Understanding how your body normally handles sugar will help you to understand what diabetes is and how it occurs.

Sugar in your blood, called glucose, comes from two major sources: the food you eat and your liver. During digestion, sugar is absorbed into your bloodstream from food particles in your stomach and small intestine. This sugar is vital to your health because it's the main source of energy for individual cells that make up your muscles and tissues. However, to do its job, glucose requires a companion called insulin. The hormone insulin comes from tiny cells in your pancreas called beta cells. These cells reside in isolated masses of tissue called islets (EYE-lets). When you eat, your pancreas responds by secreting insulin into your bloodstream. As it circulates, insulin acts like a key, unlocking microscopic doors that allow sugar to enter your cells. By allowing sugar into your cells, insulin lowers the amount of sugar in your bloodstream and prevents it from reaching high levels. As your blood sugar level drops, so does the secretion of insulin from your pancreas.

Your liver, meanwhile, acts as a glucose storage and manufactur­ing center. When the level of insulin in your blood is high, such as after a meal, your liver stores extra sugar in case your cells need it later. When insulin levels in your blood are low, such as when you haven't eaten in a while, your liver converts the stored sugar (glycogen) into glucose and releases it into your bloodstream to keep your blood sugar level within a narrow and safe range.

In addition to insulin, several other hormones affect your blood sugar level - but in the opposite manner. In certain circum­stances, hormones such as glucagon, epinephrine and cortisol counteract the effects of insulin, preventing glucose from entering your cells. The hormones also encourage your liver to release its stored sugar. As a result, your body is continuously coordinating the effects of all of these hormones to keep your blood sugar within a normal range.

In people with diabetes, this precisely balanced process runs afoul. Instead of being transported into your cells, glucose remains in your bloodstream, accumulates and eventually is excreted in your urine. This most often occurs for one of two reasons: Your pancreas is unable to produce insulin or your cells are unrespon­sive to insulin's effects.

The medical term for this condition is diabetes mellitus. Mellitus is a Latin word meaning "honey sweet," referring to excess sugar in your blood and urine. Another form of diabetes, called diabetes insipidus, is much less common. Rather than an insulin problem, it results from a hormone disorder that causes your body to lose con­trol of its water balance, producing increased urination and exces­sive thirst. In this book, we are referring to diabetes mellitus.