Dairy products, folate and magnesium may all help stave off bowel cancer, according to a new study by French, Canadian and Dutch researchers.
The study, published in the journal Gut, brought together data from 80 previous studies.
But the research found no evidence for protective effect of garlic, onions, tea, coffee, or fish
It found that a high fibre diet rich in fruit and vegetables might also reduce risk, and that aspirin and non-steroidal anti-inflammatory drugs were also probably protective.
Eating too much red or processed meat and drinking alcohol, even at modest levels, were linked to increased risk of the disease.
Bowel cancer is the second most common cancer in Australia with about 14,000 new cases diagnosed each year.
The researchers said while deaths from the disease had been falling in most developed countries, the numbers of new cases have been rising in some, including in Canada, the United Kingdom and the Netherlands.
Screening for the disease can pick up the disease at an early treatable stage, but take-up varies considerably from country to country.
As it takes more than 15 years for bowel cancer to develop, a healthy lifestyle played a key role in helping to halt or stop its progress altogether, the researchers said.
They therefore trawled relevant research databases for published systematic reviews and meta-analyses (pooled data analyses) of clinical trials and observational studies assessing the impact of dietary and medicinal factors on bowel cancer risk.
The results showed that aspirin was likely protective against bowel cancer, lowering the risk by between 14 per cent and 29pc at doses as low as 75 mg/day, with a dose-response effect reported up to 325 mg/day.
NSAID use for up to five years was associated with a significant (26pc to 43pc) fall in the incidence of bowel cancer.
Magnesium intake of at least 255 mg/day was associated with a 23pc lower risk compared with the lowest intake, and high intake of folic acid was associated with a 12-15pc lower risk, although it wasn't possible to pinpoint a threshold dose from the available data.
Similarly, eating dairy products was associated with 13-19pc lower risk of the disease.
But the small number of available meta-analyses, and the many different research outcomes and variety of dairy products included make it difficult to draw firm conclusions about the quantities required to ward off the disease, the researchers cautioned.
Fibre intake was associated with a 22-43pc lower risk, while fruit/vegetable intake was associated with up to a 52pc lower risk, with added benefit for every additional 100 g/day increase in intake.
Dietary soy intake was associated with a modest, but significant, fall (8-15pc) in risk.
But there was no evidence that vitamins E, C, or multivitamins were protective. Similarly, there was no evidence that -carotene or selenium helped stave off the disease.
The data were weak or equivocal on the impact of tea; garlic or onions; vitamin D either alone or combined with calcium; coffee and caffeine; fish and omega 3; and inconsistent on the protective effect of vitamin A and the B vitamins.
A modest protective effect was found in observational studies for high calcium intake, but a meta-analysis of clinical trial data found no protective effect, and even an increased risk.
Similarly, although meta-analyses of observational studies suggest that statins may lower cancer risk, no positive effect was noted in meta-analyses of clinical trial data.
Most of the available meta-analyses of observational studies reported an increased risk of between 12pc and 21pc for meat, particularly red and processed meat. Dose-effect studies reported a 10-30pc increased risk for each additional 100 g/day of red meat eaten.
Alcohol was associated with a significantly increased risk. The higher the intake, the greater the risk. This was evident even at the lowest level of consumption studied: 1-2 drinks/day.
The researchers cautioned that the level of evidence is low or very low in most cases, mainly due to wide differences in study design, end points, numbers of participants.
They were unable to define "an optimal dose and duration of exposure/intake for any of the products, even in the case of low dose aspirin and other compounds that have been extensively assessed," they point out.
But, they suggest that their findings could help clinicians advise patients on the best diet to lower bowel cancer risk and guide the direction of future research.
The research 'Recent advances in clinical practice: colorectal cancer chemoprevention in the average-risk population' by Chapelle N, Martel M, Toes-Zoutendijk E, et al. appeared in Gut. The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
