Sunday, September 20, 2020

Does Vitamin C Really Cause False Blood Glucose Readings? Not at an Amount I can Eat.

Disclaimer: This experiment was only done on one person (myself) and is not fully randomized, placebo controlled, or blinded. Please take all conclusions with the appropriate skepticism.


Vitamin C is reported to falsely raise blood glucose readings in both fingerstick and continuous glucometers. I wanted to know if this was something that I (and others) should be concerned about, but all the reports I found were for intravenous vitamin C and I couldn't get any data from the CGM companies. 

To get an answer, I tested it myself, monitoring blood glucose after ingesting 2-6g of vitamin C, 20-60 the FDA recommended daily amount (but known to be safe). I did not observe any consistent or sustained increase in blood glucose, so I conclude that ingested vitamin C has no measurable impact on my blood glucose measurements at any practical quantity.


When I first tried out the FreeStyle Libre CGM, I read through the manual and noticed an unusual warning:

"Taking ascorbic acid (vitamin C) while wearing the Sensor may falsely raise Sensor glucose readings. Taking salicylic acid (used in some pain relievers such as aspirin and some skin care products) may slightly lower Sensor glucose readings. The level of inaccuracy depends on the amount of the interfering substance active in the body."

Vitamin C is an essential nutrient and present in tons of foods and most multivitamins. If it falsely raises glucose readings, that's a huge flaw in the device.

I tried contacting Abbot, who confirmed that vitamin C can cause erroneously high glucose readings, but couldn't point me to any studies or say how strong the effect was. I also contacted Dexcom and there support rep. said that "Vitamin C was not contraindicated for Dexcom CGMs," but again couldn't point me to any studies or say if they had or hadn't run any tests.

Having failed at getting useful info directly from the CGM companies, I turned to the published literature. I found a number of reports of Vitamin C leading to high blood glucose readings (1, 23, 4, 5; not exhaustive, there are many more). From these, I learned the following:

  • All reports in scientific or medical journals were for patients taking intravenous vitamin C.
  • Vitamin C can be oxidized by glucose oxidase and glucose dehydrogenase, the enzymes used to detect glucose in most portable glucometers, including most fingerstick meters and both the FreeStyle Libre and Dexcom CGMs
  • The increase in reported blood glucose can be extremely large; in one case a women read 343 mg/dL on standard glucometer, while a blood test showed her actual blood glucose as 49 mg/dL.
  • The blood levels of vitamin C in these reports were extremely high, 1,000 - 5,000 ┬Ámol/L. This is much higher than the 100-200 ┬Ámol/L observed with regular oral vitamins.
  • There's only one anecdotal report on a diabetes forum of someone who saw high CGM readings after taking oral vitamin C, with numerous comments from people claiming that they do not see this.
Given all this, it seemed unlikely that regular consumption of vitamin C would significantly affect blood glucose readings, but I was left with two questions:
  • Was there any measurable effect of ingested vitamin C, even if small?
  • Would mega-doses of vitamin C have a significant effect?
    • mega-doses of vitamin C were advocated by Linus Pauling (nobel prize winner in chemistry). His theory has since been debunked, but is unfortunately still used by many people.
Since I couldn't find any direct studies of the effect of ingested vitamin C, I decided to do my own experiment. 

To quantify the effect of ingestion of vitamin C in blood glucose readings from Dexcom G6 and Freestyle Libre CGMs.

Procedure. At selected times of day, 2, 4, or 6g of vitamin C was ingested and blood glucose monitored using a continuous glucose monitor.

Measurements. Blood glucose was measured using a Dexcom G6 CGM and FreeStyle Libre 14 day CGM.

Data Processing & Visualization. Data was visualized using Tableau.

Medication. I took my normal morning, evening, and meal medications

Data: set1, set2

Results & Discussion
Figure 1. Blood Glucose vs. time for each of four vitamin C ingestion experiments. Color represents data from Dexcom G6 (blue) and FreeStyle Libre (orange)

According to the NIH, the recommended amount of vitamin C is 65-90 mg/day and the tolerable upper limit is 2,000 mg/day. However, studies have shown no serious adverse effects at much higher amounts and >10,000 mg/day is prescribed for treatment of certain cancers with no ill effects. Given that information, I tested ingestion of 2,000, 4,000, and 6,000 mg of vitamin C. 

As can be seen in Figure 1, there was no significant rise in measured blood glucose in any of the four experiments that could be attributed to vitamin C intake. While here were transient increases in measured blood glucose in the first three measurements, they were not consistent in time or magnitude. In addition, all the reports of increases from intravenous vitamin C indicated that the effect was of long-duration (such that measured blood glucose was still elevated well after the patient arrived at the hospital or clinic). 

Based on this data and the fact that typical vitamin C consumption is much less than what I took in these experiments, I conclude that ingested vitamin C has no measurable impact on blood glucose measurements at any practical quantity.

Final Thoughts & Next Experiments
This was a very satisfying experiment. Pretty quick, answered a question I wasn't able to find in the literature or from the CGM companies (though they probably knew the answer), and results were very clear. 

Next up:
  • Re-tune basal and bolus (meal) insulin doses
    • My routine has changed a lot due to working from home, changing doctors, and changing medication (due to insurance requirements). Plus, I was able to get a Dexcom G6 CGM, which is showing accuracy comparable to my blood glucose meter. Blood sugars are still good, but I think I can get them better.
    • This is going pretty well. As I expected, I had my basal:bolus ratio off. I'm steadily getting it corrected and it's making my blood sugar a lot more stable.
  • Re-measure blood sugar impact of glucose and insulin; compare to previous data
    • While working from home, I've gained some weight (and hopefully muscle). This has resulted in a change in my insulin sensitivity. Not huge, but I need to remeasure to have an accurate baseline for future experiments.
  • Test the effect of some dietary supplements that have been reported to affect blood sugar in the literature, but where data insufficient or contradictory
    • Glutamate (reported to reduce post-prandial glucose, but magnitude and timing vary widely)
    • Others tbd. 

As always, if you have any questions, comments, suggestions, or are interested in joining in future experiments, please let me know in the comments or send a PM via the contact form or to

1 comment:

  1. My blood sugars stay fairly stable during the day but spike and rise while fasting at night so I have a higher fasting reading, higher than before going to bed. Any helps or ideas to bring that down?