Your A1c rises when glucose stays elevated in your blood over weeks and months. The test measures how much sugar has permanently attached to your red blood cells during their roughly 120-day lifespan, so it reflects your average blood sugar over the past two to three months. Anything that keeps blood sugar higher for a sustained period, from diet to stress to sleep habits, will push that number up. But some factors can raise your A1c even when your blood sugar control hasn’t actually changed.
How the A1c Test Actually Works
Glucose in your blood naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. This happens in a two-step chemical reaction: glucose first loosely bonds to the hemoglobin chain, then rearranges into a permanent attachment. The process is irreversible. Once glucose latches on, it stays for the life of that red blood cell.
Because this bonding happens continuously and can’t be undone, red blood cells accumulate more and more sugar coating the longer they’re exposed to high glucose levels. Your A1c result is the percentage of hemoglobin molecules that have glucose stuck to them. A normal A1c falls below 5.7%. Between 5.7% and 6.4% indicates prediabetes, and 6.5% or higher meets the diagnostic threshold for diabetes, according to the American Diabetes Association’s current standards.
High-Glycemic Foods and Carbohydrate Load
Diet is the most direct lever. Foods that break down quickly during digestion flood your bloodstream with glucose, creating sharp spikes that contribute to a higher average over time. White bread, white rice, sugary drinks, pastries, and most processed snack foods fall into this high-glycemic category. They’re absorbed rapidly and push both blood sugar and insulin responses up fast.
Low-glycemic foods do the opposite. Legumes, lentils, oats, and most non-starchy vegetables contain carbohydrates that break down slowly and enter the bloodstream gradually. Swapping high-glycemic staples for slower-digesting alternatives is one of the most effective dietary changes for lowering A1c over time. It’s not just about cutting sugar. Refined carbohydrates that don’t taste sweet, like white flour products, can spike blood sugar just as aggressively as candy.
Chronic Stress and Cortisol
When you’re under sustained stress, your body releases cortisol and adrenaline. In the short term, these hormones are harmless. Over weeks and months, they create a cascade of metabolic problems. Cortisol stimulates your liver to produce more glucose while simultaneously blocking your muscle cells from absorbing it. It does this by interfering with the transporter that normally shuttles glucose from your blood into your muscles in response to insulin. The result is more sugar being pumped into your blood and less of it being cleared out.
Chronic stress also suppresses insulin production from the pancreas, promotes the accumulation of visceral fat, and gradually erodes your body’s sensitivity to insulin. All of these effects keep blood sugar elevated over time, which directly translates to a higher A1c. People dealing with prolonged work pressure, caregiving stress, financial hardship, or untreated anxiety often see their A1c creep up even without obvious dietary changes.
Sleep Duration and Quality
Sleeping less than six hours a night consistently raises fasting blood sugar, fasting insulin, and insulin resistance. Research shows that men without diabetes who slept under six hours had twice the risk of developing diabetes compared to those with adequate sleep, even after accounting for other risk factors. Both insufficient sleep and poor sleep quality have been directly linked to higher A1c levels in adults with type 2 diabetes.
Interestingly, sleeping too much also appears to cause problems. Regularly logging more than nine hours per night has been associated with increased insulin resistance and a higher risk of type 2 diabetes, though this link is less consistent than the one for short sleep. The metabolic sweet spot appears to be somewhere between seven and eight hours for most adults. If your A1c has gone up and nothing else in your routine has changed, your sleep patterns are worth examining.
Medications That Raise Blood Sugar
Corticosteroids are one of the most common medication-related causes of rising blood sugar. Whether taken as pills for inflammation, inhaled for asthma, or injected into a joint, steroids push glucose levels up. The effect is dose-dependent: higher doses and longer courses cause bigger spikes. For people with diabetes, even a single steroid injection into a shoulder or knee can temporarily elevate blood sugar levels enough to matter.
Other medication classes that can raise blood sugar include certain blood pressure drugs (particularly thiazide diuretics), some antipsychotic medications, and niacin supplements. If your A1c has risen after starting a new medication, that connection is worth discussing with your prescriber. The solution is rarely to stop the medication, but knowing the cause can change how you manage it.
Dehydration and Fluid Intake
Being chronically under-hydrated affects blood sugar through several pathways. When your body is low on water, plasma volume drops and glucose becomes more concentrated in your blood. Dehydration also triggers your liver to produce more glucose through a process called gluconeogenesis. Higher blood concentration (osmolality) has been shown to increase both liver glucose output and circulating glucose levels compared to well-hydrated states.
Better hydration increases plasma volume, which dilutes glucose concentrations and supports healthier glucose metabolism overall. This doesn’t mean drinking extra water will dramatically lower your A1c, but chronic mild dehydration, which is common in older adults and people who primarily drink coffee or soda, can contribute to a higher reading than your actual metabolic health warrants.
Iron Deficiency and Other Anemias
This is one of the most overlooked causes of a rising A1c, and it has nothing to do with blood sugar at all. Iron deficiency anemia can falsely elevate your A1c result. The mechanism relates to red blood cell lifespan: any condition that causes red blood cells to live longer than normal exposes them to glucose for more time, coating them with more sugar and inflating the A1c reading.
Iron deficiency slows red blood cell turnover, effectively aging your red blood cells and giving them more time to accumulate glucose. Studies show that treating iron deficiency anemia with iron supplementation lowers A1c, even in people whose actual blood sugar levels haven’t changed. Vitamin B12 deficiency and folate deficiency anemias cause the same kind of false elevation through the same mechanism. People who’ve had their spleen removed (which normally filters out old red blood cells) can also show artificially high A1c results because their red blood cells circulate longer than usual.
If your A1c seems higher than your daily blood sugar readings would suggest, untreated anemia is one of the first things to rule out.
Aging
A1c naturally drifts upward as you get older, even in people without diabetes. In non-diabetic adults, A1c increases by about 0.074% per decade. That’s a small number, but it means a healthy 60-year-old will typically have a measurably higher A1c than a healthy 30-year-old with identical diet and exercise habits. This gradual rise reflects age-related changes in insulin sensitivity and glucose metabolism rather than any disease process.
This matters most for people near diagnostic cutoffs. An A1c of 5.8% in a 25-year-old is more concerning than the same result in a 70-year-old, because the baseline has shifted. It’s one reason some clinicians interpret A1c results in context rather than relying on a single number.
Kidney Disease
Chronic kidney disease complicates A1c readings in both directions. Elevated urea levels in kidney disease create a modified form of hemoglobin called carbamylated hemoglobin, which can interfere with certain lab methods and push A1c results higher than they should be. At the same time, kidney disease often shortens red blood cell survival, which would lower A1c. The net effect depends on the severity of kidney disease and the lab method used, making A1c a less reliable marker in this population. If you have kidney disease and your A1c seems inconsistent with your glucose meter readings, the test itself may be the problem rather than your blood sugar control.