A few years back I wrote an in depth blog post about the questionable role of fiber in preventing colorectal cancer based on the results of several clinical trials. find that post here.

If you haven’t read it the synopsis is I made the argument that dietary fiber doesn’t reduce the risk of colon cancer and we should move beyond the paradigm of “fiber prevents colon cancer” to that of “some unidentified factors present in fiber rich dietary patterns reduces the risk of colon cancer”, which is admittedly not as catchy but is probably more accurate.

I wanted to make this post update with some additional information, essentially an addendum to that first post.

First, a study I didn’t cover in my first post.

The CAPP2 study (full text here) looked at the effect of resistant starch on colorectal cancer risk in people with Lynch Syndrome, a genetic condition in which individuals have a higher risk of colon cancer. Resistant starch, a form of indigestible starch (found in green bananas and beans, among other foods), is a sort of soluble fiber, notable for it’s ability to increase intestinal formation of butyrate.

Butyrate, it has been suggested, may help prevent colon cancer through a variety of mechanisms, perhaps chiefly by inhibiting something called HDAC (histone deacetylase).

Alas, things are rarely so simple. Ultimately, the CAPP2 study found supplementing 30g of resistant starch per day (compared to a placebo) had no significant effect on colorectal cancer incidence:

Beyond this I want to address the broad issue of whether the time between fiber supplementation and cancer risk in clinical studies is long enough to see a real effect.

As an example, drinking excess alcohol is pretty commonly accepted to increase the risk of certain cancers (liver, colon, breast, etc.), but this effect generally seems to take around 8-11 years.

Similarly, smoking cigarettes also seems takes a while to lead to cancer, as shown by the rise in cigarette smoking by men in the United States, which was paralleled by a similar rise in lung cancer, only the rates of lung cancer went up around 20 years later.

What about factors to reduce the risk of cancer? The results of multiple clinical trials suggest taking aspirin reduces the risk of certain cancers, such as colorectal cancer (like fiber is proposed to do). Sure enough, a number of studies (including the BDAT and UKIA trials) have found aspirin can lower the risk of colon cancer, but it can take up to 10 years before the effect appears.

In other words, are clinical trials on fiber which generally lasted around 3-4 years actually a fair assessment of the effect of fiber on colorectal cancer risk if cancer can take upwards of 8 years to actually occur? Well, I still think so and I have 3 basic reasons why.

1. The unsupportive fiber trials looked at risk of adenomas, which are essentially pre-cancerous tumors that appear early, with some of them potentially progressing to cancer over a long enough timeline. So while cancer may theoretically take upwards of a decade to manifest, adenomas may occur more quickly and thus could reasonably show up in a 3-4 year time frame. See for example this clinical trial which found aspirin seemed to lower the incidence of adenoma formation within a year:

2. Some nutrition trials have seen effects on cancer incidence early. The Nutritional Prevention of Cancer trial saw lower cancer mortality within a few years of folks with low selenium levels taking a selenium supplement.

In the Lyon Diet Heart trial, heart attack survivors following an experimental Mediterranean diet (click here for one of my longest blog posts ever talking covering this study) ended up with a lower rate of cancer, despite this study lasting under 4 years (see top row):

3. Fiber, particularly cereal fiber, is without a doubt often associated with a lower rate of colorectal cancer. Some evidence suggests if this relationship is causal we should be able to see it quickly. One study found the association between fiber and colorectal cancer was actually more apparent in the first 5 years. A 2005 analysis of 13 prospective cohort studies involving 725,628 men and women looking at the association between fiber and colorectal cancer and reported the following:

“When follow-up time was limited to only the first 5 years, there was a suggestion of an inverse association (pooled multivariate RR = 0.87; 95% CI, 0.76-1.00 in the highest quintile vs the lowest; 3257 cases). However, after a 5-year latency period between the baseline diet assessment and outcome ascertainment, no association was observed (pooled multivariate RR = 1.00; 95% CI, 0.89-1.12 in the highest quintile vs the lowest; 4824 cases)”

In other words, if the effect of fiber is real, this paper suggests we should be able to see it in short term studies.