A guide to dosing vitamin C, magnesium, and other nutrients
We often talk about taking vitamin C and magnesium to “bowel tolerance” – but what does that mean?
Dosing a supplement to bowel tolerance means taking the highest dose of that supplement possible without causing loose, watery stools (diarrhea).
If you’re taking vitamin C or magnesium with the goal of troubleshooting constipation, then you’ll want to exceed your bowel tolerance – that is, you want to take so much that you have a loose stool (caused by osmosis).
But if your aim in taking these nutrients is something other than a bowel movement, then it is typically advisable to keep dosage within your bowel tolerance. This isn’t just to spare yourself an urgent run to the bathroom: it’s also to ensure proper absorption of the nutrients you take.
Let’s say you’re taking magnesium to help with headaches. If you took a high dose and had diarrhea a few hours later, it means that your body didn’t actually absorb very much of the magnesium (since you just flushed it all out in the watery stool), and thus you may still be at risk of headaches. You may therefore want to take magnesium again – but at a lower dose.
Bowel tolerance varies from person to person, and can change over time as well.
One way to determine bowel tolerance
Here is one common method for determining “bowel tolerance” for vitamin C:
- Take 2 grams (2,000mg) of vitamin C.
- 15 minutes later, take another 1 gram of vitamin C.
- 15 minutes later, take another 1 gram.
- Keep going taking 1 gram of vitamin C every 15 minutes until you experience diarrhea.
- Take note of how many total grams of vitamin C it took to cause the loose stool(s). Then calculate 75% of that amount to determine how much vitamin C to take daily, in divided doses.
- If loose stools occur again (which sometimes happens after about a week), reduce the total daily dose by another 25%.
For example, if it took a total of 10 grams of vitamin C to stimulate diarrhea, 75% of 10 grams is 7.5 grams (or 7,500mg). Splitting this up over the course of the day could mean taking 2.5 grams three times daily, or 4 grams in the morning and 3.5 grams at night.
If this dose of 7.5 grams daily was well tolerated for a week, but then diarrhea occurred again, we would reduce the dose by another 25%: 25% of 7.5 grams is 1.875 grams (let’s just say 2 grams, for easy math). The total daily dose would now be 5.5 grams. One way to take it could be 3 grams in the morning and 2.5 grams at night.
Another way to determine bowel tolerance
Another approach for determining bowel tolerance is gentler, but more time consuming. We’ll look at magnesium dosing for this titration strategy:
- Take 100mg of magnesium twice daily for 4-7 days.
- Increase to 200mg of magnesium twice daily for 4-7 days.
- Increase to 300mg of magnesium twice daily for 4-7 days.
- Keep increasing dosage by 100mg per dose (200mg daily total) until you experience diarrhea.
- Take note of how many total daily grams of magnesium it took to cause the loose stool(s). Then calculate 75% of that amount to determine how much magnesium to take daily, in divided doses.
- If loose stools occur again, reduce the total daily dose by another 25%.
Bowel tolerance increases with illness
It has been observed that the sicker a patient is, or the more deficient they are in a particular nutrient, the higher their tolerance is for supplemental doses of that nutrient. You may therefore find that if you get sick with a cold or flu, or are wheezing through allergy season, that your bowel tolerance of vitamin C increases. Likewise, if you’re in a slump of getting lots of migraines, your bowels may be able to handle higher doses of magnesium than usual.
Ways to take higher doses – without diarrhea
If you want to take doses of vitamin C or magnesium greater than your bowel tolerance, you the following strategies may be of help:
- Break up your supplement regimen to include more doses throughout the day, with each dose staying within your bowel tolerance. (For example: instead of taking 800mg of magnesium glycinate all at once, you could take 400mg twice daily, or 200mg four times daily.)
- Use sodium ascorbate or other buffered vitamin C preparation. Buffered vitamin C is much gentler on the bowels than regular ascorbic acid.
- Of the various forms of oral magnesium available, magnesium glycinate is relatively less likely to cause diarrhea.
- Consider using topical preparations of magnesium, such as applying magnesium cream to the skin or taking baths with Epsom (magnesium sulfide) salts.
- Intravenous (IV) infusions of vitamin C and magnesium deliver high doses of nutrients to the bloodstream, bypassing the digestive tract. Work with your naturopathic physician (ND) or other orthomolecular medicine specialist to see if IV nutrient infusions are right for you.
ReferencesClick here to see References
 Cathcart RF. Vitamin C, titrating to bowel tolerance, anascorbemia, and acute induced scurvy. Med Hypotheses. 1981 Nov;7(11):1359-76. doi: 10.1016/0306-9877(81)90126-2.
 Cathcart RF. Letter to the editor: Clinical trial of vitamin C [Internet]. Naperville (IL): Vitamin C Foundation; 1975 [cited 2021 Aug 16]. Available from: http://vitamincfoundation.org/www.orthomed.com/incline.htm
 Yablon LA, Mauskop A. Magnesium in headache. In: Magnesium in the Central Nervous System [Internet]. Adelaide (AU): University of Adelaide Press; 2011 [cited 2021 Aug 16]. PMID: 29920023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507271/
 Sesso KA. Taking magnesium to bowel tolerance [Internet]. Spokane (WA): North Central Chiropractic Center; 2014 [cited 2021 Aug 16]. Available from: https://www.chiropracticspokane.com/taking-magnesium-to-bowel-tolerence/
 Cathcart RF. The method of determining proper doses of vitamin C for the treatment of disease by titrating to bowel tolerance. J Orthomolecular Psychiatry. 1981;10:125-32. Available from: http://orthomolecular.org/library/jom/1981/pdf/1981-v10n02-p125.pdf
 Medical News Today. The uses and benefits of magnesium glycinate [Internet]. Brighton (UK): Medical News Today; 2018 [cited 2021 Aug 16]. Available from: https://www.medicalnewstoday.com/articles/315372