High fiber intake is associated with improved survival among patients diagnosed with colorectal cancer (CRC), even among those who increase their fiber intake after the diagnosis, the results of an analysis of two prospective studies indicate.
In a study that included more than 1500 healthcare professionals who had been diagnosed with CRC, researchers found that each 5 g/d increase in fiber intake was linked to a 22% reduction in cancer-specific mortality and a 16% reduction in overall mortality, with the greatest effect seen with whole-grain foods. Notably, no association was seen with daily fruit fiber intake.
The study, which was published online November 1 in JAMA Oncology, found that the survival benefit occurred even among those patients who increased their fiber intake after diagnosis. The maximum effect seen for patients who consumed about 24 g/d.
Study author Andrew T. Chan, MD, MPH, of Massachusetts General Hospital and Harvard Medical School, Boston, said that the findings may influence advice given to CRC patients as well as our understanding of the disease.
The possibility that fiber or related substances may be protective "does give us some interesting biological insights into the process by which colon cancer may spread, and as result, it may provide us some new targets for new disease interventions," he told Medscape Medical News.
"The mechanism could be related to some of the effects that fiber has, for example, on insulin pathways, and this may be a potential target for treatment in the future. It's also possible that fiber may form a substrate for bacteria in the gut to produce anti-inflammatory compounds and metabolites," said Dr Chan.
Previous studies have indicated that dietary fiber protects against the development of CRC. Potential mechanisms include a reduction in exposure to intestinal carcinogens; systemic benefits on insulin sensitivity and metabolic regulation; and the fermentation of fiber in the gut into short-chain fatty acids that have tumor-suppressive effects.
However, no studies have examined the impact of fiber intake on survival in patients already diagnosed with CRC.
The researchers collated data from the prospective Nurses' Health Study (NHS) and the Health Professionals Follow-up Study (HPFS). The NHS recruited 121,700 US registered female nurses aged 30 to 55 years in 1976; the HPFS enrolled 51,529 US male healthcare professionals aged 40 to 75 years in 1986.
In both studies, the participants completed a medical history and lifestyle questionnaire, which included information on CRC status, at baseline and then every 2 years. Dietary data were also collected, and a food frequency questionnaire was completed every 4 years.
For those reporting a CRC diagnosis, the team asked for consent to acquire their medical records and pathologic test reports. A total of 1575 individuals diagnosed with stage I-III CRC completed the questionnaire, including 963 women from the NHS and 612 men from the HPFS.
The mean age of the patients was 68.6 years. During a median follow-up period of 8 years, there were 773 deaths, of which 174 (22.5%) were deemed to be CRC specific. The overall 5-year survival rate was 83% for stage I disease, 82% for stage II disease, and 72% for stage III CRC.
After taking into account potential confounding factors, fiber intake was inversely associated with mortality, at a multivariate hazard ratio (HR) per 5 g/d increase in intake of 0.78 for CRC-specific mortality (P = .006) and 0.86 for all-cause mortality (P < .001).
The team also found that mortality was lower for patients who increased their fiber intake after diagnosis: each 5 g/d increase was associated with an 18% reduction in CRC-specific mortality (P = .002) and a 14% reduction in all-cause mortality (P < .001).
The relationship between fiber intake after diagnosis and CRC-specific mortality was linear, reaching a maximum at approximately 24 g/d, beyond which there was no further mortality reduction.
Consumption of fiber from cereals was associated with a significant reduction in CRC-specific mortality, at an HR for each 5 g/d increase of 0.67 (P = .007 for trend), and all-cause mortality, at an HR of 0.78 ( P < .001 for trend).
There was no association between daily fruit fiber intake and CRC-specific or all-cause mortality. Vegetable fiber intake was associated with a significant reduction only in all-cause mortality, at an HR per 5 g/d of 0.83 increase (P = .009 for trend).
Whole-grain consumption was linked to lower CRC-specific mortality, at an HR per 20 g/d increase of 0.72 (P = .002 for trend). This was attenuated after taking into account total fiber intake, at an HR of 0.77 (P = .02). A similar effect was seen for all-cause mortality.
The researchers write: "Our present study adds to the existing literature and suggests that the effect of high fiber intake may extend beyond protection against cancer incidence and contribute to better prognosis after cancer is established."
While acknowledging that there are a number of potential limitations to their study, including the self-reported nature of the information on fiber intake and sources and lack of detailed treatment data, they note that the findings are in accord with those of previous studies.
They conclude: "Our findings provide support for the nutritional recommendations of maintaining sufficient fiber intake among CRC survivors."
Is It Fiber or Phytochemicals?
Approached for comment, Elizabeth Ryan, PhD, assistant professor, College of Veterinary Medicine and Biomedical Sciences at Colorado State University in Fort Collins, said that there have been many studies of the intake of fiber from various food sources, but the results have been mixed.
She told Medscape Medical News that one of the major limitations of such studies is their reliance on diet logs, which are yet to be fully validated.
Dr Ryan explained: "There are not a lot of biomarkers that I could look at in your blood or my blood and say: 'You've eaten oats today.' It's based on your writing, 'I ate oats yesterday,' and that's sometimes difficult when we want to make sure we want to hold people accountable for reporting what they're eating."
Nevertheless, she said that in the current analysis, the researchers address the limitations of their methodology and that the results support what is already known about the beneficial effects of fiber, particularly fiber from whole grains.
For Dr Ryan, the outstanding question is, "Do we suggest one particular type of whole grain over another? Because, is it just about meeting the [recommended] fiber intake, or are there other phytochemicals in those foods that [are also] protective against colorectal cancer?"
Do we suggest one particular type of whole grain over another? Dr Elizabeth Ryan
She believes that the best approach would be to ensure that patients eat a variety of cereals, rather than rely on a fiber supplement. However, the lack of data to support that recommendation means that the picture for patients is "confusing."
Dr Ryan also believes that individual patients are likely to respond differently to different levels of fiber intake: "Maybe I have colorectal cancer and I increase by 5 g/d. How am I going to find out if maybe it should have been 10 g/d for me, because I didn't respond as well to the 5 g/d?"
She added: "There may be a lot of individual variation in how we would respond to increased fiber intake."
