Diabetes mellitus is a group of chronic conditions in which the body cannot properly regulate blood sugar (glucose), leading to persistently elevated levels that damage organs over time. It affects roughly 589 million adults worldwide, about 11% of the global population, and that number is projected to reach 853 million by 2050. While most people have heard of “Type 1” and “Type 2,” diabetes actually encompasses several distinct diseases with different causes, all sharing the common problem of too much glucose in the blood.
How Insulin and Blood Sugar Normally Work
Your cells need glucose for energy, and insulin is the hormone that lets glucose move from your bloodstream into those cells. Insulin is produced by beta cells in the pancreas. These beta cells act as glucose sensors: when blood sugar rises after a meal, they detect the increase and release insulin in proportion to how much glucose is present. Insulin then signals muscle, liver, and fat cells to absorb that glucose, bringing blood sugar back down to a normal range.
Diabetes develops when this system breaks down. Either the beta cells are destroyed and stop making insulin, or the body’s cells stop responding to insulin properly (a state called insulin resistance), or both. Without functioning insulin signaling, glucose accumulates in the blood while cells are starved for energy.
Type 1 Diabetes
Type 1 diabetes is an autoimmune disease. The immune system mistakenly attacks and destroys the insulin-producing beta cells in the pancreas, driven by a combination of genetic susceptibility and environmental triggers. Once enough beta cells are gone, the body produces little to no insulin. People with Type 1 require insulin from the time of diagnosis and for the rest of their lives.
Type 1 most commonly appears in childhood or adolescence, though it can develop at any age. It accounts for a small fraction of all diabetes cases. Symptoms tend to come on quickly: excessive thirst, frequent urination, unintended weight loss, and fatigue that worsens over days to weeks.
Type 2 Diabetes
Type 2 diabetes is far more common and develops gradually. It begins with insulin resistance, meaning muscle, liver, and fat cells don’t respond well to insulin. In the early stages, the pancreas compensates by producing extra insulin to keep blood sugar in a normal range. Over time, this compensatory effort falters. Beta cell function declines, insulin production drops, and blood sugar rises. By the time Type 2 is diagnosed, the disease has often been progressing silently for years.
Risk factors include excess body weight, physical inactivity, family history, and age, though Type 2 is increasingly diagnosed in younger adults and even teenagers. Unlike Type 1, many people with Type 2 still produce some insulin, so treatment initially focuses on improving insulin sensitivity and lowering blood sugar through lifestyle changes and oral medications. Some people eventually need insulin as their beta cells wear out.
Gestational Diabetes
Gestational diabetes develops during pregnancy, typically detected through screening at 24 weeks of gestation or after. Hormonal changes during pregnancy can cause insulin resistance, and in some women, the pancreas can’t keep up with the increased demand. Blood sugar usually returns to normal after delivery, but having gestational diabetes significantly raises the risk of developing Type 2 diabetes later in life. Screening generally involves drinking a glucose solution and having blood sugar measured afterward.
Less Common Forms
Not all diabetes fits neatly into Type 1 or Type 2. Maturity-onset diabetes of the young (MODY) is caused by a single inherited gene mutation and typically appears before age 30 in people who are not overweight. It runs strongly in families: if one parent carries the mutation, each child has a 50% chance of inheriting it. Some forms of MODY cause only mildly elevated blood sugar that never requires treatment, while others need active management. MODY is often misdiagnosed as Type 1 or Type 2, and genetic testing is needed to confirm it.
Common Symptoms and Why They Happen
The hallmark symptoms of diabetes all trace back to excess glucose in the blood:
- Frequent urination: When blood sugar exceeds what the kidneys can reabsorb, glucose spills into the urine, pulling water along with it.
- Excessive thirst: The fluid lost through increased urination triggers dehydration and intense thirst.
- Persistent hunger: Because cells can’t access glucose for energy, the body signals for more food even after eating.
- Fatigue: Without adequate glucose entering cells, energy levels drop.
- Blurred vision: High blood sugar can cause the lens of the eye to swell, distorting focus.
- Slow-healing wounds and frequent infections: Elevated glucose impairs immune function and blood flow to tissues.
In Type 1, these symptoms tend to appear suddenly and severely. In Type 2, they often develop so gradually that people attribute them to aging or stress and go undiagnosed for years.
How Diabetes Is Diagnosed
Diagnosis relies on blood tests that measure glucose levels or a longer-term marker of blood sugar control. The American Diabetes Association recognizes several tests, each with specific cutoffs for normal, prediabetes, and diabetes:
- A1C (glycated hemoglobin): Reflects average blood sugar over the previous 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 plasma glucose: Measured after at least eight hours without eating. Normal is below 100 mg/dL, prediabetes is 100 to 125 mg/dL, and diabetes is 126 mg/dL or higher.
- Oral glucose tolerance test: Blood sugar is measured two hours after drinking a glucose solution. Normal is below 140 mg/dL, prediabetes is 140 to 199 mg/dL, and diabetes is 200 mg/dL or higher.
- Random plasma glucose: A reading of 200 mg/dL or higher at any time, combined with symptoms, indicates diabetes.
Prediabetes is a critical window. Blood sugar is elevated but not yet high enough for a diabetes diagnosis, and lifestyle changes at this stage can significantly delay or prevent progression to Type 2.
Long-Term Complications
Chronically elevated blood sugar damages blood vessels throughout the body, and the consequences are divided into two categories based on which vessels are affected.
Small Blood Vessel Damage
The eyes, kidneys, and nerves are especially vulnerable. Diabetic retinopathy, in which blood vessels in the retina leak or grow abnormally, is a leading cause of blindness in people with diabetes. Diabetic kidney disease develops in roughly one quarter of people with Type 2 diabetes, starting with small amounts of protein leaking into the urine and potentially progressing to kidney failure over years. Nerve damage, particularly in the feet, causes numbness, tingling, and pain that can lead to unnoticed injuries and infections.
Large Blood Vessel Damage
Cardiovascular disease is the leading cause of death in people with Type 2 diabetes, accounting for approximately 70% of deaths in this group. People with diabetes have a four-fold greater risk of cardiovascular events compared to those without diabetes, even after accounting for other risk factors like high blood pressure and cholesterol. The risk of a first heart attack is five times higher, and outcomes after a heart attack tend to be worse, with greater rates of heart failure.
How Diabetes Is Managed
Management looks different depending on the type and stage of diabetes, but the foundation is the same: keeping blood sugar as close to normal as possible to prevent complications.
For Type 1 diabetes, insulin replacement is essential and non-negotiable. This means multiple daily injections or an insulin pump, combined with frequent blood sugar monitoring.
For Type 2 diabetes, treatment typically starts with lifestyle changes. Regular exercise, a balanced diet, and adequate sleep (seven to eight hours per night) can meaningfully improve insulin sensitivity and lower blood sugar. When lifestyle changes alone aren’t enough, medications are added. The most widely used first-line drug works by reducing the amount of glucose the liver releases into the blood and improving insulin sensitivity. It’s effective, inexpensive, and well-tolerated for most people.
Beyond that first-line option, several other drug classes are available. Some work by helping the kidneys flush excess glucose out through urine, with added benefits for heart and kidney health. Others mimic gut hormones that stimulate insulin release after meals while also promoting weight loss. Still others boost the body’s natural hormones that regulate blood sugar, slow carbohydrate digestion to prevent post-meal spikes, or directly stimulate the pancreas to release more insulin. The choice depends on a person’s other health conditions, weight, kidney function, and risk profile.
Regardless of type, ongoing monitoring is central to diabetes management. Regular A1C testing tracks long-term blood sugar control, and many people use home glucose meters or continuous glucose monitors to track daily patterns and adjust their food, activity, or medication accordingly.