Diabetes develops when your body either stops making insulin, stops responding to it properly, or both. The specific cause depends on which type you have, but genetics, body weight, lifestyle habits, and even certain medications can all play a role. Most people who search this question are thinking about Type 2 diabetes, which accounts for roughly 90 to 95 percent of all cases, but understanding every type gives you a clearer picture of your actual risk.
Type 1: Your Immune System Attacks
Type 1 diabetes is an autoimmune condition. Your immune system mistakenly identifies the insulin-producing cells in your pancreas as threats and destroys them. Without those cells, your body can’t make insulin at all, and blood sugar builds up in your bloodstream with no way to enter your cells for energy.
The triggers for this immune response aren’t fully understood, but both genetic and environmental factors are involved. Certain viral infections are suspected of kicking off the process in people who are already genetically susceptible. Interestingly, more than 90 percent of people diagnosed with Type 1 diabetes don’t have a first-degree relative with the disease, which means family history is a weaker predictor here than most people assume. Type 1 can appear at any age but is most commonly diagnosed in children and young adults.
Type 2: Insulin Resistance Builds Over Time
Type 2 diabetes starts with insulin resistance. Your pancreas still makes insulin, but your muscles, fat, and liver cells gradually stop responding to it efficiently. To compensate, your pancreas pumps out more and more insulin. Eventually it can’t keep up, blood sugar stays elevated, and you develop diabetes.
Excess body fat, particularly around the abdomen, is the single biggest driver of this process. Fat tissue isn’t just storage; it’s metabolically active. When you carry too much of it, it promotes low-grade inflammation that interferes with insulin signaling at the cellular level. This is why weight loss has such a dramatic effect on risk. The landmark Diabetes Prevention Program study found that losing just 5 to 7 percent of body weight (10 to 14 pounds for someone weighing 200 pounds) combined with 150 minutes of weekly physical activity reduced the risk of developing Type 2 diabetes by 58 percent in high-risk adults.
Physical inactivity compounds the problem independently of weight. When muscles are sedentary, they become less efficient at pulling glucose from the blood. Regular movement, even without significant weight loss, improves how well your cells respond to insulin.
Genetics and Family History
Your genes don’t guarantee diabetes, but they load the dice. A large study examining family risk found that having any relative with diabetes roughly doubled the risk. More specifically, having a father with diabetes increased risk by about 1.8 times, while having a mother with diabetes raised it by about 2 times. Having a sibling with the condition nearly tripled the risk, and for twins the risk was nearly six times higher. People with two parents who have diabetes face up to four times the risk compared to people with no family history.
For identical twins, the concordance rate for Type 2 diabetes is approximately 50 percent. That means if one twin develops it, the other has a coin-flip chance. This tells you something important: genetics matters a great deal, but it’s clearly not the whole story. Environment, diet, and activity level fill in the other half.
What You Eat and Drink
Diet affects diabetes risk through several pathways, not just sugar intake. A diet high in refined carbohydrates and added sugars causes repeated blood sugar spikes, which forces your pancreas to work harder over years and decades. Sugary drinks are a particularly well-studied risk factor. A study following men over time found that those in the highest quarter of sugar-sweetened beverage consumption had a 24 percent higher risk of Type 2 diabetes compared to those who drank the least, even after adjusting for body weight and other factors. Replacing one daily serving of a sugary drink with coffee was associated with a 17 percent reduction in risk.
Diets high in processed foods and low in fiber tend to promote both weight gain and insulin resistance. On the flip side, diets rich in whole grains, vegetables, and lean protein are consistently associated with lower risk. The food itself matters, but so does the overall pattern of eating over years.
Sleep and Stress
Chronic sleep deprivation is an underappreciated risk factor. A study published in the journal Diabetes found that restricting sleep to five hours per night for just one week reduced insulin sensitivity by 20 percent in healthy men. That’s a significant metabolic shift from a single week of poor sleep. Cortisol, the body’s primary stress hormone, rose by 51 percent during the sleep-restricted period, though researchers couldn’t directly link the cortisol increase to the drop in insulin sensitivity, suggesting other mechanisms are also at work.
For people who routinely get fewer than six hours of sleep, this effect compounds over months and years. Shift workers, new parents, and people with untreated sleep apnea face elevated risk partly for this reason.
Gestational Diabetes
Gestational diabetes develops during pregnancy, typically around the 24th week. The placenta produces hormones that help the baby grow, but those same hormones interfere with how the mother’s cells respond to insulin. Combined with the natural weight gain of pregnancy, this creates enough insulin resistance in some women to push blood sugar above normal levels.
Most cases resolve after delivery, but having gestational diabetes significantly raises the chance of developing Type 2 diabetes later in life. The risk factors overlap with Type 2 as well: being overweight before pregnancy, having a family history, or being over 25 all increase the likelihood.
Medications That Raise Blood Sugar
Certain medications can push blood sugar high enough to trigger what’s called steroid-induced or secondary diabetes. Corticosteroids (commonly prescribed for asthma, arthritis, and autoimmune conditions) are the most well-known culprit. They raise blood sugar in three ways: they cause the liver to release more glucose, they block muscle and fat cells from absorbing glucose, and they reduce the body’s sensitivity to insulin. High doses taken orally or by injection are the most likely to cause problems.
Some antipsychotic medications and certain drugs used to treat HIV can have similar effects. Steroid-induced diabetes is more common in people who already have risk factors for Type 2, and it sometimes reverses when the medication is stopped.
Prediabetes: The Warning Stage
Prediabetes is the period when blood sugar is elevated but not yet high enough for a diabetes diagnosis. An A1C between 5.7 and 6.4 percent falls in the prediabetes range, while 6.5 percent or above indicates diabetes. It’s not a guarantee that you’ll develop the full condition, but it’s a clear signal that the process is underway.
Data from a large UK study found that about 20 percent of people with prediabetes progressed to Type 2 diabetes within four years. That leaves 80 percent who didn’t, many of whom made changes that slowed or reversed the trend. This is the stage where lifestyle changes have the most impact. The 58 percent risk reduction from the Diabetes Prevention Program was specifically in people with prediabetes, meaning the window for changing course is real and well-documented.
Multiple Factors Usually Combine
In practice, diabetes rarely has a single cause. A person with a genetic predisposition who also sleeps poorly, carries extra weight, drinks sugary beverages daily, and sits for most of the day faces a risk that’s far greater than any one of those factors alone. The reassuring flip side is that addressing even one or two of those factors can meaningfully lower your odds, particularly if you catch the process at the prediabetes stage.