Diabetes develops when your body either destroys the cells that make insulin, stops responding to insulin properly, or both. The specific process depends on the type, but in every case the end result is the same: blood sugar rises and stays elevated because insulin can no longer do its job. Globally, about 589 million adults are living with diabetes as of 2024, roughly one in nine people aged 20 to 79.
How Type 1 Diabetes Develops
Type 1 diabetes is an autoimmune disease. Your immune system mistakenly identifies the insulin-producing beta cells in the pancreas as threats and systematically destroys them. The process usually begins years before any symptoms appear.
It starts when immune cells called helper T cells encounter fragments of beta cell proteins presented by other immune cells in and around the pancreas. Once activated, these helper T cells release chemical signals that recruit a second wave of immune cells, called cytotoxic T cells, which are specifically designed to kill. These cytotoxic T cells travel into the pancreatic islets (the clusters of cells where insulin is made) and begin destroying beta cells directly by releasing toxic molecules. Activated immune cells called macrophages pile on, releasing inflammatory compounds and reactive oxygen molecules that cause further damage. The result is a coordinated, escalating attack on the very cells your body needs to regulate blood sugar.
By the time blood sugar rises enough to cause symptoms like excessive thirst, frequent urination, and unexplained weight loss, most of the insulin-producing capacity of the pancreas is already gone. Type 1 typically appears in childhood or early adulthood, though it can develop at any age.
Genetic and Environmental Triggers
Genetics load the gun, but something in the environment pulls the trigger. The strongest genetic risk factor for Type 1 sits in a region of DNA called the HLA complex, which governs how your immune system distinguishes your own cells from foreign invaders. Certain variants in this region make the immune system more likely to misrecognize beta cells as dangerous.
On the environmental side, viral infections are the leading suspects. Enteroviruses, particularly a group called Coxsackie B, have the strongest evidence linking them to the onset of beta cell autoimmunity in genetically susceptible people. The TEDDY study, a large international effort tracking children at genetic risk, found that prolonged infection with enterovirus B was associated with a higher likelihood of developing the autoimmune process that leads to Type 1. Rotavirus has also been implicated through a mechanism called molecular mimicry: the virus carries a protein that shares 56% of its structure with a protein found on beta cells, potentially confusing the immune system into attacking both.
How Type 2 Diabetes Develops
Type 2 diabetes follows a different, slower path. It begins with insulin resistance, a condition where your cells gradually stop responding to insulin’s signal to absorb sugar from the blood. Your pancreas compensates by producing more insulin, sometimes for years. Eventually the beta cells can’t keep up with demand, and blood sugar starts climbing.
The process typically starts in fat tissue, particularly the visceral fat stored deep in your abdomen around your organs. Visceral fat is metabolically active in ways that subcutaneous fat (the kind under your skin) is not. It releases elevated levels of inflammatory signaling molecules while producing less of the protective hormones that help cells stay sensitive to insulin. These inflammatory signals activate enzymes inside cells that physically alter the proteins responsible for passing insulin’s message along. The result is that cells in your muscles, liver, and fat tissue become progressively deaf to insulin, even when plenty of it is circulating.
The Liver’s Role in Rising Blood Sugar
Your liver plays a central and often underappreciated role in Type 2. One of insulin’s most important jobs is telling the liver to stop making new glucose after you eat, since there’s already plenty of sugar arriving from your meal. In insulin resistance, the liver ignores that signal. It keeps producing glucose around the clock, flooding the bloodstream with sugar you don’t need. Research has confirmed that this unchecked glucose production by the liver is the primary source of excess blood sugar in people with Type 2, more so than the liver releasing stored sugar.
Fat accumulation in the liver compounds the problem. When fat builds up in liver cells, it activates a chain reaction that further blocks insulin signaling at the molecular level, creating a vicious cycle: insulin resistance promotes fat storage in the liver, and liver fat deepens insulin resistance.
Beta Cell Decline Over Time
For a long time, the prevailing view was that Type 2 was purely about insulin resistance. That picture is incomplete. Autopsy studies across European, Asian, and North American populations have found that people with Type 2 diabetes have 20 to 65% less beta cell mass than people without diabetes. The higher end of that range, around 65% loss, appears to be the tipping point where the pancreas can no longer compensate and blood sugar control fails.
This beta cell loss doesn’t happen overnight. It’s a gradual decline driven by years of overwork, chronic inflammation, and the toxic effects of elevated blood sugar and fat on the cells themselves. The combination of rising insulin resistance and falling insulin production is what ultimately pushes someone from normal blood sugar into prediabetes and then into Type 2.
Prediabetes: The Warning Stage
Prediabetes is the metabolic middle ground where blood sugar is elevated but not yet high enough for a diabetes diagnosis. The American Diabetes Association defines it as a fasting blood sugar between 100 and 125 mg/dL, or an A1C between 5.7% and 6.4%. Full diabetes is diagnosed at a fasting blood sugar of 126 mg/dL or higher, or an A1C of 6.5% or above.
Not everyone with prediabetes progresses to diabetes. In studies of younger populations with impaired glucose tolerance, 65 to 75% reverted to normal blood sugar levels over the following three to five years without medication. About 2 to 8% progressed to Type 2 over two to three years, while the rest stayed in the prediabetes range. Physical activity, weight loss, and dietary changes have a strong track record of shifting those odds in your favor.
Gestational Diabetes
Gestational diabetes develops during pregnancy, typically in the second or third trimester, and usually resolves after delivery. The placenta produces several hormones, including human placental growth hormone, human placental lactogen, progesterone, and cortisol, that naturally make the mother’s cells less responsive to insulin. This evolved as a way to ensure enough glucose reaches the growing fetus.
In most pregnancies, the pancreas ramps up insulin production to compensate. Gestational diabetes occurs when the pancreas can’t keep pace with the increasing demand. Interestingly, no single placental hormone has been identified as the definitive cause. One study found that cortisol was the only hormone whose changes during pregnancy correlated significantly with changes in insulin sensitivity, while another pointed to triglycerides and the appetite hormone leptin. The reality is likely a combination of hormonal, metabolic, and genetic factors working together.
Genetics and Who Is at Risk
Both Type 1 and Type 2 have genetic components, but the specific genes involved are entirely different. For Type 1, the HLA region remains the most important genetic risk factor, governing how the immune system identifies targets. For Type 2, the most influential single gene discovered is TCF7L2. People who carry two copies of the risk variant in this gene face nearly double the risk of developing Type 2 compared to those with no copies, affecting roughly 10% of the population.
But genetics alone rarely cause diabetes. For Type 2 in particular, the interplay between genetic susceptibility and lifestyle factors like excess weight, physical inactivity, and diet quality determines whether the disease actually develops. Prevalence is higher in men than women (11.6% vs. 10.7%) and in urban areas compared to rural ones (12.3% vs. 9.2%), reflecting how environment and behavior shape risk on a population level. Middle-income countries currently have the highest rates globally, likely driven by rapid urbanization and shifts toward processed diets without corresponding improvements in healthcare access.