Meta-analysis questions supplements for cancer patients
'low' quality, has led to a conclusion that dietary supplements for
cancer patients do not improve survival or benefit prognosis.
But the limited evidence has led to calls from different quarters for more high-quality research before any widespread recommendations of supplements for cancer management be made.
"The large personal expenditure on supplements and dietary modifications by patients with cancer demonstrates an urgent need to understand their effects on cancer outcomes," explained lead reviewer Anna Davies from the University of Bristol, UK.
"This vulnerable group of people needs to be better informed, as diet is one of the few areas of their lives where they may feel that they have some control."
The meta-analysis, published in the July 19 issue of the Journal of the National Cancer Institute (Vol. 98, pp. 961-973), combined the results of 59 trials searcher from the four online databases: the Cochrane Library, MEDLINE, EMBASE, and AMED.
Studies were eligible for inclusion if they were randomised controlled trials with patients with cancer or preinvasive lesions, and included a nutritional intervention.
Of the 59 trials selected, 25 were for cancer patients and 34 were for people with preinvasive lesions.
Nutritional interventions included vitamin A, B6, C and E, folate, calcium, beta-carotene, selenium, fibre, and green tea. The vitamins and minerals were administered individually, or in differing combinations.
When the results were analysed in terms of death from all-cause (all-cause mortality), no significant effect was observed for any of the intervention supplements.
But while the overall view was negative, there did appear to be one or two areas of potential glimmers of hope. Indeed, for breast cancer only, healthy dietary advice intervention (with or without supplements) was associated with a 30 per cent reduction in mortality.
Similarly, for one selenium-skin cancer trial, the supplements were reported to reduce mortality by 41 per cent.
In terms of cancer recurrence, the reviewers reported no "overall effect of antioxidant, retinal, or vitamin B6".
In terms of pre-invasive lesions, no apparent benefits for any of the supplements were reported. Calcium supplements did however show "some evidence" of a reduction in the risk of recurrence for colorectal polyps, said the researchers.
Overall, the meta-analysis may come as a disappointment to the many people suffering from cancer and looking for ways to combat their disease.
But there are several limitations with the meta-analysis which means the case is definitely not closed for nutritional interventions, with the body of available literature the major limitation.
Davies and her colleagues rated many of the studies as being of 'low' quality and also acknowledged that the dietary interventions may not have been equal, nor were the stages of the cancers same.
It may also be too late to observe any effects in mortality rates in people with cancer in advanced stages, said the reviewers.
In an accompanying editorial, John Baron, MD, from Dartmouth Medical School in the US, said that while the overview came to "fairly negative conclusions", there were some, "tantalizing findings" (J. Natl. Cancer Inst., Vol 98, pp. 945-946).
"The summary estimates suggested non-statistically significant reductions in risk of all-cause mortality and cancer mortality with "healthy diet" interventions in cancer (largely breast cancer) patients," said Baron.
The article, said Baron, illustrated well the state of chemoprevention research: "Hard to summarise, many negative findings, but some hopeful nuggets of progress."
This view was echoed by the Bristol reviewers, who said that scientists planning future studies face many challenges: "A priority should be given to large-scale, high-quality trials evaluating the most promising interventions.
"The available evidence suggests that large trials of calorie and fat restriction in breast cancer and calcium in colorectal preinvasive lesions are most likely to be successful."
The limitations of the studies were also highlighted by Dr Emma Knight, science information manager at British charity, Cancer Research UK, who told NutraIngredients.com that, as the authors also pointed out, the null results may be due to the limited number of high-quality studies.
"More research is needed to tease out the role of specific dietary factors in both cancer prevention and treatment.
"The link between diet and cancer is complex. At present the best way to get the vitamins and minerals we need is through eating a balanced and varied diet with plenty of fruit and vegetables," said Knight.
