People get diabetes when their body either stops making insulin, stops responding to it properly, or both. About 15.8% of American adults now have diabetes, and roughly 4.5% of those cases are undiagnosed, meaning tens of millions of people are living with blood sugar problems they don’t know about. The reasons someone develops diabetes depend heavily on which type they have, but genetics, body composition, immune system misfires, and even gut bacteria all play a role.
Two Diseases With Different Causes
Type 1 and type 2 diabetes both result in high blood sugar, but they get there through completely different paths. Type 1 is an autoimmune disease: the immune system destroys the cells in the pancreas that produce insulin. Without insulin, sugar piles up in the bloodstream with no way to enter cells. Type 1 accounts for roughly 5 to 10% of all diabetes cases and usually appears in childhood or young adulthood, though it can strike at any age.
Type 2 is far more common and develops gradually. Your cells become resistant to insulin’s signal, so your pancreas compensates by producing more and more of it. Eventually the pancreas can’t keep up, and blood sugar rises. This process can unfold over years before symptoms appear, which is why so many cases go undiagnosed.
How Insulin Resistance Develops
Insulin works like a key. It binds to receptors on the surface of your cells and triggers a chain of signals that opens tiny gates (called GLUT4 transporters) so glucose can move from your blood into the cell. In insulin resistance, that signaling chain gets disrupted. Certain fat byproducts activate enzymes that essentially jam the lock, preventing the gates from ever reaching the cell surface. The result: insulin is present, it’s binding to the receptor, but the cell doesn’t let glucose in.
This isn’t just a muscle problem. Your liver is supposed to slow down its own glucose production when insulin levels rise after a meal. When the liver becomes insulin resistant, it keeps pumping out glucose even when blood sugar is already elevated. Fat tissue that’s become inflamed releases fatty acids and glycerol into the bloodstream, and the liver converts those raw materials into even more glucose. This is a major reason fasting blood sugar creeps up in people with type 2 diabetes: the liver is overproducing glucose around the clock.
Why Excess Body Fat Changes Your Metabolism
Carrying extra weight, especially around the midsection, doesn’t just correlate with diabetes. It actively drives the process. Visceral fat (the fat packed around your organs) behaves differently from the fat under your skin. It attracts immune cells called macrophages, which shift into an inflammatory mode and release signaling molecules like TNF-alpha, IL-1 beta, and IL-6. These inflammatory signals directly interfere with insulin’s ability to work in fat cells, muscle cells, and the liver.
Think of it as a feedback loop. More visceral fat triggers more inflammation, which worsens insulin resistance, which raises insulin levels, which promotes further fat storage. The inflammation isn’t confined to fat tissue either. Those same immune signals circulate through the bloodstream and affect metabolism throughout the body, including in organs that have nothing to do with fat storage. This is why even modest weight loss (5 to 7% of body weight) can meaningfully improve insulin sensitivity: it dials down the inflammatory signal at its source.
Genetics Load the Gun
Type 2 diabetes has a stronger genetic component than many people realize. Studies of identical twins show that if one twin develops type 2, the other has a 70 to 90% chance of developing it too. Family history matters enormously: having one parent with type 2 diabetes roughly doubles or triples your risk, and having both parents with it raises it further still.
Type 1 diabetes is also partly genetic, though the numbers look different. If a father has type 1, his child has about a 1 in 17 chance of developing it. If the mother has type 1 and gave birth before age 25, the child’s risk is about 1 in 25. When both parents have type 1, the risk jumps to between 1 in 10 and 1 in 4. But genetics alone don’t explain everything. The vast majority of people who develop type 1 have no family history of it, which points to environmental triggers.
What Triggers the Immune Attack in Type 1
Something in the environment appears to flip the switch in genetically susceptible people, causing their immune system to turn against the insulin-producing cells of the pancreas. The strongest suspects are viral infections, particularly enteroviruses. Persistent enterovirus infection in the pancreas has been linked to a nearly tenfold increase in the risk of developing type 1 diabetes. Other viruses implicated include Coxsackie B, mumps, rubella, and influenza B.
Some viruses can damage beta cells directly without any autoimmune process at all. This causes a rapid, dramatic form of diabetes where blood sugar skyrockets within a week. More commonly, though, a viral infection seems to confuse the immune system into attacking pancreatic cells that look similar to the virus, and the autoimmune destruction unfolds over months or years. SARS-CoV-2 has also been shown to infect beta cells and trigger high blood sugar in people without a prior diabetes diagnosis, though research on long-term effects is still ongoing.
Geography plays a role too. Type 1 diabetes is more common in northern latitudes, and incidence varies dramatically between countries, suggesting that climate, sunlight exposure, early childhood infections, and even vaccination patterns all shape risk in ways researchers are still untangling.
Your Gut Bacteria Play a Role
The trillions of bacteria living in your intestines influence your metabolic health more than was understood even a decade ago. A large population study found that 12 groups of gut bacteria were associated with insulin resistance or type 2 diabetes, and every one of those 12 groups produces butyrate, a short-chain fatty acid made from dietary fiber. People with higher levels of these butyrate-producing bacteria had better insulin sensitivity and lower rates of type 2 diabetes.
Butyrate helps in several ways. It fuels the cells lining your intestines, strengthens the gut barrier to prevent inflammatory molecules from leaking into the bloodstream, and appears to improve how your cells use energy. A diet low in fiber starves these beneficial bacteria, reducing butyrate production and potentially tipping the balance toward inflammation and insulin resistance. This is one reason why fiber-rich diets consistently show up as protective against type 2 diabetes: they’re feeding the bacteria that help keep your metabolism running smoothly.
Risk Factors You Can and Can’t Control
Some diabetes risk factors are fixed. Your age, ethnicity, and family history all influence your likelihood. Men have slightly higher rates than women (16.6% vs. 12.2% after adjusting for age), and risk climbs steadily after age 40. People of South Asian, African, Hispanic, and Native American descent face higher rates of type 2 diabetes than those of European descent, likely due to a combination of genetic susceptibility and disparities in access to healthy food and healthcare.
The modifiable risk factors carry enormous weight, though. Physical inactivity, a diet high in processed foods and low in fiber, excess body fat (particularly visceral fat), poor sleep, and chronic stress all independently raise risk. These factors don’t operate in isolation. Someone with a strong genetic predisposition who maintains a healthy weight, stays active, and eats a fiber-rich diet may never develop type 2 diabetes. Someone with minimal genetic risk who is sedentary and carries significant visceral fat may develop it in their 40s.
Gestational Diabetes
Pregnancy naturally increases insulin resistance because hormones from the placenta interfere with the mother’s insulin signaling. For most women, the pancreas compensates by producing more insulin. When it can’t keep up, blood sugar rises and gestational diabetes develops, typically in the second or third trimester. It usually resolves after delivery, but it’s a significant warning sign. Women who develop gestational diabetes have roughly a 50% chance of developing type 2 diabetes within the next 5 to 10 years, making it one of the strongest predictors of future metabolic problems.