Key Point:

A diet high in fiber may reduce the incidence of cough with phlegm.

SINGAPORE—A diet high in fiber may do more than help foster a healthy digestive system. New research suggests that it may reduce the cough and phlegm associated with respiratory disease.¹

Lesley M. Butler, PhD, and colleagues examined fiber intake and cough with phlegm incidence in an ongoing prospective cohort study of 49,140 Singaporean Chinese ages 45 to 74, with an average of five years of follow-up. “Promoting a diet high in fiber and flavonoids, such as [from] fruit and soy, may be an important contribution to the prevention of chronic respiratory symptoms, both in smokers and nonsmokers,” asserted Dr. Butler. At the time the research was conducted, Dr. Butler was affiliated with the Epidemiology Branch of the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina. She is now an Assistant Professor of Epidemiology at the University of California in Davis.

FRUIT AND SOY: MOST BENEFICIAL FOODS

The researchers identified 571 incident cases of cough with phlegm. Nonstarch polysaccharides, major components of dietary fiber, were inversely associated with cough plus phlegm: The odds ratio (OR) for the symptoms was 0.61 (95% CI, 0.47 to 0.78) in the highest quartile compared with the lowest quartile of intake. Fruit, grain, and vegetables were the greatest food sources of nonstarch polysaccharides, composing 34.3%, 24.6%, and 22.2% of intake, respectively.

When specific foods were analyzed, soy products were found to protect against cough with phlegm (OR, 0.69; 95% CI, 0.53 to 0.90). The soy isoflavones genistein, daidzein, and glycitein had almost identical ORs and thus seemed to confer similar amounts of protection. Total fruit intake was also inversely associated with cough and phlegm (OR, 0.67; 95% CI, 0.52 to 0.87). When individual fruits were assessed, the strongest associations were seen with apples, pears, and grapes—fruits that have high amounts of flavonoids. Only the associations with apples and pears remained statistically significant after adjusting for nonstarch polysaccharide intake. When individual vitamins and antioxidants were analyzed, there were no associations found between reduced cough and phlegm and carotenoids or vitamin E; and the correlation between reduced risk and vitamin C disappeared after adjusting for nonstarch polysaccharide intake.

Although there was an inverse association between fruit intake and cough with phlegm risk, there was no association found for grain or vegetable intake. Dr. Butler acknowledged that this could be a false-negative finding. Alternately, she suggested that the fiber from fruit could be more beneficial than fiber from vegetables and grains in reducing cough with phlegm. Yet another possible explanation is that there may be other components in fruit, such as flavonoids, that provide protection. “In other words,” she said, “fiber [may be] a marker for other important nutrients in fruit.”

FIBER ALSO BENEFITS SMOKERS

The high fiber diet appears to be protective, regardless of smoking history. Ever smokers had ORs of 0.45 (95% CI, 0.29 to 0.70), comparing fourth with first quartile, and never smokers had ORs of 0.70 (95% CI, 0.50 to 0.97), comparing fourth with first quartile.

The protective effect of the high fiber diet may come from its “antioxidant, anti-inflammatory, and blood glucose reducing properties,” suggested Dr. Butler. Although clinicians could counsel patients to eat more high fiber foods, Dr. Butler emphasized that counseling patients against smoking in the first place, or counseling smokers to quit, are the most important things that clinicians can do to prevent or mitigate respiratory symptoms.

—Tamara Gibb

Reference
1. Butler LM, Koh W-P, Lee H-P, et al. Dietary fiber and reduced cough with phlegm. Am J Respir Crit Care Med. 2004;170:279-287.

Return to table of contents