Dietary supplement use has become increasingly popular among breast cancer patients [1]. Recent research has raised safety concerns suggesting the use of antioxidants such as vitamins A, C, and E; carotenoids and coenzyme Q10; and vitamin B12 supplements may be associated with increased recurrence and mortality rates in patients with early breast cancer [2]. It has been postulated that the antioxidant properties of certain supplements may interfere with the oxidising free radicals of chemotherapy and radiotherapy, reducing their ability to target cancer cells [3]. Conversely, several publications indicate that dietary supplement use may play a role in optimising cancer treatments and improving breast cancer outcomes [4]. The contradictory findings to date, in combination with data largely drawn from observational cohort studies, limit their validity. Consequently, this study aimed to characterise dietary supplement behaviours, perceptions, and attitudes among breast cancer patients and to assess the prevalence of supplement use both pre- and post-breast cancer diagnosis.

This was a multicentre study conducted between August, 2021, and November, 2022, in adult patients diagnosed with early stage breast cancer at Cork University Hospital. An anonymous patient administered questionnaire was devised by the authors and circulated to those attending clinics. The study was approved by the institutional ethics committee and all participants provided informed consent. Behaviours, attitudes, and perceptions regarding supplements, as well as sociodemographic and lifestyle data were recorded. Supplement use patterns both pre- and post-breast cancer diagnosis were collected. Regular use was defined as greater than 3 different dietary supplements taken at least once weekly, generating two distinct groups. Data analysis was conducted using IBM SPSS V28.0.0.0. Demographic characteristics associated with regular use were analysed via logistic regression. Multiple chi-squared tests were performed to determine the association between post-diagnosis regular use and non-regular use and survivors’ attitudes and perceptions, and healthcare provider advice. Statistical significance was assumed by a p-value < 0.05.

A total of 185 questionnaires were completed, with baseline demographics shown in Table 1. Regular supplement use following breast cancer diagnosis was reported by 45% of participants, with 16% being new regular users. Regular use prior to diagnosis was associated with increased age, higher education levels, and lower BMI (p < 0.05), while regular use following diagnosis was associated with higher education level (p = 0.05). The most commonly used dietary supplements were multivitamins, vitamin C, vitamin D, calcium, and omega 3 (Fig. 1). The use of calcium and vitamin D was considerably increased following diagnosis.

Table 1 Summary of participant characteristics
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Fig. 1
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Most commonly used dietary supplements and proportion of use by breast cancer patients pre- and post-breast cancer diagnosis

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The majority of patients (77%) desired more information regarding safe dietary supplement use in survivorship. Many were uncertain as to whether dietary supplement intake affected their risk of breast cancer development, recurrence and mortality, and about the safety of supplements when used concurrently with cancer treatments (Fig. 2). These participants were more likely to be non-regular users. A significant proportion believed dietary supplement use had no effect on breast cancer outcomes, a belief evenly distributed between groups. Those who perceived supplement use carried a reduced risk of poorer outcomes were more likely to be regular users. Very few participants felt supplement use increased the risk of poor outcomes (Table 2).

Fig. 2
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Attitudes toward and perceptions of dietary supplement use in breast cancer patients

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Table 2 Association of post-diagnosis regular and non-regular supplement use with attitudes and perceptions
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Over half of the study participants (54%) received no recommendations or advice regarding dietary supplements from healthcare providers, 21% were recommended to use dietary supplements, while the remainder were recommended to avoid supplement use.

Our findings highlight that while many patients believe dietary supplements are worthwhile, the majority of study participants did not receive advice regarding supplement use from healthcare providers. Dangerous misconceptions regarding ‘cancer-beating diets’ are widespread on social media and potentially harmful for patients with cancer especially those with lower health literacy, reinforcing the need for robust evidence-based information [5].

The most commonly used supplements in this study were multivitamins, vitamin C, vitamin D, calcium, and omega 3. The increase in vitamin D and calcium use may reflect guidelines supporting adjuvant bisphosphonates with calcium and vitamin D to improve outcomes and alleviate the side effects of treatments [6]. Other supplements commonly used by breast cancer patients to mitigate side effects of therapy include vitamin E for taxane-induced peripheral neuropathy, glutamine for mucositis, and selenium for post-operative upper limb lymphoedema [7]. Large randomised trials have previously demonstrated increased risk of lung and gastric cancer with beta-carotene use, as well as an association between vitamin E and prostate cancer, with mixed effects of antioxidants on chemotherapy toxicity [8]. Unmonitored use of herbal medicines is becoming a more frequent phenomenon [9]. Additionally in the era of immunotherapy, the evolving role of the gut microbiome and interactions between what we ingest and treatment outcomes will be a major research area [10]. Taking all of the above into consideration, open communication between patient and physician regarding dietary supplement use is critical.

The strengths of this study include a substantial data collection window,, as well as broad selection criterion, with no exclusion of particular demographic characteristics, current disease activity, or co-morbidities, generating a study sample more representative of the ‘real-world’. Limitations include the self-reporting of supplement use, particularly pre-diagnosis use, creating the potential for recall bias. Our initial ambition was to perform a survey of healthcare professional attitudes in parallel with this study, in order to assess healthcare provider advice concerning dietary supplements in breast cancer survivorship; however, unfortunately we did not have adequate response rates for this to be feasible.

In conclusion, dietary supplement use is prevalent among breast cancer patients; however, attitudes and perceptions suggest a lack of awareness of potential consequences of dietary supplements in breast cancer survivorship. Patients also report a lack of sufficient guidance from healthcare providers regarding dietary supplement use. Future steps include exploration of healthcare provider attitudes and knowledge of dietary supplements and recommendations. Large-scale randomized controlled trials (RCTs) to compile safety data on individual dietary supplements in order to provide robust evidence would be optimal to guide clinical practice. Recommendations for nutritional supplements and dietary advice should be integrated into oncology patient care plans and survivorship guidelines.