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How to Take Vitamin C To Bowel Tolerance

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By Dr. John Neustadt

Your immune system is an intricate network of specialized organs, cells, proteins, and chemicals. It protects you from an endless onslaught of disease-causing bacteria, viruses, fungi, and parasites, as well as rogue cells that could turn into cancer. More than 50 years of research have shown vitamin C plays a crucial role in various aspects of the immune system. 

The amount of oral vitamin C someone can tolerate depends on the person and their health. Studies indicate that when someone gets an infection, their ability to absorb vitamin C increases. This is because the body uses up a lot more vitamin C when it’s under stress and is an example of how your physiology adapts to meet your body’s needs. This phenomenon of increased vitamin C absorption is often called “to bowel tolerance,” and it’s a technique I shared with many of my patients.1-3 

Vitamin C is safe, even at high doses. When taken orally, if someone gets too much vitamin C, it creates diarrhea. This happens because the amount of vitamin C exceeds the ability of the small intestines to absorb the nutrient. At that point, you’ve reached bowel tolerance and simply need to decrease the amount of vitamin C to the dose that doesn’t cause loose stool.

To Bowel Tolerance

If you’re starting to feel a bit under the weather, or know you’re already fighting something, try this technique to promote a healthy immune system. 

Currently, there is no best test to know how much vitamin C you need. One way to estimate it, however, is based on your bowel tolerance.  Here’s how you determine it.

      • Begin by taking 1000 mg of Vitamin C per day, which is one capsule of NBI’s Vitamin C 1000.
      • The next day, take 1000 mg of vitamin C twice a day, for a total of 2000 mg.
      • The next day, add another capsule, increasing your dose to 3000 mg of vitamin C. You can try this by taking two NBI Vitamin C capsules in the morning and one at night.
      • The day after that, add another 1000 mg. You can take two NBI Vitamin C capsules in the morning and two at night, for a total of 4000 mg for the day.
      • Continue adding one capsule a day until you experience loose or burning stools. At this point, you’ve reached your tolerance of vitamin C and have taken a bit too much.
      • Decrease your dosage of vitamin C one capsule (1000 mg). If loose stools do not reoccur, this is your maximal dosage of vitamin C.
      • If loose stools continue, decrease your dosage of vitamin C until they stop. 

Keep in mind that if you try this when you’re sick, your need for vitamin C may decrease as you recover. You will then experience bowel tolerance at a much lower dose. You will need to slowly lower your dosage until you find your new maximum dosage of vitamin C.


1 Cathcart RF. 1980;9(4):9-13. 

2 Cathcart RF 1986;21(3):307-21. 

3 Holford P, Carr AC, Jovic TH, et al. 2020;12(12).

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