Eating a banana a day may help keep cancer away — but only if its green, a study suggests.
Scientists gave people with a high hereditary risk of cancer a diet high in resistant starch, which is abundant in the fruit when unripe.
The diet reduced their risk of some forms of the disease by more than half, results showed.
Resistant starch, a form of carbohydrate, is also commonly found in foods like oats, cereals and beans.
The study, led by UK experts, looked at almost 1,000 patients with Lynch syndrome – a genetic condition which leaves people predisposed to a range of cancers.
Around 200,000 Britons thought to have the genetic condition, though less than 5 per cent know they are a carrier.
All participants were given a daily dose of resistant starch, equivalent to one unripe banana, for an average of two years.
Experts found while it did not reduce the risk of bowel cancers the dose did reduce the risk of cancers developing in other parts of the body by up to 60 per cent.
This protective effect was particularly strong for upper gastrointestinal cancers including oesophageal, gastric, biliary tract, pancreatic and duodenum cancers, which can be difficult to detect.
Protection was observed to last for 10 years after people stopping taking the supplement and experts hope the findings could also be beneficial to the general population, not just people with Lynch syndrome.
The study was led by experts at the Universities of Newcastle and Leeds and published in the journal Cancer Prevention Research.
Between 1999 and 2005, nearly 1,000 participants began either taking resistant starch in a powder form every day for two years or a placebo.
At the end of the treatment stage, there was no overall difference between those who had taken resistant starch and those who had not.
However, the research team anticipated a longer-term effect and designed the study for further follow-up.
In the period of follow-up, there were just five new cases of upper gastrointestinal cancers among the 463 participants who had taken the resistant starch compared with 21 among the 455 who were on the placebo.
Professor John Mathers, an expert in human nutrition at Newcastle, said: ‘We found that resistant starch reduces a range of cancers by over 60 per cent.’
‘The effect was most obvious in the upper part of the gut.’
‘This is important as cancers of the upper gastrointestinal tract are difficult to diagnose and often are not caught early on.’
He added that the dose of resistant starch used in the trial was roughly equivalent to that found in an unripe banana.
‘Resistant starch can be taken as a powder supplement and is found naturally in peas, beans, oats and other starchy foods,’ he said.
‘The dose used in the trial is equivalent to eating a daily banana; before they become too ripe and soft, the starch in bananas resists breakdown and reaches the bowel where it can change the type of bacteria that live there.’
Professor Mathers said researchers suspected resistant starch, which isn’t digested in the small intestine but instead ferments in the large intestine, feeds beneficial gut bacteria altering the production of bile acids.
‘We think that resistant starch may reduce cancer development by changing the bacterial metabolism of bile acids and reducing those types of bile acids that can damage our DNA and eventually cause cancer,’ he said.
‘However, this needs further research.’
Professor Tim Bishop, an expert in genetics at Leeds who was also involved in the study, said the results were exciting but added further research was needed.
‘The results are exciting but the magnitude of the protective effect in the upper gastrointestinal tract was unexpected so further research is required to replicate these findings,’ he said.
Lynch syndrome is a genetic condition that leads to errors in DNA replication being less likely to be repaired.
This means mutations can accumulate over time, potentially leading to cancer, particularly in the bowel.
Around half of people with Lynch syndrome will go on to develop bowel cancer, according to the NHS.
Charity, Bowel Cancer UK, estimates 1,200 cases of bowel cancer are caused by Lynch syndrome each year.
The condition is also responsible for causing about 1,000 other cases of cancer in the UK each year.
In the US, Lynch syndrome is estimated to cause about 4,200 bowel cancers and 1,800 uterus cancers per year.