Blood sugar is the amount of glucose (a simple sugar) circulating in your bloodstream at any given moment. It’s your body’s primary fuel source, and keeping it within a healthy range is one of the most important things your metabolism does. A normal fasting blood sugar is below 100 mg/dL, while levels between 100 and 125 mg/dL signal prediabetes.
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 simple sugars. That breakdown starts in your mouth, where enzymes in saliva begin working on starches, and finishes in your small intestine, where individual sugar molecules pass through the intestinal wall into your bloodstream. From there, they travel to the liver.
The liver acts as a sorting station. It either lets glucose pass through into your general circulation so cells throughout your body can use it for energy, or it stores the excess as a compact storage form called glycogen. When you haven’t eaten in a while, the liver reverses the process, converting that stored glycogen back into glucose and releasing it into your blood. If your stores run low enough, during fasting or on a very low-carb diet, for example, the liver can even build new glucose from protein and other raw materials. This is how your body keeps blood sugar from crashing between meals or overnight.
How Your Body Keeps Blood Sugar Stable
Two hormones from the pancreas work as a balancing act. When blood sugar rises after a meal, the pancreas releases insulin. Insulin acts like a key, unlocking cells so they can pull glucose out of the blood and use it for energy. When blood sugar drops too low, the pancreas releases glucagon, which signals the liver to release its stored glucose back into the bloodstream. Glucagon also tells the liver to stop absorbing glucose so more stays available in circulation.
These two hormones counterbalance each other constantly throughout the day. In a healthy person, this system keeps blood sugar in a surprisingly tight range. Problems start when the system breaks down: either the pancreas can’t produce enough insulin (type 1 diabetes), or the body’s cells stop responding to insulin effectively (type 2 diabetes), and glucose builds up in the blood instead of entering cells.
Normal, Prediabetic, and Diabetic Ranges
Blood sugar numbers depend on when you last ate. Here are the standard thresholds used for diagnosis:
- Fasting blood sugar (no food for 8+ hours): Normal is below 100 mg/dL. Prediabetes falls between 100 and 125 mg/dL. Diabetes is 126 mg/dL or higher.
- Two hours after a glucose drink (oral glucose tolerance test): Normal is below 140 mg/dL. Prediabetes is 140 to 199 mg/dL. Diabetes is 200 mg/dL or higher.
- A1C (a longer-term average): Normal is below 5.7%. Prediabetes is 5.7% to 6.4%. Diabetes is 6.5% or higher.
These numbers come from the American Diabetes Association’s diagnostic criteria. If you’ve had blood work done, your fasting glucose is typically included in a standard metabolic panel.
What the A1C Test Tells You
While a regular blood sugar reading captures a single moment, the A1C test reveals your average blood sugar over the past two to three months. It works by measuring the percentage of your red blood cells that have glucose attached to them. The higher your blood sugar has been running, the more sugar-coated cells you’ll have.
To give you a sense of scale: an A1C of 6% corresponds to an average blood sugar of about 126 mg/dL. At 7%, that average jumps to 154 mg/dL. By the time A1C reaches 10%, average blood sugar is around 240 mg/dL. This test is especially useful because it isn’t thrown off by what you ate the night before, making it a more reliable picture of how well blood sugar has been controlled over time.
What High Blood Sugar Feels Like
Mildly elevated blood sugar often produces no symptoms at all, which is why many people with prediabetes or early type 2 diabetes don’t realize anything is wrong. Many people with diabetes don’t notice symptoms until blood sugar exceeds 250 mg/dL. People who haven’t been diagnosed yet tend to feel symptoms at lower levels because their bodies aren’t accustomed to the elevation.
Early signs include increased thirst, frequent urination, headaches, and blurred vision. The frequent urination happens because your kidneys work harder to filter out the excess glucose, pulling extra water along with it. That water loss triggers the thirst. When blood sugar stays elevated over weeks or months, you may notice fatigue, unexplained weight loss, slow-healing cuts, and recurring infections, particularly yeast infections.
What Low Blood Sugar Feels Like
Blood sugar below 70 mg/dL is considered low. The symptoms tend to come on quickly: a fast heartbeat, shaking, sweating, sudden hunger, anxiety, and dizziness. Your body is essentially sounding an alarm that its fuel supply is running short, especially for the brain, which depends almost entirely on glucose.
If blood sugar drops below 54 mg/dL, the situation becomes more serious. At that level you may feel weak, have trouble walking or seeing clearly, act confused, or even lose consciousness. Severe low blood sugar can cause seizures. This is more common in people taking insulin or certain diabetes medications, but it can occasionally happen in people without diabetes after prolonged fasting or intense exercise.
How Blood Sugar Is Measured
There are two main ways to track blood sugar at home. A traditional blood glucose meter requires a small finger prick to produce a drop of blood, which you place on a test strip. The meter gives you a single number representing your blood sugar at that exact moment.
Continuous glucose monitors (CGMs) take a different approach. A small sensor, typically worn on the back of your arm or your abdomen, checks glucose levels automatically throughout the day and night, sending readings to your phone or a receiver. The key advantage is that a CGM doesn’t just show you a number. It shows a trend line: whether your blood sugar is rising, falling, or holding steady. That context helps you see how specific meals, physical activity, stress, and sleep affect your levels in real time, something a single finger-stick reading can’t provide.
CGMs were originally designed for people managing diabetes with insulin, but they’ve become increasingly popular among people with prediabetes and even those simply curious about how their bodies respond to different foods.