Dr. Kumar’s Take:
This landmark NIH study shows that plasma vitamin C levels rise steeply with low-to-moderate doses, then plateau at higher intakes. Most people reach near-maximal blood levels with just 200 mg daily, and above 400 mg/day, your body starts dumping the extra in urine. For optimal absorption and benefit, aim for 200 mg daily from food or supplements.
Key Takeaways:
✔ Plasma vitamin C increases quickly with doses up to 100 mg/day, then plateaus at higher intakes.
✔ Maximal white blood cell (immune cell) saturation occurs at 100 mg/day; plasma is near-maximal at 200 mg/day.
✔ No vitamin C is lost in urine until about 100 mg/day; after 400 mg/day, most extra is excreted.
✔ High doses above 1000 mg/day may increase urine oxalate and uric acid, with no added benefit.
Actionable Tip:
If you want to optimize your vitamin C status, aim for around 200 mg daily—easily achieved with several servings of fruits and vegetables. Doses above this are mostly excreted in urine and provide little extra benefit.
Brief Summary:
This NIH study explored how different vitamin C intakes affect blood (plasma) and immune cell levels, as well as how much is lost in urine. Seven healthy young men were depleted of vitamin C, then given increasing daily doses (30, 60, 100, 200, 400, 1000, and 2500 mg) while closely monitoring their blood and urine. Blood levels rise sharply with small-to-moderate doses and level off at higher doses. Extra vitamin C above about 400 mg is mostly lost in urine.
Study Design: Vitamin C Dosing, Absorption, and Excretion
- Seven healthy male volunteers were kept on a vitamin C–restricted diet in a hospital for 4–6 months.
- Subjects were given increasing daily doses of vitamin C (30 mg to 2500 mg), with blood and urine levels closely monitored.
- Plasma (blood) and immune cell vitamin C levels were measured at each dose.
- Urinary excretion was tracked after single oral and intravenous doses to see how much was absorbed and lost.
Results: Plasma Levels and Urinary Loss at Different Doses
Plasma Vitamin C Levels by Dose
- 30 mg/day: Plasma vitamin C averaged 7–12 µM.
- 60 mg/day: Plasma averaged 15–24 µM.
- 100 mg/day: Plasma jumped to 50–62 µM.
- 200 mg/day: Plasma reached 57–75 µM.
- 400 mg/day: Plasma reached 60–80 µM.
- 1000 mg/day: Plasma plateaued around 70–85 µM.
- 2500 mg/day: Only slight increase; plasma near max (75–92 µM).
Most of the increase occurs between 30–200 mg/day. After 400 mg/day, plasma levels barely increase.
Urinary Excretion at Different Doses
- Below 100 mg/day: No measurable vitamin C in urine; the body holds onto nearly all of it.
- 100 mg/day (single dose): Around 25 mg excreted in urine.
- 200 mg/day (single dose): Small but detectable excretion.
- 500 mg and higher (single dose): Nearly all of the extra is lost in urine. At 500 mg and 1250 mg, the body excretes most of the absorbed vitamin C.
- Above 400 mg/day: Urine oxalate and uric acid start to rise—no clear health benefit, just more loss in urine.
Absorption is nearly 100% up to 200 mg but drops to about 33–73% at 1250 mg doses (the rest is not absorbed or is quickly lost in urine).
Vitamin C: Absorption vs. Excretion Explained
When you take more than your body can use, your kidneys rapidly remove the excess. This is why urine loss of vitamin C sharply increases at doses above 200–400 mg/day—your plasma (blood) levels can’t go any higher, so your body dumps the rest.
Related Studies and Research
Vitamin C and the Common Cold: Leucocyte Ascorbic Acid Study – Links vitamin C status to immune cell saturation and cold frequency.
Vitamin C Deficiency: Clinical Signs, Risk Factors, and Treatment – Summarizes how inadequate intake leads to subclinical and clinical deficiency.
Vitamin C Physiology: The Goldilocks Review – Highlights the balance needed for optimal vitamin C utilization and storage.
Plasma and Leucocyte Vitamin C Relationship: Insights from BMJ 1971 – Explains how plasma and immune cell vitamin C are interconnected.
Frequently Asked Questions
What blood levels of vitamin C should I aim for?
Most health benefits and immune cell saturation are achieved with plasma levels around 60–70 µM, which occurs at 100–200 mg/day.
Is there any benefit to taking more than 200–400 mg/day?
Not really. Your blood levels will barely rise, and your kidneys will excrete the excess. Doses above 1000 mg/day may increase urine oxalate and uric acid.
Does this mean megadoses of vitamin C are a waste?
For healthy people, yes—your body just gets rid of the extra. There may be exceptions in illness or special cases, but for everyday use, more is not better.
Can high doses cause harm?
Doses above 1000 mg/day can raise urine oxalate and uric acid, potentially increasing kidney stone risk in susceptible people.
How can I get 200 mg of vitamin C naturally?
Eat a variety of fruits and vegetables daily—for example, an orange, a cup of strawberries, and a cup of broccoli will easily get you there.
Conclusion
This careful clinical study confirms: 200 mg per day is enough to fully saturate your blood with vitamin C. More is not better for most people, and excess is simply lost in urine. Focus on getting enough from your diet, and don’t rely on megadoses for extra benefit.