Blood sugar, or blood glucose, levels are typically measured using a scale of grams per deciliter (g/dL) or millimoles per liter (mmol/L). This level will tend to fluctuate throughout the course of a day, with the lowest readings during periods of fasting and the highest coming shortly after a meal. The normal blood sugar range for a healthy person is about 83 mg/dl (4.6 mmol/L) to 120 mg/dl (6.6 mmol/L).
A healthy person's body is able to regulate blood glucose levels very tightly, resulting in a predictable normal blood sugar range. This means that blood glucose levels will tend to normalize fairly quickly, even though they can rise sharply after a meal, during what is known as the postprandial period. For a person without a blood glucose disorder, a fasting blood sugar level should be about 83 mg/dl (4.6 mmol/L). This means that his or her blood glucose should be at, or below, this level when he or she first wakes up in the morning. For many healthy people, the fasting blood glucose level is lower, at around 70 mg/dl (3.9 mmol/L).
One to two hours after eating a meal, the blood sugar will typically spike. Within the normal blood sugar range, this shouldn't go any higher than 120 mg/dl (6.6 mmol/L). Many people experience an even lower postprandial blood sugar level, with readings of less than 100 mg/dl (5.5 mmol/L) two hours after eating. Meals that are higher in sugar or starch can affect the amount of the postprandial spike in blood glucose, or cause the increased level to last longer.
There is some disagreement in the scientific community as to what exactly constitutes a normal blood sugar range, and what might be a precursor to developing diabetes later on. While a fasting blood glucose level of 100 mg/dL (5.6 mmol) is often considered normal, it may also be an indicator of problems down the road. This may be useful as an early warning indicator, allowing an individual to made dietary and other lifestyle changes to avoid developing type 2 diabetes later in life.
Maintaining a normal blood sugar range can be much more difficult for an individual with diabetes. Diabetics may develop a resistance to insulin, which is the hormone in the blood that allows glucose to be broken down, or their bodies may simply not make enough of the compound. Those with diabetes will typically see much higher blood glucose levels both at fasting and postprandial states, as their body is not able to break down the glucose in the blood.