Some 40% of individuals living with or beyond breast, prostate, or colorectal cancer were taking at least one dietary supplement each day, according to a new study from the United Kingdom.

In addition, one third of those taking supplements said they believed that supplements reduce the risk of cancer recurrence.

However, there is no evidence to support such a belief.

The World Cancer Research Foundation as well as the American Institute for Cancer Research specifically state that dietary supplements are not recommended for cancer prevention because randomized controlled trials have generally shown no benefit from supplement use in this regard and in some cases, unexpected adverse effects have been found.

The authors of the new study, Rana Conway, PhD, University College London, London, UK, and colleagues comment that “a belief that dietary supplements reduce the risk of cancer recurrence is common” and is positively associated with their use.

The authors suggest that further research is needed to explore why cancer survivors believe that dietary supplements reduce the risk of cancer recurrence and how practitioners could provide appropriate advice to patients living with and beyond cancer.

The study was published online December 20, 2021, in Cancer.

ASCOT Study

For their study, Conway and colleagues used data from the Advancing Survival Cancer Outcomes Trial (ASCOT). Ten hospital sites across London and Essex sent a survey to patients who had been diagnosed with breast, prostate, or colorectal cancer between 2021 and 2015, and patients were asked to complete a 24-hour dietary recall questionnaire online or by telephone via the My Food24 program.

The first baseline 24-hour recall was completed by 1049 participants, and it was these participants who were included in the currently analysis. The majority of respondents were White (94%), and more than half (62%) were female, with a mean age of 64 years.

Among participants who reported using dietary supplements, 40% took more than one supplement, while almost 10% took more than three supplements a day.

Fish oil supplements, including cod liver oil, omega 3s, and docosahexaenoic acid, were the most commonly used supplements, with 13% of participants reporting taking such products.

Next commonly used were calcium supplements with or without vitamin D, multivitamins and minerals, vitamin D, and herbal supplements, researchers add.

However, among the subgroup of patients with or after breast cancer, 15% took calcium with or without vitamin D, so in this subgroup, these were more commonly used than fish oils.

In the group overall, 19% believed that dietary supplements reduce the risk of cancer recurrence.

Females were almost 2.5 times more likely to take a supplement than were males. Participants who managed to reach the five-a-day fruit and vegetable recommendation were 1.3 times more likely to take a supplement than those who did not.

But the factor that had the greatest influence on whether or not supplements were taken was whether or not the participant believed that supplements can reduce the risk of cancer recurrence.

Those who had such a belief were three times more likely to take a supplement than those who did not, the researchers found.

In contrast, being obese reduced the likelihood that patients would take a dietary supplement by almost half, they add.

Conway and colleagues note that previous studies have reported an even higher use of dietary supplements among patients with cancer and survivors. For example, in a systematic review of 32 studies, 64% to 81% of cancer survivors reported taking either a vitamin or mineral supplement.

However, methodologic difference might explain the lower use of dietary supplements in this particular survey, as only supplements taken the day before the 24-hour recall were included in the analysis. “By recording only DS [dietary supplement] use on the previous day, we may have reduced recall error but missed some intermittent supplement use,” researchers surmise.

The study was funded by Cancer Research UK. Conway reports no conflicts of interest.

Cancer. Published online December 20, 2021. Full text

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