What Is Diabetes? Sugar, Insulin and the Two Diseases That Share a Name
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Glucose is the body's fuel, and it is also, in the wrong amount, a slow poison. Every cell needs it, and a hormone called insulin, made in the pancreas, is the key that lets it in; when the key is missing or the lock has stiffened, sugar builds up in the blood while the cells go hungry, and the excess over years damages eyes, kidneys, nerves and arteries. That condition is diabetes, and about one adult in ten in the world now has it, most of them a form that barely existed a century ago and that is largely a consequence of how people now live.
Insulin and the lock
After a meal, glucose from digested food enters the blood and the pancreas responds by releasing insulin from clusters of cells called the islets. Insulin travels to muscle, fat and liver cells and binds to receptors on their surface, which signal the cell to bring transporters to its membrane and take glucose in, to burn or to store. Between meals insulin falls, the liver releases stored glucose, and the level in the blood stays within a narrow band. The name of the disease comes from the Greek for siphon, because the ancient physicians who described it noticed that patients passed enormous quantities of urine, and the mellitus, honeyed, was added when someone tasted it. The kidneys, unable to hold back the excess sugar, let it out with water, which is why unchecked diabetes brings thirst, constant urination and weight loss.
Two diseases
The two main types share the symptom and differ in the cause:
- •Type 1: the immune system destroys the insulin-producing cells, usually in childhood or youth, for reasons not fully understood; the body makes no insulin at all, and without injections the patient dies within weeks. It accounts for about one case in ten and is not caused by diet or weight
- •Type 2: the cells stop responding properly to insulin, a state called insulin resistance, and the pancreas, working harder to compensate, eventually cannot keep up. It develops slowly in adults, is strongly linked to excess body fat, inactivity, age and genes, and is nine cases in ten
- •Gestational diabetes: insulin resistance during pregnancy, which usually resolves at birth and raises the mother's later risk of type 2
- •Rarer forms from single-gene defects, pancreatic disease or steroid drugs
What high sugar does
Glucose in excess sticks to proteins throughout the body, a process related to the browning of toast, and the altered proteins stiffen blood vessels and trigger inflammation. Over years the smallest vessels suffer first: in the retina, where leaking and abnormal growth make diabetes the leading cause of blindness in working-age adults; in the kidneys, where it is the leading cause of kidney failure; and in the nerves, where loss of sensation in the feet, combined with poor circulation, lets small injuries become ulcers and is the reason for most non-traumatic amputations. The large vessels suffer too, and heart attack and stroke are two to four times more common. All of this is a matter of degree and time, and the single most important finding in the disease's modern history is that keeping blood sugar close to normal cuts every one of these complications sharply.
Treatment
Before 1922 type 1 diabetes was fatal, and the only treatment was a starvation diet that postponed death by a year or two. In January that year a fourteen-year-old boy in Toronto received an extract of dog pancreas prepared by Frederick Banting and Charles Best, and his blood sugar fell to normal; within months insulin was being made from cattle and pigs at industrial scale, and the discoverers won the Nobel Prize in 1923, having sold the patent to the university for a dollar so that it could not be monopolised. Type 1 is treated with insulin, now made by genetically engineered bacteria and delivered by pens or pumps, with continuous sensors under the skin that read the sugar every few minutes, and the first systems that close the loop, adjusting the dose automatically, are in use. Type 2 begins with weight loss, diet and exercise, which can put it into remission if caught early, and then with tablets, of which metformin has been the first choice since the 1950s; the newer drugs that mimic a gut hormone, developed for diabetes and now famous for weight loss, lower sugar, weight and heart risk at once.
Why it is spreading
The number of adults with diabetes has quadrupled since 1980, to over half a billion, and the rise is almost entirely type 2, tracking the rise in obesity and the fall in physical work. Countries that have industrialised fastest have been hit hardest: China and India together hold a third of the world's cases, and in some Pacific islands a third of adults are affected. Genes set the vulnerability, and some populations develop the disease at lower weights than others, but the trigger is environmental, and the same person who develops type 2 in a city with cars and processed food would in most cases not have developed it on a farm. It is the clearest example in medicine of a disease made by prosperity, and the one where the arithmetic of prevention, a walk a day and a smaller plate, is best established and least followed.
The takeaway
Diabetes is chronically high blood sugar caused either by the loss of the pancreatic cells that make insulin, in type 1, or by the body's cells ceasing to respond to insulin, in type 2, which is nine cases in ten and tied to excess weight and inactivity. Excess glucose damages the vessels of the eyes, kidneys, nerves and heart over years, and controlling it prevents most of that harm; type 1 is treated with insulin, discovered in 1922, and type 2 with weight loss, exercise, metformin and newer hormone-mimicking drugs.