The Fiber Files

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First published 2/18/11 on The Doctor Weighs In

Back in the 1970s when I was in my medical training, I took time off from a grueling schedule to see with my own eyes one of the great physician/scientists of the day. Dr Denis Burkitt was a Christian missionary who decided to forgo his love of physics and engineering and go to medical school so he could serve humanity in Africa. Indeed, after WWII he settled in Uganda and practiced good old fashioned general medicine for 20 years. This in itself would have been an admirable record were it not for his acute powers of observation. This ever observant country doc discovered a new childhood cancer that was later named Burkitt’s lymphoma. So you can imagine the excitement when this legendary figure came to UCSF to deliver a grand rounds lecture (a lecture to the whole campus community) on his work.

May I have the first slide please

After the requisite glowing introduction a short silver-haired gentleman stepped up to the podium (short indeed; his head was barely visible over the podium). He started with the usual introductory banter, delivered in an Irish brogue and a mischievous smile.

First slide: a view of the African bush; grasses, a few acacia trees in the distance, and… a pile of sh…t in the center! Embarrassed laughter, gaining momentum into gales of guffaws –is this an Irish or a Ugandan joke? No, he said, it is a representative sample of a Ugandan boy daily output. To make it more scientific, in deference to the audience, he plied us with the vital statistics of the Ugandan excrement: volume, weight, composition, water content.

Next slide: the beautiful English countryside and, you guessed it, a pile of sh..t in the center. Only this one belonged to an English schoolboy, and it looked more like goat turds. Indeed, volume, weight, composition, water content corroborated the visual impression.

Funny, I thought, but what’s the point? The answer wasn’t late in coming: the difference between the African and the Western diets was in the percentage of fiber in them. But he didn’t stop there: the fiber content accounts for the “diseases of civilization” of hypertension, heart disease, stroke, diabetes –and that ain’t chicken shit of a claim. He followed with comparative data of disease prevalence, mortality rates, and their correlation with diet. Interesting hypothesis, was the general feeling, but a bit far-fetched, going far beyond any available data.

The train was set in motion

A stubborn Irish Christian missionary like Dr. Burkitt was not about to wind down his fiber crusade just because some hard-nosed skeptical scientists demanded data. His relentless prodding  encouraged a few research projects that with time grew into an avalanche of data.  There have been ups and downs, bad studies and good studies, confirmed results and unconfirmed ones, but a picture started to emerge.

  • Fiber is made up of a metabolizable part, and a non-metabolizable part.
  • Its presence in the diet causes a shortened transit time in the digestive tract, resulting in shortened exposure of colon cells to carcinogenic compounds. It is also responsible for increased bulk of the stool, which lowers the incidence of diveriticulosis (an outpouching of a weak section of the colon) and hemorrhoids (an outpouching of a weak segment of a blood vessel wall). The latter two form as a consequence of straining at stool. The high water content provided by the presence of fiber obviates the need for that.
  • The metabolizable (soluble) portion provides short-chain fatty acids, which are important in a healthy diet. For instance, one of those, butyrate, is essential for the normal development of colonic cells. Others inhibit the absorption of bile acids, and thereby reduce blood levels of LDL  cholesterol. This, in turn, results in lower heart disease, hypertension and stroke.
  • It also smoothes out the absorption of glucose, avoiding the peaks and valleys of insulin release. It results in the reduction of diabetes and metabolic syndrome.

So what’s there not to like, you might ask? Nothing, except that all of those effects are not huge (except of course the effect on stool volume). And furthermore, many of the studies are far from definitive.

Nutritional studies are notorious for their flaws, some avoidable other less so. Epidemiological studies are difficult because we are complex animals, living in a complex world, having complex behaviors, belonging to different cultures-it makes it almost hopeless to discern the effect of an isolated factor on the subject. There are statistical ways to try and remove the confounding factors that tend to muddy the picture, but these methods are valid only when the sample size is huge. Hence the need for those gargantuan-size studies, and their accompanying massive costs.

Another drawback of nutritional studies is their duration. Nutritional effects are almost by definition very small, incremental and cumulative. Only with the passage of time can we expect to see measurable, significant results. But the longer the study, the more expensive. So how can we get around these massive hurdles?

The breakthrough study

Yikyung Park, Sc.D., of the National Cancer Institute, Rockville, Md., and colleagues analyzed data from 219,123 men and 168,999 women in the National Institutes of Health-AARP Diet and Health Study. Participants completed a food frequency questionnaire at the beginning of the study in 1995 and 1996. Causes of death were determined by linking study records to national registries.

Participants’ fiber intake ranged from 13 to 29 grams per day in men and from 11 to 26 grams per day in women.

Over an average of nine years of follow-up, 20,126 men and 11,330 women died. Fiber intake was associated with a significantly decreased risk of total death in both men and women; the one-fifth of men and women consuming the most fiber (29.4 grams per day for men and 25.8 grams for women) were 22 percent less likely to die than those consuming the least (12.6 grams per day for men and 10.8 grams for women).

The risk of cardiovascular, infectious and respiratory diseases was reduced by 24 percent to 56 percent in men and 34 percent to 59 percent in women with high fiber intakes. Dietary fiber from grains, but not from other sources such as fruits, was associated with reduced risks of total, cardiovascular, cancer and respiratory disease deaths in men and women.

Some comments and questions

First, notice the large number of participants, 388,122 in all. This is about 0.15% of the U.S.

Dov Michaeli MD PhD opines deeply from The Doctor Weighs In.

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