High blood sugar, called hyperglycemia, means there’s too much glucose circulating in your bloodstream. A fasting blood sugar level of 126 mg/dL or higher on two separate tests indicates diabetes, while levels between 100 and 125 mg/dL fall into the prediabetes range. Normal fasting blood sugar sits below 100 mg/dL. Understanding where your numbers fall helps you know whether your body is processing sugar the way it should.
The Numbers That Define High Sugar
Blood sugar is measured in a few different ways, and each has its own set of thresholds. The simplest is a fasting blood sugar test, taken after you haven’t eaten for at least eight hours. Below 100 mg/dL is normal. Between 100 and 125 mg/dL is prediabetes. At 126 mg/dL or above on two separate tests, it’s diabetes.
A second common test measures blood sugar two hours after drinking a sugary liquid. Below 140 mg/dL is normal, 140 to 199 mg/dL is prediabetes, and 200 mg/dL or higher is diabetes. This test captures how well your body handles a sudden influx of sugar, which can reveal problems that a fasting test might miss.
There’s also the A1C test, which reflects your average blood sugar over the past two to three months. A normal A1C is below 5.7%. Between 5.7% and 6.4% signals prediabetes. An A1C of 6.5% or higher means diabetes. Because it captures a longer window, A1C is useful for seeing the bigger picture rather than a single snapshot.
What Happens in Your Body
When you eat, your digestive system breaks carbohydrates down into glucose, which enters your bloodstream. Your pancreas responds by releasing insulin, a hormone that acts like a key, unlocking your cells so they can absorb that glucose and use it for energy. In a healthy person, blood sugar peaks about 46 to 50 minutes after a meal and drops back toward baseline within two hours.
High blood sugar develops when this system breaks down. The most common path starts with insulin resistance: your muscle, liver, and fat cells stop responding efficiently to insulin. Your pancreas compensates by pumping out more and more insulin, but over time it can’t keep up. The insulin-producing cells in the pancreas become exhausted, and blood sugar begins to climb. This progression, from insulin resistance to pancreatic exhaustion, is the core mechanism behind type 2 diabetes.
In type 1 diabetes, the problem is different. The immune system destroys the insulin-producing cells entirely, so the body can’t make insulin at all. Without insulin, glucose builds up in the blood regardless of how much or how little you eat.
How High Sugar Feels
Mild elevations often produce no symptoms at all, which is why many people walk around with prediabetes or early diabetes for years without knowing it. As levels rise higher, a few hallmark symptoms appear.
Excessive thirst is one of the earliest signs. When blood sugar is too high, your kidneys work overtime to filter out the extra glucose, pulling water along with it. That leads to frequent urination, which in turn dehydrates you and makes you thirstier. It becomes a cycle: you drink more, you urinate more, and your body keeps losing fluid.
Increased hunger is another classic symptom. Even though there’s plenty of glucose in your blood, your cells can’t access it without adequate insulin. Your body interprets this energy shortage as starvation and ramps up hunger signals. You may feel tired despite eating more than usual, because the fuel isn’t reaching the cells that need it. Blurred vision, slow-healing cuts, and tingling or numbness in your hands and feet can also develop as blood sugar stays elevated.
Foods That Spike Blood Sugar Fastest
Not all foods raise blood sugar equally. The glycemic index ranks carbohydrates by how quickly they cause a spike. A serving of white rice, for example, raises blood sugar almost as fast as eating pure table sugar. Other high-glycemic foods include white bread, bagels, rice cakes, most packaged breakfast cereals, doughnuts, and croissants. These foods are digested rapidly, flooding the bloodstream with glucose in a short window.
Foods with more fiber, fat, or protein slow that process down. Whole grains, legumes, non-starchy vegetables, and most fruits produce a more gradual rise. Pairing a high-glycemic food with protein or healthy fat (like toast with peanut butter instead of toast alone) can blunt the spike. The goal isn’t necessarily to avoid all carbohydrates but to choose ones that your body can process at a manageable pace.
Long-Term Damage From Chronic High Sugar
When blood sugar stays elevated for months or years, it damages blood vessels throughout the body. The smallest vessels are hit first, which is why the eyes, kidneys, and nerves are especially vulnerable.
In the eyes, chronic high sugar damages the tiny blood vessels in the retina, a condition called diabetic retinopathy. The severity depends on how high blood sugar runs and whether high blood pressure is also present. Over time, this can lead to vision loss. In the kidneys, excess glucose triggers oxidative stress and the accumulation of harmful compounds that gradually impair the organ’s filtering ability. Left unchecked, this can progress to kidney failure.
Nerve damage, particularly in the feet and legs, causes numbness, tingling, and pain. Because you lose sensation, small injuries can go unnoticed and become serious infections. On the larger-vessel side, people with diabetes face a significantly higher risk of heart attack, stroke, and peripheral artery disease. High blood sugar accelerates the buildup of fatty deposits in artery walls, and having microvascular complications (eyes, kidneys, nerves) further increases the risk of these cardiovascular events.
When High Sugar Becomes an Emergency
Blood sugar above 250 mg/dL combined with certain other warning signs can signal diabetic ketoacidosis, or DKA. This happens when the body, unable to use glucose for fuel, starts breaking down fat at a dangerous rate. The byproducts of that fat breakdown (ketones) build up in the blood and make it acidic. DKA is most common in type 1 diabetes but can occur in type 2 as well.
Symptoms of DKA include nausea, vomiting, abdominal pain, fruity-smelling breath, rapid breathing, and confusion. It can develop over hours and is life-threatening without treatment. Notably, DKA can sometimes occur even when blood sugar is below 250 mg/dL in people taking certain medications or those with alcohol use disorder, so the other symptoms matter as much as the number on the meter.
Bringing Blood Sugar Back Down
For people with prediabetes, lifestyle changes alone can often reverse the trajectory. Losing 5% to 7% of body weight, choosing lower-glycemic foods, and getting regular physical activity all improve the way your cells respond to insulin. Exercise is particularly effective because working muscles pull glucose out of the bloodstream even without much insulin.
For people already diagnosed with diabetes, management typically combines dietary changes with medication. Some medications help your body use insulin more effectively, others prompt the pancreas to produce more, and some slow the absorption of carbohydrates from food. People with type 1 diabetes, and some with advanced type 2, need insulin delivered by injection or pump. Regular monitoring, whether through finger-stick tests or a continuous glucose monitor worn on the skin, helps you see how specific foods, activities, and stressors affect your levels in real time.
The A1C test remains the standard way to track long-term control. Keeping A1C below 7% significantly reduces the risk of complications affecting the eyes, kidneys, and nerves, though the ideal target varies from person to person depending on age, other health conditions, and how long they’ve had diabetes.