Vitamin C has been studied in cancer for decades, often with mixed or disappointing results. A new clinical trial does not settle the question, but it gives researchers a reason to keep looking.
In the study, 109 adults with precancerous blood disorders or lower-risk blood cancers were randomly assigned to take either 1,000 milligrams of vitamin C or a placebo each day for one year. Vitamin C did not significantly change the study’s main measure of disease progression. But people in the vitamin C group had fewer serious health events, and an exploratory analysis found they were more likely to be alive after about three years of follow-up. Because the study was small and survival was not its primary focus, that result needs to be confirmed in a much larger trial.
The study, published in the American Cancer Society journal Cancer, included people with clonal cytopenia of undetermined significance, or CCUS, along with lower-risk myelodysplastic syndromes and related blood disorders. These conditions affect the blood-forming cells and can sometimes progress to acute myeloid leukemia.
Participants were mostly in their 60s and 70s, and none were receiving cancer treatment when they enrolled. Neither the participants nor the researchers knew who was taking vitamin C and who was taking the placebo until the trial ended.
More than half of the participants started with vitamin C levels that were considered low or inadequate. In the vitamin C group, those levels rose into the adequate range during the study.
But raising vitamin C levels did not improve the trial’s primary outcome.
Researchers tracked the growth of abnormal blood-cell populations over the course of the year. Those cells grew at similar rates in both groups, with no statistically significant difference between vitamin C and placebo.
That is an important result because it means the study did not show that vitamin C slowed the underlying blood disorder.
Other findings were more encouraging.
Serious adverse events occurred in 18 of 55 people in the vitamin C group, compared with 30 of 53 people in the placebo group. Researchers also saw differences in several markers of inflammation, suggesting that vitamin C was having measurable effects in the body even though it did not change the main disease measure.
The most striking result came from the longer follow-up.
Over a median of about 34 months, 24 people in the placebo group died compared with 11 in the vitamin C group. The researchers found a statistically significant difference in overall survival between the groups.
But that survival analysis was exploratory. The trial was not designed or large enough specifically to test whether vitamin C helps people live longer.
That makes the result promising, but far from conclusive.
The researchers also looked at whether participants progressed to more advanced blood cancer. Nine people in the placebo group and five in the vitamin C group did, but that difference was not statistically significant.
Vitamin C was generally well tolerated, though not completely free of side effects. Gastrointestinal problems were somewhat more common in the vitamin C group, and one participant developed kidney stones that required treatment.
The findings should not be interpreted as evidence that vitamin C supplements prevent leukemia or cancer in healthy people.
This study involved people who already had diagnosed blood abnormalities or lower-risk blood cancers, and many started out with low vitamin C levels. The 1,000-milligram daily dose was also much higher than most people would get from food alone.
For now, the clearest takeaway is that vitamin C corrected low vitamin C levels and appeared safe enough to study further, but it did not improve the trial’s main measure of disease progression.
The survival difference is the part researchers are most interested in pursuing next.
“We are tremendously excited and cautiously optimistic that our results could one day translate into a simple way to improve care and outcomes for people with pre-leukemia disorders,” said study co-author Peter A. Jones of Van Andel Institute.
A larger phase 3 trial will be needed to determine whether the survival signal and other encouraging findings hold up.
Several authors reported professional relationships with pharmaceutical or research companies, including consulting, advisory work and research grants. None of those disclosures involved a commercial vitamin C product.
The study was funded by Van Andel Institute through the Van Andel Institute–Stand Up To Cancer Epigenetics Dream Team, the Danish Cancer Society and Greater Copenhagen Health Science Partners.