March 5, 2011

033 Vitamin C & Immunity [13 Oct 2009]

In last week’s column Vitamin C was suggested as important in the prevention of colds and flu. Is there any evidence to support this belief?

Most medical sources will tell you that there is no proof and that studies have been disappointing for Vitamin C. But those studies used low doses of about 500 mg. What is an optimal dose for humans?

Unlike most mammals, humans and the apes do not synthesize their own vitamin C so must get it in their diet. Species that do create their own vitamin C produce much higher amounts than the Canadian adult RDI of 60 mg. A healthy 45 pound goat will produce about 6 grams (6,000 mg) per day in normal situations and 20 to 30 times as much when fighting an infection. So 2 to 4 grams when unstressed and up to 15 grams when stressed would not be out of line for an adult human.

Unfortunately the digestive tract cannot tolerate the higher amounts. Vitamin C researchers refer to “bowel tolerance” as the level of vitamin C that can be taken without causing diarrhea.  This is usually between 5 and 10 grams (more than this amount must be taken by IV infusion) which is sufficient for most therapeutic effects. At doses over 1 gram a buffered form of C is recommended to reduce acidity.

But are these levels safe? Dr. Carol Johnston published an article in Nutrition Review (March 1999) on the safety of high intakes of vitamin C. She examined the evidence on “rebound scurvy”, kidney stones, hemolytic anemia, enhanced iron absorption, the destruction of vitamin B12, and other possible effects. She concluded that “the available data indicate that very high intakes of vitamin C (2-4g/day) are well tolerated biologically…”

In the nearly 40 years since 1970 when Linus Pauling published his book on Vitamin C and the Common Cold, there have been many clinical studies looking at its effectiveness. Many showed little or no effect, others only limited benefits. A review of these studies by Dr. Harri Hemila in 1994 showed an overall reduction in the duration of colds by 23% but no consistent effect on incidence (number of colds). A 2004 meta-analysis by Dr Hemila again found little effect on incidence but significant reduction in duration and symptoms. A 2006 Japanese 5 year trial did find a significant reduction in incidence by the group taking 500 mg. Would a higher dose result in even better results? We still don’t know for sure.

Enhanced immunity however is only one of many physiological functions and health benefits of vitamin C. These may form the basis for a future column.

This article is intended for educational purposes only; for medical advice consult your licensed health practitioner.

No comments:

Post a Comment