Higher Vitamin C Levels May Protect Against Certain Types of Cancer

Previous studies have already observed that people who regularly consume vitamin C are more protected against certain types of cancer, although it is possible that this could be due to factors independent of vitamin C. Now, a lesser-known research method supports that higher vitamin C levels alone may be responsible.

Why is the relationship between vitamin C and cancer hard to study?

The most reliable research on health issues has long been randomized controlled trials and meta-analyses summarizing their results. The problem is that these studies are very expensive and cannot be relied upon for certain questions for ethical or practical reasons. It would be unethical, for example, to ask people to smoke or overeat, and it would be difficult to check whether recommendations are followed.
In this way, the link between vitamin C intake and cancer cannot be effectively investigated. It is a disease that has many different types and usually develops over decades; so many participants would need to be followed over a long period of time, which is incredibly expensive. 
Consequently, similar questions rely mainly on observational studies, where other influencing factors can never be completely ruled out, and it is difficult to estimate people's vitamin C intake accurately.

Advantages of the new experiment

A few decades ago, a research method was discovered that uses genetic differences to investigate different health-related patterns. Its main advantage is that it can rule out external influences or reverse causality more effectively than other observational studies.
Mendelian randomization is similar in many ways to randomized trials, in that genetic variants are inherited randomly from parents and are independent of external factors.
In the present study, for example, circulating vitamin C was not associated with fruit and vegetable consumption, alcohol consumption, physical activity, smoking, vitamin C supplement use, body mass index or education. 
In a hierarchy of scientific evidence, this method scores lower than randomized trials but higher than observational studies. 

Previous studies on the connection between vitamin C and cancer risk

Previous research has shown that vitamin C may play a role in the prevention of cancer, particularly digestive cancers. The main causes of small bowel cancer remain largely unknown, but a recent study provides evidence that vitamin C may play a preventive role in this rare form of cancer. Experimental studies have also shown that vitamin C supplementation reduces the growth of colon polyps.
Case-control and observational studies have demonstrated a possible protective effect of high vitamin C intake against colorectal, gastric and pancreatic cancers.
So far, there have been only a few randomized trials on the effect of vitamin C on cancers of the digestive tract, and because these included very few cases, the association was not found to be significant (although the results for colorectal cancer showed a slight protective effect here too). Unfortunately, in almost all cases, vitamin C was supplemented with other active substances, such as high doses of vitamin E (alpha-tocopherol) or beta-carotene, which have been found to be negative.
A recent meta-analysis summarizing the results of observational studies also found that high intakes of dietary vitamin C and vitamin C supplementation were associated with a lower risk of colorectal cancer, although the result was not considered significant.
A meta-analysis of two randomized trials also suggested a reduced incidence of colorectal cancer in participants taking vitamin C supplements compared with the placebo group, but the 16% relative risk reduction was not considered significant because of the small number of cancer cases.

Results of the new study

In this new study, the association between lifetime higher vitamin C levels and the risk of malignant gastrointestinal tumors was examined, based on data from more than 650,000 people. (1)
Ten genetic variants previously found to be significantly associated with vitamin C levels were used. Genetically higher vitamin C levels were associated with a lower risk of small bowel and colon cancer, and a one-unit increase in circulating vitamin C levels reduced the risk of small bowel cancer by 45% on average and colon cancer by 16%. No association was found for esophageal, gastric and pancreatic cancer, but an association was found for liver cancer, however only in one study.

Vitamin C is effective against other diseases as well

This new research also shows that higher vitamin C levels offer general protection against certain cancers, so as there is no known downside; it is worth aiming for higher intakes. 
A previous huge summary has already shown that higher levels of vitamin C reduce the risk of all-cause mortality, so it reduces the risk not only of cancer but of most chronic diseases. (2)
It is important to note that inadequate supplementation of certain antioxidants, including vitamin C, during cancer treatment can reduce the effectiveness of therapy, so always check with your doctor about which supplements to take.