Can Vitamin C or Exercise Distort a Glucose Reading? A Trial Takes a Closer Look

Could taking a vitamin C supplement or exercising before checking glucose lead to an inaccurate reading? For people managing type 1 diabetes, even small differences in glucose measurements can influence treatment decisions. A randomized trial examined whether vitamin C supplements and exercise could affect the accuracy of continuous glucose monitors (CGMs). The study evaluated a monitoring system using GDH-FAD technology. Its findings highlight why interference risks should be assessed for individual devices rather than generalized across all glucose monitors.

Table of Contents

1. Why Vitamin C and Exercise May Affect Glucose Readings
2. How Researchers Evaluated GDH-FAD Glucose Monitoring
3. What the Clinical Trial Found About CGM Accuracy
4. What the Findings Mean for Diabetes Management
5. Conclusion
6. Frequently Asked Questions

Why Vitamin C and Exercise May Affect Glucose Readings

Accurate glucose monitoring helps people with diabetes make informed decisions about insulin, food, and physical activity. However, several factors can influence glucose measurements, including medications, supplements, and changes in the body's internal conditions.

Vitamin C, also known as ascorbic acid, is a common supplement that supports normal immune function and tissue repair. Although it is generally considered safe at recommended doses, higher amounts can interfere with certain glucose monitoring systems.

The concern involves how glucose sensors detect blood sugar. Some devices use chemical reactions that generate electrical signals. Because vitamin C can also participate in these reactions, it may produce falsely elevated readings in susceptible systems.

The U.S. Food and Drug Administration provides information about blood glucose monitoring devices and factors affecting their performance. Therefore, patients should review the instructions for their specific device rather than assume all sensors respond similarly.

Exercise introduces another challenge. During physical activity, glucose levels may change quickly as muscles use glucose for energy. Meanwhile, changes in circulation and tissue conditions can influence sensor measurements.

Unlike traditional finger-stick meters, CGM systems measure glucose in the fluid surrounding cells. Consequently, readings may temporarily differ from blood glucose values, especially during rapid changes.

As discussed in Diabetes in Control's coverage of CGM use during exercise, understanding this delay is important when interpreting glucose trends.

How Researchers Evaluated GDH-FAD Glucose Monitoring

A clinical trial published in Diabetes Therapy investigated whether vitamin C supplementation and moderate exercise affected the performance of the iCan continuous glucose monitoring system.

The study involved 16 adults with type 1 diabetes, ages 18 to 65, who were receiving stable insulin treatment. Researchers followed participants during the system's approximately 15-day sensor wear period.

The device uses glucose dehydrogenase with flavin adenine dinucleotide, commonly called GDH-FAD technology. This sensing approach differs from glucose oxidase technology used in many other CGM systems.

Researchers assessed the sensors during three clinical visits. Participants consumed meals designed to produce changes in glucose levels. During the second and third visits, they also completed 30 minutes of moderate exercise on a treadmill or reclining bicycle.

In addition, participants were randomly assigned to receive a 1,000-milligram oral dose of vitamin C during either the second or third visit. This arrangement allowed researchers to compare measurements under conditions with and without supplementation.

The team compared CGM readings against laboratory glucose measurements obtained using the Yellow Springs Instrument 2300 analyzer.

Importantly, the trial examined the specific iCan system rather than every device using GDH-FAD technology. Its findings therefore cannot establish universal accuracy across different sensor brands.

The complete research report is available in Diabetes Therapy.

What the Clinical Trial Found About CGM Accuracy

The results were encouraging for the system evaluated. Across the overall glucose measurement range, 92.0% of CGM readings fell within 20% of the laboratory reference value, or within 20 mg/dL when the reference value was 100 mg/dL or lower.

Researchers also examined mean absolute relative difference (MARD), a measure commonly used to evaluate CGM accuracy. Lower MARD values generally indicate closer agreement between sensor readings and reference measurements.

When comparing vitamin C exposure with no vitamin C exposure, the overall difference in MARD was -1.6 percentage points. Similarly, the comparison between exercise and nonexercise conditions showed a difference of -1.5 percentage points.

The corresponding changes in mean absolute difference were -1.1 mg/dL for vitamin C and -1.3 mg/dL for exercise.

According to the investigators, these differences were not clinically meaningful. Therefore, the tested vitamin C dose and moderate exercise did not produce clinically significant interference in this particular monitoring system.

No serious adverse events were reported. Overall, the findings suggest that this GDH-FAD-based system maintained consistent performance under the conditions studied.

Why These Findings Require Careful Interpretation

Although the trial provides valuable evidence, several limitations deserve attention.

First, only 16 participants completed the study. A larger and more diverse population would provide stronger evidence about device performance.

Second, researchers tested a single 1,000-milligram vitamin C dose. Consequently, the findings cannot determine whether larger doses, repeated supplementation, or different exposure patterns would produce similar results.

Third, exercise testing involved moderate activity under supervised conditions. More intense workouts, prolonged physical activity, and everyday environmental conditions may affect sensor performance differently.

The study was also funded by Sinocare Meditech, the manufacturer associated with the evaluated device. Independent studies would help strengthen confidence in the findings.

What the Findings Mean for Diabetes Management

The key takeaway is that vitamin C's effect on glucose monitor accuracy depends largely on the specific device and its sensing technology.

For example, some glucose sensors carry warnings about interference from vitamin C supplements. Others may use sensing methods that respond differently to the same substance.

Therefore, clinicians should not assume that favorable findings from one GDH-FAD system apply to every continuous glucose monitor.

Patients should also understand that a sensor's resistance to chemical interference does not eliminate every source of measurement error. Rapid glucose changes, sensor placement, and other device-specific factors can still influence readings.

During exercise, trend arrows and glucose alerts remain valuable. Nevertheless, patients should follow device instructions and confirm unexpected readings with an appropriate finger-stick meter when symptoms do not match sensor values.

Healthcare professionals can also help patients review supplement use, interpret glucose patterns, and select suitable monitoring technologies. Individuals seeking personalized medical guidance can consult a qualified healthcare professional through Healthcare.pro.

Ultimately, effective diabetes management requires understanding both the benefits and limitations of the chosen device.

Conclusion

This randomized trial found no clinically significant interference from a 1,000-milligram vitamin C dose or moderate exercise when using the tested iCan CGM system with GDH-FAD technology.

However, the small study population and controlled testing conditions limit how widely the findings can be applied.

For clinicians and patients, the central lesson is straightforward: glucose monitoring accuracy should be evaluated according to the specific device, its sensing technology, and its documented interference warnings.

Frequently Asked Questions

Can vitamin C cause falsely high glucose readings?

Yes. Vitamin C can cause falsely elevated readings in certain glucose monitoring systems. However, the effect depends on the device and sensing technology.

Does exercise affect continuous glucose monitor accuracy?

Exercise may affect CGM accuracy, particularly when glucose changes rapidly. The trial found no clinically significant exercise-related interference in the specific GDH-FAD system tested.

What is GDH-FAD technology in glucose monitors?

GDH-FAD technology uses a glucose-sensitive enzyme and an associated molecule to help detect glucose. Its sensing approach differs from that of glucose oxidase-based systems.

Does the study prove that vitamin C is safe for every CGM?

No. The study evaluated one CGM system and a specific vitamin C dose. Results should not be generalized to other devices without supporting evidence.

What should patients do when a CGM reading seems incorrect?

Patients should follow their device instructions and use a finger-stick glucose meter for confirmation when readings do not match symptoms. They should seek medical guidance when discrepancies persist.

Reference

Brazg R, Mitchell T, Fei J, et al. Impact of Ascorbic Acid and Exercise on Accuracy of a Glucose Monitoring System with GDH-FAD Technology in Adult Patients with Type 1 Diabetes Mellitus: A Double-Blinded, Randomized Trial. Diabetes Therapy. 2026. https://doi.org/10.1007/s13300-026-01918-9

Disclaimer: This content is not medical advice. For any health issues, always consult a healthcare professional. In an emergency, call 911 or your local emergency services.