The dullest finding in nutrition is also the largest
A hundred and eighty-five studies, a hundred and thirty-five million person-years, and a dose-response curve that keeps going down. Nobody sells it, because nobody owns it.
If you asked us to name the single best-evidenced thing a person can change about what they eat, it would not be a supplement, a superfood or a protocol. It would be fibre, and it is almost impossible to make that sound interesting, which is most of the reason you have not been told.
The study
In January 2019 The Lancet published a series of systematic reviews and meta-analyses on carbohydrate quality, led by Andrew Reynolds with Jim Mann as corresponding author, both at the University of Otago. The scale is unusual:
- 185 prospective observational studies, covering just under 135 million person-years.
- 58 clinical trials with 4635 adult participants.
Two different kinds of evidence, pointing the same way, is worth more than either alone. The observational studies say what happens to people who already eat this way. The trials say what happens when you change it.
What they found
- Comparing the people who ate the most fibre with those who ate the least, all-cause and cardiovascular mortality were 15% to 30% lower.
- Incidence of coronary heart disease, type 2 diabetes and colorectal cancer was 16% to 24% lower.
- For every extra 8 grams of fibre a day, deaths and the incidence of those conditions fell by 5% to 27%.
- Across 1000 people, the difference between the highest and lowest intakes worked out at 13 fewer deaths and 6 fewer cases of coronary heart disease.
- In the trials, higher fibre intake was associated with lower body weight and lower cholesterol.
The dose-response is the part that makes this hard to dismiss. It is not a threshold you cross. More was better across the range studied, and the curve did not flatten at the recommended intake.
How much
The authors put the useful target at 25 to 29 grams a day, and noted that more than that may confer further benefit. Most people in wealthy countries eat well under 20.
The same series looked at glycaemic index and glycaemic load, which have had far more attention and far more products built on them. They found limited support, for protection against type 2 diabetes and stroke only. Foods can score well on those measures and still be high in added sugar, saturated fat and sodium. Fibre was the measure that held.
What the authors warn about
- The findings apply to healthy participants, not to people already managing a chronic condition.
- They concern fibre occurring naturally in food, not fibre supplements, which were not what was studied.
- Dietary intake in the observational studies is self-reported, which is prone to error and misreporting. This is the standing weakness of all nutritional epidemiology and it applies here too.
- Very high intakes can reduce mineral absorption in people who are already deficient in iron or zinc.
What we do in the kitchen
We do not sell fibre and there is nothing to link to. Whole grains, pulses, vegetables, fruit and nuts are the foods the studies were measuring, and they are the cheapest aisle in the shop.
It pairs with the other two pieces here. Ultra-processed food made people eat 500 calories a day more without choosing to. Fermented food shifted the gut in ten weeks. And fibre, which is duller than either, has by far the most evidence behind it. The pattern across all three is the same: the strongest findings in food are about what is on the plate, and none of them is for sale.
