Eating dairy products may reduce bowel cancer risk, but there’s no evidence that garlic or onions, fish, tea or coffee protect against the disease, say researchers.

For the findings, published in the journal Gut, the research team McGill University in Canada, trawled relevant published systematic reviews and meta-analyses of clinical trials and observational studies assessing the impact of dietary and medicinal factors on bowel cancer risk.

The medicinal factors included aspirin, non-steroidal anti-inflammatory drugs (NSAIDs), such as paracetamol and statins.

The dietary factors included: vitamins or supplements (magnesium, calcium, folic acid, vitamin A, B, C, E, D, ß-carotene and coffee, tea, fish and omega 3 fatty acids, dairy products, fibre, fruit and vegetables, meat and alcohol

The results showed that aspirin is likely protective against bowel cancer, lowering the risk by between 14 per cent and 29 per cent at doses as low as 75 mg/day, with a dose-response effect reported up to 325 mg/day.

NSAID use for up to 5 years was associated with a significant (26 per cent to 43 per cent) fall in the incidence of bowel cancer.

Magnesium intake of at least 255 mg/day was associated with a 23 per cent lower risk compared with the lowest intake, and high intake of folic acid was associated with a 12-15 per cent per cent lower risk.

Similarly, eating dairy products was associated with 13 per cent to 19 per cent lower risk of the disease

Fibre intake was associated with a 22 per cent-43 per cent lower risk, while fruit/vegetable intake was associated with up to a 52 per cent lower risk.

“But there was no evidence that vitamins E, C, or multivitamins were protective. Similarly, there was no evidence that ß-carotene or selenium helped stave off the disease,” the study authors wrote.

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 12 per cent and 21 per cent for meat, particularly red and processed meat.

Alcohol was associated with a significantly increased risk. The higher the intake, the greater the risk.

The researchers caution that the level of evidence is low or very low in most cases, mainly due to wide differences in study design, endpoints, numbers of participants, etc.