A1C rises when your blood sugar stays elevated over weeks and months. The test measures how much sugar has attached to your red blood cells during their roughly three-month lifespan, so anything that keeps blood sugar high (or changes how long your red blood cells live) can push your number up. The main drivers fall into a few categories: what you eat, how you sleep, your stress levels, certain medications, and a handful of medical conditions that can skew results even when your blood sugar is reasonably well controlled.
How the A1C Test Works
Your red blood cells carry hemoglobin, and glucose in your bloodstream gradually sticks to it. The more glucose circulating over time, the more hemoglobin gets coated. Because red blood cells regenerate roughly every three months, the A1C test captures your average blood sugar across that window. The formula to convert A1C to an estimated average glucose is: (28.7 × A1C) − 46.7 = average glucose in mg/dL. So an A1C of 7% translates to an average blood sugar around 154 mg/dL.
For reference, the American Diabetes Association’s 2025 standards define normal A1C as below 5.7%, prediabetes as 5.7% to 6.4%, and diabetes as 6.5% or higher. Even small shifts within these ranges reflect meaningful changes in your average blood sugar over the preceding weeks.
High-Glycemic Foods and Refined Carbohydrates
Diet is the most direct lever. Foods that spike blood sugar quickly, like white bread, sugary drinks, white rice, and pastries, contribute more to A1C than the same number of calories from slower-digesting sources. A meta-analysis in Diabetes Care found that switching from a high-glycemic-index diet to a low-glycemic-index diet lowered A1C by 0.43%. That’s a clinically meaningful difference from food choices alone. The EURODIAB study of nearly 3,000 people with type 1 diabetes confirmed the pattern: the higher the glycemic index of someone’s usual diet, the higher their A1C.
It’s not just about sugar, though. Total carbohydrate load matters, and so does the type. Whole grains and cereal fiber are consistently linked to lower diabetes risk and better blood sugar control. Interestingly, some sugars behave differently than you’d expect. Table sugar (sucrose) and fructose actually have a lower glycemic response than some starches, despite being “simple” sugars. This is why glycemic index matters more than whether a food is labeled “complex” or “simple.”
Replacing high-glycemic carbohydrates with either low-glycemic options or healthy fats (like olive oil, nuts, and avocado) produces similar improvements in average blood sugar. You don’t necessarily need to go low-carb. You need to go lower on the glycemic index.
Poor Sleep
Chronic short or poor sleep raises A1C through several pathways at once. Sleep deprivation reduces your muscles’ ability to absorb glucose, triggers your liver to release more glucose into the bloodstream, and impairs insulin secretion. The result mimics the metabolic pattern of type 2 diabetes, even in otherwise healthy people.
The hormonal side makes it worse. Sleep loss raises evening cortisol levels and ramps up your sympathetic nervous system, both of which push blood sugar higher. And there’s a behavioral layer on top: people who are sleep-deprived eat roughly 20% more calories, with a strong preference for carbohydrate-heavy and fatty foods. So poor sleep raises blood sugar directly through hormones and indirectly by changing what and how much you eat.
Chronic Stress and Cortisol
When you’re under sustained stress, your adrenal glands can produce up to 20 times the normal amount of cortisol. Cortisol raises blood sugar in multiple ways: it stimulates the liver to produce and release glucose, reduces glucose uptake in your muscles and fat tissue, suppresses insulin secretion, and promotes insulin resistance. Under acute stress, this is a useful survival response. Under chronic stress, it’s a recipe for persistently elevated blood sugar and a climbing A1C.
The severity matters. Cortisol output rises in proportion to the intensity and duration of stress. Ongoing work pressure, caregiving strain, financial worry, or chronic illness can keep cortisol elevated enough to meaningfully shift your three-month average.
Medications That Raise Blood Sugar
Corticosteroids are the most common medication-related culprit. Even a single steroid injection can spike fasting blood sugar by 30 to 40 mg/dL in people with well-controlled diabetes, with that elevation lasting about four days before returning to baseline around day eight. For people with poorly controlled diabetes or those using insulin, the spike can exceed 80 mg/dL and take more than 10 days to normalize. Oral steroids taken over weeks or months, such as prednisone for autoimmune conditions, have a larger cumulative effect on A1C.
Niacin (vitamin B3), sometimes prescribed for cholesterol, raises fasting glucose by about 4% to 5% and can increase A1C by up to 0.3 percentage points. The effect is generally modest and reversible after stopping or adjusting the dose.
Other medications that can raise blood sugar include certain antipsychotics, some blood pressure drugs (particularly thiazide diuretics), and immunosuppressants used after organ transplants. If your A1C climbed after starting a new medication, that connection is worth exploring.
Iron Deficiency Anemia
This one is frequently overlooked: iron deficiency can raise your A1C even if your blood sugar is normal. When your body is low on iron, it produces fewer red blood cells. The ones in circulation live longer than usual, giving glucose more time to attach to hemoglobin. The result is a falsely elevated reading that doesn’t reflect your actual blood sugar control.
The effect is substantial. In one study, people with iron deficiency anemia had an average A1C of 6.87%, compared to 5.65% in non-anemic individuals, despite similar blood sugar levels. Among those with normal fasting glucose (under 100 mg/dL), iron-deficient individuals still averaged an A1C of 6.43% versus 5.46% in controls. That’s nearly a full percentage point of artificial elevation. Women were affected more, averaging an A1C of 7.02%. If your A1C seems higher than your daily blood sugar readings suggest, iron deficiency is one of the first things to investigate.
Kidney and Liver Disease
Chronic kidney disease complicates A1C in both directions. End-stage kidney disease typically causes falsely low A1C readings because the associated anemia shortens red blood cell survival, reducing the time glucose has to attach. However, uremia (the buildup of waste products in the blood) can create a modified form of hemoglobin that some lab methods misread as glycated hemoglobin, producing a false high. The net effect depends on which factor dominates and which lab assay is used.
Severe liver disease can similarly distort results by altering red blood cell turnover and protein levels. For people with significant kidney or liver problems, alternative markers of blood sugar control may give a more accurate picture than A1C alone.
Sedentary Lifestyle and Weight Gain
Physical inactivity reduces your muscles’ sensitivity to insulin, meaning glucose lingers in the bloodstream longer after meals. This effect compounds over time. Excess body fat, particularly visceral fat around the organs, promotes chronic low-grade inflammation and insulin resistance, both of which keep blood sugar elevated between meals. Even modest weight gain of 5 to 10 pounds, if it’s primarily fat, can nudge A1C upward over the following months.
The reverse is also true. Regular physical activity improves insulin sensitivity for 24 to 48 hours after each session, which is why consistent exercise (not just occasional bursts) has the most impact on A1C.
What Recent Weeks Matter Most
Although A1C reflects three months of blood sugar history, it’s not weighted equally across that period. More recent weeks contribute more heavily to the result, because a larger proportion of your circulating red blood cells were produced recently. A stretch of high blood sugar in the two to four weeks before your test will have a disproportionate effect compared to an equivalent stretch three months earlier. This is why a sudden dietary change, a stressful period, a round of steroids, or a bout of illness right before testing can push the number higher than your overall trend would predict.