A blood sugar level is considered high when it rises above 100 mg/dL after fasting or stays above 140 mg/dL two hours after eating. These thresholds mark the point where glucose in your bloodstream starts exceeding what your body can efficiently process, and the higher it climbs, the more damage it can cause over time.
Normal, Prediabetic, and Diabetic Ranges
Blood sugar isn’t a single number. It fluctuates throughout the day depending on when and what you ate, how active you’ve been, and how well your body produces and responds to insulin. That’s why doctors use several different measurements to assess where you fall on the spectrum.
A fasting blood sugar test, taken after at least eight hours without food, is the most straightforward snapshot. Normal is below 100 mg/dL. Between 100 and 125 mg/dL is prediabetes. At 126 mg/dL or higher on two separate tests, the diagnosis is diabetes.
Your A1C, a blood test that reflects your average blood sugar over the past two to three months, gives a broader picture. The CDC uses these ranges: below 5.7% is normal, 5.7% to 6.4% is prediabetes, and 6.5% or above is diabetes. To put those percentages in concrete terms, an A1C of 6% corresponds to an average blood sugar of about 126 mg/dL, while an A1C of 8% means your blood sugar has been averaging around 183 mg/dL.
What Happens Inside Your Body
When you eat, your digestive system breaks carbohydrates 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 for energy. In a healthy system, insulin peaks within about 30 minutes of eating, blood sugar rises modestly (to roughly 140 mg/dL at most), and levels return to normal within two hours.
High blood sugar happens when this system breaks down. In type 1 diabetes, the pancreas produces little or no insulin. In type 2 diabetes, cells become resistant to insulin’s signal, and the pancreas can’t keep up with demand. The result is the same: glucose stays trapped in the bloodstream instead of entering cells. Your liver also plays a role. It normally releases stored glucose between meals to keep your brain fueled, then dials back production after you eat. In people with type 2 diabetes, the liver keeps producing glucose even when blood sugar is already elevated, and it can take roughly six hours to slow down instead of doing so promptly.
This delayed insulin response creates a distinctive pattern. Instead of peaking at 30 minutes, insulin doesn’t reach its highest level until about two hours after a meal. Meanwhile, blood sugar climbs higher and stays elevated longer. Someone with mild type 2 diabetes might see their blood sugar rise to about 200 mg/dL after eating, while someone with more advanced disease can spike well above that and stay elevated for hours.
Early Signs of High Blood Sugar
Mildly elevated blood sugar often produces no symptoms at all, which is why millions of people have prediabetes without knowing it. As levels climb higher, the body starts showing predictable signs:
- Increased thirst and frequent urination. Your kidneys work overtime to filter excess glucose, pulling more water with it. You pee more, get dehydrated, and feel thirstier.
- Blurred vision. High glucose causes the lens of your eye to swell, temporarily distorting your focus.
- Headaches. Dehydration and shifting fluid balance in the brain contribute to persistent headaches.
- Increased hunger. When glucose can’t get into your cells, your body signals that it needs more fuel, even though there’s plenty of sugar in your blood.
When blood sugar stays elevated for weeks or months, the symptoms shift. Fatigue becomes constant because your cells are chronically starved for energy. Unexplained weight loss can occur as your body starts breaking down fat and muscle for fuel. Cuts and sores heal slowly, skin infections become more frequent, and vaginal yeast infections may recur. These longer-term symptoms often prompt the doctor visit that leads to a diabetes diagnosis.
Factors Beyond Food
Diet is the most obvious driver of blood sugar, but it’s far from the only one. Stress triggers the release of hormones like cortisol and adrenaline, which tell your liver to dump more glucose into your bloodstream. This is a survival mechanism designed for short bursts of danger, but chronic stress keeps those levels elevated day after day. Even the stress of a sunburn can raise your blood sugar noticeably.
Illness has a similar effect. When you’re fighting an infection, your body releases stress hormones as part of the immune response, pushing blood sugar up even if you’re eating less than usual. Certain medications can do the same. Steroids prescribed for conditions like asthma or autoimmune diseases are well-known blood sugar elevators. Even some nasal sprays contain chemicals that trigger the liver to produce extra glucose. Poor sleep, dehydration, and skipping physical activity all contribute to higher readings as well.
Long-Term Damage From Chronic High Blood Sugar
The real danger of high blood sugar isn’t any single spike. It’s what happens when levels stay elevated over months and years. Excess glucose in the bloodstream damages blood vessels throughout the body, and the consequences show up in nearly every organ system.
Small blood vessels are especially vulnerable. Damage to the tiny vessels in the eyes (retinopathy) is one of the leading causes of vision loss in adults. The kidneys, which depend on millions of microscopic blood vessels to filter your blood, gradually lose function, potentially progressing to chronic kidney disease. Nerve damage, particularly in the feet and hands, causes numbness, tingling, and pain. Because you may not feel injuries to your feet, small cuts or blisters can develop into serious infections and ulcers.
Larger blood vessels suffer too. Chronic high blood sugar accelerates atherosclerosis, the buildup of fatty deposits in artery walls. This raises the risk of heart attack, stroke, and poor circulation in the legs. Heart disease is the leading cause of death in people with diabetes, and the risk is roughly double that of someone without diabetes.
The effects extend beyond the cardiovascular system. High blood sugar impairs immune function, making bacterial and fungal infections more common and harder to fight. It’s associated with fatty liver disease, which can progress to scarring and cirrhosis. Depression, cognitive decline, gum disease, and sexual dysfunction, including erectile dysfunction, are all more common in people with chronically elevated blood sugar.
When High Blood Sugar Becomes an Emergency
Blood sugar above 250 mg/dL with symptoms is a warning sign that requires prompt attention. At these levels, especially in type 1 diabetes, the body may start breaking down fat so rapidly that it produces acids called ketones. When ketones build up in the blood, the result is diabetic ketoacidosis (DKA), a life-threatening condition that can develop within hours.
The warning signs of DKA are distinct from ordinary high blood sugar symptoms. Nausea and vomiting, stomach pain, fruity-smelling breath, rapid breathing, a racing heartbeat, and confusion all signal that ketone levels are dangerously high. Loss of consciousness can follow. If your blood sugar is consistently above 250 mg/dL, checking for ketones with an over-the-counter urine test strip is a practical step. Positive results, combined with any of those symptoms, warrant emergency care.
It’s worth noting that DKA can occasionally occur even when blood sugar is below 250 mg/dL, a situation called euglycemic ketoacidosis. This is less common but means that symptoms like nausea, stomach pain, and fruity breath should always be taken seriously, regardless of the number on your meter.
How Blood Sugar Is Monitored
If you’re tracking your blood sugar, the numbers you see on a glucose meter reflect a single moment in time. A fasting reading taken first thing in the morning tells you how well your body managed glucose overnight. A reading two hours after a meal shows how effectively your system handled the carbohydrates you just ate. Both pieces of information matter.
The A1C test fills in the gaps by averaging your blood sugar over roughly 90 days. An A1C of 7% translates to an average of about 154 mg/dL, while 9% means an average around 212 mg/dL. For context, most treatment guidelines aim for an A1C below 7%, though individual targets vary based on age, how long you’ve had diabetes, and other health conditions. Continuous glucose monitors, small sensors worn on the skin, now allow people to see their blood sugar in real time, revealing patterns that finger-stick tests alone can miss, like overnight dips or post-meal spikes that resolve before the next scheduled check.