Adult-onset diabetes, now called type 2 diabetes, develops when your body loses the ability to use insulin effectively and your pancreas can no longer produce enough insulin to compensate. It accounts for roughly 90 to 95% of all diabetes cases, and its prevalence climbs sharply with age: about 3.6% of adults in their twenties and thirties have diabetes, compared to 27.3% of adults 60 and older, according to CDC survey data from 2021 to 2023. The causes are a combination of insulin resistance in key organs, gradual pancreatic decline, genetics, and lifestyle factors that interact over years before a diagnosis.
Insulin Resistance: The Core Problem
The central feature of type 2 diabetes is insulin resistance, a state where your cells stop responding normally to insulin’s signal to absorb glucose from the bloodstream. Skeletal muscle is the biggest glucose consumer in the body, so when muscle tissue becomes resistant to insulin, blood sugar rises significantly. At the cellular level, the chain of events that normally shuttles glucose into muscle cells breaks down at several points: the docking proteins that relay insulin’s signal become less active, and the glucose transporter proteins that physically move sugar from the blood into the cell fail to reach the cell surface in adequate numbers.
This isn’t an all-or-nothing switch. Insulin resistance develops gradually. In the early stages, your pancreas compensates by pumping out more insulin, which keeps blood sugar in a normal range. Many people live in this compensated state for years, sometimes decades, without knowing anything is wrong. The problem worsens over time as the pancreas wears down from the sustained effort, and insulin production eventually drops below what’s needed to overcome the resistance. That tipping point is when blood sugar starts climbing into the prediabetes and then diabetes range.
How the Liver Contributes
Your liver plays a major role in blood sugar regulation. It stores glucose after meals and releases it between meals to keep your brain and organs fueled. When the liver accumulates excess fat, a condition called nonalcoholic fatty liver disease, it becomes resistant to insulin’s signal to stop releasing glucose. The result is that your liver keeps dumping sugar into the bloodstream even when levels are already high.
Research published in the Proceedings of the National Academy of Sciences identified the specific fat molecule driving this process: a compound called diacylglycerol that accumulates inside liver cells. The amount of this fat in the liver explained 64% of the variability in insulin sensitivity among study participants, making it the single strongest predictor of insulin resistance in the liver. The more fat that builds up in the liver, the more it interferes with insulin signaling, and the harder it becomes to control blood sugar.
What Excess Weight Actually Does
Carrying extra body fat, particularly around the abdomen, is the strongest modifiable risk factor for type 2 diabetes. But it’s not simply that fat tissue is “there.” Visceral fat, the fat packed around your liver and other organs, is metabolically active. It releases inflammatory signals and fatty acids into the bloodstream, both of which directly impair insulin signaling in muscle, liver, and other tissues. This chronic, low-grade inflammation is one of the key bridges between weight gain and insulin resistance.
Fat also accumulates where it shouldn’t. When excess fatty acids spill into muscle cells and liver cells, they disrupt insulin’s normal signaling pathways from the inside. This is why some people who appear relatively thin can still develop type 2 diabetes if they carry disproportionate fat in their liver or around their organs, and why weight loss of even 5 to 10% of body weight can dramatically improve insulin sensitivity.
Why Aging Raises Your Risk
Age is one of the strongest risk factors for type 2 diabetes, and the biology explains why. As you age, several things happen in your muscles simultaneously. Mitochondria, the structures inside cells that burn fuel for energy, become less efficient and produce more damaging byproducts called reactive oxygen species. The number of mitochondria in muscle cells also declines. Studies show a direct positive correlation between the number of mitochondria in skeletal muscle and insulin sensitivity in older adults.
At the same time, muscle mass itself decreases with age, a process called sarcopenia. Since skeletal muscle is the primary tissue responsible for clearing glucose from the blood, losing muscle mass means less capacity to absorb sugar after a meal. Inflammation in muscle tissue also increases with age, and the enzymes that regulate insulin sensitivity become less active. These changes stack on top of each other: less muscle, worse mitochondria, more inflammation, and weaker enzyme activity all push your body toward insulin resistance as the decades pass.
Genetics and Family History
Type 2 diabetes has a strong genetic component. If one parent has the condition, your lifetime risk roughly doubles. If both parents have it, the risk climbs even higher. Research on the children of parents with type 2 diabetes found that even before any blood sugar abnormalities appear, these individuals already show measurable defects in insulin signaling in their muscle cells. The docking proteins that relay insulin’s message are significantly less active compared to people without a family history, and these signaling defects correlate closely with the severity of insulin resistance.
No single gene causes type 2 diabetes. Instead, dozens or even hundreds of small genetic variations each contribute a modest increase in risk, affecting everything from how well your pancreas produces insulin to how your cells respond to it. Genetics loads the gun, but environment and lifestyle pull the trigger. Many people with a strong family history never develop diabetes because they maintain habits that keep insulin resistance in check.
Medications That Can Raise Blood Sugar
Certain medications are known to push blood sugar higher and, in some cases, trigger diabetes in people who are already on the edge. The most well-established offenders include:
- Glucocorticoids (prescribed for inflammation, autoimmune conditions, and asthma) are the most common drug-related cause of high blood sugar
- Antipsychotic medications, particularly newer-generation drugs used for schizophrenia and bipolar disorder
- Statins, widely used for cholesterol management, carry a modest but real increase in diabetes risk
- Thiazide diuretics and beta blockers, commonly prescribed for high blood pressure
- Immunosuppressive drugs, used after organ transplants or for autoimmune diseases
These medications impair glucose metabolism through different pathways. Glucocorticoids, for instance, directly increase the liver’s glucose output while making muscle and fat tissue more resistant to insulin. The risk tends to be dose-dependent, meaning higher doses and longer courses carry greater risk. If you take any of these medications and have other risk factors for diabetes, your doctor will likely monitor your blood sugar more closely.
LADA: When It Looks Like Type 2 but Isn’t
Some adults diagnosed with what appears to be type 2 diabetes actually have a slower-moving autoimmune form called latent autoimmune diabetes in adults, or LADA. In LADA, the immune system gradually destroys insulin-producing cells in the pancreas, similar to type 1 diabetes but at a much slower pace. People with LADA tend to be leaner, progress to needing insulin faster (often within a few years), and don’t respond as well to the standard oral medications used for type 2 diabetes.
The key difference is the presence of autoantibodies, immune proteins that target the pancreas. The most commonly tested is an antibody called GADA. By definition, people with true type 2 diabetes do not have these autoantibodies. LADA is worth considering if you’re diagnosed with type 2 diabetes but are relatively lean, under 50, and find that your blood sugar becomes harder to control quickly despite treatment.
How Type 2 Diabetes Is Diagnosed
Diagnosis relies on blood sugar measurements crossing specific thresholds. The current standards use three main tests: a fasting blood sugar of 126 mg/dL or higher, an A1C of 6.5% or higher (this reflects your average blood sugar over the past two to three months), or a two-hour glucose reading of 200 mg/dL or higher during an oral glucose tolerance test. A result in the abnormal range is typically confirmed with a repeat test on a separate day before a formal diagnosis is made.
What these numbers don’t capture is how long the process has been underway. Insulin resistance and elevated blood sugar can simmer for 5 to 10 years before reaching the diagnostic threshold. That window is the prediabetes stage, defined as a fasting blood sugar between 100 and 125 mg/dL or an A1C between 5.7% and 6.4%. Catching the condition during this phase, when lifestyle changes are most effective, is one of the strongest arguments for routine screening, especially if you have a family history or other risk factors.