A1C levels reflect your average blood sugar over the past two to three months, but dozens of factors beyond diet can push that number up or down. Some change your actual blood sugar, while others distort the test itself, giving a reading that doesn’t match reality. Understanding the difference matters, especially if your A1C result doesn’t line up with your day-to-day glucose readings.
How A1C Works (And Why It’s a Weighted Average)
A1C measures the percentage of hemoglobin, the oxygen-carrying protein in your red blood cells, that has glucose permanently attached to it. Glucose binds to hemoglobin through a chemical reaction that happens naturally in everyone’s bloodstream. The more glucose circulating in your blood, the more hemoglobin gets coated.
Red blood cells live about 120 days, so your A1C captures a rolling window of blood sugar history. But it’s not an even average. More recent weeks are weighted more heavily than older ones, with the past six to eight weeks contributing most of the signal. That’s why a sudden change in diet or medication can start shifting your A1C within a month or two, even though the full picture takes three months to turn over.
Blood Sugar: The Biggest Driver
The most straightforward thing that affects A1C is how much glucose is in your blood on a daily basis. Anything that raises or lowers your blood sugar over weeks and months will move your A1C in the same direction. The diagnostic thresholds reflect this: below 5.7% is considered normal, 5.7% to 6.4% falls in the prediabetes range, and 6.5% or higher indicates diabetes.
This means the factors you’d expect matter most. What you eat, how active you are, how well your body produces and responds to insulin, and whether you take blood sugar-lowering medication all directly influence A1C by changing your actual glucose levels. These are “real” effects on A1C, meaning the number accurately reflects what’s happening in your body.
Exercise and Physical Activity
Regular exercise lowers A1C by improving how efficiently your muscles use glucose and by increasing your cells’ sensitivity to insulin. The effect is dose-dependent. A study in people with type 2 diabetes found that every 20% increase in exercise adherence (roughly two additional sessions per month) was associated with a 0.15% decrease in A1C. That may sound small, but it adds up: someone who goes from sporadic activity to consistent exercise several times a week can see a meaningful drop over a few months.
Both aerobic exercise (walking, cycling, swimming) and resistance training (weights, bodyweight exercises) contribute. The combination tends to be more effective than either alone. The key variable is consistency over weeks, not intensity on any single day, since A1C is a long-term average.
Medications That Raise Blood Sugar
Corticosteroids like prednisone are one of the most common medication-related causes of A1C increases. Steroids raise blood sugar starting four to eight hours after you take them, and a higher dose produces a larger spike. If you’re on steroids for weeks or months to manage inflammation, asthma, or an autoimmune condition, your A1C will climb to reflect that sustained elevation. The effect is usually temporary and reverses after you stop taking them, though it can take a full red blood cell cycle (two to three months) for A1C to normalize.
Other medications that can raise blood sugar over time include certain antipsychotics, some blood pressure medications (particularly thiazide diuretics), and niacin supplements at high doses. If your A1C rises after starting a new medication, that’s worth discussing with your provider.
Iron Deficiency and Anemia
Iron deficiency anemia is one of the most clinically significant non-glucose factors that affects A1C, and it pushes the number falsely high. When your body is low on iron, a chemical byproduct that accumulates in iron-deficient blood actually enhances the glucose-binding process on hemoglobin, making it look like your blood sugar has been higher than it really was.
This matters because iron deficiency is common, particularly in women of reproductive age, vegetarians, and people with chronic digestive conditions. If you have iron deficiency anemia, your A1C could overestimate your true blood sugar average, potentially leading to an inaccurate prediabetes or diabetes diagnosis. Treating the iron deficiency with supplementation has been shown to lower both A1C and other glycemic markers in diabetic and non-diabetic people alike, confirming that the original reading was artificially inflated.
Hemoglobin Variants and Genetic Factors
Inherited variations in the structure of hemoglobin can interfere with the lab methods used to measure A1C. People of sub-Saharan African, Mediterranean, South Asian, or Southeast Asian descent are more likely to carry hemoglobin variants S and E, among others. Depending on the specific variant and the testing method used, A1C results can be falsely high or falsely low.
A falsely high result could lead to unnecessary treatment or a misdiagnosis of diabetes. A falsely low result could mean diabetes goes undetected or undertreated. For people with certain combinations of hemoglobin variants (such as sickle cell disease with HbSS, or HbCC or HbSC), the National Institute of Diabetes and Digestive and Kidney Diseases recommends that A1C testing not be used at all. Alternative measures of blood sugar control, like fructosamine or continuous glucose monitoring, are more reliable in these cases.
If you know you carry sickle cell trait or another hemoglobin variant, make sure your provider is aware before interpreting A1C results.
Kidney Disease
Advanced chronic kidney disease, particularly stages 4 and 5 or dialysis, reduces the reliability of A1C measurements. People with severe kidney disease often have anemia and altered red blood cell production, both of which shorten how long red blood cells survive. Since A1C depends on red blood cells living their full 120-day lifespan, a shorter lifespan means less time for glucose to accumulate on hemoglobin, pulling A1C falsely low.
Alternative markers like glycated albumin are increasingly available and may provide a more accurate picture for people on dialysis, though they come with their own limitations in people who lose protein through their urine.
Pregnancy
Pregnancy changes red blood cell dynamics in ways that make A1C unreliable, especially in the second and third trimesters. Pregnant women produce red blood cells faster and experience hemodilution (the blood volume expands, diluting the concentration of hemoglobin). Both effects shorten the effective exposure time of hemoglobin to glucose.
The result is that A1C tends to underestimate actual blood sugar levels in mid-to-late pregnancy, and the underestimation is more pronounced in women with lower hemoglobin levels. This is why gestational diabetes is diagnosed using oral glucose tolerance tests rather than A1C, and why pregnant women with pre-existing diabetes typically rely on frequent glucose monitoring rather than A1C alone.
Age
A1C drifts upward with age even in people who don’t have diabetes. Data from the National Health and Nutrition Examination Survey spanning 15 years found that among non-diabetic adults with normal fasting glucose, A1C increased by roughly 0.008% to 0.009% per year. That translates to about a 0.1% rise per decade.
The practical effect: the average A1C in healthy adolescents and young adults is around 5.18%, while healthy older adults average about 5.51%. This age-related creep is modest, but it means a 70-year-old with an A1C of 5.7% (the prediabetes threshold) may have a meaningfully different metabolic picture than a 30-year-old with the same number. The diagnostic cutoffs don’t currently adjust for age, which is a limitation worth keeping in mind.
Vitamin C and Antioxidants
High vitamin C intake is inversely associated with A1C levels. Analysis of national survey data found a statistically significant relationship: higher blood concentrations of vitamin C correlated with lower A1C readings. The mechanism appears to involve vitamin C chemically interfering with the glycation process, essentially competing with glucose for binding sites on hemoglobin.
This means that someone taking high-dose vitamin C supplements could see an A1C reading that underestimates their true average blood sugar. The effect from normal dietary intake of vitamin C-rich foods is likely small, but mega-dose supplementation (1,000 mg or more daily) could be enough to skew results.
Anything That Changes Red Blood Cell Lifespan
Because A1C depends entirely on hemoglobin spending 120 days in circulation, any condition that shortens or lengthens red blood cell survival will affect the result independent of blood sugar. Conditions that destroy red blood cells faster (hemolytic anemias, significant blood loss, recent blood transfusions, spleen enlargement) tend to lower A1C falsely. Conditions that extend red blood cell lifespan (spleen removal, certain vitamin deficiencies like B12 or folate that slow red blood cell production) can push A1C falsely higher.
This is the single most important principle for understanding A1C accuracy: the test assumes your red blood cells live a normal lifespan. When that assumption breaks down, so does the test. If your A1C doesn’t match your home glucose readings or your clinical picture, altered red blood cell turnover is often the explanation.