Blood glucose is produced by digesting food, specifically carbohydrates. It circulates in the blood until it is absorbed by the brains, muscles and other tissues as a source of energy. Insulin is required for the glucose to be absorbed. If there’s not enough insulin, some of the glucose is not absorbed and remains in the blood.
Blood glucose is either measured in mg/dl (milligrams per deciliter) or mmol/L (millimoles per liter) units.
The normal blood glucose range while fasting (about 8 hours after a meal) is 70 to 100 mg/dl, or 4.5 to 6 mmol/L.
During and after a meal, the concentration of blood glucose in the blood temporary rises above 100 mg/dL, or 6 mmol/L, as nutrients are being digested. The pancreas then releases just enough insulin for the glucose to be absorbed by the body. Although muscles can store glucose for later use, the brain cannot store any, and constantly absorbs glucose in order to work. For that reason, there’s always some glucose (the liver can produce some between meals) and insulin in the blood stream, although the levels are low while we sleep. The pancreas constantly monitors the level of glucose and releases insulin accordingly.
The picture below shows the glucose (solid red line) and insulin (solid blue line) levels throughout the day. As food is digested, glucose is released in the blood stream, and the pancreas produces insulin for the body to absorb the glucose. The pancreas and the liver ensure that there’s always glucose in the blood stream, although in lower quantity during sleep. The figure also shows the the amount of glucose and insulin when consuming simple carbs (dotted lines) as opposed to complex carbs (solid lines): simple carbs are absorbed faster (more glucose in the blood in a shorter period of time) and results in a strong insulin response, which leads to a low glucose level within 1-2 hours (sometimes called as a “low” where people feel tired). Instead, consuming complex carbs leads to a much more moderate glucose release, providing energy throughout the day and avoid “lows”. Complex carbs (e.g. whole wheat bread) should be preferred over simple carbs (e.g. white bread) as repeated glucose swings from simple carbs consumption can lead to Type 2 diabetes over time.