NEW YORK - Adding extra daily servings of fruits and vegetables didn't improve lung function or other markers of lung health in people with chronic obstructive pulmonary disease (COPD) in a new study from Northern Ireland.
The lung disease, which includes emphysema and chronic bronchitis, is one of the most common causes of death in the US, with a yearly death toll of more than 100,000.
Previous research has shown that people who eat lots of fruits and veggies may have less severe lung symptoms, or be less likely to die from COPD -- possibly because of the antioxidants in that food group and its anti-inflammatory effects, the theory goes.
But those studies were observational, meaning that scientists compared people who were already downing fruits and vegetables to those who avoided them. That kind of research doesn't prove that nutritional habits, themselves, are driving lung changes, because scientists can't take into account every other health and lifestyle variable that affects breathing and airway function.
"You can't control for everything that might confound or bias the results," said Dr. Don Sin, a respiratory specialist at the University of British Columbia in Vancouver who wasn't involved in the new research.
"People taking a lot of antioxidants may also exercise a lot more than people who don't eat a lot of fruits and vegetables. Maybe they smoke less, maybe they are more compliant with their medications, and the list goes on," he told Reuters Health.
For the new study, researchers tried to get around that hurdle by recruiting 75 people with COPD who all skimped on fruits and veggies and randomly assigned half of them to up their consumption from less than two servings per day to at least five. The rest of the patients were instructed not to eat more than two daily portions.
Participants in both groups had fruits and vegetables delivered to their home each week for 12 weeks, and were also advised on storing and cooking them.
On average, people assigned to a diet high in fruit and vegetables increased their consumption to more than six servings per day, compared to just under two in the group not prescribed diet changes.
That improvement was impressive on its own, researchers said, but it didn't lead to any improvement in lung function or measures of inflammation in the airway.
Thirty-five patients saw their COPD worsen during the study, including six who needed to be hospitalized. People in both diet groups were equally likely to have worsening symptoms, according to findings published in the European Respiratory Journal.
Michelle McKinley from Queen's University Belfast and her colleagues said it's possible that a fruit and vegetable boost that lasted longer than just a few months would lead to improvements in lung function -- or that a broader dietary change might be necessary.
Another explanation for the lack of airway benefit is that patients' disease might have been too far along for diet changes to make a difference, said Irfan Rahman, who studies COPD at the University of Rochester Medical Center in New York and wasn't involved in the research.
All patients in McKinley's study had moderate or severe lung disease and were in their early 60s, on average.
"This is too late to give fruits and vegetables to these patients," Rahman told Reuters Health.
He added that younger people who have smoked but don't have emphysema or chronic bronchitis should up the amount of fruits and vegetables in their diet -- especially green, leafy vegetables. They can also work out their lungs by getting aerobic exercise, such as brisk walking.
And for those who are still smoking, Rahman said, "the first message is: quit."