Blood Sugar and Insulin: A Plain-English Guide to Glucose Metabolism

Blood sugar and insulin are discussed constantly in nutrition — but often inaccurately. Understanding how glucose metabolism actually works clarifies why some dietary approaches succeed, what causes energy crashes, and how to prevent the long-term consequences of chronically poor blood sugar control.

The Basics: What Is Blood Sugar?

Blood glucose — "blood sugar" — is the concentration of glucose in your bloodstream, measured in mmol/L (or mg/dL in the US). Your body maintains blood glucose within a tight range (4–6 mmol/L fasting in healthy adults) because glucose is the primary fuel for your brain, red blood cells, and during intense exercise, your muscles.

Normal reference ranges:

• Fasting (8+ hours): 3.9–5.6 mmol/L (70–100 mg/dL)
• 2 hours post-meal: <7.8 mmol/L (<140 mg/dL)
• Prediabetes (fasting): 5.6–6.9 mmol/L
• Diabetes (fasting): ≥7.0 mmol/L

What Is Insulin?

Insulin is a peptide hormone produced by beta cells in the pancreas. When blood glucose rises (after eating carbohydrates), insulin is secreted in proportion to the glucose load. Insulin acts as a "key" that unlocks cells — allowing glucose to enter muscle cells (for energy), liver cells (for glycogen storage), and fat cells (for lipogenesis/fat storage).

Without insulin, cells cannot absorb glucose effectively — this is the situation in type 1 diabetes (insulin production fails) and eventually type 2 diabetes (cells become resistant to insulin's signal).

The Blood Sugar Cycle

1. You eat carbohydrates → they're digested to glucose → absorbed into the bloodstream
2. Blood glucose rises → pancreas secretes insulin
3. Insulin drives glucose into cells → blood glucose falls
4. As glucose returns to baseline, insulin drops → glucagon rises
5. Glucagon signals liver to release stored glycogen (glycogenolysis) → maintains blood sugar between meals
6. Extended fasting → liver begins producing glucose from non-carbohydrate sources (gluconeogenesis)

Energy Crashes Explained

Post-meal energy crashes ("reactive hypoglycaemia") occur when a large carbohydrate load — especially from refined, low-fibre sources — spikes blood glucose rapidly. The pancreas overresponds with a large insulin surge, which drives blood glucose below baseline. The result: fatigue, irritability, and cravings within 1–2 hours of eating.

How to Maintain Stable Blood Sugar

• Eat protein and fat with every meal — both slow gastric emptying and blunt glucose spikes
• Choose high-fibre carbohydrates — fibre slows carbohydrate absorption significantly
• Don't skip meals — long gaps increase counter-regulatory hormone release and promote overeating at the next meal
• Exercise regularly — muscle contractions drive glucose uptake independently of insulin
• Manage sleep and stress — both cortisol and sleep deprivation impair insulin sensitivity

The Bottom Line

Blood sugar regulation is the foundation of energy, weight management, and metabolic health. Prioritise whole foods, fibre, protein, and regular exercise — and your body's glucose management systems will do the rest effectively.