How the kidneys keep vitamin C and pass high doses into urine

The body tightly controls the level of vitamin C in the blood and tissues.^[1] The small intestine absorbs 70% to 90% of 30 to 180 mg a day, but less than 50% of doses above 1 g a day.^[1] The kidneys take back all the vitamin C they filter until the blood level reaches the renal threshold. On average, this threshold is 48.5 µmol/L (micromoles per litre) in men and 58.3 µmol/L in women. Above it, vitamin C goes into urine.^[2] As intake rises, the steady blood level reaches a maximum of about 70 to 80 µmol/L.^[3]

How high does vitamin C in the blood go?

The steady level stops at about 70 to 80 µmol/L. About 200 to 400 mg a day is enough to fill the blood in healthy people.^[3]

The maximum of 70 to 80 µmol/L comes from studies in which people lived in hospital and took set doses.^[3][4][8] In the 1996 study, 7 healthy volunteers stayed in hospital for 4 to 6 months. They ate a diet with less than 5 mg of vitamin C a day and took 7 daily doses in turn, from 30 to 2500 mg. At each dose, the researchers measured the steady level in the blood.^[4]

The blood level and the dose formed an S-shaped curve:

• The level rose steeply between 30 and 100 mg a day.^[4]
• 200 mg a day was the first dose past the steep part.^[4]
• At 1000 mg a day the plasma was completely full.^[4]
• White blood cells were full at 100 mg a day. Their level of vitamin C was at least 14 times that of the plasma.^[4]

A study of the same kind in healthy young women found plasma and blood cells full at 400 mg a day. Higher doses left in urine.^[8]

A low level has its own limits. Severe deficiency, which can end in scurvy, is a plasma level below 11 µmol/L. Marginal deficiency is a level below 23 µmol/L.^[9]

Does a bigger tablet raise the level much more?

Only a little. In a study of 17 healthy volunteers in hospital, 1.25 g of vitamin C by mouth gave an average peak of 134.8 µmol/L.^[7] This is about twice the level from 200 to 300 mg a day of vitamin C from food.^[1]

In the same study, a computer model predicted a peak of only 220 µmol/L for 3 g by mouth every 4 hours. The same 1.25 g dose given into a vein gave a peak of 885 µmol/L. The authors concluded that vitamin C by mouth gives tightly controlled blood levels.^[7]

When does vitamin C appear in urine?

When the blood level passes the renal threshold.^[2] In the 1996 study, 6 of the 7 volunteers had no vitamin C in their urine until the dose reached 100 mg a day. At single doses of 500 mg and more, the amount that the gut absorbed was passed in urine.^[4]

The share of filtered vitamin C that leaves in urine changes with how full the body is. In people with low vitamin C, the kidneys take back all of it. In people whose blood is full, all of the extra leaves.^[3] Below 38.1 µmol/L in men and 43.2 µmol/L in women, a healthy person's urine is not expected to hold any vitamin C.^[2]

After a large dose into a vein, the kidneys filter vitamin C out and take almost none of it back. Half of it is gone from the blood in about 2 hours.^[3]

Where does the vitamin C from a tablet go?

It follows this path:

1. The small intestine absorbs part of the dose with a transporter. The rest stays in the gut.^[1]
2. The blood carries vitamin C to the cells, which take it up with a second transporter.^[1]
3. The kidneys filter it and take it back with SVCT1, up to a limit.^[3]
4. What the kidneys cannot take back leaves in urine.^[2]

The body holds about 300 mg of vitamin C when it is close to scurvy and about 2 g when it is full. Cells keep much higher levels than the plasma. White blood cells, the eyes, the adrenal glands, the pituitary gland and the brain hold the most.^[1]

Do women have a different threshold?

Yes, a higher one on average. In the 2022 analysis, the average renal threshold was 58.3 µmol/L in women and 48.5 µmol/L in men.^[2] In young women who lived in hospital, the plasma was full at 400 mg a day.^[8]

Can the kidneys lose too much vitamin C?

Yes, in some people, and more often in people with diabetes. A 2022 study looked for a "renal leak": vitamin C in urine while the blood level is below 38.1 µmol/L in men or 43.2 µmol/L in women. Below these levels, a healthy person's urine is not expected to hold any vitamin C. It found a leak in 33% of 82 people with diabetes and in 9% of 80 people without diabetes. On average, the people with diabetes had 30% less vitamin C in the blood: 40.9 against 53.1 µmol/L.^[2]

Are high doses safe for the kidneys?

High doses of vitamin C are not believed to cause serious harm. High intakes may raise oxalate and uric acid in urine, which could add to kidney stones, mainly in people with kidney disorders. Studies of this give conflicting results.^[1] In the 1996 study, oxalate and uric acid in urine were higher at 1000 mg a day than at lower doses.^[4]

In the US, the recommended intake for adults is 90 mg a day for men and 75 mg for women. People who smoke need 35 mg more. The upper limit for adults is 2000 mg a day.^[1] The concern about kidney stones applies mainly to people with kidney disorders, so they should ask a doctor before they take high doses.^[1]

In short

• The gut absorbs 70% to 90% of 30 to 180 mg a day, and less than 50% of doses above 1 g a day.^[1]
• The kidneys filter vitamin C and take it back with the transporter SVCT1, up to a limit.^[3]
• Vitamin C appears in urine above the renal threshold: 48.5 µmol/L in men and 58.3 µmol/L in women.^[2]
• The steady blood level stops at about 70 to 80 µmol/L. About 200 to 400 mg a day fills the blood.^[3]
• Taken by mouth, 1.25 g gave an average peak of 134.8 µmol/L. The same dose given into a vein gave 885 µmol/L.^[7]
• From doses of about 500 mg, nearly all of the vitamin C that the gut absorbs leaves in urine.^[4][3]

What this figure leaves out

The shapes come from the Human Reference Atlas, shared under the CC BY 4.0 licence: the small intestine (Small Intestine, Female, v1.2), the left kidney (Kidney, Female, Left, v1.3), its artery and vein (Blood Vasculature, Female, v1.3) and the first 4 cm of the ureter (Ureter, Female, Left, v1.2).^{[10][11][12][13]} The dots and the glow are signs, not single molecules.

The motion and the numbers come from a small model. Its kidneys filter 120 mL of plasma a minute,^[5] and it uses the renal threshold of men, 48.5 µmol/L.^[2] Its absorption and the amount the body uses each day (about 75 mg when full) are fitted to the 1996 study. They are not measured values.^[4]

The model leaves out women, whose threshold is higher^[2] and whose plasma was full at 400 mg a day.^[8] It also leaves out what changes vitamin C status from person to person: genes, smoking, diet and disease.^[3] For education; not for diagnosis.