Perspectives with Dr Alan Barclay

Alan Barclay

Australia's Dietary Fibre Paradox. 

Bowel cancer, or colorectal cancer (CRC) to use its more scientific name, is one of the most common cancers in Australian men and women with 62 out of every 100,000 people diagnosed each year. It is also a leading cause of cancer in the USA (44 diagnosed per 100,000 people) and UK (47 per 100,000 people) and many other countries that enjoy the so-called Western diet.

Systematic reviews of observational studies around the world tell us that a high intake of dietary fibre, in particular cereal fibre and whole grains, is associated with a reduction in the risk of developing colorectal cancer. Hence the common dietary guideline: enjoy daily “Grain (cereal) foods, mostly wholegrain and/or high cereal fibre varieties”.

“Australians have taken the advice to eat more fibre very seriously and consume fibre at high levels, largely as cereal fibres. This has not led to the anticipated fall in CRC rates, which remain high” say CSIRO’s Drs David Topping and Michael Conlon in the Medical Journal of Australia. They have coined the phrase the Australian Fibre Paradox to describe this unexpected outcome. Indeed, in 1983, the average Australian adult (25–64 years of age) consumed just under 22 grams of dietary fibre each day and in 2011–12, they consumed just over 23 grams each day – not a huge increase, but an increase nevertheless. And from a Western perspective, Australians are relatively high fibre consumers, with our UK and US cousins consuming on average around 14 grams and 16 grams, respectively. Despite this, Australians have higher rates of bowel cancer than people living in both the UK and US. A fibre paradox indeed.

Topping and Conlon go on to say: “Emerging evidence now shows that many of the actions of fibre are mediated through short chain fatty acids (SCFAs), principally acetate, propionate and butyrate, produced by the fermentation of its constituents by beneficial large-bowel bacteria ... Resistant starch [a kind of dietary fibre] is fermented extensively in the large bowel, producing SCFAs, which promote a normal colonocyte phenotype – possibly lowering CRC ... Levels of resistant starch in most Australian processed consumer foods are low, suggesting a general deficiency of fermentable substrate – and thus a collectively hungry microbiome. We suggest that filling this gap by increasing the availability of new fermentable fibre-rich foods to feed the colonic microbiota and raise large-bowel SCFA levels has considerable potential to improve the health of the population.”.

Or in other words decrease the risk of colorectal cancer in Australia.

Carisma potato salad
Carisma potato salad with lemon-yogurt dressing

One way of increasing the resistant starch content of foods is eating foods cold after cooking them – potato, pasta and rice salads are classic examples, but even reheated starchy foods are thought to provide some additional benefits if you don’t like to eat them cold. Other good sources of resistant starches include legumes, rolled oats, barley and bananas (not over ripe). Regular GI News readers will recognise that many of the foods higher in resistant starch have a lower GI and that heating then cooling common starchy foods is one way of lowering the GI (e.g., potatoes).

What the emerging evidence is suggesting is that all dietary fibres are not the same and that simply eating more fibre in total may not provide all of the expected health benefits.

Unfortunately, it’s more complicated than that.
  
Alan Barclay (BSc; Grad Dip; PhD, APD, AN) is a consultant dietitian and Chief Scientific Officer at the Glycemic Index Foundation. He worked for Diabetes Australia (NSW) from 1998-2014 and is a member of the editorial boards of Diabetes Australia’s consumer magazine, Conquest, and health professional magazine, Diabetes Management Journal. He is coauthor of The Low GI Diet: Diabetes Handbook, The Low GI Diet: Managing Type 2 Diabetes, and The Ultimate Guide to Sugars and Sweeteners. Contact: alan.barclay@gisymbol.com or 0416 111 046