Diabetes develops through different pathways depending on the type. Type 1 diabetes results from the immune system destroying insulin-producing cells in the pancreas. Type 2 diabetes, which accounts for the vast majority of cases, develops when your body gradually loses the ability to use insulin effectively, usually over years of metabolic stress. As of 2024, an estimated 589 million adults worldwide are living with diabetes, and the number continues to climb.
How Type 1 Diabetes Develops
Type 1 diabetes is an autoimmune condition. Your immune system mistakenly targets and kills the beta cells in your pancreas, the only cells that produce insulin. This process begins months or even years before symptoms appear. Researchers can detect autoantibodies (immune proteins directed against beta cell components) in a person’s blood long before blood sugar levels become abnormal.
What’s unusual about type 1 compared to other autoimmune diseases is how little immune activity it takes. Fewer than 10% of the insulin-producing cell clusters in the pancreas become infiltrated by immune cells, and the number of attacking cells is only about twice what you’d find in a healthy person. Despite this relatively quiet assault, it’s enough to wipe out insulin production over time.
Type 1 has both genetic and environmental triggers. A person needs a genetic predisposition, but something in the environment appears to set the process in motion. Enteroviruses, particularly a group called coxsackieviruses, are among the strongest viral suspects. These viruses can infect beta cells directly, impairing their function and even killing them. Rotaviruses, certain herpesviruses, and even SARS-CoV-2 have been linked to increased risk. A meta-analysis of over 38,000 newly diagnosed cases in children found that the incidence of type 1 diabetes was 14% higher in the first year and 27% higher in the second year after the start of the COVID-19 pandemic. Other possible environmental triggers include early introduction of gluten in infancy and respiratory infections during the first year of life.
Type 1 can appear at any age, though it’s most commonly diagnosed in children and young adults. It is not caused by diet or lifestyle choices.
How Type 2 Diabetes Develops
Type 2 diabetes is a slow-building condition driven by insulin resistance. Your cells, particularly in your muscles and liver, stop responding normally to insulin. Insulin is the hormone that signals cells to absorb sugar from the blood. When cells ignore that signal, your pancreas compensates by producing more and more insulin. Eventually, the pancreas can’t keep up, and blood sugar rises.
The liver plays a central role. Normally, insulin tells the liver to stop releasing stored sugar into the bloodstream. In someone with insulin resistance, that signal doesn’t get through. The liver keeps producing glucose even when blood sugar is already high, which is the primary reason fasting blood sugar levels are elevated in type 2 diabetes.
Fat buildup in the liver and muscles is a key driver of this resistance. When excess fat accumulates in these organs (rather than staying in fat tissue where it’s meant to be stored), it generates lipid byproducts that interfere with insulin signaling at the molecular level. This is why you don’t need to be severely overweight to develop type 2 diabetes. What matters is where fat accumulates, not just how much you carry.
The Role of Belly Fat and Inflammation
Visceral fat, the deep abdominal fat surrounding your organs, is especially problematic. Unlike fat stored under the skin, visceral fat tissue becomes inflamed as it expands. Immune cells called macrophages infiltrate the fat tissue and release inflammatory molecules. This chronic, low-grade inflammation directly worsens insulin resistance throughout the body. Both the innate and adaptive branches of the immune system participate in this process, creating a feedback loop: more fat leads to more inflammation, which leads to worse insulin sensitivity, which leads to higher blood sugar and more fat storage.
Genetics vs. Lifestyle in Type 2
Type 2 diabetes is not purely a lifestyle disease. Twin and family studies estimate its heritability at 31% to 72%, meaning genetics play a substantial role in determining who is vulnerable. If you have a parent or sibling with type 2, your risk is significantly higher regardless of your habits.
That said, environmental factors are what push a genetically susceptible person across the threshold. A diet high in processed foods, physical inactivity, and weight gain (particularly around the midsection) are the most well-established triggers. Age is another major factor. Older adults are at higher risk because insulin sensitivity naturally declines over time, and pancreatic beta cell function gradually weakens. The interplay is straightforward: genetics loads the gun, and lifestyle pulls the trigger.
The Prediabetes Stage
Most people who develop type 2 diabetes pass through a stage called prediabetes first. During this window, blood sugar is higher than normal but not yet in the diabetic range. The American Diabetes Association defines prediabetes as a fasting blood sugar of 100 to 125 mg/dL, a two-hour glucose tolerance reading of 140 to 199 mg/dL, or an A1C between 5.7% and 6.4%.
Prediabetes is not a guaranteed path to diabetes. In a large pooled analysis of 19 studies, about 8% of younger men with prediabetes progressed to type 2 diabetes within five years. Roughly 38% reverted to normal blood sugar levels, and just over half stayed in the prediabetic range. The trajectory depends heavily on what you do during this window, particularly whether you increase physical activity and reduce excess weight. Even modest changes (losing 5% to 7% of body weight) can dramatically reduce the odds of progression.
Other Causes of Diabetes
Some people develop diabetes through mechanisms that aren’t neatly type 1 or type 2. Gestational diabetes occurs during pregnancy when hormonal changes increase insulin resistance beyond what the pancreas can handle. It usually resolves after delivery but significantly raises the mother’s long-term risk of developing type 2.
Certain medications can push blood sugar into the diabetic range. Antipsychotic drugs, both first- and second-generation, are well known for causing weight gain and metabolic disruption. Some antiretroviral medications used in HIV treatment, particularly older protease inhibitors, can do the same. Chronic conditions that damage the pancreas directly, such as pancreatitis or cystic fibrosis, can also lead to diabetes by reducing the organ’s ability to produce insulin.
How Diabetes Is Diagnosed
Diabetes is confirmed through blood tests that measure how well your body manages sugar. The three standard tests each have clear cutoffs:
- A1C: This reflects your average blood sugar over the past two to three months. An A1C of 6.5% or higher indicates diabetes. Between 5.7% and 6.4% is prediabetes.
- Fasting blood sugar: A reading of 126 mg/dL or higher after an overnight fast means diabetes. Between 100 and 125 mg/dL is prediabetes.
- Oral glucose tolerance test: After drinking a sugary solution, a two-hour blood sugar of 200 mg/dL or higher confirms diabetes. Between 140 and 199 mg/dL is prediabetes.
Doctors typically repeat the test on a separate day to confirm the diagnosis unless symptoms are already obvious (extreme thirst, frequent urination, unexplained weight loss). Type 1 is further distinguished from type 2 through autoantibody testing, which detects the immune markers specific to autoimmune beta cell destruction.