Diabetes affects nearly every major body system. While it starts as a problem with insulin and blood sugar, the damage spreads over time to your heart, blood vessels, kidneys, nerves, eyes, skin, bones, and immune system. The common thread is chronic high blood sugar, which quietly injures tissues throughout the body, often years before symptoms appear.
The Endocrine System: Where It Starts
Diabetes is fundamentally a breakdown in how your body produces or responds to insulin, a hormone made by specialized cells in the pancreas. In type 1 diabetes, the immune system destroys these cells, cutting off insulin production almost entirely. In type 2, the problem is twofold: the pancreas gradually loses its ability to sense blood sugar levels and release insulin properly, while cells in muscle, fat, and the liver stop responding to insulin the way they should.
Inherited differences in how well the pancreas functions are actually the critical starting point for most people with type 2 diabetes. Insulin resistance compounds the problem, but it’s the declining ability of the pancreas to compensate that tips blood sugar into the diabetic range. A fasting blood sugar of 126 mg/dL or higher, an A1C of 6.5% or above, or a two-hour glucose tolerance reading of 200 mg/dL or more confirms the diagnosis.
Heart and Blood Vessels
Cardiovascular disease is the leading cause of death in people with diabetes. People with diabetes are twice as likely to have heart disease or a stroke compared to those without it.
Chronically elevated blood sugar damages blood vessels through several overlapping processes. High glucose triggers oxidative stress, essentially an overproduction of harmful molecules that injure the delicate lining of blood vessel walls. Once those walls are damaged, white blood cells stick to them and build up, accelerating the kind of plaque formation that leads to heart attacks and strokes. At the same time, diabetes activates hormonal systems that raise blood pressure and stiffen arteries, compounding the damage.
This vascular injury happens in both large vessels (the ones supplying your heart and brain) and tiny ones (the capillaries feeding your eyes, kidneys, and nerves). That’s why so many of the complications below trace back to the same root cause: blood vessel damage from high blood sugar.
The Nervous System
Nerve damage, called diabetic neuropathy, comes in four forms, and it’s possible to develop more than one at the same time.
Peripheral nerve damage is the most common type. It typically begins in the feet and can spread to the legs, hands, and arms. You might feel tingling, “pins and needles,” burning pain that worsens at night, or gradually increasing numbness. That numbness is particularly dangerous because you may not notice cuts, blisters, or sores on your feet, which can progress to serious infections.
Autonomic nerve damage affects the nerves that control automatic body functions. This can show up as bladder problems, chronic constipation or diarrhea, nausea and vomiting from slowed stomach emptying, sexual dysfunction (erectile problems or vaginal dryness), and changes in how your eyes adjust between light and dark.
Proximal nerve damage targets the thighs, hips, buttocks, and sometimes the stomach or chest. It can cause severe pain in these areas and make it difficult to stand up from a seated position. Focal nerve damage strikes a single nerve, sometimes causing double vision, facial paralysis on one side, or sudden weakness in a hand.
Kidneys
About 1 in 3 adults with diabetes develops chronic kidney disease. The damage starts at the kidney’s filtering units, tiny clusters of blood vessels that clean your blood. High blood sugar forces these filters to work harder than normal, a state called hyperfiltration, which is one of the earliest detectable changes. Over time, the filtering membranes thicken and scar, letting protein leak into the urine.
The progression is gradual. Early kidney damage shows up as small amounts of protein in the urine (30 to 300 mg per day), often with no symptoms at all. As damage worsens, protein leakage increases above 300 mg per day, and kidney function begins to decline more noticeably. In advanced stages, more than half of the filtering units can be scarred shut, potentially leading to kidney failure and the need for dialysis.
Eyes and Vision
Diabetic retinopathy is the most common complication of diabetes and a leading cause of blindness. It develops in stages that can span years. In the early stage, small bulges form in retinal blood vessels, along with tiny hemorrhages and spots. Most people don’t notice any vision changes at this point.
In the later stage, the retina responds to its damaged blood supply by growing new blood vessels. These new vessels are extremely fragile. They rupture easily, causing severe bleeding inside the eye. Growth factors that drive this new vessel formation also promote scar tissue, which can pull the retina away from the back of the eye. Once retinal detachment or major hemorrhaging occurs, vision loss can be irreversible. Researchers also increasingly recognize diabetic retinopathy as a neurodegenerative disease, meaning the nerve cells in the retina deteriorate alongside the blood vessel damage.
Skin
Because skin is rich in both nerves and blood vessels, it’s one of the most visibly affected organs. Several skin conditions are closely linked to diabetes:
- Diabetic dermopathy (shin spots): Red or brown round patches, usually on the shins. Painless and harmless.
- Acanthosis nigricans: Dark, velvety patches in skin folds like the neck, armpits, or groin. Often one of the earliest visible signs of insulin resistance.
- Necrobiosis lipoidica: Yellow-brown patches that start as small bumps and become hard, swollen areas of skin. Can be itchy and painful.
- Digital sclerosis: Thick, waxy, tight skin on the fingers that limits joint movement and can spread elsewhere on the body.
- Diabetic blisters: Painless blisters resembling burns, typically on the lower legs, feet, or hands. They usually heal on their own.
People with diabetes are also more prone to bacterial and fungal infections. Fungal infections thrive in warm, moist skin folds, showing up as jock itch, athlete’s foot, or vaginal yeast infections. Bacterial infections commonly target eyelids, hair follicles, and fingernails. Poor circulation from vascular damage makes dry, itchy skin more likely as well.
Immune System and Wound Healing
High blood sugar fundamentally weakens your body’s ability to fight infection and repair itself. Neutrophils, the white blood cells that are your first line of defense against bacteria, lose their ability to produce the reactive molecules they normally use to kill pathogens. Their internal stores of germ-fighting enzymes become depleted. Monocytes and macrophages, the cells responsible for engulfing debris and bacteria, also become sluggish in their movement toward wounds and less effective at cleaning up damaged tissue.
The adaptive immune system suffers too. The cells responsible for presenting foreign invaders to the rest of the immune system don’t activate properly in a high-sugar environment. The combined result is that bacteria persist longer in wounds, often forming protective biofilms that are even harder to clear. Wounds heal with thinner, weaker skin and carry a higher risk of reinfection. This is why foot ulcers in people with diabetes can become such serious, slow-healing problems.
Bones
Both types of diabetes increase fracture risk, but through different pathways. In type 1, the lack of insulin impairs bone formation directly, leading to reduced bone density at all skeletal sites. In type 2, bone density is often normal or even higher than average, yet fracture risk paradoxically goes up. The likely explanation is that diabetes changes bone quality, not just quantity: the internal structure becomes more brittle even when overall mineral content looks fine on a scan.
In one multicenter study, 9.9% of people with type 2 diabetes reported fall-related fractures, compared to 5.4% of those with type 1. Major osteoporotic fracture risk was also higher in the type 2 group (5.4% vs. 4.3%). Older age, female sex, lower body weight, reduced calcium levels, and the presence of kidney-related protein leakage all independently predicted worse bone health in people with diabetes.