Diet as a risk factor

Even in developed societies, populations and individuals are exposed to two main potential nutritional hazards: over-nutrition and under-nutrition. In 2013, a large meta-analysis of data on healthy cohorts, which included 2.88 million subjects, showed that a body mass index (BMI) of up to 30 kg·m -2 – the boundary between ‘overweight’ and ‘obese’ – could have a protective effect against death. Furthermore, in a large population-based cohort of more than 1 million South Koreans aged 30–95 years, who were tracked for 12 years, over-nutrition had clear protective effects against death from respiratory causes in nonsmoking women and in both smoking and nonsmoking men. Diet surveys and, more recently, dietary pattern analysis have shown that in addition to abnormal overall calorie intake, the intake of individual nutrients and certain dietary behaviours can have protective or harmful effects. This approach has been used to investigate the role of diet in several chronic diseases, but so far there have been few studies relating to respiratory diseases. Study of dietary patterns may provide insight into combinations of foods and/or nutrients that have a positive or negative impact on the prevalence or severity of respiratory diseases. It should, however, be understood that the existence of an epidemiological link between a risk factor and a respiratory phenotype does not necessarily mean that interventions aimed at modifying diet are justified: large prospective and randomised controlled studies must be carried out first (table 1).

Conditions Epidemiological study data Effectiveness of intervention,
primary prevention #
Effectiveness of intervention,
secondary prevention #
Available
recommendations
Allergy and asthma Risk factors: low level of antioxidants, low vitamin D status, high ratio of omega-6 to omega-3 polyunsaturated fatty acids
Protective factors:
Mediterranean dietary
pattern during childhood (but conflicting results in adults)
NA NA No specific recommendations
COPD and chronic respiratory failure Risk factors: related to
‘Western’, ‘traditional’ dietary patterns, cured meat
Protective factors: fruit,
vegetables and fibre
NA Of three RCTs, one was positive for slowing the decline in FEV 1 A meta-analysis (2012) showed a positive impact for oral supplementation on body composition and functioning in under-nourished patients with COPD WHO recommendations on
chronic diseases (2009)
ATS/ERS guideline on
pulmonary rehabilitation
(updated 2013)
Lung cancer Protective factors: fruit and vegetables   Negative results for nutritional
support
No specific recommendations
Venous
thromboembolism
Risk factor: high BMI NA NA Obesity prevention in general
population
Lung infections Risk factors: low vitamin D status, under-nutrition   RCTs positive for vitamin D
intake
General recommendations
for vitamin D intake
Cystic fibrosis NA NA Positive cohort studies but no RCTs Specific recommendations
for nutritional support to
prevent under-nutrition
Table 1 – Dietary risk factors, protective factors and interventions in respiratory diseases. COPD: chronic obstructive pulmonary disease; NA: not available; RCT: randomised controlled trial; FEV1: forced expiratory volume in 1 second; WHO: World Health Organization; ATS: American Thoracic Society; ERS: European Respiratory Society; BMI: body mass index. # : primary prevention is the prevention of a disease in healthy people; secondary prevention aims to halt or slow worsening of a disease in people who already have the disease.

Obstructive lung diseases

Asthma

In asthma, both dietary exposures themselves (food, nutrients and dietary patterns) and the periods of exposure (antenatal, childhood, adulthood) are relevant to the pathogenesis and progress of the disease.

Dietary antioxidant intake has been associated with asthma throughout life. Cohort studies covering the antenatal period have suggested a link between childhood asthma and reduced maternal consumption of vitamin E, zinc, fruit and vegetables; however, randomised controlled trials (RCTs) are lacking. A recent meta-analysis of 62 studies on the effect of childhood food and nutrient intake on the risk of developing wheezing or asthma concluded that there was some evidence (although weak) of protective effects from vitamins A, D, and E, zinc, fruit and vegetables, and of a Mediterranean diet, against the development of asthma. In adults, an association between asthma and lower intake of antioxidants has been reported but data from RCTs do not support supplementation with vitamin C or selenium.

During the antenatal period, and throughout life, observational studies have reported associations between asthma and a high intake of omega-6 polyunsaturated fatty acids, as well as a decreased intake of omega-3 polyunsaturated fatty acids. However, RCTs are either lacking or show minimal or no benefit for dietary intervention in adults.

Birth cohort studies to examine the association between maternal dietary intake of vitamin D during pregnancy and wheezing and asthma have yielded conflicting results. Studies on children already diagnosed with asthma have shown an association between low levels of vitamin D and asthma exacerbations. These studies have also shown that in patients with mild or moderate persistent asthma treated with inhaled corticosteroids, vitamin D sufficiency is associated with better lung function. These results suggest that vitamin D levels should be monitored in asthmatic children. A longitudinal study conducted in children in Australia reported that low vitamin D levels measured at 6 and 14 years of age were predictive of allergy phenotypes and bronchial hyperresponsiveness, but not of asthma, at those ages. However, low vitamin D levels at 6 years of age were predictive of later asthma and atopy phenotypes at 14 years of age. One RCT using data from the Northern Finland Birth Cohort 1966 reported that high-dose vitamin D supplementation in infancy was associated with an increased risk of atopy, allergic rhinitis, and asthma later in life (at age 31 years). Few studies have been conducted in adults, but a recent US study reported a strong protective effect of vitamin D against wheezing and exacerbations of asthma, supporting the notion that vitamin D status might influence the risk of respiratory disease.

Studies do not support an association between dietary pattern during pregnancy and asthma or wheezing in the child. Although research suggests an association between children’s dietary patterns and asthma and wheezing, all such studies have been cross-sectional and therefore, in theory, prone to incorrect attribution of cause and effect. In adults, the overall results are conflicting: cross-sectional studies suggest that certain dietary patterns are associated with asthma, but these findings have not been confirmed in longitudinal surveys. Few studies relating asthma severity and control  to diet have been performed, but ‘Western’-type dietary patterns ( e.g . refined grains, cured and red meats, desserts, French fries) are reported to be associated with a higher frequency of asthma attacks and with poor asthma control.

Chronic obstructive pulmonary disease

The main risk factor for COPD in the developed world is cigarette smoking, but up to one-third of patients with COPD (especially in developing countries) have never smoked, implying that other factors are also important. Diet is probably one such factor, but data on a diet–COPD association remain scarce.

Over the past decade, there has been growing research into dietary factors with antioxidant or anti-inflammatory properties that might affect lung function or COPD symptoms. Most of these epidemiological studies have been cross-sectional, but a few longitudinal studies have reported associations between specific dietary factors and a slower decline in lung function. In large prospective epidemiological studies in the USA, it was reported that a high intake of a ‘prudent’ dietary pattern ( e.g . fruit, vegetables, fish, whole-grain products) decreased the risk of newly diagnosed COPD, whereas a high intake of a Western-type pattern increased risk. More recently, it has been reported that a ‘traditional’ dietary pattern (with high intake of red meat, processed meat, boiled vegetables, added fat, coffee, beer, and potatoes, but reduced consumption of soy products, low-fat dairy products, tea, breakfast cereal, brown rice, pizza, juice and fruit) was associated with reduced lung function and a higher prevalence of COPD. A high intake of refined foods was associated with an accelerated decline in lung function over 5 years.

In relation to specific foods, special attention has been paid to fibre intake. It has been reported among several groups that dietary fibre intake has independent inverse associations with the incidence and symptoms of COPD and with decline in lung function. In addition to foods and nutrients with potential beneficial effects, several studies have focused on those with potential harmful effects. Two studies have reported associations between frequent or high consumption of cured meats and the risk of developing COPD. A recent study has extended this association to include the evolution of the disease, revealing that high cured meat consumption is linked to a higher risk of readmission to hospital with COPD.

Nutritional factors are also important in relation to mortality from COPD, with reports from diverse clinical settings showing that a low BMI is a major predictor of mortality among patients with COPD. The poor prognosis for COPD patients with a low BMI has been confirmed by a study showing that mortality is higher among COPD patients with recent weight loss.

Other respiratory diseases

Lung cancer

Lung cancer is the leading cause of cancer-related death worldwide in men and the second-commonest cause in women. Evidence from several observational, retrospective and prospective studies strongly suggests that high consumption of fruit or vegetables, or both, reduces the risk of lung cancer by approximately 20–30%, with a similar magnitude of reduction for current smokers, ex-smokers and never-smokers.

However, vitamin supplementation has not been shown to decrease lung cancer risk. In fact, the results of two major primary randomised prevention trials of vitamin supplementation showed a higher lung cancer incidence in the group receiving high doses of beta-carotene. A re-analysis of data from the beta-Carotene and Retinol Efficacy Trial (CARET) showed that a high intake of fruit and vegetables decreased the risk of lung cancer in the placebo arm after 12 years of follow-up. Similarly, in the Alpha-Tocopherol Beta-Carotene (ATBC) trial, after 14 years of follow-up, higher dietary intake and serum levels of carotenoids, including carotene, were related to a lower risk of lung cancer. These findings suggest that other potentially protective dietary factors associated with fruit and vegetable intake are playing a part.

Venous thromboembolism

In a prospective cohort study of 87 226 female nurses, the risk of new cases of pulmonary embolism was nearly six-fold higher among those with a BMI of ≥35 kg·m -2 . The risk was present in multiple subgroups and increased in linear fashion with BMI.

Respiratory infections

Although the respiratory tract is only the third-commonest site of infection in the body, it is the commonest site of fatal infections, which often represent the ‘final common pathway’ complicating the many effects of under-nutrition. For example, deaths among the malnourished are frequently due to pneumonia. The D vitamins have a demonstrated protective effect in preventing lung infections.

See the entire Diet and nutrition Chapter