A systematic review of studies over almost 40 years, commissioned to inform new World Health Organisation guidelines, found that higher levels of fibre and whole grain intake cut all-cause and cardiovascular-related mortality by 15-30% and reduced incidence of coronary heart disease, stroke, type 2 diabetes and colorectal cancer by 16-24%. In addition, a meta-analysis of clinical trials suggested that increasing fibre intake was associated with lower body-weight and cholesterol.
The authors of the review say their findings are good news but incompatible with fashionable low-carb diets. According to The Guardian, the same research team also carried out the major review that informed WHO guidance on curbing sugar in the diet, leading to sugar taxes around the world.
People who eat higher levels of dietary fibre and whole grains have lower rates of non-communicable diseases compared with people who eat lesser amounts, while links for low glycaemic load and low glycaemic index diets are less clear. Observational studies and clinical trials conducted over nearly 40 years reveal the health benefits of eating at least 25g to 29g or more of dietary fibre a day, according to a series of systematic reviews and meta-analyses.
The results suggest a 15-30% decrease in all-cause and cardiovascular related mortality when comparing people who eat the highest amount of fibre to those who eat the least. Eating fibre-rich foods also reduced incidence of coronary heart disease, stroke, type 2 diabetes and colorectal cancer by 16-24%. Per 1,000 participants, the impact translates into 13 fewer deaths and six fewer cases of coronary heart disease.
In addition, a meta-analysis of clinical trials suggested that increasing fibre intakes was associated with lower body-weight and cholesterol, compared with lower intakes.
The study was commissioned by the World Health Organisation to inform the development of new recommendations for optimal daily fibre intake and to determine which types of carbohydrate provide the best protection against non-communicable diseases (NCDs) and weight gain.
Most people worldwide consume less than 20g of dietary fibre per day. In 2015, the UK Scientific Advisory Committee on Nutrition recommended an increase in dietary fibre intake to 30g per day, but only 9% of UK adults manage to reach this target. In the US, fibre intake among adults averages 15g a day. Rich sources of dietary fibre include whole grains, pulses, vegetables and fruit.
“Previous reviews and meta-analyses have usually examined a single indicator of carbohydrate quality and a limited number of diseases so it has not been possible to establish which foods to recommend for protecting against a range of conditions,” says corresponding author Professor Jim Mann, the University of Otago, New Zealand. “Our findings provide convincing evidence for nutrition guidelines to focus on increasing dietary fibre and on replacing refined grains with whole grains. This reduces incidence risk and mortality from a broad range of important diseases.”
The researchers included 185 observational studies containing data that relate to 135m- person-years and 58 clinical trials involving 4,635 adult participants. They focused on premature deaths from and incidence of coronary heart disease, cardiovascular disease and stroke, as well as incidence of type 2 diabetes, colorectal cancer and cancers associated with obesity: breast, endometrial, oesophageal and prostate cancer. The authors only included studies with healthy participants, so the findings cannot be applied to people with existing chronic diseases.
For every 8g increase of dietary fibre eaten per day, total deaths and incidence of coronary heart disease, type 2 diabetes and colorectal cancer decreased by 5-27%. Protection against stroke, and breast cancer also increased. Consuming 25g to 29g each day was adequate but the data suggest that higher intakes of dietary fibre could provide even greater protection.
For every 15g increase of whole grains eaten per day, total deaths and incidence of coronary heart disease, type 2 diabetes and colorectal cancer decreased by 2-19%. Higher intakes of whole grains were associated with a 13-33% reduction in NCD risk – translating into 26 fewer deaths per 1,000 people from all-cause mortality and seven fewer cases of coronary heart disease per 1,000 people. The meta-analysis of clinical trials involving whole grains showed a reduction in bodyweight. Whole grains are high in dietary fibre, which could explain their beneficial effects.
The study also found that diets with a low glycaemic index and low glycaemic load provided limited support for protection against type 2 diabetes and stroke only. Foods with a low glycaemic index or low glycaemic load may also contain added sugars, saturated fats, and sodium. This may account for the links to health being less clear.
“The health benefits of fibre are supported by over 100 years of research into its chemistry, physical properties, physiology and effects on metabolism. Fibre-rich whole foods that require chewing and retain much of their structure in the gut increase satiety and help weight control and can favourably influence lipid and glucose levels. The breakdown of fibre in the large bowel by the resident bacteria has additional wide-ranging effects including protection from colorectal cancer.” says Mann.
While their study did not show any risks associated with dietary fibre, the authors note that high intakes might have ill-effects for people with low iron or mineral levels, for whom high levels of whole grains can further reduce iron levels. They also note that the study mainly relates to naturally-occurring fibre rich foods rather than synthetic and extracted fibre, such as powders, that can be added to foods.
Commenting on the implications and limitations of the study, Professor Gary Frost, Imperial College London, UK, says: “(The authors) report findings from both prospective cohort studies and randomised controlled trials in tandem. This method enables us to understand how altering the quality of carbohydrate intake in randomised controlled trials affects non-communicable disease risk factors and how these changes in diet quality align with disease incidence in prospective cohort studies. This alignment is seen beautifully for dietary fibre intake, in which observational studies reveal a reduction in all-cause and cardiovascular mortality, which is associated with a reduction in bodyweight, total cholesterol, LDL cholesterol, and systolic blood pressure reported in randomised controlled trials…
“There are some important considerations that arise from this article. First, total carbohydrate intake was not considered in the systematic review and meta-analysis… Second, although the absence of association between glycaemic index and load with non-communicable disease and risk factors is consistent with another recent systematic review, caution is needed when interpreting these data, as the number of studies is small and findings are heterogeneous. Third, the absence of quantifiable and objective biomarkers for assessing carbohydrate intake means dietary research relies on self-reported intake, which is prone to error and misreporting. Improving the accuracy of dietary assessment is a priority area for nutrition research.
“The analyses presented by Reynolds and colleagues provides compelling evidence that dietary fibre and whole grain are major determinants of numerous health outcomes and should form part of public health policy.”
Background: Previous systematic reviews and meta-analyses explaining the relationship between carbohydrate quality and health have usually examined a single marker and a limited number of clinical outcomes. We aimed to more precisely quantify the predictive potential of several markers, to determine which markers are most useful, and to establish an evidence base for quantitative recommendations for intakes of dietary fibre.
Methods: We did a series of systematic reviews and meta-analyses of prospective studies published from database inception to April 30, 2017, and randomised controlled trials published from database inception to Feb 28, 2018, which reported on indicators of carbohydrate quality and non-communicable disease incidence, mortality, and risk factors. Studies were identified by searches in PubMed, Ovid MEDLINE, Embase, and the Cochrane Central Register of Controlled Trials, and by hand searching of previous publications. We excluded prospective studies and trials reporting on participants with a chronic disease, and weight loss trials or trials involving supplements. Searches, data extraction, and bias assessment were duplicated independently. Robustness of pooled estimates from random-effects models was considered with sensitivity analyses, meta-regression, dose-response testing, and subgroup analyses. The GRADE approach was used to assess quality of evidence.
Findings: Just under 135 million person-years of data from 185 prospective studies and 58 clinical trials with 4635 adult participants were included in the analyses. Observational data suggest a 15–30% decrease in all-cause and cardiovascular related mortality, and incidence of coronary heart disease, stroke incidence and mortality, type 2 diabetes, and colorectal cancer when comparing the highest dietary fibre consumers with the lowest consumers Clinical trials show significantly lower bodyweight, systolic blood pressure, and total cholesterol when comparing higher with lower intakes of dietary fibre. Risk reduction associated with a range of critical outcomes was greatest when daily intake of dietary fibre was between 25 g and 29 g. Dose-response curves suggested that higher intakes of dietary fibre could confer even greater benefit to protect against cardiovascular diseases, type 2 diabetes, and colorectal and breast cancer. Similar findings for whole grain intake were observed. Smaller or no risk reductions were found with the observational data when comparing the effects of diets characterised by low rather than higher glycaemic index or load. The certainty of evidence for relationships between carbohydrate quality and critical outcomes was graded as moderate for dietary fibre, low to moderate for whole grains, and low to very low for dietary glycaemic index and glycaemic load. Data relating to other dietary exposures are scarce.
Interpretation: Findings from prospective studies and clinical trials associated with relatively high intakes of dietary fibre and whole grains were complementary, and striking dose-response evidence indicates that the relationships to several non-communicable diseases could be causal. Implementation of recommendations to increase dietary fibre intake and to replace refined grains with whole grains is expected to benefit human health. A major strength of the study was the ability to examine key indicators of carbohydrate quality in relation to a range of non-communicable disease outcomes from cohort studies and randomised trials in a single study. Our findings are limited to risk reduction in the population at large rather than those with chronic disease.
Funding: Health Research Council of New Zealand, WHO, Riddet Centre of Research Excellence, Healthier Lives National Science Challenge, University of Otago, and the Otago Southland Diabetes Research Trust.
The authors of the review say their findings are good news – but incompatible with fashionable low-carb diets. According to a report in The Guardian, Mann’s team also carried out the major review that informed WHO guidance on curbing sugar in the diet, leading to sugar taxes around the world.
Sugar is a “bad” carbohydrate, but fibre is found in “good” carbohydrates such as wholegrain bread and oat-based muesli. However, the overwhelming backlash against sugar has led to popular diets that reject carbohydrates, including the fibrous sort that can, say the scientists, save lives.
Mann is quoted in the report as saying that the research “does contribute to the debate considerably. Here we have got very strong evidence that a high-fibre diet, which for the majority of people is at least high-ish in carbohydrates, has an enormous protective effect – a wide range of diseases including diabetes, cardiovascular disease and cancer benefit from a high-carbohydrate diet.”
But he said it would not end the “diet wars” because there were so many vested interests involved. “It’s twofold. There is the commercial vested interest, which there is an enormous amount of from chefs and celebrity chefs and so on. And there is also the professional vested interest.” This included some doctors and scientists, he said.
“Fibre-rich whole foods that require chewing and retain much of their structure in the gut increase satiety and help weight control and can favourably influence lipid and glucose levels,” said Mann. It was very difficult to have high levels of fibre on a low-carbohydrate diet unless you took fibre supplements, said Mann. And “there isn’t the huge body of evidence that we’re talking about” for supplements being beneficial, he said, adding that “it’s pretty well impossible” to get enough fibre from fruit and vegetables alone.
John Cummings, emeritus professor of experimental gastroenterology at the University of Dundee, one of the authors, said the study was of huge importance and the conclusions should not be thought of as “just a fad”. “This is the end of 50 years of researching dietary fibre. It is a defining moment,” he said. The research brings together population epidemiological studies and feeding studies and, he said, “we now know that fibre does things in the body which give us a credible explanation for how this works”.
“We need to get this written in stone and part of people’s lives.”
Other scientists backed the findings and said the public should eat more fibre. “It is a concern that the fibre consumption in the UK is on average, currently much less than (30g a day). It is also worrying that otherwise healthy consumers who try to follow popular diets low in carbohydrate will find it very difficult to achieve a healthy level of fibre intake,” said Dr Ian Johnson, emeritus fellow at the Quadram Institute Bioscience.
Professor Nita Forouhi of Cambridge University’s MRC epidemiology unit said the findings “do imply that, though increasingly popular in the community at large, any dietary regimes that recommend very low-carbohydrate diets should consider the opportunity cost of missing out on fibre from whole grains”.
When it came to carbohydrates, she said, “the quality matters very much, over and above the debate on quantity. Wholegrain foods are typically high in fibre, and this research provides further evidence to highlight their importance and support a shift in our diets from processed and refined foods in the food supply chain towards more fibre-rich wholegrain foods.”
BBC News reports that the research suggests if you shifted 1,000 people from a low fibre diet (less than 15g) to a high-fibre one (25-29g), then it would prevent 13 deaths and six cases of heart disease. That’s during the course of these studies, which tended to follow people for one to two decades. It also showed lower levels of type-2 diabetes and bowel cancer as well as lower weight, blood pressure and cholesterol levels.
And the more fibre people ate, the better.
The report says there used to be a view that fibre didn’t do much at all – that the human body could not digest it and it just sailed through. But fibre makes us feel full and affects the way fat is absorbed in the small intestine – and things really become interesting in the large intestines, when your gut bacteria get to have their dinner. The large intestines are home to billions of bacteria – and fibre is their food.
It’s a bit like a brewery down there where bacteria are fermenting fibre to make a whole load of chemicals, the report says. This includes short-chain fatty acids, which are absorbed and have effects throughout the body. “We have this organ set up to digest fibre, which a lot of people just don’t use very much,” says Cummings.
The report says the fact fibre and whole-grains and fruit and vegetables are healthy should not come as a surprise. But there is concern people are turning their back on fibre, with the popularity of low-carb diets.
Forouhi says: “We need to take serious note of this study. Its findings do imply that, though increasingly popular in the community at large, any dietary regimes that recommend very low-carbohydrate diets should consider the opportunity cost of missing out on fibre from whole-grains. This research confirms that fibre and whole-grain intakes are clearly important for longer term health.”