A New Zealand-led scientific review has found people with a diet high in fibre - specifically whole grains - have lower rates of a range of chronic diet-related diseases.
The study showed rates of heart disease - including death, colon cancer and type 2 diabetes were lower in people who eat at least 25g of fibre each day - and benefits increased with intake.
Most people consume about 20g of fibre each day. Researchers said the study, which looked at 40 years of data, looked to inform new global recommendations on fibre intake.
The series of systematic reviews and meta-analyses published in The Lancet suggested a 15-30 per cent 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 cent. Per 1000 participants, the impact translated into 13 fewer deaths and six fewer cases of coronary heart disease.
In addition, a meta-analysis of clinical trials suggested increasing fibre intakes was associated with lower body weight and cholesterol, compared with lower intakes.
One of the authors, Professor Jim Mann from the University of Otago, said: "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."
Dr Kathryn Bradbury, a nutritional epidemiologist at the University of Auckland, who was not involved with the study, said there was good evidence that higher intakes of wholegrains were associated with a lower risk of bowel cancer, and lower body weight.
"These findings are particularly relevant for New Zealand, because we have one of the highest rates of bowel cancer in the world. Also, with the rise in popularity of so-called Paleo and low carbohydrate diets, this study reminds us that dietary fibre (from fruit, vegetables and wholegrains) reduces the risk of chronic disease."
The study was commissioned by the World Health Organization to inform the development of new recommendations for optimal daily fibre intake and to determine which types of carbohydrate provided the best protection against non-communicable diseases (NCDs) and weight gain.
It was funded by the Health Research Council of New Zealand, the WHO, the Riddet Centre of Research Excellence, the Healthier Lives National Science Challenge, the University of Otago and the Otago Southland Diabetes Research Trust.
Researchers said most people worldwide consumed 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 per cent of UK adults managed to reach this target. In the US, fibre intake among adults averaged 15g a day. Rich sources of dietary fibre include whole grains, pulses, vegetables and fruit.
The researchers included 185 observational studies containing data that relate to 135 million person years and 58 clinical trials involving 4635 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 could not be applied to people with existing chronic diseases.
For every 8g increase of dietary fibre eaten per day, total deaths and incidences of coronary heart disease, type 2 diabetes and colorectal cancer decreased by 5-27 per cent. 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 incidences of coronary heart disease, type 2 diabetes and colorectal cancer decreased by 2-19 per cent. Higher intakes of whole grains were associated with a 13-33 per cent reduction in NCD risk – translating into 26 fewer deaths per 1000 people from all-cause mortality and seven fewer cases of coronary heart disease per 1000 people. The meta-analysis of clinical trials involving whole grains showed a reduction in body weight. 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." said Professor Jim 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 could 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.
Professor Gary Frost, Imperial College London, UK, said the analyses provided compelling evidence that dietary fibre and whole grain were major determinants of numerous health outcomes and should form part of public health policy.
Dr Elaine Rush, Professor of Nutrition at the Auckland University of Technology, who was not involved with the study, said the research presented a substantial body of evidence that we should be eating more whole plants to delay dying and reduce disease.
"Dietary fibres are constituents of all plants. Animal products including meat and dairy do not contain fibre. Whole grains, commonly consumed by and easily available to New Zealanders at a reasonable price and will store well include whole grain wheat, corn kernels (tinned), brown rice and oats. Uncooked, every 100g of these foods contains more than 10g of dietary fibre. We also need to eat more vegetables and fruit and legumes such as peas, beans and lentils.
Rush said it was not easy to increase fibre in the diet, but an example of how we could achieve 25 to 29 grams of fibre a day was to eat:
- half a cup of rolled oats (9g fibre), - 2 Weet-Bix (3g), - 1 thick slice of wheatmeal bread (2g), - 1 cup of cooked lentils (4g) - a potato cooked with the skin on (2g), - half a cup of silverbeet (1g), - a carrot (3g), - an apple with the skin on (4g).
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