Vitamin C
Also known as: Ascorbic Acid
What Does Vitamin C Measure?
Vitamin C, also known as ascorbic acid, is a water-soluble vitamin measured in the blood (typically as plasma or serum ascorbic acid) to assess the body's current vitamin C status. The test reflects recent dietary intake and absorption rather than long-term stores, since the body cannot synthesize vitamin C on its own and must obtain it entirely through food or supplements. Blood levels are reported in micromoles per liter (µmol/L) or milligrams per deciliter (mg/dL), and results can indicate deficiency, adequacy, or excess.
Why Does Vitamin C Matter?
Vitamin C is essential for a wide range of physiological functions, including the synthesis of collagen (the structural protein in skin, blood vessels, and connective tissue), immune system support, and the neutralization of harmful free radicals as a potent antioxidant. It also enhances the absorption of non-heme iron from plant-based foods and plays a role in the synthesis of neurotransmitters such as norepinephrine. Chronically low levels lead to scurvy, characterized by fatigue, bleeding gums, poor wound healing, and joint pain. Even subclinical deficiency has been linked to increased susceptibility to infections, cardiovascular disease, and impaired collagen repair, making this marker clinically relevant across a wide range of health conditions.
Normal Ranges
Males
50–90 µmol/L (0.9–1.6 mg/dL); deficiency below 11 µmol/L
Females
50–90 µmol/L (0.9–1.6 mg/dL); deficiency below 11 µmol/L
Children
Varies by age; generally 20–80 µmol/L; consult pediatric reference ranges
Causes of High Levels
- Excessive supplementation with high-dose vitamin C pills or IV infusions
- Over-consumption of vitamin C-rich foods in combination with supplementation
- Renal impairment reducing excretion of excess ascorbic acid
- Certain laboratory errors or sample handling issues causing falsely elevated results
- Hemolytic conditions that can affect plasma measurement accuracy
Causes of Low Levels
- Inadequate dietary intake of fruits and vegetables (most common cause)
- Smoking, which dramatically increases oxidative stress and vitamin C utilization
- Chronic alcohol use impairing absorption and increasing excretion
- Inflammatory conditions and serious illness increasing metabolic demand
How to Improve Your Vitamin C
Diet
- Eat bell peppers (especially red and yellow), which provide over 150 mg of vitamin C per 100g serving
- Include citrus fruits such as oranges, grapefruits, and lemons daily as snacks or juice
- Add kiwi fruit to your diet — one medium kiwi provides approximately 70 mg of vitamin C
- Consume cruciferous vegetables like broccoli, Brussels sprouts, and kale, lightly steamed to preserve vitamin C content
- Eat strawberries, guava, and papaya regularly, as they are among the richest natural sources of vitamin C
Supplements
- Standard supplementation: 500–1000 mg of ascorbic acid daily for adults with deficiency
- Buffered vitamin C (calcium ascorbate or sodium ascorbate) at 500 mg daily for those with sensitive stomachs
- Liposomal vitamin C at 500–1000 mg daily for enhanced bioavailability and absorption
- Time-released vitamin C supplements at 500–1000 mg to maintain steadier plasma levels throughout the day
- Do not exceed 2000 mg per day (the Tolerable Upper Intake Level) without medical supervision to avoid gastrointestinal side effects
Related Biomarkers
Frequently Asked Questions
What are the symptoms of low vitamin C that might prompt a blood test?
Common symptoms of low or deficient vitamin C include persistent fatigue, bleeding or swollen gums, slow-healing wounds, easy bruising, dry and rough skin, frequent infections, and joint or muscle pain. In severe deficiency (scurvy), corkscrew body hair and petechiae (tiny skin hemorrhages) can appear. If you experience several of these symptoms, particularly with a diet low in fresh fruits and vegetables, your doctor may order a plasma vitamin C test to confirm deficiency.
Can you have too much vitamin C, and is it dangerous?
While vitamin C is generally safe because excess amounts are excreted in urine, very high supplemental doses (above 2000 mg per day) can cause gastrointestinal side effects including diarrhea, nausea, and stomach cramps. In people with a history of kidney stones, high doses may increase the risk of oxalate kidney stones, since ascorbic acid is metabolized to oxalate. Individuals with hemochromatosis (iron overload disorder) should also be cautious, as vitamin C enhances iron absorption and could worsen the condition. Toxicity from food sources alone is essentially impossible.
How does vitamin C affect iron absorption?
Vitamin C significantly enhances the absorption of non-heme iron, which is the form of iron found in plant-based foods like legumes, tofu, and leafy greens. It does this by converting iron from its less absorbable ferric form (Fe³⁺) to the more readily absorbed ferrous form (Fe²⁺) in the gut. Consuming a vitamin C-rich food or supplement alongside iron-rich plant foods or iron supplements can increase absorption by up to 67%. This interaction is especially important for vegetarians, vegans, and individuals with iron deficiency anemia.