MARIKA SBOROS: Can eating fibre protect one from heart attack?
Some experts say a new study is a landmark and compelling evidence of benefit, while others say it’s dangerous and the authors are spreading a false public health message
Your parents probably often told you to eat fibre to stay “regular”. Now, the authors of a major study in The Lancet say that fibre from wholegrain cereals, breads, pasta, fruit, nuts and legumes will protect you from heart attack and premature death.
The New Zealand researchers say fibre has “enormous protective effects” and also reduces your risk of type 2 diabetes and specific cancers, particularly colon cancer.
They say their study is a —“defining moment” that should be “written in stone” in public health policy. It’s also “good news” for high-carbohydrate, high-fibre diets and “bad news” for “fashionable”, low-carb diets.
Some experts say the study is a “landmark” and “compelling evidence” of benefit. Others say it’s “dangerous” and the authors are “cheating” and spreading a “false public health message”.
The study is a meta-analysis of 185 observational studies with data on 135-million person-years and 58 clinical trials involving 4,635 adults. Low-carb diets were not included.
The World Health Organisation (WHO) commissioned the study to inform its new recommendations for optimal daily fibre intake and determine which types of carbohydrate provide the best protection against non-communicable diseases (NCDs). NCDs include obesity, type 2 diabetes, heart disease and cancer and are epidemic globally, including in SA.
The study authors say we should all eat 25g- 29g of “good” fibre a day, preferably more. They say their results show a 15% -30% decrease in all-cause and cardiovascular-related mortality from increased fibre intake.
The study’s co-author is South African Jim Mann, professor of human nutrition and medicine at Otago University in Auckland, and a University of Cape Town graduate.
“We all knew dietary fibre was good for us,” Mann said in an Otago University release. “We didn’t know the extent to which the old mantra was true.”
Co-author John Cummings, emeritus professor of experimental gastroenterology at the University of Dundee, has called the study a “defining moment” heralding “the end of 50 years of researching dietary fibre”.
“We need to get this written in stone and part of people’s lives,” Cummings said in a Guardian report.
Also in the Guardian, Nita Forouhi, of Cambridge University’s MRC epidemiology unit, (who was not involved in the study) said fans of low-carb diets don’t count “the opportunity cost of missing out on fibre from wholegrains”.
In a commentary in The Lancet, Gary Frost, of Imperial College, London, called the meta-analysis “compelling evidence”.
New Zealand public health professor Grant Schofield, of Auckland University of Technology, and chief education adviser, health and nutrition, for the New Zealand government, is less complimentary.
“The Lancet study is useful and we need more meta-analyses,” Schofield said via e-mail. These “bring together what we know in the scientific community at that point in a way we can understand”.
However, we should treat prospective studies with caution unless the effect size is “very large”, Schofield said, especially where food intakes are self-reported and there are many confounding variables.
If the media have quoted Mann correctly, his statements about low-carb diets are “unfounded and dangerous”, Schofield said. Thus, Mann went “beyond his data, failed his duty as a scientist and public health advisor and demonstrated long-time prejudice against [low-carb] diets”, Schofield said.
The researchers have also reported relative not absolute risk.
“Once you see that, you know that the authors are cheating,” Cape Town scientist Tim Noakes said.
They claim an average 20% risk reduction. They say that for every 1,000 participants in the studies, the impact of higher-fibre intakes translates into 13 fewer deaths and six fewer heart disease cases compared to those consuming lower-fibre diets.
Therefore, of those who increase their fibre intake to 25g-29g or more daily over a lifetime, 1% will benefit, Noakes said. In other words, 99% will not benefit.
No one has challenged the authors on how that’s “an enormous protective effect”, he said.
The researchers are giving “a false public message” because “20% sounds much better than 1%”, Noakes said.
For anyone with insulin resistance and type 2 diabetes, increasing fibre from carbohydrate is risky, Noakes said: “What if the 1% effect masks a detrimental effect in those with insulin resistance?”
South African-born New Zealand dietitian academic Caryn Zinn said scientists have “long suspected that dietary fibre is a protective nutrient against chronic disease”. The Lancet study “consolidates that, despite much of the data coming from observational research”.
Wholegrains may simply be a marker of better diet, Zinn said. Alongside overall dietary fibre, researchers often overlook that a diet rich in wholegrains is likely to be generally low in refined, poor-quality foods. That leaves the question: is the actual health benefit from wholegrains themselves or a “generally tidy diet devoid of junk”?
It is “irresponsible” for the reviewers to extrapolate their findings into a warning about low-carb diets simply because they are low or even devoid of wholegrains, Zinn said.
“A low-carb diet is not synonymous with low-fibre,” she said. “Any eating style can be low or high in overall dietary fibre if there’s care in constructing it.”
Zinn’s 2018 research in the BMJ Open demonstrates that a well-planned low-carb diet provides plenty of fibre, soluble and insoluble. Meal plans provided 38g- 39g fibre for females and 44g- 45g for males, superseding the New Zealand recommendations, she said.