Science backs the sweet smell of beans
The importance of pulses (legumes) in gut function and bowel health is largely misunderstood in Australia, where the rate of colorectal cancer is the second highest in the world
Average dietary fibre consumption in Australia has increased over the past 20 years and is much higher than in the US and the UK yet, paradoxically, the rate of bowel cancer, responsible for about 10 per cent of all cancer deaths nationally, has not dropped.
Cancer Australia figures indicate that about 17,000 Australians would have been diagnosed with bowel cancer in 2014 and anticipates almost 20,000 will be diagnosed in 2020.
Dr David Topping, chief research scientist at CSIRO’s Food and Nutrition Flagship in Adelaide, says that in Australia most of the fibre is consumed as (insoluble) cereal fibre, with the increasing disease rate due to a deficiency of fermentable fibres such as resistant starch – a component of pulses.
Dietary fibre largely comprises indigestible plant carbohydrates and was previously thought to act primarily through its faecal bulking action as ‘roughage’. However, research undertaken by Dr Topping and colleagues at CSIRO has produced evidence that it is not the bulk cereal fibre that reduces cancer risk but the lesser-known fermentable fibres – especially resistant starch.
“A combination of fibres is important for gut health,” he says. “Most people know that eating insoluble fibre improves regular bowel movements, but are unaware of the benefits of soluble fibre in slowing glucose release and resistant starch in promoting beneficial bacteria in the large bowel.”
CSIRO experiments using baked beans show that resistant starch is metabolised by colonic bacteria, producing short-chain fatty acids including butyrate – the preferred energy source for cells lining the large bowel.
“If we don’t eat enough resistant starch, the good bacteria in our large bowel get hungry and feed on other things including protein, releasing potentially damaging products such as phenols [digestion products of aromatic amino acids] instead of beneficial short-chain fatty acids,” Dr Topping says.
“Eating more resistant starch protects the bowel from the damage associated with having a hungry microbiome and can also prevent damage to the DNA of colon cells, which is a prerequisite for bowel cancer.”
Dr Topping says levels of resistant starch in processed consumer foods are low in Australia, which is also one of the world’s lowest consumers of pulses.
Combined, he says these factors suggest “a general deficiency of fermentable substrate – and thus a collectively hungry microbiome”.
The world’s oldest known food group, pulses are ubiquitous in the Middle East, North Africa, India and Europe, where they have provided a wholesome, inexpensive protein source throughout history.
Dr Topping says it is not surprising that the rate of bowel cancer and associated diseases is lower in these parts of the world, and he is puzzled as to why Australians have not embraced pulses to the same extent.
“Legumes are somehow seen as poor food in Australia and for a long time they have suffered from a degree of public scepticism for their perceived connection to flatulence, but this is now costing Australians their health,” he says.
Anecdotally, Australians do report the major reason for avoiding pulses is a fear of intestinal gas, but Dr Topping says the bacteria that break down resistant starch fibres are anaerobic and produce carbon dioxide, methane and hydrogen – “so the gases are odourless”.
“Oxygen is harmful to these beneficial bacteria so the gases protect them,” he says. “The smells come from other undigested food components, such as proteins.”
Research undertaken by Australian nutritionist Rosemary Stanton and gastroenterologist Professor Terry Bolin also acquitted pulses of any responsibility for malodorous gas, shifting the blame to spicy foods, onions, garlic and beer.
Grains & Legumes Nutrition Council research has confirmed that ignorance of pulses’ health benefits, and unfamiliarity with preparation and cooking techniques, are also barriers in Australia and other western markets.
The 2016 United Nations International Year of Pulses aims to address these issues.
With the exception of soybeans, Dr Topping says pulses produce a very favourable profile for resistant starch, which he believes to be “a very important type of fibre for human health and one that is deficient in western diets”.
“Beans, lentils and chickpeas are an excellent source of fermentable fibre. They’re cheap, highly nutritious and agronomically they have a lot going for them,” he says.
Resistant starch is also found in whole grains including barley, root vegetables (including cooked potatoes eaten cold), pasta cooked al dente and served cold, and green bananas.
The National Health and Medical Research Council recommends adults eat between 25 and 30 grams of dietary fibre daily, but Dr Topping says consuming at least 20 grams of resistant starch alone each day is necessary for optimal bowel health.
“This is almost four times more than a typical western diet provides; it’s the equivalent to eating three cups of cooked lentils,” he says.
An editorial in the October issue of the Journal of the National Cancer Institute referred to a study that suggests eating resistant starch may also lower the risk of colon cancer associated with a diet high in red meat.
Author Dr Karen Humphreys, a research associate at Flinders University’s Flinders Centre for Innovation in Cancer, said: “Red meat and resistant starch have opposite effects on the colorectal cancer-promoting microRNAs – a finding that supports consumption of resistant starch as a means of reducing the risk.”
The study found that short-chain fatty acids such as butyrate reduce expression of RNA molecules that increase cancer cell proliferation.
More information:Dr David Topping
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