (sugar) and releases it into your bloodstream. Your pancreas then produces insulin, a hormone that acts like a key, unlocking your muscle and fat cells so glucose can enter and be used for energy. This process depends on specialized transporters sitting on the surface of your cells. Insulin triggers these transporters to move to the cell membrane and pull glucose inside.
In Type 1 diabetes, your immune system attacks and destroys the cells in your pancreas that make insulin. Your body produces no insulin at all, so glucose has no way to get into cells and accumulates in the blood instead. This is an autoimmune condition, and it typically appears in childhood or early adulthood.
In Type 2 diabetes, your pancreas still makes insulin, at least initially. But your cells stop responding to it properly. The signaling chain that moves those glucose transporters to the cell surface breaks down, so even though insulin is present, the door to your cells stays partially shut. Your pancreas compensates by producing more and more insulin, but eventually it can’t keep up. Blood sugar climbs and stays elevated.
The First Symptoms You Notice
High blood sugar triggers a chain reaction that produces the classic early symptoms. When there’s too much glucose in your blood, your kidneys try to filter it out. Glucose in the kidney fluid pulls extra water along with it through osmosis, which is why frequent urination is often the first sign. You’re losing more water than usual, so you feel intensely thirsty. You drink more, urinate more, and the cycle continues.
Because your cells aren’t getting the glucose they need for fuel, you may also feel exhausted, hungry even after eating, and notice unexplained weight loss. Blurred vision can occur when fluid shifts affect the shape of your eye’s lens. These symptoms can develop quickly in Type 1 (over days to weeks) or so gradually in Type 2 that you don’t notice them for months or years.
What Happens If Blood Sugar Spikes Dangerously
Diabetes can produce two acute emergencies, both of which require immediate medical attention.
Diabetic ketoacidosis (DKA) occurs most often in Type 1 diabetes. Without any insulin, your body can’t use glucose at all, so it starts burning fat for energy. That fat breakdown produces acids called ketones, which build up in the blood and make it dangerously acidic. Warning signs include fruity-smelling breath, nausea and vomiting, stomach pain, fast heavy breathing, and severe fatigue. Left untreated, DKA can be fatal within hours.
Hyperosmolar hyperglycemic state (HHS) is more common in Type 2 diabetes. Blood sugar climbs extremely high, sometimes above 600 mg/dL, causing severe dehydration. Unlike DKA, ketone levels stay relatively low, but the neurological effects are more pronounced: confusion, drowsiness, hallucinations, loss of vision, and in severe cases, coma. HHS develops more slowly than DKA, typically over days to weeks, which means it’s sometimes caught late.
How High Blood Sugar Damages Small Blood Vessels
The long-term damage from diabetes comes down to what chronically elevated blood sugar does to your blood vessels. Small blood vessels are hit first, and the organs that depend on them suffer the consequences.
Eyes (retinopathy): The tiny blood vessels in your retina weaken and leak, or new fragile vessels grow abnormally. Over time this can cause vision loss and is one of the leading causes of blindness in adults. The damage is gradual and often painless, which is why regular eye exams matter.
Kidneys (nephropathy): Your kidneys filter blood through millions of tiny clusters of blood vessels. High blood sugar causes the filtering membranes to thicken and the surrounding tissue to expand with accumulated protein. This progressively reduces the kidneys’ filtering capacity. Early on, small amounts of protein leak into your urine. Over years, kidney function can decline to the point of requiring dialysis.
Nerves (neuropathy): Excess sugar damages the small vessels that supply blood to your nerves, particularly in your feet and hands. This starts as tingling or numbness and can progress to pain, burning sensations, or a complete loss of feeling. Losing sensation in your feet is especially dangerous because you may not notice cuts, blisters, or pressure sores.
Heart Disease and Stroke Risk
Diabetes doesn’t just damage small vessels. It accelerates damage to larger arteries throughout your body. Adults with diabetes are nearly twice as likely to develop heart disease or have a stroke compared to adults without diabetes. High blood sugar promotes the buildup of fatty deposits inside artery walls, stiffens blood vessels, and increases inflammation. These changes reduce blood flow to the heart, brain, and legs. Many people with Type 2 diabetes also have high blood pressure and elevated cholesterol, which compound the vascular damage.
Why Wounds Heal Slowly
If you’ve heard that people with diabetes have trouble healing from cuts or injuries, there are several reasons working together. Nerve damage means you might not feel an injury when it happens, so it goes untreated longer. Reduced blood flow to the extremities means less oxygen and fewer immune cells reach the wound. And at the cellular level, diabetes creates a state of chronic low-grade inflammation in the skin. The body overproduces enzymes that break down the structural proteins needed to rebuild tissue, while simultaneously becoming resistant to the growth factors that drive healing.
This combination is why diabetic foot ulcers are a serious concern. They develop in roughly 15 to 25 percent of people with diabetes over their lifetime and are the leading cause of non-traumatic lower limb amputations. The same vascular problems that raise cardiovascular risk also contribute to poor wound healing in the feet.
How Diabetes Is Diagnosed
Diabetes is confirmed through blood tests that measure how much sugar is circulating. A diagnosis is made when any of these thresholds are met:
- A1C test: 6.5% or higher. This reflects your average blood sugar over the past two to three months.
- Fasting blood glucose: 126 mg/dL or higher after not eating for at least eight hours.
- Oral glucose tolerance test: 200 mg/dL or higher two hours after drinking a standardized sugar solution.
Results between normal and diabetic ranges indicate prediabetes, a stage where blood sugar is elevated but not yet high enough for a diabetes diagnosis. Prediabetes significantly increases your risk of progressing to Type 2 diabetes, but it’s also the stage where lifestyle changes have the most impact.
Managing Blood Sugar and the Possibility of Remission
Type 1 diabetes requires insulin replacement for life, since the body no longer produces any. People with Type 1 use insulin injections or an insulin pump and monitor blood sugar levels multiple times a day.
Type 2 diabetes is managed through a combination of dietary changes, physical activity, weight loss, and often medication. The goal is to keep blood sugar levels as close to normal as possible, which slows or prevents the vascular damage described above. Even modest improvements in blood sugar control reduce the risk of complications significantly.
For some people with Type 2 diabetes, remission is possible. Clinical guidelines define remission as achieving a fasting blood sugar below 126 mg/dL or an A1C below 6.5%, sustained for at least six months after stopping all blood sugar-lowering medications. Weight loss is the primary driver. Studies have shown that meaningful, sustained weight loss can restore the body’s ability to use insulin effectively and bring blood sugar back into a normal range. Remission is not a cure, though. The underlying tendency toward insulin resistance remains, and blood sugar can rise again if the weight is regained or metabolic conditions change.
What This Looks Like Day to Day
Living with diabetes means paying consistent attention to things most people take for granted. You learn to track how different foods affect your blood sugar, how exercise lowers it, and how stress or illness can spike it. You check your feet regularly for sores you might not feel. You keep up with eye exams, kidney function tests, and cardiovascular screenings because the damage from high blood sugar is silent long before symptoms appear.
The gap between well-managed and poorly managed diabetes is enormous. Uncontrolled blood sugar over years leads to the cascade of complications: vision loss, kidney failure, nerve damage, heart attacks, slow-healing wounds. Well-managed diabetes, where blood sugar stays close to target, dramatically reduces these risks and allows most people to live full, active lives without significant complications.