What Is Blood Sugar and Why Does It Matter?
Blood sugar, or blood glucose, is the amount of sugar (glucose) circulating in your bloodstream at any given time. Your body uses glucose as its primary fuel source — every cell, tissue, and organ depends on it to function.
When you eat carbohydrates, your digestive system breaks them down into glucose, which enters the bloodstream. In response, your pancreas releases insulin — a hormone that helps cells absorb glucose for energy. When this system works correctly, blood sugar rises after meals and returns to normal within 1–2 hours.
When blood glucose stays persistently high (hyperglycemia) or drops dangerously low (hypoglycemia), it signals a problem — either with insulin production, insulin sensitivity, or both. Over time, uncontrolled high blood sugar causes serious complications including nerve damage, kidney disease, and cardiovascular problems.
Normal Blood Sugar Levels by Age Group
Blood sugar reference ranges are consistent across most age groups for healthy individuals, but context matters. Fasting levels, post-meal levels, and HbA1c each tell a different part of the story.
Adults (18–64 years)
| Test Type | Normal | Prediabetes | Diabetes |
|---|---|---|---|
| Fasting (mg/dL) | 70–99 | 100–125 | 126+ |
| Fasting (mmol/L) | 3.9–5.5 | 5.6–6.9 | 7.0+ |
| 2-hr Post-meal (mg/dL) | Below 140 | 140–199 | 200+ |
| 2-hr Post-meal (mmol/L) | Below 7.8 | 7.8–11.0 | 11.1+ |
| HbA1c (%) | Below 5.7% | 5.7–6.4% | 6.5%+ |
Older Adults (65+ years)
For seniors, many clinical guidelines — including those from the American Geriatrics Society — recommend slightly less aggressive blood sugar targets. This is because very low blood sugar (hypoglycemia) in older adults carries higher risks, including falls and confusion.
| Test Type | Healthy Senior | Complex/Fragile Health |
|---|---|---|
| Fasting (mg/dL) | 80–130 | 90–150 |
| HbA1c target | Below 7.0–7.5% | Below 8.0% |
| Bedtime (mg/dL) | 90–150 | 100–180 |
Children and Teenagers (under 18)
| Age Group | Fasting (mg/dL) | Before Meals | Bedtime |
|---|---|---|---|
| Toddlers (0–5 yrs) | 100–200 | 100–180 | 110–200 |
| School age (6–12 yrs) | 90–180 | 90–180 | 100–180 |
| Teenagers (13–19 yrs) | 90–130 | 90–130 | 90–150 |
Fasting vs Post-Meal Blood Sugar: What's the Difference?
Understanding when you measure your blood sugar is just as important as the number itself.
Fasting blood sugar is measured after at least 8 hours without food or drink (other than water). It reflects your body's baseline glucose management and is the standard test used to screen for diabetes during routine checkups.
Post-meal (postprandial) blood sugar is typically measured 2 hours after starting a meal. It shows how well your body handles the glucose from food. Even people with normal fasting levels can have abnormally high post-meal spikes — a condition called impaired glucose tolerance.
HbA1c — Your 3-Month Blood Sugar Average
The HbA1c test (glycated haemoglobin) measures your average blood sugar over the past 2–3 months. Unlike a finger-prick test that shows a single moment in time, HbA1c reveals the full picture of your glucose control.
It works by measuring the percentage of haemoglobin proteins in your red blood cells that are coated with sugar. The higher your average blood glucose, the higher the percentage.
| HbA1c (%) | HbA1c (mmol/mol) | Interpretation |
|---|---|---|
| Below 5.7% | Below 39 | Normal — no diabetes |
| 5.7% – 6.4% | 39–47 | Prediabetes — act now |
| 6.5% and above | 48+ | Diabetes diagnosed |
| Below 7.0% | Below 53 | Good control (if diabetic) |
Signs Your Blood Sugar Is Too High or Too Low
- Excessive thirst and dry mouth
- Frequent urination
- Blurred vision
- Fatigue and weakness
- Headaches
- Slow-healing wounds
- Shakiness and trembling
- Sweating and chills
- Rapid heartbeat
- Irritability or anxiety
- Pale skin
- Confusion or difficulty concentrating