Blood sugar can go up during fasting due to several physiological reasons:
- When you fast, your body needs to maintain a steady supply of glucose to fuel your cells and organs, especially the brain. Since no food is coming in, your body starts to release stored glucose from the liver into the bloodstream. This process is triggered by hormones like glucagon.
- Several counter-regulatory hormones such as cortisol, growth hormone, adrenaline (noradrenaline), and glucagon increase during fasting. These hormones signal the liver to release glucose by breaking down glycogen (stored glucose) and also produce glucose through gluconeogenesis (making glucose from non-carbohydrate sources). This raises blood sugar even without eating.
- Insulin levels drop during fasting, which reduces cellular uptake of glucose, keeping more glucose available in the blood for energy.
- In people with diabetes or insulin resistance, this hormone response can be exaggerated or poorly regulated. For example, the "dawn phenomenon" is a natural early-morning increase in blood sugar due to hormonal changes, and people with diabetes may experience higher fasting blood sugar because their insulin response is impaired.
- Additional factors like stress hormones released during fasting and dehydration can also increase blood sugar concentration.
Overall, the rise in blood sugar during fasting is a normal response to maintain energy balance, primarily driven by hormone signals to release glucose from the liver and reduced insulin action. In healthy individuals, this is well controlled, but in diabetes or insulin resistance, it can cause fasting hyperglycemia (high blood sugar when not eating). This explanation aligns with medical sources that discuss fasting hyperglycemia and the hormonal mechanisms behind it.
