When your blood sugar climbs too high, your body starts showing signs almost immediately: excessive thirst, frequent urination, fatigue, and blurred vision. A fasting blood sugar above 126 mg/dL on two separate tests indicates diabetes, while readings between 100 and 125 mg/dL fall into the prediabetes range. But the real concern isn’t just a single high reading. It’s what persistently elevated glucose does to your blood vessels, nerves, kidneys, eyes, and immune system over time.
Why High Blood Sugar Makes You Thirsty and Tired
Your kidneys act as a filter, pulling waste from your blood while keeping useful molecules like glucose. But kidneys can only reabsorb so much sugar. When blood glucose gets too high, the excess spills into your urine, and it pulls water along with it through a process called osmotic diuresis. That’s why frequent urination is one of the earliest and most noticeable signs of high blood sugar. Your body is literally flushing out the extra glucose, losing water in the process.
This water loss triggers intense thirst. You drink more, you urinate more, and the cycle continues. Meanwhile, because your cells aren’t absorbing glucose properly (either due to insufficient insulin or insulin resistance), they’re starved for energy even though your bloodstream is flooded with sugar. That disconnect is what causes the deep fatigue many people feel. Your cells are surrounded by fuel they can’t use.
What High Glucose Does to Your Blood Vessels
Persistently high blood sugar damages the inner lining of your blood vessels. Excess glucose triggers a cascade of chemical reactions inside vessel walls, ramping up the production of harmful molecules called free radicals. Under normal conditions, your blood vessels produce nitric oxide, a compound that keeps them relaxed and flexible. High glucose disrupts this process. Instead of producing nitric oxide, the vessel lining starts generating more free radicals, which stiffen and inflame the vessel walls.
Research from the American Heart Association shows that in diabetes, the expression of one key enzyme involved in this damage increases roughly sevenfold compared to healthy controls. Over months and years, this damage leads to two broad categories of vascular disease. Large vessel damage (macroangiopathy) raises the risk of heart attack and stroke, the leading causes of death in people with diabetes. Small vessel damage (microangiopathy) harms the eyes, kidneys, and nerves. Both start with the same root cause: glucose quietly corroding your circulatory system from the inside.
How Nerves Get Damaged
Your nerve cells absorb glucose without needing insulin, which means they’re especially vulnerable when blood sugar is high. Normally, cells break down glucose through a standard energy pathway. When there’s too much glucose, the overflow gets diverted into an alternative route called the polyol pathway, where it’s converted into sorbitol and fructose. These sugar alcohols accumulate inside nerve cells, drawing in water, causing swelling, and eventually degrading the nerve fibers.
At the same time, excess glucose binds to proteins and fats through a non-enzymatic reaction, forming compounds called advanced glycation end products. These interfere with the nerve cell’s ability to repair itself and transport signals along its length. Free radical production adds a third layer of damage by reducing blood flow to the nerves themselves. The result is diabetic neuropathy, which typically starts in the feet and hands as tingling, numbness, or burning pain, and can progress to a complete loss of sensation.
Eyes and Kidneys Take the Hit
The tiny blood vessels in your retinas and kidneys are particularly fragile. In diabetic retinopathy, high blood sugar causes microaneurysms (tiny bulges in blood vessel walls), dot and blot hemorrhages, and eventually the growth of abnormal new blood vessels on the retina’s surface. These new vessels are weak and leak easily, which can lead to vision loss if untreated.
In the kidneys, the damage targets the glomeruli, the microscopic filtering units. Chronic high glucose thickens the membrane that these filters rely on. In men, a glomerular basement membrane thicker than 430 nanometers (395 nm in women) signals diabetic kidney disease. As the membrane thickens, the kidneys become less efficient at filtering waste while simultaneously letting protein leak into the urine. A long-term study of 232 patients with type 2 diabetes found that retinopathy progression and kidney disease tracked closely together, meaning worsening eye damage often signals worsening kidney damage too.
Your Immune System Weakens
High blood sugar impairs the white blood cells responsible for fighting infection. Neutrophils, the most abundant type of immune cell, rely on chemical signals to mobilize and reach infection sites. Elevated glucose reduces the secretion of a key mobilization signal (G-CSF), which means fewer neutrophils arrive where they’re needed. This is why people with poorly controlled diabetes are more prone to skin infections, urinary tract infections, and slow wound healing.
The good news from the same research: normalizing blood glucose with insulin rescued neutrophil recruitment in experimental models, restoring immune function. This underscores that the immune suppression isn’t permanent. It’s directly tied to how well blood sugar is managed at any given time.
When High Blood Sugar Becomes an Emergency
Two life-threatening conditions can develop when blood sugar spikes severely.
Diabetic ketoacidosis (DKA) occurs most often in type 1 diabetes. Without enough insulin, your body can’t use glucose for fuel at all and starts breaking down fat at a dangerous rate. This produces acidic compounds called ketones. DKA is diagnosed when blood sugar is at or above 200 mg/dL, blood ketone levels reach 3 mmol/L or higher, and blood pH drops below 7.3. Symptoms come on fast: nausea, vomiting, abdominal pain, fruity-smelling breath, and confusion. Without treatment, it can be fatal within hours.
Hyperosmolar hyperglycemic state (HHS) is more common in type 2 diabetes, particularly in adults over 65. Blood sugar in HHS climbs above 600 mg/dL, sometimes exceeding 1,000 mg/dL. Unlike DKA, ketone levels stay low because the body still produces some insulin. Instead, the extreme blood sugar causes severe dehydration, confusion, and can progress to seizures or coma. HHS develops more slowly than DKA, often over days or weeks, which means it’s sometimes missed until it’s advanced.
Bringing Blood Sugar Back Down
For mildly elevated blood sugar, physical activity is one of the most effective immediate responses. Exercise increases your muscles’ demand for glucose, pulling it out of your bloodstream without requiring extra insulin. Walking for even 15 to 20 minutes after a meal can meaningfully reduce a post-meal spike. Staying hydrated helps too, since water supports the kidneys in flushing out excess glucose.
There’s an important exception: if your blood sugar is above 270 mg/dL (15 mmol/L) and you have ketones in your blood or urine, exercise can actually make things worse. When ketones are present, your body needs insulin to clear them first. Exercising in this state can drive blood sugar and ketone levels higher. If you have type 1 diabetes and feel unwell, check for ketones before reaching for your running shoes.
For people on insulin, a correction dose based on your prescribed ratio can bring levels down over one to three hours. The key is avoiding the temptation to “stack” doses by taking more insulin before the first dose has had time to work, which can cause a dangerous low-sugar crash later.
The Numbers That Matter
Understanding blood sugar thresholds helps you know where you stand:
- Normal fasting blood sugar: below 100 mg/dL
- Prediabetes range (fasting): 100 to 125 mg/dL
- Diabetes (fasting): 126 mg/dL or higher on two separate tests
- Normal after a meal (2 hours): below 140 mg/dL
- Prediabetes after a meal: 140 to 199 mg/dL
- Diabetes after a meal: 200 mg/dL or higher
A single high reading after a large meal doesn’t mean you have diabetes. But if your fasting numbers consistently land above 100, or your post-meal readings regularly exceed 140, that pattern of elevated glucose is already doing low-grade damage to your vessels, nerves, and organs. The earlier you catch it, the more reversible the damage tends to be.