Supplements Taken to Prevent Cancer Recurrence Despite a Lack of Evidence

According to a new study conducted in the United Kingdom, 40% of patients with breast, prostate or colorectal cancer, or people who have survived these cancers, take at least one dietary supplement (vitamins, minerals, antioxidants, fish oils, etc.) every day. A third of supplement users also say they believe these supplements reduce the risk of cancer recurrence. However, there is no evidence to support such beliefs.

Both the American Institute for Cancer Research and the World Cancer Research Fund state that these dietary supplements should not be recommended specifically for cancer prevention. In general, randomized controlled trials in this setting have not shown any benefit from supplement use, and unexpected side effects have been found in some cases (for example, B6 and B12 or antioxidants).

Rana Conway of the University of London, who was involved in this new study, commented, “the belief that dietary supplements reduce cancer recurrence is very common and is directly associated with their use”. The researchers say more research is needed to clarify why cancer survivors believe dietary supplements reduce recurrence risk and how professionals should advise cancer patients and survivors.

The Study to Improve Cancer Outcomes

For their study, Conway and colleagues used data from the Advancing Survival Cancer Outcomes Trial (ASCOT), a study to improve cancer-survival outcomes. In London and Essex, 10 hospitals sent a survey to patients diagnosed with breast, prostate or colorectal cancer between 2015 and 2021 and asked them to use the “My Food24 program” to complete 24-hour dietary recall questions by telephone or online.

The initial baseline survey covering 24 hours was completed by 1049 patients. The large majority were white (94%), their average age was 64, and more than half (62%) were women.

Survey Results

• Among participants who reported using dietary supplements, 40% took more than one a day, while almost 10% took more than three a day.
• Based on the products reported by 13% of participants, Omega 3 and docosahexaenoic acid-containing fish oils were the most commonly used supplements.
• Calcium supplements with or without vitamin D, multivitamins and minerals, vitamin D, and herbal supplements were the commonly used supplements following fish oils.
• However, among participant subgroups, 15% of people with breast cancer or who had survived breast cancer took calcium supplements with or without vitamin D, and these were used more commonly than fish oils in this patient group.
• Overall, 19% of the group believed dietary supplements reduced the risk of cancer recurrence.
• Women took almost 2.5 times more dietary supplements than men.
• Those following the recommendation to eat five portions of fruit and vegetables a day used dietary supplements 1.3 times more than those taking no supplements.
• However, the biggest factor influencing whether people took dietary supplements was whether participants believed supplements could reduce the risk of cancer recurrence. The researchers also found that people who believed this used dietary supplements 3 times more than nonusers.
• The researchers added that being obese also halved patients’ likelihood of taking dietary supplements.

Conway and colleagues also emphasized that previous studies had reported even greater dietary supplement use among cancer patients and survivors. For example, a review of 32 studies reported that between 64% and 81 percent of cancer survivors took either vitamin or mineral supplements. However, methodological differences may explain the lower supplement use in this particular survey, because supplements taken earlier than the day before the 24-hour recall were not included in the analysis.

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