Your glucose, or blood sugar, is the concentration of sugar circulating in your bloodstream at any given moment. It’s your body’s primary fuel source, and for most healthy adults, a normal fasting level falls below 100 mg/dL. That number rises after you eat and dips between meals, but your body works constantly to keep it within a tight range of roughly 70 to 140 mg/dL throughout the day.
Why Glucose Matters to Your Body
Glucose is the universal fuel for human cells. When you eat carbohydrates or proteins, your digestive system breaks them down into glucose, which then enters your bloodstream. From there, it needs to get inside individual cells to be converted into the energy molecule ATP, which powers virtually everything your cells do.
Glucose can’t simply pass through cell walls on its own because the molecule is too large. Instead, it relies on protein carriers embedded in cell membranes to shuttle it inside. Most tissues need insulin to make this process efficient. Insulin increases glucose uptake into muscle and fat cells by roughly ten times. The brain and liver are exceptions: they can absorb glucose more freely without as much insulin, which makes sense given the brain’s constant, high energy demands.
How Your Body Keeps Glucose Stable
Your pancreas produces two key hormones that act like a thermostat for blood sugar. When glucose rises after a meal, beta cells in the pancreas release insulin, which signals muscle and fat tissue to pull glucose out of the blood and store it. When glucose drops between meals or overnight, alpha cells release glucagon, which tells the liver to break down its stored sugar and release it back into the bloodstream. These two hormones work in opposition, keeping blood glucose in a narrow window of about 70 to 110 mg/dL in healthy people.
This balancing act is called glucose homeostasis. During a prolonged fast, glucagon also triggers the liver and kidneys to manufacture new glucose from non-sugar sources like amino acids, ensuring your brain never runs dry. After a large meal, insulin does the opposite: it promotes storage of glucose as glycogen in the liver and muscles, and as fat in adipose tissue.
Normal Blood Sugar Ranges
Glucose levels shift throughout the day, so the numbers that matter depend on when you measure them.
- Fasting (no food for 8+ hours): Below 100 mg/dL is normal. Between 100 and 125 mg/dL signals prediabetes. At 126 mg/dL or above on two separate tests, the threshold for a diabetes diagnosis is met.
- Two hours after eating: Below 140 mg/dL is normal for people without diabetes. Levels between 140 and 199 mg/dL suggest impaired glucose tolerance.
- A1c (a 2- to 3-month average): Below 5.7% is normal. Between 5.7% and 6.4% falls in the prediabetes range. At 6.5% or above, diabetes is diagnosed.
These thresholds come from major clinical guidelines and are used consistently across most labs. Your doctor may interpret your specific results in context, especially if you’re pregnant, elderly, or managing another condition that affects blood sugar.
How Blood Sugar Is Measured
The most common method is a finger-prick test using a portable glucose meter. You lance the side of a fingertip, place a drop of blood on a test strip, and the meter displays your level in mg/dL within seconds. Fasting glucose and A1c tests done at a lab use a blood draw from a vein and tend to be slightly more precise.
Continuous glucose monitors (CGMs) have become increasingly popular, even among people without diabetes. These small sensors sit just under the skin and measure glucose in the fluid surrounding your cells, called interstitial fluid, every few minutes. When your blood sugar is relatively stable, CGM readings closely match a finger-prick result. But when glucose is changing rapidly, such as right after a meal or during exercise, there’s a lag of about 5 to 25 minutes between what’s happening in your blood and what the sensor detects. This is a normal limitation of the technology, not a malfunction.
When Glucose Drops Too Low
Hypoglycemia is generally defined as a blood glucose level below 70 mg/dL. The symptoms tend to arrive in two waves. First come the warning signs driven by your nervous system’s alarm response: shakiness, a pounding heart, sweating, sudden hunger, and anxiety. If glucose continues to fall, the brain itself starts running short on fuel, leading to confusion, difficulty concentrating, fatigue, slurred speech, and in severe cases, seizures or loss of consciousness.
Most episodes of low blood sugar happen in people taking insulin or certain diabetes medications. But it can also occur in people without diabetes after prolonged fasting, intense exercise, or heavy alcohol consumption. Eating a fast-acting carbohydrate, like juice or glucose tablets, typically reverses mild symptoms within 15 minutes.
Surprising Factors That Affect Your Levels
Food is the most obvious driver of blood sugar, but several non-dietary factors can push your numbers up or down in ways you might not expect.
Stress, whether physical or emotional, triggers the release of cortisol and adrenaline, both of which prompt the liver to dump stored glucose into the bloodstream. Even something as simple as a bad sunburn can raise blood sugar because the pain itself generates a stress response. Sleep plays a significant role too. Just one night of poor sleep can reduce your body’s ability to use insulin effectively the next day, leading to higher-than-usual readings.
Illness and infection also raise glucose levels as part of the immune response. If you’re tracking your blood sugar and notice an unexplained spike, it’s worth considering whether stress, sleep, or a brewing illness might be the cause rather than something you ate.
What Prediabetes Means in Practice
A fasting glucose between 100 and 125 mg/dL, or an A1c between 5.7% and 6.4%, places you in the prediabetes category. Roughly 1 in 3 American adults meets this threshold, and most don’t know it. Prediabetes means your body is starting to struggle with glucose regulation, but the process is far from irreversible.
Lifestyle changes are remarkably effective at this stage. The landmark Diabetes Prevention Program trial found that participants who adopted moderate exercise and dietary improvements had a 58% reduction in their risk of developing type 2 diabetes over about three years. A broader analysis of 11 clinical trials found an even stronger effect: a 64% lower risk of progressing to diabetes with lifestyle interventions. Specific dietary patterns matter too. One large trial showed that a Mediterranean-style diet reduced new diabetes cases by 53% compared to a standard low-fat diet over four years.
The core changes that drive these results are consistent but not extreme: regular physical activity (about 150 minutes per week of brisk walking or similar), modest weight loss of 5% to 7% of body weight, and a diet emphasizing whole grains, vegetables, healthy fats, and lean protein. These shifts improve how your cells respond to insulin, often enough to bring glucose levels back into the normal range.