Recently, a reader wanted to know what we thought about a couple of vitamin C studies. These studies found reductions in glucose levels when people with type 2 diabetes took vitamin C supplements. After a literature search, it turns out there were a lot more studies than anticipated, and we didn’t have most of them on the vitamin C page.

Consequently, many studies have been added to the page, specifically to the page’s Human Effect Matrix (HEM). Here’s an overview of what they say.

What’s the deal with vitamin C?

Vitamin C has been one of the most researched nutrients for a long time. There was a great deal of hype surrounding it decades ago because it’s an antioxidant, and oxidative stress damages cells and contributes to a wide variety of diseases. Unfortunately, large doses of vitamin C didn’t turn out to be the panacea many had hoped for, and in fact, it may have some downsides.

That doesn’t mean supplements can’t be useful in specific contexts. Vitamin C levels tend to be lower in type 2 diabetes,^[1][2][3][4] a disease with an abundance of oxidative stress, both as a result of the disease and an exacerbating influence.^[5] Observations like this make it plausible that vitamin C could improve glycemic control in type 2 diabetes. But to settle the matter, we need to look at the trials.

Taking vitamin C for “general health” hasn’t shown the benefits that were hoped decades ago, but supplementation might be helpful in specific cases, such as with type 2 diabetes.

What was added to our page?

The vitamin C page now has 19 trials that look at its effects on blood glucose levels. There are also 10 trials that look at insulin and eight that look at HbA1c, most of which are the same studies that looked at blood glucose.

Here are some things to know about these studies. (Warning: we’re about to get a bit nerdy)

The majority of studies were on type 2 diabetics but didn’t necessarily treat glucose-related measures as primary outcomes. Sometimes they were more interested in lipids, but mostly with metabolic syndrome in general. Most didn’t declare primary outcomes at all, which reduces the level of confidence we have in the findings. Two of the trials evaluated vitamin C only when taken acutely and intravenously, one during surgery and the other during an intravenous dose of fatty acids. One study was on ultramarathon athletes, and vitamin C was taken for seven days prior to a race and on the day of the race. The largest of the double-blind RCTs had 70 participants among the vitamin C and placebo groups. Most studies had far fewer subjects. Studies were very mixed, and the majority didn’t find notable reductions in glucose, HbA1c, or insulin, though the next section will explore why studies differed.

Despite “19 trials” sounding impressive, many of the studies don’t provide a great deal of evidence for vitamin C supplementation helping blood sugar control.

We also added a meta-analysis for vitamin C

Meta-analyses can be useful tools for quantitatively summarizing the findings from a group of trials. One conducted in 2017 contained most of the studies that are now in our HEM.^[6] When looking at all of the studies together, it didn’t find a reduction in glucose, HbA1c, or insulin levels. But there are a few issues with this part of the study.

For one, it combined fasting and postprandial (after eating a meal or undergoing a glucose tolerance test) glucose, which won’t give an accurate estimate of either, since their normal ranges differ too much. Granted, there were only a few studies where postprandial levels were included, but they still muddy the waters.

It also doesn’t make much sense to combine data from type 2 diabetics with healthy people, because what qualifies as a large effect in people with normal glucose levels might be very small for someone with type 2 diabetes. And remember those studies that used intravenous doses of vitamin C? They’re not applicable to most people.

While studies in healthy people didn’t suggest effects, the authors did a subgroup analysis on studies looking at type 2 diabetics. There was a reduction in fasting glucose levels that was a small but statistically significant (-0.44 mmol/L). Notable effects weren’t found for insulin or HbA1c, however.

Studies that lasted longer than 30 days seemed to bring larger effects, but this analysis was limited by its inclusion of acute dosing studies in nondiabetics. It’s also unclear how study duration affected outcomes in type 2 diabetics specifically, and especially at durations longer than 2 months.

Hopefully, future research will be more precise and investigate vitamin C for type 2 diabetes in the long term. The longest studies were two tiny, four-month studies, so it can’t be said that vitamin C has been adequately tested in the long term for type 2 diabetes.

Based on the meta-analysis, vitamin C supplements might have a small effect on fasting glucose in type 2 diabetes, but most likely not on other parameters. More research is needed before we can have confidence in that small effect, or know when vitamin C is most useful.

What might the future look like for vitamin C and blood sugar?

One study (shared by the aforementioned helpful reader) published after the meta-analysis may give us a sense of where research on vitamin C for type 2 diabetes is headed.^[7]

It was a randomized, double-blind, placebo-controlled crossover trial in type 2 diabetics. Thirty-one participants took a placebo or 500 milligrams of vitamin C twice daily for a total for four months and then crossed over to the other treatment for another four months. Interestingly, the researchers used an unconventional method to measure postprandial glucose levels: a leave-in glucose monitor that relays information to a computer throughout the day. While the placebo group’s average postprandial glucose increased, the vitamin C group’s decreased, and the difference between them was fairly notable. Fasting glucose, insulin, and HbA1c were only a little lower than on the placebo, and it’s unclear if vitamin C had a genuine effect. This study doesn’t alter the conclusions from previous research very much, but it’s possible that more studies like it could.

It should be noted that none of these studies comment on the ultimate type 2 diabetes outcomes: mortality and complications. Those are the real endgame. Such studies require a lot of people, a lot of time, and a lot of money, so it would make little sense to spend the money while we’re still speculating over ambiguous results. Vitamin C supplementation doesn’t appear to reduce mortality in the general population,^[8] but type 2 diabetes might be a different story. Time may tell.

It would be premature to start thinking of vitamin C as definitely useful in type 2 diabetes, but it might be something to look out for in the future.

For practical recommendations, check out our Diabetes & Blood Sugar Supplement Guide

Some people like to dive into the details of each study, and they enjoy digging through the Human Effect Matrix on our pages.

But that’s a bit too advanced for most people. If you’re short on time, and want a guide that gives you actionable recommendations in a practical and convenient way, check out our Diabetes & Blood Sugar Guide. It goes into dosages and timing of heavily-researched supplements, along with promising supplements, and also goes over supplements to stay away from because of safety or other concerns.

Vitamin C levels are low in type 2 diabetes and diabetics have an abundance of oxidative stress, so it makes sense that antioxidants could help. And some studies found improvements in blood glucose when taking vitamin C supplements. Despite this, the effects seem to be small, and the evidence isn’t particularly strong. Future research with better methods might tell us whether vitamin C is useful for this purpose.