High blood glucose, also called hyperglycemia, means there is too much sugar circulating in your bloodstream. A normal fasting blood sugar is below 100 mg/dL. Once it reaches 100 to 125 mg/dL, you’re in the prediabetes range, and a reading of 126 mg/dL or higher on two separate tests means diabetes.
How Blood Sugar Is Regulated
Every time you eat, your body breaks down carbohydrates into glucose, which enters the bloodstream. Your pancreas responds by releasing insulin, a hormone that acts like a key, unlocking cells so they can absorb that glucose and use it for energy. When this system works properly, blood sugar rises after a meal and then drops back to normal within a couple of hours.
High blood glucose develops when this process breaks down. In some cases, the pancreas doesn’t produce enough insulin. In others, your cells stop responding to insulin efficiently, a condition known as insulin resistance. When cells resist insulin’s signal, the pancreas has to work harder and produce more of it to achieve the same effect. Over time, the insulin-producing cells in the pancreas can burn out under that extra workload, and blood sugar stays elevated.
What the Numbers Mean
There are several ways to measure blood glucose, and the thresholds differ slightly depending on the test. For a fasting blood sugar test (taken after at least eight hours without eating), under 100 mg/dL is normal, 100 to 125 mg/dL indicates prediabetes, and 126 mg/dL or higher on two separate occasions confirms diabetes. A random blood sugar reading of 200 mg/dL or higher at any time of day also suggests diabetes, regardless of when you last ate.
The A1C test takes a different approach. Rather than capturing a single moment, it measures your average blood sugar over the past three months. An A1C below 5.7% is normal, 5.7% to 6.4% falls in the prediabetes range, and 6.5% or higher indicates diabetes. The American Diabetes Association recommends that most adults with diabetes aim for an A1C below 7%.
If you use a glucose meter or continuous glucose monitor day to day, the targets look like this: 80 to 130 mg/dL before meals, and under 180 mg/dL at the peak after eating. People who wear continuous monitors generally aim to spend more than 70% of the day in the 70 to 180 mg/dL range.
Common Symptoms
Mild or moderate high blood sugar often produces no symptoms at all, which is why many people have prediabetes for years without knowing. When levels climb high enough, the body tries to flush out excess glucose through the kidneys. That leads to frequent urination, which in turn pulls water from your tissues and triggers persistent thirst. This cycle of urinating more and drinking more is one of the earliest and most recognizable signs of uncontrolled blood sugar.
Other common symptoms include blurry vision, unusual fatigue, and excessive hunger. You may also notice slow-healing cuts or frequent infections. These symptoms tend to develop gradually in type 2 diabetes and more rapidly in type 1.
Causes Beyond Diet
Food is the most obvious factor, but plenty of non-dietary triggers can push blood sugar up. Stress is a major one. Physical stress from illness, injury, or even sunburn triggers the release of cortisol and other hormones that raise glucose levels. Emotional and mental stress does the same thing.
Sleep deprivation makes your body use insulin less effectively, and even a single night of poor sleep can have a measurable impact. Dehydration concentrates the sugar already in your blood, so not drinking enough water can make readings appear worse. Caffeine raises blood sugar in some people, even without any sweetener added. Certain medications, particularly steroids and some nasal sprays, can also spike glucose by signaling the liver to release more of it.
There’s also a natural hormonal surge in the early morning hours, sometimes called the dawn phenomenon. Your body releases hormones that prepare you to wake up, and for people with diabetes, this can cause blood sugar to be noticeably higher at breakfast time than it was at bedtime.
When Blood Sugar Gets Dangerously High
Severely elevated blood sugar can lead to two medical emergencies. The first, diabetic ketoacidosis, occurs most often in type 1 diabetes when the body has almost no insulin available. Without insulin, cells can’t access glucose, so the body starts burning fat at a rapid rate. This produces acids called ketones that build up in the blood. It typically involves blood sugar above 250 mg/dL along with nausea, vomiting, abdominal pain, and a fruity smell on the breath. Left untreated, it can lead to coma.
The second emergency, hyperosmolar hyperglycemic state, is more common in type 2 diabetes and involves blood sugar soaring above 600 mg/dL. At that level, the blood becomes extremely concentrated, pulling water out of organs including the brain. Symptoms range from confusion and lethargy to seizures and coma. The mortality rate from this condition ranges from 10% to 50%, making it one of the most dangerous acute complications of diabetes. Both emergencies require immediate hospital treatment.
Long-Term Damage From Chronic High Blood Sugar
When blood sugar stays elevated over months and years, it damages blood vessels throughout the body. The smallest vessels are affected first, which is why the eyes, kidneys, and nerves are particularly vulnerable. In the eyes, high glucose damages the tiny capillaries of the retina, causing them to thicken, leak, or grow abnormally. This is diabetic retinopathy, and it’s the leading cause of new blindness in working-age adults.
In the kidneys, chronically high glucose damages the filtering units, gradually reducing the kidneys’ ability to clean the blood. Early-stage kidney damage produces no symptoms, which is why regular urine and blood tests are important for anyone with diabetes. Over time, untreated damage can progress to kidney failure.
Nerve damage, or neuropathy, most commonly affects the feet and legs first. High blood sugar interferes with nerve fiber health and disrupts blood flow to the nerves. This can cause tingling, numbness, burning pain, or a total loss of sensation. Losing feeling in the feet is especially risky because small injuries can go unnoticed and become serious infections.
Larger blood vessels suffer too. Chronic hyperglycemia accelerates the buildup of plaque in arteries, raising the risk of heart attack and stroke. Cardiovascular disease is the leading cause of death among people with diabetes, and keeping blood sugar closer to target is one of the most effective ways to reduce that risk.
Lowering Blood Sugar Through Daily Habits
Physical activity is one of the most effective tools for bringing blood sugar down. When your muscles contract during exercise, they pull glucose out of the blood for fuel, and regular activity also makes your cells more responsive to insulin. The general recommendation is at least 150 minutes per week of moderate aerobic exercise, things like brisk walking, cycling, or swimming. Strength training two to three times a week adds an additional benefit by building muscle tissue, which uses glucose more efficiently even at rest.
What you eat matters, but the key factor is less about eliminating specific foods and more about choosing ones that release glucose slowly. Fiber-rich foods like vegetables, whole grains, beans, and fruits help stabilize blood sugar because fiber slows digestion and prevents sharp spikes. Pairing carbohydrates with protein or fat has a similar buffering effect.
Staying hydrated helps keep glucose from concentrating in the blood. Getting consistent, adequate sleep (seven or more hours for most adults) supports insulin sensitivity. Managing stress through whatever works for you, whether that’s exercise, social connection, or breathing techniques, keeps stress hormones from pushing glucose up unnecessarily.
For people with prediabetes, these lifestyle changes alone can prevent or significantly delay the progression to type 2 diabetes. For those already managing diabetes, they work alongside medication to keep blood sugar in a safer range and reduce the risk of complications.