We all understand that eating too much of the wrong foods – those that are high in energy and low in nutrients, such as fast foods, processed foods and takeaways – causes weight gain and can lead to obesity. These foods are often high in saturated fat, refined carbohydrates (or sugars) and sodium, which increase the risk of developing diabetes, heart disease and some cancers.
But eating poorly has other, somewhat more surprising ramifications. Recently we have come to understand that unhealthy eating patterns can affect our lungs. Switching your diet to one rich in fruit and vegetables could help you breathe easier.
Healthy diets and healthy lungs
Most of the epidemiological evidence linking diet with lung function has focused on chronic obstructive pulmonary disease (COPD). Linked to smoking, COPD causes progressive lung deterioration and asthma.
Several large studies have observed people over time, and found that an unhealthy eating pattern (including refined grains, cured and red meats, desserts and French fries) increases the risk of lung function decline and COPD onset, compared to a healthy eating pattern (including fruit, vegetables, fish and wholegrains).
A recent study followed more than 40,000 men for 13 years, and found a high fruit and vegetable intake was associated with reduced risk of COPD. Current and ex-smokers eating five or more serves a day of fruit and vegetables were 30 to 40% less likely to develop COPD compared to those eating fewer than two serves per day.
A three year study in patients with existing COPD revealed those consuming a high fruit and vegetable diet had an improvement in lung function.
We have tested the effect of a high fruit and vegetable diet in asthma sufferers over three months. We found people consuming seven or more servings of fruit and vegetables per day had a reduced risk of asthma attacks, compared to people who consumed a low fruit and vegetable diet (fewer than three servings per day).
Another intervention study in asthma used a diet originally designed to reduce high blood pressure – the Dietary Approaches to Stop Hypertension (DASH) diet – for six months. One of the DASH dietary goals was to consume seven to 12 servings of fruit and vegetables, as well as two to four servings of low-fat/fat-free dairy products, and limiting daily fat and sodium intake. This led to improvements in asthma control and quality of life.
How do fruit and vegetables improve lung health?
People with respiratory diseases such as COPD and asthma typically suffer from inflamed airways. The airway tissue becomes swollen and hypersensitive, excess mucus is produced and the breathing tubes become damaged, sometimes irreversibly. The resulting narrowing of the airways makes it difficult for air to pass in and out of the lungs.
Failure to breathe freely can very quickly become life threatening. Restricted airflow can also have a debilitating effect on day-to-day activities, causing symptoms such as coughing, wheezing, breathlessness and chest tightness in people with asthma and COPD.
Fruit and vegetables are a rich source of several nutrients, in particular soluble fibre and antioxidants, that have been shown to reduce inflammation in the airways.
Dietary fibre reduces lung inflammation
Dietary fibre exists in soluble and insoluble forms. Soluble fibre is fermented by gut bacteria to produce short chain fatty acids. These can bind to specific receptors on the surface of immune cells, which suppress airway inflammation. We have shown a single dose of soluble fibre activates these receptors and reduces inflammation in human airways within just four hours.
Short chain fatty acids can also inhibit expression of the genes that cause airway inflammation, through a process known as epigenetic modification. So a high soluble-fibre intake has the potential to protect against airway inflammation through both activation of anti-inflammatory immune receptors, and inhibition of genes controlling inflammation.
Antioxidants are also anti-inflammatory
Antioxidants present in fruit and vegetables – such as vitamin C, carotenoids and flavonoids – are also beneficial, as they can protect against the damaging effects of free radicals, which are highly reactive molecules produced by activated inflammatory cells that can damage asthmatic airways. Many observational studies have linked antioxidants with lung health.
However, data from antioxidant supplementation trials in asthma are not convincing. Few studies show a beneficial effect, likely due to the use of individual nutrients. Multiple antioxidants exist together in fruit and vegetables, which have interdependent roles that are likely to be critical for their protective effects. So dietary modifications using whole fruit and vegetables is a better strategy.
Sometimes we can become overwhelmed by the nutrition messages in the media, which tell us to eat this and not eat that. Sometimes the advice seems contradictory and confusing. So here is a very simple and focused message for people with respiratory disease - eat more fruit and vegetables!
There’s really nothing to lose and everything to gain. As well as helping to maintain or achieve a healthy weight and reducing the risk of heart disease, diabetes and cancer, you will also be improving your lung health.
Lisa Wood receives funding from the National Health and Medical Research Council of Australia.
Authors: Lisa Wood, Professor, University of Newcastle