Diabetes is a chronic metabolic disease in which your blood sugar (glucose) stays too high because your body either doesn’t produce enough insulin or can’t use the insulin it makes. As of 2024, about 589 million adults worldwide are living with diabetes, roughly one in nine people. That number is projected to reach 853 million by 2050.
How Insulin and Blood Sugar Work Together
Every cell in your body needs glucose for energy. When you eat, your digestive system breaks carbohydrates into glucose and releases it into your bloodstream. Your pancreas responds by releasing insulin, a hormone that acts like a key: it signals cells in your muscles, liver, and fat tissue to open up and absorb that glucose. Specifically, insulin triggers transport proteins inside those cells to move to the cell surface, where they pull glucose in from the blood.
When this system breaks down, glucose builds up in the bloodstream instead of entering cells. That excess blood sugar is what defines diabetes, and over time it damages blood vessels, nerves, and organs throughout the body.
Types of Diabetes
Type 1
In type 1 diabetes, the immune system mistakenly attacks and destroys the insulin-producing cells in the pancreas. The result is little to no insulin production, which means the body has no way to move glucose into cells on its own. People with type 1 diabetes need to take insulin every day to survive. It can appear at any age but is most often diagnosed in children and young adults.
Type 2
Type 2 is the most common form, typically developing in adults. People with type 2 diabetes do produce insulin, at least initially, but their cells stop responding to it properly. This is called insulin resistance: the cells in muscles, liver, and fat tissue essentially ignore insulin’s signal. The pancreas tries to compensate by making more insulin, but over time it can’t keep up. The combination of resistance and declining production causes blood sugar to rise.
Gestational Diabetes
Gestational diabetes develops during pregnancy in people who didn’t previously have diabetes. Hormonal changes during pregnancy can make cells more resistant to insulin. It usually resolves after delivery, but it significantly raises the risk of developing type 2 diabetes later in life. Screening typically happens between 24 and 28 weeks of pregnancy, though people with a history of gestational diabetes or other risk factors may be tested earlier.
LADA (Type 1.5)
Latent autoimmune diabetes in adults, sometimes called type 1.5, shares features of both type 1 and type 2. Like type 1, it’s caused by immune system antibodies that attack insulin-producing cells in the pancreas. But like type 2, it develops gradually and is usually diagnosed in adults between ages 30 and 50. Because the progression is slow, it’s often initially misdiagnosed as type 2. A blood test that checks for specific antibodies can distinguish LADA from type 2 diabetes.
Diagnostic Thresholds
Diabetes is diagnosed using blood tests that measure how well your body processes glucose. Three tests are commonly used, each with specific cutoffs:
- A1C test: Reflects your average blood sugar over the past two to three months. Normal is below 5.7%, prediabetes falls between 5.7% and 6.4%, and diabetes is diagnosed at 6.5% or higher.
- Fasting blood sugar: Measures glucose after an overnight fast. Normal is below 100 mg/dL, prediabetes is 100 to 125 mg/dL, and 126 mg/dL or above indicates diabetes.
- Glucose tolerance test: Measures blood sugar two hours after drinking a sugary solution. Normal is below 140 mg/dL, prediabetes is 140 to 199 mg/dL, and 200 mg/dL or above means diabetes.
What Prediabetes Means
Prediabetes is the stage where blood sugar is higher than normal but hasn’t crossed into diabetes territory. It’s a warning sign that the body’s insulin system is struggling. The ranges sit between normal and diabetic thresholds on all three tests listed above. Prediabetes doesn’t always progress to type 2 diabetes. Changes to diet, physical activity, and weight can bring blood sugar back to normal levels and significantly reduce the risk of progression.
Common Symptoms
Many of diabetes’s hallmark symptoms trace directly back to excess glucose in the blood. When blood sugar rises too high, your kidneys work to filter out the extra glucose by producing more urine. That frequent urination leads to fluid loss, which triggers increased thirst as your body tries to compensate. Other common signs include unexplained weight loss, blurred vision, fatigue, and slow-healing wounds.
Type 1 symptoms tend to come on suddenly over days or weeks. Type 2 often develops so gradually that people live with it for years without knowing. LADA follows a similarly slow pattern, which is part of why it gets mistaken for type 2.
Long-Term Complications
Poorly controlled blood sugar damages blood vessels over time. The consequences fall into two broad categories based on which blood vessels are affected.
Damage to small blood vessels can lead to three major problems. The first is eye disease: high blood sugar weakens the tiny blood vessels in the retina, initially causing small bulges in vessel walls and potentially progressing to abnormal blood vessel growth that threatens vision. The second is kidney damage, where filtering structures in the kidneys gradually lose function, potentially leading to kidney failure. The third is nerve damage, which most commonly causes numbness, tingling, or pain in the hands and feet but can also affect digestion, heart rate, and other automatic body functions.
Damage to large blood vessels accelerates the buildup of fatty deposits in artery walls, raising the risk of heart attack, stroke, and poor circulation in the legs. Heart disease is the leading cause of death among people with diabetes.
Beyond blood vessel damage, chronically high blood sugar weakens the immune system’s ability to fight infection, making bacterial and fungal infections more common. It can also cause fat buildup in the liver, which in some cases leads to inflammation and scarring of liver tissue.
How Each Type Is Managed
Type 1 diabetes requires insulin replacement for life, delivered through injections or an insulin pump. People with type 1 also monitor their blood sugar multiple times a day, often with a continuous glucose monitor worn on the skin that tracks levels in real time.
Type 2 diabetes management usually starts with lifestyle changes: regular physical activity, dietary adjustments that reduce blood sugar spikes, and weight loss when needed. Many people with type 2 also take oral medications that help the body use insulin more effectively or prompt the pancreas to produce more. Some eventually need insulin as the disease progresses and the pancreas produces less on its own.
Gestational diabetes is typically managed with dietary changes and blood sugar monitoring. If blood sugar stays above target levels, insulin may be added during the pregnancy. After delivery, blood sugar usually returns to normal, but ongoing screening for type 2 diabetes is recommended.