It started, as many issues do, because we didn’t get enough roughage in our diets. Before dietary fiber gained currency in the ‘70s as a way to protect against serious disease, people who believed we were eating ourselves into early graves weren’t taken very seriously. We’ll meet the interesting Irishman who turned that around shortly.
In the years since then, the claim that getting more fiber can reduce our chances of colorectal cancer has taken a solid scientific battering. On the side of the unconvinced are the National Cancer Institute and an old meta-analysis of randomized trials – with later trials from Japan and from England (on resistant starch) strengthening that position.
Dueling meta-analysts don’t make life easy, do they? There’s a new interesting piece of “meta-epidemiology” that shows just how common this situation is when it comes to claims of nutrients’ ability to prevent disease.
Researchers commissioned by the U.S. Agency for Healthcare Research and Quality recently published an analysis of 34 cases of studying whether or not “nutrient X prevents disease Y.” For these, they compared the conclusions of randomized trials (where groups of people randomized to consuming more nutrient X were compared with groups who weren’t) and epidemiological studies (no randomized comparison group).
In only 6 of these 34 cases did both the randomized and epidemiological study pools give “thumbs up” or “thumbs down” to nutrient X, with a statistically significant result from both. Whereas in 11 cases, there was at least a maybe “thumbs up” from one (usually the epidemiological pool) but more of a “thumbs down” from the other – including dietary fiber to prevent colorectal cancer. The researchers concluded that to be sure about the effects of nutrients, “plausible epidemiological support is probably not enough.”
They were looking for signs to suggest when a body of evidence may be getting to a definitive answer. But mapping the research showed no clear pathway. There wasn’t a translational science trajectory from non-randomized studies to randomized trials, for example. Researchers are doing non-randomized studies at the same time – and after – strong trials.
Trials are the only way to remove other dietary, lifestyle and environmental differences from the mix of factors that could be muddying the waters, making nutrient X look better (or worse) than it really is. They will sometimes point to possible harm from tinkering with something as complex as the diet.
But the evidence from trials on nutrients is criticized for a range of reasons. For example, people didn’t get nutrient X in the right form or enough of it, or the trial didn’t go on long enough or at the right time in people’s lives.
The National Cancer Institute points out that even if there is an influence from dietary fiber on its own, it’s unlikely that it could be a large difference.
The idea that you could prevent yourself getting colorectal cancer by piling on the bran is appealing, though. It captured people’s imaginations in the ‘70s, and I don’t suppose it’s going to let go easily.
The fiber theory was the child of an earlier “nutrient X” hypothesis by an English naval surgeon, Captain Thomas Latimer Cleave. His little sister had died of appendicitis when they were young. He later became convinced over-refined diets were responsible for this disease and much more. He was nicknamed “the bran man” during World War II, for all the bran he had put on naval vessels to prevent constipation.
It wasn’t bran that he thought was the really big culprit, though. He believed it was refined sugar, publishing a book in the ‘60s on “the saccharine disease.” Cleave had convinced another physician who became involved, Hubert Trowell, but wasn’t breaking through on a large scale.
Sir Richard Doll – the epidemiologist famous for his work on establishing the link between smoking and disease – said although the data behind the theory could be torn into strips, there might nevertheless be something to the idea that western diets were contributing to diseases more common in western countries.
Doll introduced Cleave to former surgeon turned researcher, Denis Burkitt. That was the turning point. Burkitt, like Trowell, had worked in Africa for years, and was then at the Medical Research Council. His credibility, network and energy were immense. He was responsible for making the connection between a virus and a kind of non-Hodgkin’s lymphoma which was named after him (Burkitt lymphoma and leukemia).
Burkitt explains on YouTube what happened next: “I can remember walking along the sands one time in Mombasa, and the thought coming to me: you can’t have sugar unless you take the fiber out in the first case, to refine it and give you sugar. And could it be the fiber that is taken out that we are short of, rather than the sugar we are feeding…And this sent me worldwide on measuring stools, because the amount of stool people pass is related to their fiber intake.” So they got people weighing stools, in Africa and India – and in Seventh Day Adventists who had high-fiber diets.
Burkitt published his seminal paper linking lack of dietary fiber to the development of colorectal cancer in 1971, having an immediate major impact. A best seller, “Don’t forget fibre in your diet,” followed in 1979. Burkitt’s advocacy for diet’s contribution to health, along with Linus Pauling’s advocacy for mega-doses of vitamin C, helped set off a spate of theories and beliefs about dietary supplements that shows no signs of letting up.
The recent analysis showing the difficulty of reconciling epidemiological and randomized studies suggests the debate over evidence isn’t going to let up any time soon either.
You can read more about prevention of colorectal cancer here.
You might also be interested in my post on 5 things to know about meta-analysis.
My thanks to @Profmicro who pointed out an imprecise use of “mosquito-borne” with Burkitt Lymphoma in the original text (via Twitter). More on this.
The painting at the top of this post is Piantagione di grano, Tacuinum sanitatis Casanatense from the 14th century, via Wikimedia Commons.
* The thoughts Hilda Bastian expresses here are personal, and do not necessarily reflect the views of the National Institutes of Health or the U.S. Department of Health and Human Services.