When your blood sugar climbs too high, your body sends increasingly urgent signals that something is wrong. Early on, you might notice excessive thirst, frequent urination, and blurred vision. Many people with diabetes don’t feel symptoms until their blood sugar reaches 250 mg/dL or higher, while people who haven’t been diagnosed tend to notice problems at lower levels. Left unchecked, high blood sugar can progress from uncomfortable to dangerous within hours, and over months or years, it quietly damages blood vessels, nerves, kidneys, and the brain.
The First Warning Signs
The earliest symptoms of high blood sugar are your body’s attempt to flush out the excess glucose. Your kidneys work overtime to filter it from your blood, pulling extra water along with it. That’s why you urinate more frequently, which in turn makes you intensely thirsty. You may also feel hungrier than usual because your cells aren’t absorbing glucose efficiently, even though there’s plenty circulating in your blood.
Headaches and blurred vision often show up alongside the thirst and urination. The blurred vision happens because high glucose levels cause fluid shifts in the lens of your eye, temporarily changing its shape. These symptoms can develop gradually over days, making them easy to dismiss as dehydration or fatigue. If your blood sugar stays elevated, the symptoms get worse and new ones appear: fatigue, difficulty concentrating, and slow-healing cuts or infections.
What Happens Inside Your Body
At the cellular level, persistently high glucose does three things that damage the lining of your blood vessels. First, the sugar itself directly injures the delicate inner walls of small blood vessels. Second, it triggers an overproduction of harmful molecules called free radicals, creating oxidative stress. Third, the excess glucose gets converted into compounds that accumulate in tissues and stiffen blood vessel walls. Together, these processes alter blood flow, make vessel walls leakier, and promote abnormal clotting. This is the foundation for nearly every long-term complication of high blood sugar.
How It Affects Your Brain
Your brain is especially vulnerable to the vascular damage caused by high blood sugar. Over time, damaged blood vessels deliver less oxygen-rich blood to brain tissue, and brain cells can die as a result. The CDC notes that these effects build gradually and aren’t obvious right away, so most people don’t realize their brain is being affected.
In the short term, high blood sugar causes brain fog, difficulty concentrating, and mood shifts. Over the long term, the damage can lead to problems with memory and learning, and it increases the risk of both vascular dementia and Alzheimer’s disease. Repeated episodes of hyperglycemia essentially stress the brain, even when each individual episode feels manageable.
Two Dangerous Emergencies
When blood sugar stays very high without treatment, it can tip into one of two life-threatening crises that require hospitalization.
Diabetic Ketoacidosis (DKA)
DKA happens most often in people with type 1 diabetes, though it can occur in type 2 as well. Without enough insulin, your body can’t use glucose for energy, so it starts breaking down fat at a rapid pace. That fat breakdown floods your bloodstream with acidic compounds called ketones, which make your blood dangerously acidic. Warning signs include nausea, vomiting, abdominal pain, a fruity smell on the breath (from the ketones), and a distinctive pattern of deep, labored breathing as your body tries to counteract the acidity. Confusion or altered mental status signals a critical stage.
Hyperosmolar Hyperglycemic State (HHS)
HHS is more common in type 2 diabetes and develops when blood sugar climbs above 600 mg/dL, sometimes much higher. Unlike DKA, the body still produces enough insulin to prevent significant ketone buildup, so blood acidity stays closer to normal. The real danger is extreme dehydration. Blood becomes so concentrated that it pulls water out of organs, including the brain. The hallmark of HHS is altered mental status caused by this severe dehydration, ranging from confusion to seizures to coma. Both DKA and HHS rarely resolve outside a hospital. Patients typically need hourly blood sugar monitoring and neurological checks until they stabilize.
Long-Term Damage to Your Organs
Even when blood sugar doesn’t spike high enough to cause an emergency, staying consistently elevated above normal ranges causes progressive organ damage. A key measurement for tracking this is the A1C test, which reflects your average blood sugar over the past two to three months. A large cohort study found that every 1% increase in A1C was associated with a 20% higher risk of cardiovascular disease and a 14% higher risk of death. Those percentages compound over years of poor blood sugar control.
Kidney Damage
Your kidneys contain millions of tiny blood vessels that filter waste from your blood. High glucose damages these filters, causing them to leak protein into your urine. The earliest detectable sign is a slight increase in a protein called albumin. Normal levels are below 30 mg/g in a urine test. When levels rise to 30 to 299 mg/g, it signals early kidney damage and also serves as a marker that blood vessels elsewhere in your body are being harmed. Without intervention, kidney function continues to decline, and the rate of that decline is strongly predicted by how much protein is leaking at baseline. In advanced stages, the kidneys can fail entirely, requiring dialysis or a transplant.
Eyes, Nerves, and Heart
The same small-vessel damage that harms the kidneys also targets the retina (leading to diabetic eye disease and potential vision loss), the peripheral nerves (causing numbness, tingling, or pain, usually starting in the feet), and the cardiovascular system broadly. Heart attack and stroke risk climb substantially with sustained high blood sugar, driven by the combination of blood vessel stiffening, inflammation, and abnormal clotting that hyperglycemia promotes.
The Numbers That Matter
Understanding where normal ends and diabetes begins helps you interpret blood work. A fasting blood sugar below 100 mg/dL is normal. Between 100 and 125 mg/dL is considered prediabetes. At 126 mg/dL or above on two separate tests, the diagnosis is diabetes. For A1C, normal is below 5.7%, prediabetes falls between 5.7% and 6.4%, and 6.5% or higher indicates diabetes. A random blood sugar reading of 200 mg/dL or above, combined with classic symptoms like excessive thirst and frequent urination, is also sufficient for a diabetes diagnosis.
These thresholds aren’t arbitrary. They reflect the glucose levels at which the risk of complications, particularly to the eyes, kidneys, and cardiovascular system, begins to climb meaningfully.
How Your Body Can Bring Blood Sugar Down
Physical activity is one of the fastest ways to lower blood sugar without medication. During aerobic exercise, your muscles increase glucose uptake by up to fivefold through a mechanism that doesn’t even require insulin. After you stop exercising, that enhanced glucose absorption continues for roughly two hours through insulin-independent pathways. If the exercise session is prolonged, insulin-dependent glucose uptake in the muscles stays elevated for up to 48 hours as the muscles replenish their energy stores.
This is why a brisk walk after a meal can noticeably blunt a blood sugar spike. For people with diabetes or prediabetes, regular physical activity is one of the most effective tools for keeping blood sugar in a safer range day to day. Combined with dietary changes, particularly reducing refined carbohydrates, and weight loss when appropriate, many people with type 2 diabetes or prediabetes can meaningfully lower their A1C and slow or prevent the progression of complications. Weight reduction through various approaches has been associated with improved kidney outcomes specifically, partly by reducing the excess pressure that obesity places on the kidney’s filtering units.