When blood sugar climbs too high in someone with diabetes, the body goes through a predictable cascade of problems. In the short term, you feel it: excessive thirst, constant trips to the bathroom, fatigue, and blurred vision. If levels keep rising without treatment, the situation can escalate to life-threatening emergencies within hours. And when blood sugar stays elevated over months or years, it quietly damages blood vessels, nerves, kidneys, and eyes. Here’s what’s actually happening at each stage.
How High Is Too High?
Blood sugar is considered elevated when it exceeds 125 mg/dL while fasting or rises above 180 mg/dL within two hours of eating. Those are the clinical cutoffs for hyperglycemia. But the real danger zones are much higher. Once readings pass 250 mg/dL, you should check for ketones in your urine. Above 600 mg/dL, you’re in territory for a severe emergency called hyperosmolar syndrome, which can lead to coma.
A single high reading isn’t necessarily a crisis. Blood sugar fluctuates throughout the day, and occasional spikes happen even with careful management. The concern is when levels stay elevated for hours, climb very high very fast, or remain persistently above target over weeks and months.
What You Feel First
The earliest and most noticeable symptom is usually frequent urination. When blood sugar is high, your kidneys can’t reabsorb all that excess glucose, so it spills into the urine. That glucose pulls water along with it through a process called osmotic diuresis, essentially forcing your body to flush out extra fluid. The result is that you urinate far more than normal, sometimes waking multiple times at night.
All that fluid loss triggers intense thirst and a very dry mouth. You drink more, urinate more, and the cycle continues. Meanwhile, your cells aren’t getting the energy they need from glucose (either because there isn’t enough insulin to move it in, or because cells are resisting insulin’s signal), so you feel tired, weak, and sometimes irritable. Blurred vision is common too, because shifts in fluid balance change the shape of the lens in your eye.
Other symptoms that show up as blood sugar stays elevated:
- Headaches from dehydration and fluid shifts
- Nausea and vomiting
- Unexplained weight loss, because the body starts breaking down fat and muscle for energy it can’t get from glucose
- Slow-healing cuts or frequent infections, since high sugar impairs immune function
When It Becomes an Emergency
If high blood sugar isn’t corrected, two dangerous conditions can develop. Which one depends largely on the type of diabetes involved.
Diabetic Ketoacidosis (DKA)
DKA is most common in Type 1 diabetes but can happen in Type 2 as well. When the body has little or no insulin, cells are completely locked out from using glucose. The body shifts to burning fat for fuel, which produces acidic byproducts called ketones. As ketones build up, the blood becomes dangerously acidic.
Warning signs include a fruity smell on the breath, stomach pain, nausea, vomiting, shortness of breath, and confusion. DKA is diagnosed when blood sugar is above 200 mg/dL, ketone levels are significantly elevated, and blood pH drops below 7.3 (normal is around 7.4). Severe cases push blood pH below 7.0, which is immediately life-threatening. If you notice a fruity breath odor, persistent vomiting, or any level of ketones in your urine, that warrants urgent medical attention.
Hyperosmolar Hyperglycemic Syndrome (HHS)
HHS is more common in Type 2 diabetes, particularly in older adults. Blood sugar climbs extremely high, often above 600 mg/dL, causing severe dehydration. Unlike DKA, significant ketone buildup typically doesn’t occur because there’s still enough insulin to prevent the body from switching fully to fat-burning. But the extreme dehydration alone is dangerous, leading to confusion, seizures, and coma. HHS develops more slowly than DKA, sometimes over days, which means it can sneak up on people who aren’t checking their blood sugar regularly.
Both DKA and HHS can lead to diabetic coma. If someone with diabetes becomes confused, has trouble staying awake, or passes out, call emergency services immediately.
How High Sugar Damages Blood Vessels Over Time
The long-term consequences of chronically elevated blood sugar come down to what glucose does to proteins in your body. When there’s too much sugar in the bloodstream, glucose molecules stick to proteins through a chemical reaction, creating compounds called advanced glycation end products (AGEs). Think of it like a slow caramelization happening inside your blood vessels.
These AGEs cause damage in several ways. They cross-link with collagen and elastin, the structural proteins that keep blood vessels flexible, making arteries stiff. They trigger inflammation in the vessel walls, attracting immune cells that cause further injury. They also interfere with nitric oxide, a molecule your blood vessels need to relax and dilate properly. On top of that, AGEs modify LDL cholesterol (the “bad” kind) in ways that accelerate plaque buildup. The modified LDL is harder for the body to clear, and it accumulates inside vessel walls, forming the fatty deposits that lead to atherosclerosis.
This vascular damage is why people with poorly controlled diabetes face a significantly higher risk of heart attack and stroke. It’s not just about the big arteries either. The tiny capillaries that supply your eyes, kidneys, and nerve endings are especially vulnerable.
Nerve Damage and Loss of Sensation
Diabetic neuropathy, or nerve damage from high blood sugar, affects nerve fibers in a characteristic pattern. It typically starts in the longest nerves first, which is why symptoms begin in the feet and work their way upward. You might notice tingling, numbness, burning pain, or a “pins and needles” sensation in your toes and feet before it ever reaches your hands.
The underlying mechanism involves multiple overlapping processes triggered by prolonged hyperglycemia: the same AGE formation that damages blood vessels, increased oxidative stress that injures nerve cells directly, and damage to the tiny blood vessels that supply nerves with oxygen and nutrients. Over time, nerve fibers progressively degenerate. The loss follows what researchers describe as a “proximo-distal gradient,” meaning the farthest points from the spine are affected first and most severely.
The danger of losing sensation in the feet is practical: you may not feel a blister, cut, or infection developing. Combined with the poor circulation from vascular damage, minor injuries can become serious wounds that heal slowly or not at all.
What Happens to Your Eyes
Diabetic retinopathy is one of the most common complications of sustained high blood sugar. The retina, the light-sensitive tissue at the back of the eye, is fed by a dense network of tiny capillaries. Chronic hyperglycemia damages these capillaries in stages. First, the walls of the vessels thicken and weaken, forming small bulges called microaneurysms. These can leak fluid and blood into the retina.
As damage progresses, capillaries become blocked entirely, cutting off blood supply to parts of the retina. The eye responds by growing new blood vessels, but these replacement vessels are fragile and abnormal. They leak easily, causing swelling in the central part of the retina (macular edema) and, in advanced cases, severe vision loss. The progression from early to advanced retinopathy typically takes years, which means regular eye exams can catch it before it threatens your sight.
How the Kidneys Are Affected
Your kidneys filter about 50 gallons of blood per day through microscopic structures called glomeruli. High blood sugar damages these filters gradually. The earliest sign is a small amount of protein (albumin) leaking into the urine, something you can’t see or feel. This stage, called moderately increased albuminuria, is defined as 30 to 300 milligrams of albumin in a 24-hour urine sample.
If blood sugar remains poorly controlled, the damage worsens. Albuminuria exceeding 300 milligrams per day signals more advanced kidney disease. The structural damage progresses through recognizable stages, from thickening of the filter membranes to scarring of the filtering units themselves. In advanced diabetic kidney disease, more than half of the glomeruli are scarred shut, and kidney function declines to the point where dialysis or transplant becomes necessary. This process usually takes 10 to 20 years of poorly controlled diabetes, which is why regular urine and blood tests to check kidney function matter so much.
What to Watch For
The tricky part of chronic high blood sugar is that much of the damage is silent for years. You won’t feel your kidneys leaking protein or your retinal capillaries weakening. The symptoms that do show up early are the ones worth paying close attention to:
- Persistent thirst and frequent urination that don’t match your fluid intake
- Tingling or numbness in your feet or hands
- Vision changes, even subtle ones like difficulty reading or increased floaters
- Wounds that heal slowly, especially on the feet
- Fruity breath odor, nausea, or stomach pain, which may signal ketone buildup
Blood sugar above 250 mg/dL on two consecutive tests is the point at which checking urine for ketones becomes important, especially if you’re feeling unwell. Any reading above 600 mg/dL, or symptoms like confusion and extreme drowsiness, is a medical emergency regardless of whether ketones are present.