By Marika Sboros
A large New Zealand-led study on health benefits of dietary fibre is making waves in scientific circles.
The authors of a meta-analysis in The Lancet claim “enormous protective effects” from eating more fibre found in wholegrain bread, cereals, pasta, nuts and pulses. They say it significantly cuts risk of heart disease and premature death.
They say that a high-carb, high-fibre diet also protects against type 2 diabetes and certain cancers. And that we should all eat between 25-29g of fibre a day, preferably more. They call their study “good news” for high-carb, high-fibre diets and “bad news” for “fashionable” (ie “faddish”) low-carb diets. They say it’s a “defining moment” that should be “written in stone” in public health policy.
Such scientific certainty has raised eyebrows among other experts. They say it’s a “false message to the public”.
The study is “useful” but reflects “poor public health practice”, say critics. And while the media say it’s a landmark, critics say it’s more likely a mark of what’s to come from establishments in 2019. That’s likely to be more industry-led studies supporting the high-carb, low-fat (HCLF) status-quo and knocking low-carb, high-fat (LCHF).
The study’s co-author is Jim Mann, professor of Human Nutrition and Medicine at Otago University in Auckland. Mann has come under special scrutiny for his media statements knocking low-carb diets. More so, since the study says nothing at all about low-carb diets.
The real target?
Critics say it can look like the study authors’ aim from the outset was to discredit LCHF diets. The New Zealand researchers wouldn’t be the first to do that. There are distinct parallels with the Naudé Review published in PLoS One in 2014.
The authors, from the Universities of Stellenbosch and Cape Town, are all staunch opponents of LCHF. They issued public statements about the dangers of low-carb diets . Yet their review doesn’t cover real low-carb diets. The media quickly trumpeted the research as “debunking” LCHF. The researchers did nothing to disenchant the public of that notion.
The Naudé Review remains mired in a long list of identified errors, amid accusations of unethical conduct, bias and “scientific fraud”.
Critics say that Mann is “hopelessly conflicted” as lead author of The Lancet review. Certainly, his support for the high-carb status quo, is well-documented. In an editorial in The Lancet in 2016, Mann knocks scientific criticism of official high-carb dietary guidelines, claiming that it is “not evidence-based”.
At first glance, his study’s numbers in The Lancet look impressive enough. It’s a review of 185 observational studies containing data relating to 135 million person years and 58 clinical trials involving 4,635 adults.
The World Health Organisation (WHO) added gravitas by commissioning the review. The WHO will use Mann’s study to “inform development of new recommendations for optimal daily fibre intake”. And to determine which types of carbohydrate best protect against non-communicable diseases (NCDs, aka chronic lifestyle diseases) and weight gain.
In The Guardian newspaper in the UK, Mann defined a high-fibre diet for most people as “at least high-ish in carbohydrates”. He claimed that it was difficult to get enough fibre from a low-carb diet and fruit and vegetables alone – unless you take supplements.
Prof Gary Frost from Imperial College London is similarly supportive in a commentary in The Lancet. He called Mann’s study “compelling evidence that dietary fibre and wholegrain are major determinants of numerous health outcomes”. These should form part of public health policy, Frost said.
Prof Nita Forouhi of Cambridge University’s MRC epidemiology unit is similarly on board. In the same Guardian report, she took a sideswipe at low-carb diets. Forouhi claimed that followers don’t consider “the opportunity cost of missing out on fibre from wholegrains”.
Mann has said that his study will not end the “diet wars” as there are “so many vested interests involved”. That much certainly is true.
COIs in fibre studies?
The vexed questions of vested interests and undeclared conflicts of interest (COIs) continue to plague nutrition science. The Lancet study is no exception.
New Zealand public health professor Grant Schofield sees major problems with it.
Schofield is professor of public health at Auckland University of Technology and director of the school’s Human Potential Centre. He is also Chief Education Advisor Health and Nutrition for the New Zealand government.
He does not dismiss Mann’s study altogether. It is “useful”, Schofield told me via email, and “we do need more meta-analyses”. These “bring together what we know in the scientific community at that point in a way we can understand”.
If the media reflected what Mann actually said about low-carb diets, Schofield said that his statements are “unfounded and dangerous”. If so, Mann has “failed his duty as a scientist and public health advisor”. He demonstrates “long-time prejudice against (low-carb) diets” and has “gone beyond his data”.
Treat with caution
We should all treat prospective studies “with caution”, unless the size of the effect is the effect size is “very large”, Schofield said. Using the work of Dr Austin Bradford-Hill, Schofield would consider hazard and risk ratios of greater than 2 to be “large” in such studies. (Under the Bradford-Hill criteria, the effect in Mann’s study is, therefore, not “large”.)
This is especially the case where food intakes are self-reported and there are many confounding variables, Schofield said.
Mann and his team have summarised their findings well, he said. They state that the “certainty of evidence for relationships between carbohydrate quality and critical outcomes was graded as moderate for dietary fibre, low to moderate for wholegrains, and low to very low for dietary glycaemic index and glycaemia”.
This summarises statistical certainty that any of the results are “real” or an artefact of chance, Schofield noted. It is “telling” in terms of public health. Based on the study results, no one can make any recommendations for anything except fibre, Schofield said.
He found it difficult to “imagine why (Mann) would consider a diet high in fibrous vegetables, such as those used in typical LCHF diets to be low in fibre.” Schofield and his team’s research shows this “not to be the case”.
South African scientist and LCHF pioneer and specialist Prof Tim Noakes was similarly critical. “When all is said and done, (Mann’s) data add very little,” Noakes told me.
Where’s the effect?
The major study finding turns out to be that 1% of people will benefit over a lifetime from an increased, high-fibre intake of 30g a day. In other words, 99% will not benefit. No one has challenged Mann on how he finds that “an enormous protective effect”, Noakes said.
If the authors were honest, they should have told the public that if they changed their diet by eating 30g of fibre a day for life, 1 in 100 people would achieve some marginal benefit. In that case, people would have asked: “So what? Are you for real?” Noakes said.
As well, increasing fibre intake without appropriate care would increase carbohydrate intake, he said. That would be harmful for those with insulin resistance and type 2 diabetes.
“What if the 1% effect masks a detrimental effect of an increased carbohydrate intake in those with insulin resistance?”
Another glaring weakness is that researchers have reported relative rather than absolute risk. “As soon as you see that, you know the authors are cheating,” Noakes said.
‘False message’ on fibre
They are giving “a false message” to the public because “20% sounds much better than 1%”, he said.
South African-born New Zealand dietitian academic Dr Caryn Zinn is also critical of the study.
Zinn told me via email that scientists have “long suspected that dietary fibre is a protective nutrient against chronic disease”. The new study outcome consolidates that, despite much of the data coming from observational research.
However, it is “irresponsible to extrapolate these findings into a warning about low-carb diets simply because they are low or even devoid of whole grains”.
Two key points undermine the study’s conclusions, Zinn said: The first is Mann’s claim that it’s “pretty well impossible to get enough fibre from fruit and vegetables alone”.
“A low-carb diet is not synonymous with a low-fibre diet,” Zinn said. “Any style of eating can be low or high in overall dietary fibre. It simply depends on whether there is some care in constructing it.”
Mann also appears not to have read Zinn’s most recent research on the fibre topic in the BMJ Open in 2018.
The study assessed the nutrient intake of an LCHF diet in a hypothetical case study design. It shows, she said, that a well-planned low-carb diet provides plenty fibre, both soluble and insoluble. In fact, the meal plans provided 38-39g of dietary fibre for females and 44-45g for males.
These, therefore, supersede recommendations of 25g and 30g, for females and males, respectively.
There’s also the question of whether wholegrains are simply a marker of a better diet and researchers often overlook an important fact. Alongside overall dietary fibre, a diet that is truly rich in wholegrains and overall dietary fibre is also likely to be ow in refined, poor quality foods in general, Zinn said.
That leaves unanswered the question: Is the benefit for health the wholegrains themselves or a “generally tidy diet devoid of junk”?
“Food production methods still expose foods containing wholegrains to varying levels of food processing,” Zinn said.
She will not alter her advice to the public, based on Mann’s study. Zinn will continue to promote a high-fibre intake from vegetables, mainly non-starchy, some fruit, nuts and seeds without the added carbohydrate load, particularly for those who are insulin-resistant.
Another major problem for Mann, according to critics, is his die-hard support for the diet-heart hypothesis that saturated fat causes heart disease. The diet-heart hypothesis is the foundation on which public health authorities in the US built the country’s influential – and now thoroughly discredited – low-fat, high-carb guidelines.
The hypothesis is also currently unproven, despite protestations to the contrary from cardiologists and other medical experts.
Mann’s enduring support for the hypothesis is most likely the legacy of his links with the University of Cape Town, Noakes said. South African-born Mann is a graduate and former lecturer of UCT. The university that has a “long investment” in the diet-heart hypothesis, as the late UCT professor of medicine Max Klein pointed out in an email to UCT in 2014.
Klein was objecting vigorously to UCT’s treatment of Noakes over his support for LCHF and his criticism of the lipid hypothesis. Noakes and I noted it and the background in our book, Lore of Nutrition, Challenging Conventional Dietary Advice (Penguin 2017).
Mann’s mentor is another South African UCT graduate, Prof Stewart Truswell, an emeritus professor of Sydney University. Mann and Truswell have co-authored studies supporting the diet-heart hypothesis and high-carb diets. They are co-authors of a dietary textbook, Essentials of Human Nutrition (Oxford University Press).
Truswell was a contemporary of Prof Brian Bronte-Stewart, a UCT graduate and stanch supporter of the work of US physiologist Dr Ancel Keys. Keys was the man who gave birth to the hypothesis, despite his own research disproving it.
Fighting fat with fibre?
While at UCT in 1955, Bronte-Stewart and Keys did the world’s first research into the diet-heart hypothesis, an inter-racial survey in the Cape Peninsula, which The Lancet published. Noakes says that the study makes some naive arguments. The researchers used differences in blood cholesterol concentrations to explain differences in three populations of Capetonians living quite different lives in quite different socio-economic classes and from different population groups.
Truswell has lived his academic life defending Keys and looking very much like an apologist for sugar, Noakes said. Mann is similarly steeped in the diet-heart hypothesis and may have “lived the same deception” around sugar.
Mann is also author or co-author of a series of studies in the 1970s and 80s apparently showing that patients with type 2 diabetes benefit from eating a high-carb diet. However, these findings have not been replicated.
Instead, studies from the Virta Health company in the US, published in 2017 and 2018, have shown the opposite. They show that LCHF diets “reverse” type 2 diabetes in as many as 60% of patients within the first year.
The Virta studies raise questions of how Mann could have found the opposite in his studies, Noakes said. Mann’s research has had a major effect in “delaying the adoption of curative LCHF diets for persons with type 2 diabetes by perhaps as much as 40-50 years”, he said.
Another weakness of Mann’s study is that it ignores a shaky foundation. The fibre hypothesis on which it rests has never been properly tested, Noakes said.
The father of the fibre hypothesis was Dr Denis Burkitt. He was “a missionary surgeon in Africa who turned to research in his late 40’s and came up with important findings on cancer and nutrition”, according to a New York Times obituary.
In particular, his “championing of a thesis that high fiber protected against colon cancer and many other diseases led millions of people to change their diet”.
Anointed at work?
Nutrition history is unlikely to be kind to Burkitt’s work, some experts say. They say global epidemics of obesity, type 2 diabetes, heart disease and cancer bear testimony.
Still, US cardiovascular researcher Dr James Dinicolantonio sees merit in Mann’s study.
“The findings from the clinical trials included in this meta-analysis combined with population data and prospective data do hint of benefit for incorporating more fibre from wholefoods,” Dinicolantonio said via email.
There needs to be a large clinical trial where participants actually consumed a healthy wholefood high-fibre diet on hard endpoints to know for sure, he said. But overall evidence is “fairly consistent” with suggesting benefit from a diet high in wholefood fibre.
Noakes is not so sanguine. He says it’s another case of the “Anointed” unable ever to admit that they are wrong.
Mann will “go to his grave proving that he and Keys are correct. And Yudkin and everyone behind the LCHF movement are wrong,” Noakes said.