Supplements
Dietary supplements through the lens of evidence: what may help, what is unproven, and which ones can interact with cancer treatment. Always inform your oncology team about every supplement you take.
Niacin (vitamin B3) in carcinoid syndrome
Neuroendocrine tumours causing carcinoid syndrome divert tryptophan into serotonin production, which can leave the body short of the substrate needed to make niacin. The result may be niacin deficiency and, in its extreme form, pellagra - skin lesions, diarrhoea and cognitive disturbance. The evidence comes from small patient series rather than randomised trials: fully symptomatic pellagra is rare, whereas biochemical deficiency was found in a substantial proportion of those tested. Diagnosing the deficiency and deciding on supplementation rests with the treating team, not least because high-dose nicotinic acid causes flushing - a symptom that is misleading in this particular disease.
Read more — evidence, cautions, sources
WHERE THE DEFICIENCY COMES FROM
Niacin (vitamin B3) is obtained not only from food but also from tryptophan, an amino acid supplied with dietary protein. Neuroendocrine tumours causing carcinoid syndrome divert tryptophan elsewhere: they consume it to produce serotonin. When the tumour takes up a large share of the tryptophan pool, the substrate for niacin synthesis may run short - this is the mechanism of deficiency described in the European guidelines [1]. The extreme form of that deficiency is pellagra, classically described as a triad: skin lesions, diarrhoea and cognitive disturbance [1].
WHAT THE EVIDENCE SHOWS
A 2018 review of carcinoid syndrome brings together figures that are worth reading side by side, because they describe two different things. Full clinical pellagra is rare - "usually<1%, but up to 3% in one study" of patients [2]. When niacin was actually measured, however, deficiency proved far more common: in one study it affected 10 of 36 patients, that is 28% [2]. Among 42 supplemented patients, niacin levels normalised, and before supplementation urinary niacin metabolites were below normal in 45% of those tested [2].
Put differently: symptomatic pellagra is uncommon in this disease, but biochemical deficiency - invisible without testing - is frequent.
STRENGTH OF EVIDENCE
Limited. All the figures quoted come from small patient series and review articles, not from randomised controlled trials [2]. There is no study comparing supplementation with no supplementation in terms of survival, quality of life or complication rates. What is documented is that supplementation normalises niacin levels [2] - an effect on a laboratory measure, not on hard endpoints. The mechanism of tryptophan depletion, by contrast, is well described and consistently presented in society guidelines [1].
INTERACTIONS WITH CANCER TREATMENT
Two points matter in practice.
The first concerns somatostatin analogues, the mainstay of symptomatic treatment in carcinoid syndrome. The ENETS guidelines state that in patients previously treated with somatostatin analogues, deficiencies of fat-soluble vitamins should be looked for and adequately substituted [1]. Cancer treatment therefore changes nutritional status itself and can be a reason for monitoring, not merely a background factor.
The second concerns the form and dose of the vitamin. The literature describes nicotinamide at 250-500 mg per day orally for treating deficiency [3], whereas high doses of niacin (of the order of 3000 mg per day) may cause flushing, jaundice, impaired vision and abdominal discomfort, and sustained high doses may cause liver injury [3]. The tolerable upper intake level for adults is 35 mg per day [3]. In carcinoid syndrome this carries extra weight: flushing is the most prevalent symptom of the disease itself, reported in approximately 85% of patients [4]. A drug-induced symptom may therefore be mistaken for the disease or for its exacerbation - and vice versa. This is one reason why the choice of preparation and dose is not a neutral matter and belongs to the physician.
WHAT THIS ENTRY DOES NOT SAY
It does not say that every patient with a neuroendocrine tumour should take vitamin B3, nor what dose is appropriate in an individual case. The figures quoted describe groups of patients studied in the literature, not a recommendation for one person.
WHAT TO DISCUSS WITH THE TREATING TEAM
Whether, in a given situation, it is reasonable to check nutritional status and vitamin sufficiency - particularly during long-term somatostatin analogue treatment [1], chronic diarrhoea, or skin and cognitive symptoms that could correspond to the described triad [1]. Diagnosing a deficiency requires testing, and diagnostic and therapeutic decisions are made solely by a specialist physician.
Omega-3 fatty acids (EPA and DHA)
Omega-3 fatty acids — EPA and DHA — are studied in cancer patients mainly as support in cachexia, the wasting that accompanies advanced disease. A meta-analysis of 12 controlled trials (1,184 patients with cancer cachexia) showed improved quality of life (SMD 0.70; 95% CI 0.01–1.40; p=0.048) and longer survival (median survival ratio 1.10; 95% CI 1.02–1.19; p=0.014), but showed NO gain in body weight (SMD 0.10; 95% CI −0.06 to 0.26; p=0.236) or lean body mass (SMD −0.17; p=0.095). That distinction matters: the evidence supports well-being and survival, not the rebuilding of body mass that patients most often hope for.
Read more — evidence, cautions, sources
WHAT THE EVIDENCE SAYS The strongest available appraisal is a 2022 meta-analysis (Nutrition Research) of 12 controlled trials and 1,184 patients with cancer cachexia [1]. The result is split: quality of life improved (standardised mean difference 0.70; 95% CI 0.01–1.40; p=0.048) and survival was longer (median survival ratio 1.10; 95% CI 1.02–1.19; p=0.014), while body weight did not increase (SMD 0.10; 95% CI −0.06 to 0.26; p=0.236) and lean body mass did not change significantly (SMD −0.17; 95% CI −0.36 to 0.03; p=0.095) [1]. The authors' conclusion is literal: n-3 PUFAs improved quality of life and survival, but not body weight [1].
STRENGTH OF EVIDENCE — MODERATE, RESULTS DIVERGENT The upper confidence bound for quality of life (1.40) and the lower bound for survival (1.02) show these are borderline findings drawn from trials of varying methodology [1]. Cancer cachexia cannot be reversed by nutritional measures alone — we write about this separately in the entry on nutrition in cachexia — and omega-3 does not change that picture.
INTERACTIONS AND SAFETY The most common concern is bleeding. A meta-analysis of 11 randomised trials covering 120,643 participants (Journal of the American Heart Association, 2024) found no increase in bleeding events with omega-3 supplementation (rate ratio 1.09; 95% CI 0.91–1.31; p=0.34); haemorrhagic stroke, intracranial bleeding and gastrointestinal bleeding were likewise similar [2]. The caveat concerns high-dose purified EPA: in a prespecified analysis the relative risk of bleeding rose by 50%, with only a very modest rise in absolute risk (0.6%) [2]. Risk tracked the dose of EPA (risk difference 0.24; 95% CI 0.05–0.43; p=0.02), not the background use of antiplatelet therapy (risk difference −0.01; 95% CI −0.02 to 0; p=0.056) [2]. AN IMPORTANT CAVEAT: the abstract of this meta-analysis does not separate out cancer patients, so carrying its result over to people on chemotherapy or before surgery is extrapolation rather than direct data [2].
A separate, unsettled question is the use of antioxidant preparations during chemotherapy and radiotherapy — the literature calls the risk–benefit balance in that situation a controversial topic. The National Cancer Institute states the general rule: “Tell your doctor if you're taking any dietary supplements, even vitamins, no matter how safe you think they are” [3].
THE SCOPE OF WHAT WE WRITE HERE We give no target doses or dosing schedule, because that would be advice for an individual rather than a description of the state of knowledge. Nor did we find data on patients taking warfarin or direct oral anticoagulants concurrently — the meta-analysis cited examined only antiplatelet background therapy [2]. This page is not advice for any individual.
Sources
- Jin X et al. — Omega-3 polyunsaturated fatty acids improve quality of life and survival, but not body weight in cancer cachexia: a systematic review and meta-analysis of controlled trials, Nutrition Research 2022: europepmc.org ↗
- Javaid M et al. — Bleeding Risk in Patients Receiving Omega-3 Polyunsaturated Fatty Acids: A Systematic Review and Meta-Analysis of Randomized Clinical Trials, Journal of the American Heart Association 2024: europepmc.org ↗
- National Cancer Institute — Complementary and Alternative Medicine (CAM): cancer.gov ↗
This page is educational — it is not medical advice and does not replace consultation with an oncologist. Diagnostic and treatment decisions are made solely by specialist physicians.