From the Research
Vitamin C (ascorbic acid) cannot be absorbed in amounts exceeding 500mg within a 12-hour period. When a person consumes more than 500mg of vitamin C at once, the body's absorption efficiency decreases significantly. This occurs because vitamin C is water-soluble and absorption happens primarily in the small intestine through active transport mechanisms that become saturated at higher doses. Once these transporters are saturated, additional vitamin C simply passes through the digestive system unabsorbed and is excreted in urine. This is why people taking high-dose vitamin C supplements often experience diarrhea and gastrointestinal discomfort - the unabsorbed excess creates an osmotic effect in the intestines. For those needing higher daily intake of vitamin C, it's more effective to split doses throughout the day rather than taking one large dose. In contrast, vitamin D is fat-soluble and follows different absorption mechanisms that don't have the same strict upper limit within a short timeframe.
The most recent and highest quality study, 1, supports this recommendation, as it found that bioavailability of vitamin C declines and the absorbed amount is excreted at single doses of 500 mg and higher. Additionally, study 2 highlights the importance of regulating plasma levels of vitamin C, which is meticulously controlled on the level of absorption and excretion in the kidney.
Key points to consider:
- Vitamin C absorption is limited by saturation of active transport mechanisms in the small intestine
- High doses of vitamin C can cause gastrointestinal side effects due to unabsorbed excess
- Splitting doses throughout the day is more effective for achieving higher daily intake of vitamin C
- Vitamin D has different absorption mechanisms and does not have the same strict upper limit within a short timeframe, as noted in studies 3, 4, 5.