Diabetes is a disease where your body can’t move sugar (glucose) out of your blood and into your cells properly. The result is chronically high blood sugar, which over time damages blood vessels, nerves, kidneys, and eyes. Global diabetes prevalence in adults doubled from 7% to 14% between 1990 and 2022, making it one of the fastest-growing health conditions worldwide. To understand how diabetes works, you first need to understand what’s supposed to happen when everything goes right.
How Your Body Normally Handles Sugar
When you eat, your digestive system breaks carbohydrates down into glucose, which enters your bloodstream. Your pancreas detects the rising blood sugar and releases insulin, a hormone that acts like a key for your cells. Insulin travels through the blood and latches onto receptors on the surface of muscle, fat, and liver cells. This binding triggers a chain reaction inside the cell that ultimately pulls glucose transporters to the cell’s outer membrane, creating doorways for sugar to enter.
Without that signal from insulin, those transporters stay locked away inside the cell, and glucose has no way in. The system is elegant: eat food, blood sugar rises, insulin opens the doors, cells absorb glucose for energy, blood sugar falls back to normal. Diabetes breaks this cycle in one of two fundamentally different ways.
Type 1: The Immune System Destroys Insulin Production
Type 1 diabetes is an autoimmune disease. Your immune system, which normally fights infections, mistakenly identifies the insulin-producing cells in your pancreas as threats and destroys them. These are called beta cells, and they’re clustered in tiny groups called islets scattered throughout the pancreas.
The attack is coordinated. One type of immune cell (CD8 T cells) directly kills beta cells through contact, puncturing their membranes with toxic molecules. Another type (CD4 T cells) orchestrates the assault by activating other immune players, including macrophages that flood the islets with inflammatory signals. These inflammatory chemicals are directly toxic to beta cells and create a feedback loop: more inflammation leads to more cell death, which triggers more inflammation. B cells join in by presenting beta cell proteins to the T cells, essentially pointing out new targets.
By the time symptoms appear, a significant portion of your beta cells are already gone. With few or no beta cells left, your body produces little to no insulin. Without insulin, glucose piles up in the blood while your cells starve for energy. This is why people with type 1 diabetes need external insulin to survive, and why the condition typically appears suddenly, often in childhood or adolescence.
Type 2: Your Cells Stop Responding to Insulin
Type 2 diabetes develops differently. Your pancreas still makes insulin, at least initially, but your cells gradually stop responding to it. This is called insulin resistance, and it unfolds across multiple organs simultaneously.
In muscle tissue, which normally absorbs the majority of blood glucose after a meal, the signaling pathway between the insulin receptor and the glucose transporters becomes impaired. Obesity plays a direct role here: excess fat deposited between and around muscle fibers attracts immune cells that release inflammatory molecules, which interfere with insulin signaling inside the muscle cells themselves.
Fat tissue develops its own problems. Enlarged fat cells become resistant to insulin’s signal to stop releasing fatty acids into the blood. These excess fatty acids flood the liver and muscles, worsening insulin resistance in both. Overgrown fat cells also become oxygen-starved, triggering an inflammatory response that spills into the rest of the body.
The liver, meanwhile, loses its ability to respond to insulin’s instruction to stop producing glucose. Normally, after a meal, insulin tells the liver to store glucose and stop making new glucose. In type 2 diabetes, the liver ignores that signal and keeps pumping glucose into the blood even when levels are already high. It also ramps up fat production, contributing to fatty liver disease.
For a while, the pancreas compensates by producing more and more insulin. But over years, this overwork can exhaust the beta cells, and insulin production eventually declines. That’s why some people with long-standing type 2 diabetes eventually need insulin injections too.
Why High Blood Sugar Causes Damage
The real danger of diabetes isn’t the high blood sugar itself in the moment. It’s what persistently elevated glucose does to your body over months and years. One of the most important mechanisms involves a chemical process where excess glucose in the blood reacts with proteins in your tissues, forming compounds called advanced glycation end-products (AGEs). Think of it like caramelization: sugar molecules bond to proteins and permanently alter their structure.
AGEs wreak havoc on blood vessels. They cross-link collagen and elastin, the structural proteins that keep vessel walls flexible, making arteries stiff. They bind to receptors on the cells lining your blood vessels and trigger a cascade of inflammation, oxidative stress, and damage. This process reduces the availability of nitric oxide, a molecule your vessels need to relax and dilate, while simultaneously promoting blood clot formation and attracting immune cells that build up into plaques. The net result is accelerated atherosclerosis, the buildup of fatty deposits inside arteries that leads to heart attacks and strokes.
AGEs also modify LDL cholesterol (the “bad” kind), making it harder for your body to clear it from the bloodstream and easier for it to lodge in artery walls. This is a major reason why people with diabetes face two to four times the cardiovascular risk of people without it.
How Diabetes Damages Nerves
Nerve damage, or neuropathy, affects roughly half of all people with diabetes over time. The mechanism involves an alternate route your body uses to process glucose when there’s too much of it. Normally, glucose enters the main energy-producing pathway. But when glucose levels are chronically high, the excess spills into a secondary pathway where an enzyme converts it to sorbitol, a sugar alcohol.
Certain tissues, including the cells that insulate your nerves (Schwann cells), the retina, and the kidneys, lack the enzyme needed to convert sorbitol into something else. So sorbitol accumulates, drawing water into the cells by osmosis and creating damaging swelling. In Schwann cells specifically, this buildup causes them to revert to an immature state, degrading the protective insulation around nerve fibers. The result is numbness, tingling, and pain, typically starting in the feet and hands and working inward.
Beyond the sorbitol buildup itself, the metabolic detour depletes a critical molecule (NADPH) that your cells need for antioxidant defense and fat metabolism. This leaves nerve cells more vulnerable to oxidative damage.
How Diabetes Damages Kidneys
Your kidneys filter your entire blood volume roughly 40 times per day. In diabetes, high blood sugar causes the kidneys to work harder than they should. Enhanced reabsorption of glucose and sodium in the kidney tubules disrupts the feedback system that normally regulates how much blood the kidneys filter. The result is hyperfiltration: each kidney unit processes more blood at higher pressure than it was designed to handle.
This elevated pressure inside the filtering units (glomeruli) forces not just waste products but also proteins, particularly albumin, through the filter and into the urine. Protein in the urine is one of the earliest detectable signs of diabetic kidney disease. Over time, the sustained pressure damages the delicate filtering structures, scarring them and reducing the kidneys’ capacity until, in severe cases, they can no longer function.
Classic Warning Signs and Why They Happen
The hallmark symptoms of uncontrolled diabetes, frequent urination and intense thirst, are directly caused by the physics of glucose in your blood. Glucose is osmotically active, meaning it pulls water toward itself. When blood sugar exceeds the kidneys’ ability to reabsorb it (around 180 mg/dL), glucose spills into the urine and drags water along with it. This osmotic diuresis produces large volumes of urine, dehydrating you and triggering thirst.
Other common symptoms follow logically from the core problem. Your cells can’t access glucose for energy, so you feel fatigued and hungry even after eating. Your body may start breaking down fat and muscle for fuel, causing unexplained weight loss, which is especially common in type 1. Blurred vision occurs because high blood sugar changes the water content of your eye’s lens, distorting its shape. Slow-healing wounds result from impaired blood flow and immune function.
Type 1 symptoms tend to appear rapidly over days to weeks. Type 2 develops so gradually that many people have it for years before diagnosis. An estimated one in five people with diabetes don’t know they have it.
How Diabetes Is Diagnosed
The American Diabetes Association’s 2025 diagnostic criteria use three primary tests. A fasting blood sugar of 126 mg/dL or higher indicates diabetes, while 100 to 125 mg/dL falls in the prediabetes range. The A1C test, which reflects your average blood sugar over the past two to three months, diagnoses diabetes at 6.5% or above, with 5.7% to 6.4% considered prediabetes. A glucose tolerance test, where you drink a sugary solution and have your blood tested two hours later, diagnoses diabetes at 200 mg/dL or above.
Unless your blood sugar is unambiguously high with obvious symptoms, diagnosis requires two abnormal results, either from two different tests or the same test repeated on a different day.
How Treatments Target the Problem
Treatment strategies differ based on which type of diabetes you have, because the underlying mechanisms are so different. Type 1 always requires insulin replacement, delivered through injections or a pump, because the body no longer makes its own.
Type 2 treatment typically starts with lifestyle changes and medications that address insulin resistance or reduce blood sugar through other pathways. One class of newer medications works by targeting the kidneys directly. Your kidneys normally reabsorb almost all the glucose they filter back into the blood using specialized transport proteins. These medications block those transporters, lowering the threshold at which glucose spills into the urine. The result is that your kidneys excrete excess glucose instead of recycling it, bringing blood sugar down through a mechanism completely independent of insulin. These drugs have also shown unexpected benefits for heart and kidney health beyond glucose control.
Other medication classes work by helping the pancreas release more insulin, slowing glucose production in the liver, improving insulin sensitivity in muscle and fat tissue, or mimicking gut hormones that regulate appetite and insulin secretion. Many people with type 2 diabetes use combinations of these approaches, and the specific strategy often changes over time as the disease progresses.