Diet High in Fruit Fiber and Flavonoids May Prevent Chronic Productive Cough

Yael Waknine

August 03, 2004

Aug. 3, 2004 -- A diet high in fruit fiber and soy products may reduce the incidence of respiratory symptoms such as productive cough, according to results of a prospective cohort study published in the August issue of the American Journal of Respiratory and Critical Care Medicine.

"Cough and phlegm are frequently associated with chronic obstructive pulmonary disease, which may be caused by oxidative stress-mediated inflammation and tissue damage in the lung," write lead author Lesley M. Butler, MD, and colleagues, from the Epidemiology Branch at the National Institutes of Health in Research Triangle Park, North Carolina. "Fruits and vegetables are the major food sources of antioxidants that may protect the lung from oxidative stress. Fruit intake...[has] been associated with higher lung function and reduced symptoms of cough with phlegm."

Using questionnaires, investigators followed a cohort of 49,140 men and women aged 45 to 74 years from the Singapore Chinese Health Study. During a mean period of more than five years, 571 incident cases of cough with phlegm were identified as initiating after baseline interview. Odds ratios were determined by comparing highest and lowest quartiles after adjusting for age, sex, dialect group, total energy intake, and smoking.

Statistical analysis showed daily intake of nonstarch polysaccharides to be inversely associated with incidence of chronic productive cough (odds ratio [OR], 0.61; 95% confidence interval [CI], 0.47 - 0.78 for the highest vs. lowest quartiles; P < .001). The main sources of these polysaccharides included fruit (34.3%), grain products (24.6%), and vegetables (22.2%), of which only fruit intake was likewise inversely related to case incidence (OR, 0.67; 95% CI, 0.52 - 0.87; P = .006).

"Given the association with total fruit consumption, we examined associations with individual fruits," the authors note. "We observed the strongest inverse dose-response associations for apples, pears, and grapes."

An inverse relationship was also found between soy isoflavone intake and chronic productive cough (OR, 0.67; 95% CI, 0.53 - 0.86; P = .001). Odds ratios were similar in the individual isoflavones genistein (OR, 0.68; 95% CI, 0.53 - 0.87), daidzein (OR, 0.67; 95% CI, 0.53 - 0.86), and glycetein (OR, 0.68; 95% CI, 0.54 - 0.87).

"Our findings suggest that flavonoids may play a crucial role in the relation between fruit and soyfood intake and development of chronic cough with phlegm," the authors comment. "Flavonoids may protect the lung on the basis of their antioxidant and anti-inflammatory properties." Citing smoking prevention as generally being the most important method in preventing chronic respiratory symptoms and obstructive pulmonary disease, the authors note that, "[A] diet high in sources of fiber and flavonoids, such as fruit and soy, may be an important contribution to primary prevention strategies for chronic respiratory symptoms, both in smokers and non-smokers."

This study was supported by the National Institutes of Health.

Am J Respir Crit Care Med. 2004;170:279-287

Reviewed by Gary D. Vogin, MD

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