Blood sugar, also called blood glucose, is the main sugar found in your bloodstream. It comes primarily from the food you eat and serves as your body’s primary fuel source. Your cells rely on it to produce energy, and your body works constantly to keep it within a tight range of roughly 70 to 100 mg/dL when you haven’t eaten recently.
How Your Body Uses Blood Sugar
Nearly everything you eat eventually breaks down into glucose. Carbohydrates convert most directly, but proteins also contribute. Once glucose enters your bloodstream, it travels to cells throughout your body, where it’s converted into ATP, the molecule your cells use as energy currency. Think of glucose as the raw material and ATP as the usable electricity your cells actually run on.
Your body doesn’t always need energy right away, though. When glucose is available but not immediately needed, it gets stored in your liver and muscles as glycogen, a compact form your body can quickly break back down later. This stored reserve is what keeps you fueled between meals and overnight while you sleep.
How Insulin and Glucagon Keep Levels Stable
Your pancreas acts as the control center for blood sugar, releasing two hormones with opposite jobs. After you eat, rising glucose triggers your pancreas to release insulin, which essentially unlocks the door to your muscle and fat cells so glucose can move from the blood into those tissues. Insulin also signals your liver to store extra glucose as glycogen. The net effect: blood sugar comes back down.
Between meals or during sleep, the process reverses. Your pancreas releases glucagon, which tells the liver to break down stored glycogen and release glucose back into the bloodstream. During prolonged fasting, glucagon also prompts your liver and kidneys to manufacture new glucose from non-carbohydrate sources. This back-and-forth between insulin and glucagon keeps blood sugar within a remarkably narrow band of about 70 to 110 mg/dL in healthy people, a balance called glucose homeostasis.
Normal Blood Sugar Ranges
The numbers that define “normal” depend on when you last ate:
- Fasting (no food for 8+ hours): Below 100 mg/dL (5.6 mmol/L) is normal.
- Two hours after a meal: Below 140 mg/dL for most healthy adults. For people with diabetes, the target is typically below 180 mg/dL.
A fasting level between 100 and 125 mg/dL signals prediabetes. At 126 mg/dL or above on two separate tests, the diagnosis is diabetes.
There’s also a longer-term measurement called A1c (or HbA1c), which reflects your average blood sugar over roughly three months. An A1c below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher points to diabetes. Because it captures a broader picture, A1c is useful for detecting patterns that a single fasting test might miss.
What Happens When Blood Sugar Goes Too Low
Blood sugar below 70 mg/dL is considered low, a condition called hypoglycemia. It most commonly affects people taking insulin or certain diabetes medications, but it can also happen after intense exercise or prolonged fasting. Symptoms come on quickly: shakiness, sweating, confusion, irritability, a fast heartbeat, and in severe cases, loss of consciousness.
The standard approach for treating a mild low is the 15-15 rule. You consume 15 grams of fast-acting carbohydrates (four glucose tablets, half a cup of juice, or a tablespoon of honey), wait 15 minutes, then recheck. If your level is still below 70 mg/dL, you repeat. The goal is to bring blood sugar back into a safe range without overcorrecting.
What Happens When Blood Sugar Goes Too High
High blood sugar, or hyperglycemia, develops when the body can’t produce enough insulin or can’t use it effectively. In the short term, rising levels cause frequent urination, intense thirst, and unexplained weight loss. As blood sugar climbs further, neurological symptoms can follow: lethargy, difficulty concentrating, and altered mental status. In extreme cases, particularly in type 1 diabetes, the body starts breaking down fat for fuel and produces acidic byproducts, a dangerous state that can cause nausea, vomiting, abdominal pain, and a distinctive fruity breath odor.
The long-term damage from chronically elevated blood sugar is where the real danger lies. Persistently high levels injure small blood vessels, leading to damage in the eyes (retinopathy), kidneys (nephropathy), and nerves (neuropathy, often felt as tingling or numbness in the feet). Larger blood vessels suffer too, raising the risk of heart disease, stroke, and poor circulation in the legs. These complications develop gradually over years, which is why catching and managing high blood sugar early matters so much.
One notable threshold: at roughly 180 mg/dL, your kidneys can no longer reabsorb all the glucose being filtered from your blood, and glucose starts spilling into your urine. This is called the renal threshold, and it’s one reason frequent urination is such a hallmark of uncontrolled diabetes. Your body is literally flushing excess sugar out.
How Food Affects Blood Sugar
Not all foods raise blood sugar at the same speed or to the same degree. The glycemic index (GI) scores foods on a scale of 0 to 100 based on how quickly they cause blood sugar to rise, with pure glucose scoring 100. White bread and white rice score high, while most vegetables, legumes, and whole grains score lower.
But the glycemic index only tells half the story, because it doesn’t account for portion size. That’s where glycemic load comes in. Glycemic load factors in both how fast a food raises blood sugar and how much carbohydrate a typical serving actually contains. Watermelon is a good example: it has a high glycemic index of 80, but a serving contains so little carbohydrate that its glycemic load is only 5. In practice, eating a slice of watermelon barely moves the needle. Paying attention to both measures gives you a much more accurate sense of how a particular food will affect your blood sugar in real life.
How Blood Sugar Is Measured
The most common method is a fingerstick glucose meter. You prick your fingertip, place a drop of blood on a test strip, and the meter gives a reading in seconds. This measures glucose directly in capillary blood and provides a snapshot of your level at that exact moment.
Continuous glucose monitors (CGMs) take a different approach. A small sensor inserted just under the skin measures glucose in the interstitial fluid (the liquid surrounding your cells) every few minutes and transmits the data to a phone or receiver. This creates a real-time graph of your glucose trends throughout the day and night, which is far more informative than isolated fingerstick readings. One thing to know: because CGMs measure interstitial fluid rather than blood, there’s a 5- to 20-minute lag before the reading catches up with your actual blood glucose level. This gap matters most when blood sugar is changing rapidly, like right after a meal or during exercise.
For the three-month average, your doctor orders an A1c blood draw, which measures how much glucose has attached to your red blood cells over their lifespan. No fasting required, and it’s a single number that summarizes weeks of blood sugar behavior in one result.