Monday February 18 2008
A study has claimed that “cancer patients who use vitamin and mineral supplements could risk them interfering with treatment or even making the disease worse”, the Daily Mail reported today. The newspaper says there is a lack of evidence about the usefulness of supplements and that people with cancer are unaware of side effects.
The story is based on a study that reviewed research on supplement use in cancer patients. It reportedly found that between two-thirds and three-quarters of cancer patients take some form of vitamin supplement, compared to about 50% of the general population.
Although the Daily Mail reported that the researchers were concerned about the lack of evidence for the effectiveness of supplements, and the possibility of causing adverse side effects in a vulnerable group, this review only examined how common the use of vitamin and mineral supplements is among cancer patients. There is no report of any harms or benefits of supplement use in the studies included, and the study was not set up to explore a causal link between vitamin use and cancer.
The Daily Mail quoted the lead researcher as saying, “the jury was still out on whether supplements were good or bad for cancer survivors”. This seems to be a statement representative of the current level of evidence and understanding around this issue.
Where did the story come from?
Christine Velicer and Cornelia Ulrich of the Cancer Prevention Program, University of Washington, US, carried out the research. No external sources of funding were reported. The study was published in the peer-reviewed medical journal: Journal of Clinical Oncology.
What kind of scientific study was this?
In this systematic review, the researchers carried out a search of the literature in order to try to quantify the usage of vitamin and mineral supplements in cancer patients and survivors. They also wanted to establish if there are any trends by cancer type, sex, etc, and to identify the areas where further evidence is needed.
According to the researchers, many of the 10 million adults with cancer in the US are choosing to use supplements despite the lack of any clear evidence-based guidance.
The researchers searched electronic databases to find any studies published between January 1999 and December 2006 on the prevalence of vitamin and mineral supplement use among cancer patients and survivors. Selected publications listed in the references of the retrieved articles were also included.
The researchers only included studies in adults in the US, in which supplement use had been measured and that had been published in the seven-year time period. The researchers then looked at the percentage of survivors in each study who reported using supplements and also looked at the details of each individual study, including population size, the way the patients were selected for the study, cancer diagnoses, etc.
The individual studies also looked at characteristics that have been associated with supplement use (e.g. age, educational level). However, these have not been discussed here.
What were the results of the study?
The researchers included 32 studies in their analysis. The studies were carried out in various ways, and the type of supplement, cancer type, and patients that took part also varied. The use of vitamin and mineral supplements differed depending on the site of their cancer.
- Nine of the studies examined breast cancer, with 67 to 87% of patients using supplements and 57 to 62% using multivitamins in particular. One of the breast cancer studies reported a 32% increase in the use of supplements following diagnosis.
- The disease stage at which supplements were used also varied between studies, e.g. some investigated use alongside cancer treatment, others at several years post diagnosis. One study looked at the use of megavitamins as an independent form of treatment, and others did not define when vitamins had been used. Most of the participants in these studies were white and had early stage breast cancer.
- Nine studies examined men undergoing treatment for prostate cancer and found that 26 to 35% of patients used supplements and 13 to 23% used multivitamins in particular. One of the studies that investigated supplement use before and after diagnosis found that their use increased from 57 to 72% after diagnosis.
- Three studies examined the use of supplements during colorectal cancer. One study reported a 49% prevalence of any vitamin use while another reported patients were 33 to 59% more likely to use folic acid, iron or vitamin A compared to those without cancer. One of two lung cancer studies reported a prevalence of 60% use of any vitamin or mineral.
- In the 11 studies that were not restricted to cancer type and had included any cancer, supplement use varied between 64 to 81% and multivitamin use from 26 to 77%.
What interpretations did the researchers draw from these results?
The authors conclude that “vitamin and mineral supplement use is widespread among cancer patients and longer-term cancer survivors and that, after a diagnosis of cancer, individuals tend to increase use of vitamin and mineral supplements”.
What does the NHS Knowledge Service make of this study?
This review has been carefully conducted in a field where reliable evidence is limited and commercial sales messages are common. However, care should be taken when drawing conclusions from this research.
- It is an inaccurate to report that the study claims that “cancer patients who use vitamin and mineral supplements could risk them interfering with treatment or even make the disease worse”. Although it is known that certain alternative therapies can interact with prescription medications (most notably St John’s Wort), that is not the focus of this research. The review has only examined how common the use of vitamin and mineral supplements is among cancer patients and there is no report of any harms or benefits related to supplement use in the studies identified.
- The studies included in the analyses were varied and not necessarily directly comparable to each other in terms of patients, vitamins used and doses taken. This limits the conclusions that can be drawn from the studies as a whole.
- There is also limited information available on the characteristics of the studies, such as how they defined vitamin use, how long the patients were followed for, or how they were included in the study. It is therefore not possible to account for any bias that may have been introduced into the studies. For example, patients selecting themselves for entry into a study on vitamin use may have been more likely to use vitamin and mineral supplements and the numbers may not be representative of all cancer patients and survivors.
- The search for eligible studies had restrictions that could mean there are other studies on supplement use that have not been included. Therefore, the findings from this research cannot be widely generalised.
As the authors acknowledge, the review has highlighted the need for further research to better understand the role of supplement use in cancer patients and how it relates to treatment and survival.