The main cause of diabetes depends on the type. Type 2 diabetes, which accounts for roughly 90% to 95% of all cases, is primarily caused by insulin resistance, a condition where the body’s cells gradually stop responding to insulin. Type 1 diabetes is caused by an autoimmune attack that destroys the cells in the pancreas responsible for making insulin. These are fundamentally different diseases that share a common result: blood sugar levels that climb too high and stay there.
As of 2024, diabetes affects about 589 million adults worldwide, or just over 11% of the global adult population. That number is projected to reach 853 million by 2050.
Type 2 Diabetes: Insulin Resistance
In a healthy body, your pancreas releases insulin after you eat, and that insulin acts like a key, unlocking your cells so they can absorb sugar from the bloodstream for energy. In Type 2 diabetes, those cells become resistant to the signal. Your pancreas responds by pumping out even more insulin to compensate. For a while, this works. But eventually the pancreas can’t keep up with the demand, and blood sugar stays elevated.
Excess body fat, particularly fat stored around the abdomen and internal organs, is the strongest modifiable driver of this process. Visceral fat isn’t just passive storage. It actively releases inflammatory molecules that interfere with insulin signaling. Immune cells called macrophages accumulate in fatty tissue and release compounds that break down fat stores in a way that floods the bloodstream with free fatty acids, which further block insulin’s ability to do its job. Visceral fat also shifts the balance of hormones that regulate insulin sensitivity, producing more of the ones that promote resistance and less of the ones that protect against it.
Genetics play a major role in who develops Type 2 diabetes. When one identical twin has the disease, the other twin develops it up to 75% of the time, one of the highest concordance rates for any common disease. Family history, ethnicity, and age all factor in. But genetics alone don’t explain the global surge in cases. Physical inactivity, diets high in processed foods, and rising obesity rates have created an environment where genetically susceptible people are far more likely to cross the threshold into disease.
Type 1 Diabetes: Autoimmune Destruction
Type 1 diabetes has a completely different origin. The immune system, which normally targets viruses and bacteria, mistakenly attacks the insulin-producing beta cells in the pancreas. The main attackers are T cells, a type of white blood cell. Both helper T cells and killer T cells work together to destroy beta cells, releasing toxic compounds that punch holes in cell membranes or trigger programmed cell death. Macrophages are often the first immune cells to infiltrate the pancreas, arriving before T cells and setting the stage for the broader assault.
Once enough beta cells are destroyed, the pancreas can no longer produce sufficient insulin, and blood sugar rises sharply. Unlike Type 2, where insulin production declines gradually over years, Type 1 often progresses to a crisis point relatively quickly. People with Type 1 diabetes require lifelong insulin therapy from the point of diagnosis.
What triggers the immune system to turn on the pancreas isn’t fully understood, but it involves a combination of genetic susceptibility and environmental triggers. Certain inherited immune system genes (known as HLA types) significantly increase risk. Still, genetics alone aren’t enough. When one identical twin has Type 1, the other develops it only about half the time, suggesting that something external has to flip the switch.
Environmental Triggers for Type 1
Several viruses have been linked to the onset of Type 1 diabetes in people who carry the high-risk genes. Enteroviruses are the most studied culprit. Prolonged or repeated enterovirus infections in early childhood are strongly associated with the development of autoimmunity against beta cells. The theory is that viral proteins resemble proteins on the surface of beta cells closely enough that the immune system, while fighting the virus, begins targeting the pancreas as well. This process is called molecular mimicry, and people with certain HLA gene variants appear especially vulnerable to it.
Other viruses implicated include rotavirus, cytomegalovirus, and human herpesvirus 6, which has been found more frequently in the pancreatic tissue of people with Type 1 diabetes. The COVID-19 pandemic also appears to have had an effect. A large analysis of over 38,000 newly diagnosed cases in children found that the rate of new Type 1 diagnoses was 14% higher in the first year after the pandemic began and 27% higher in the second year. Respiratory infections in the first year of life and early introduction of gluten have also been investigated as potential triggers.
Gestational Diabetes
Gestational diabetes develops during pregnancy and has its own distinct cause. As pregnancy progresses, the placenta releases increasing levels of hormones, particularly progesterone, cortisol, and placental growth hormone, that interfere with insulin signaling in fat and muscle tissue. This creates a state of insulin resistance similar to Type 2 diabetes, but driven by pregnancy hormones rather than long-term metabolic changes. Most women’s pancreases can ramp up insulin production to compensate. When they can’t, blood sugar rises and gestational diabetes results. The condition typically resolves after delivery, but it significantly increases the risk of developing Type 2 diabetes later in life.
How Prediabetes Fits In
Type 2 diabetes rarely appears out of nowhere. It’s almost always preceded by prediabetes, a stage where blood sugar is elevated but not yet high enough for a diabetes diagnosis. Prediabetes is defined as an A1C between 5.7% and 6.4%, or a fasting blood sugar between 100 and 125 mg/dL. A diabetes diagnosis comes at an A1C of 6.5% or higher, or a fasting blood sugar of 126 mg/dL or higher.
Without changes to diet, activity level, or body weight, many people with prediabetes progress to full Type 2 diabetes within five years. This window matters because prediabetes is the stage where the trajectory is most reversible. The insulin resistance is already underway, but the pancreas hasn’t yet been pushed past its ability to compensate. Weight loss of even 5% to 7% of body weight, combined with regular physical activity, can significantly reduce the likelihood of progression.
Why the Distinction Matters
People often talk about “diabetes” as a single disease, but the causes are so different that the prevention strategies and treatment approaches are entirely separate. Type 2 is driven by how the body uses insulin and is heavily influenced by weight, activity, and diet. Type 1 is an immune system malfunction that can’t currently be prevented through lifestyle changes. Gestational diabetes is hormonally driven and temporary, but serves as a warning signal for future metabolic risk.
If you’re concerned about your risk, the most relevant factor for the most common form of the disease is straightforward: insulin resistance fueled by excess body fat, physical inactivity, and genetic predisposition. That combination accounts for the vast majority of diabetes cases worldwide.