Blood sugar, also called blood glucose, is the main sugar found circulating in your bloodstream. It serves as the primary energy source for nearly every cell in your body. In a healthy adult, fasting blood sugar stays within a tight range of about 70 to 99 mg/dL, maintained by a constant balancing act between hormones that raise and lower glucose levels throughout the day.
How Glucose Gets Into Your Blood
When you eat carbohydrates, whether from bread, fruit, rice, or a candy bar, your digestive system breaks them down into glucose. Specialized transport proteins on the lining of your small intestine absorb that glucose and shuttle it into your bloodstream. From there, it travels to muscles, your brain, your liver, and other tissues that need fuel.
Glucose isn’t just an energy source. It’s also a building block for other biochemical reactions, including the storage of energy in your liver and muscles for later use. Your brain is especially dependent on a steady glucose supply, which is why drops in blood sugar can quickly affect your concentration and mood.
How Your Body Keeps Blood Sugar Stable
Your pancreas is the control center. It produces two opposing hormones, insulin and glucagon, that work together to keep blood glucose within a narrow window of roughly 70 to 110 mg/dL at any given time.
After you eat, rising glucose triggers your pancreas to release insulin. Insulin acts like a key, unlocking muscle and fat cells so they can pull glucose out of the bloodstream. It also signals your liver to store excess glucose for later. This is why blood sugar drops back down within a couple of hours after a meal in a healthy person.
Between meals or while you sleep, the opposite happens. Glucose levels start to dip, and your pancreas releases glucagon instead. Glucagon tells your liver to break down its stored glucose and release it back into the blood. During longer periods without food, glucagon also prompts your liver and kidneys to manufacture new glucose from non-carbohydrate sources like amino acids. This back-and-forth between insulin and glucagon is what keeps your energy supply remarkably steady around the clock.
Normal, Prediabetes, and Diabetes Ranges
Blood sugar levels fall into well-defined categories based on a fasting test, meaning you haven’t eaten for at least eight hours:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher on two separate tests
A second common measurement is the A1C test, which reflects your average blood sugar over the previous three months rather than a single moment in time. It works by measuring how much glucose has attached to your red blood cells. An A1C below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or above indicates diabetes. Diagnosing diabetes typically requires two abnormal results, either from the same blood draw or on separate occasions.
How Blood Sugar Is Measured
A finger-prick glucose meter gives you a snapshot of your blood sugar right now. It’s useful for seeing how a specific meal, exercise session, or stressful day affects your levels. Many people with diabetes check this way several times a day.
The A1C test fills a different role. Because it captures a three-month average, it reveals patterns that daily readings can miss. A person might have normal fasting numbers but still show a high A1C if their blood sugar spikes significantly after meals. Continuous glucose monitors, small sensors worn on the skin, offer yet another view by tracking glucose every few minutes and showing real-time trends.
What Makes Blood Sugar Rise and Fall
The type of carbohydrate you eat matters as much as the amount. The glycemic index (GI) is a scale from 0 to 100 that ranks foods by how quickly they raise blood sugar, with pure glucose scoring 100. Foods with a high GI, like white bread, instant potatoes, and fruit juice, cause a rapid spike. Low-GI foods, like whole grains, legumes, and most vegetables, raise glucose more gradually.
Several factors shift a food’s GI in ways you might not expect. Processing generally raises it: fruit juice spikes blood sugar faster than whole fruit, and instant mashed potatoes score higher than a baked potato. Cooking matters too. Al dente pasta has a lower GI than soft-cooked pasta because the firmer starch is harder for your body to break down quickly. Adding fat or fiber to a meal slows digestion and blunts the glucose spike, which is why eating an apple with peanut butter produces a gentler rise than eating the apple alone.
Physical activity pulls glucose into your muscles without needing as much insulin, which is why a walk after dinner can noticeably lower a post-meal spike. Stress, illness, poor sleep, and certain medications can push blood sugar in the opposite direction, even if your diet hasn’t changed.
Signs of High Blood Sugar
When blood sugar climbs well above normal (a state called hyperglycemia), the body tries to flush the excess glucose through the kidneys. This leads to a recognizable pattern of symptoms: extreme thirst, frequent urination, tiredness, nausea, and dizziness. You may feel listless or foggy. If levels climb dangerously high, confusion, drowsiness, and even loss of consciousness can follow.
Many people with mildly elevated blood sugar feel no symptoms at all, which is why prediabetes often goes undetected for years. The damage, however, can be accumulating silently.
Signs of Low Blood Sugar
Low blood sugar (hypoglycemia) tends to announce itself more urgently. Because your brain is being starved of fuel, the symptoms come on fast: a racing pulse, cold sweats, pale skin, shakiness, intense hunger, and difficulty concentrating. You might feel anxious or restless for no apparent reason. Mild episodes resolve quickly with a few grams of fast-acting carbohydrate, like juice or glucose tablets. Severe hypoglycemia, though less common, can cause loss of consciousness and is a medical emergency.
Why Chronically High Blood Sugar Causes Damage
When blood sugar stays elevated over months and years, the excess glucose triggers a cascade of chemical reactions that generate harmful molecules called free radicals. These molecules damage cells throughout the body, particularly in blood vessels and nerves. The process works through several overlapping pathways: glucose molecules attach directly to proteins and alter their function, mitochondria (your cells’ power plants) leak more damaging byproducts, and additional stress pathways in cells get switched on.
The blood vessel damage is especially consequential. Persistently high glucose activates enzymes that stiffen and inflame vessel walls, which over time can harm the heart, kidneys, eyes, and extremities. Small blood vessels in the retina and kidneys are particularly vulnerable, which is why vision loss and kidney disease are among the most common long-term complications of poorly managed diabetes. Nerve damage, often felt as tingling or numbness in the hands and feet, follows a similar pattern of glucose-driven injury to the tiny vessels that supply nerves with blood.
This is the core reason blood sugar management matters so much. The goal isn’t just avoiding symptoms today. It’s preventing the slow, cumulative damage that high glucose inflicts on tissues over years, often before any symptoms appear.