Asthma has increased in prevalence over the last few decades. Several factors have been suggested to play a role in the increase of asthma prevalence and encompass environmental and lifestyle factors such as changes in dietary habits, physical activity, as well as the emergence of the obesity epidemic.
A growing body of evidence has suggested that obesity is likely a sizeable risk factor, however, the mechanisms that link this characteristic to the development of the disease are clear.
With regards to diet and exercise, the literature also remains inconclusive, however, the investigation of the nutritional impact holistically – in the context of physical activity, body composition, and overall diet – is relevant in understanding the underlying mechanisms as well as designing effective public health interventions.
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What is asthma?
Asthma is a chronic inflammatory disease of the Airways. It affects approximately 330 million people worldwide. In a list of 359 diseases, asthma has been classified as the 30th and 32nd cause of disability-adjusted life years (DALY) in females and males, respectively as well as being the most common chronic condition in children.
DALY reflects the sum of the years of life lost due to premature mortality (YLLs) and the years lived with a disability (YLDs) due to prevalent cases of the disease or health condition in a population.
There is a large societal and individual version of asthma, which is becoming increasingly problematic as the prevalence of the disease increases with major consequences and health status, economic burden, and quality of life. Investigating the role of modifiable lifestyle factors such as diet is necessary for both primary and secondary prevention of asthma.
What is contributing to the increased incidence of asthma?
Twentieth-century changes in dietary habits, namely increased consumption of ready-to-eat meals on decreased intake of fruits and vegetables, decreased physical activity, and the obesity epidemic is believed to be causes of asthma prevalence and uncontrolled asthma worldwide.
Several definitions and approaches have been used in observational studies of asthma and its control by way of modifiable nutritional factors. This makes comparisons of studies a challenge, and therefore the results of studies examining the individual impact of diet and physical activity remain inconclusive
The role of nutritional factors in asthma and asthma control
Nutritional factors are highly variable and will change with an individual's life, therefore diet may influence asthma in different ways and by different mechanisms depending on how long an individual is exposed to a specific nutritional factor. It is therefore essential to distinguish between the prenatal period, childhood, and adulthood when literature outcomes are compared.
The majority of the research investigating the role of nutritional factors in the development of asthma and asthma control has focused on single dietary variables while considering other nutritional factors as confounding variables when available.
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Diet and asthma
During the prenatal period, maternal intake of vitamin E and D, fruits, vegetables, zinc, omega-3 polyunsaturated fatty acids, or an overall healthy diet, during pregnancy are shown to produce a protective effect. This protective effect is related to decreased risk of wheezing in the first years of life, however evidence for a lower risk of the development of asthma, or even weaving later in childhood, is lacking.
Very few studies have investigated the role and diet during childhood, although these studies are limited as they are not all longitudinal. Overall, the association between childhood asthma and diet is moderate; observational studies suggest a protective effect for fruits, vegetables, and fish intake.
In more recent studies, including one longitudinal study, findings suggest a deleterious effect of high fructose and sugar-sweetened beverages on the development of asthma. Moreover, a longitudinal study has suggested a deleterious effect of a western diet, characterized by a high intake of sugar-containing beverages, meat, refined grains, and snacks.
With regards to asthma and diet in adults, there are very few longitudinal studies that have investigated the correlation between the two. Like with childhood asthma, the association is moderate, with observation and studies reporting a protective association for fruits and vegetables as well as vitamin E.
Recent studies have shown evidence for a role of diet at the nutritional level, citing a high fiber intake as being protective. At the food level, there is a protective association associated with healthy diets, where is a deleterious role of highly processed meat intake.
Among the few interventional studies that have considered several factors in the development of asthma, one randomized control trial which was conducted in 330 obese adults with uncontrolled asthma through a 12-month intervention of calorie reduction and moderate-intensity physical activity did not report any benefits with regards to the control of asthma.
Contrastingly, however, another randomized controlled trial conducted in 125 non-obese patients with asthma who were prescribed a high protein, though by scenic index, on team inflammatory diet as well as high-intensity interval training over 8 weeks reported improved asthma control and asked more related quality of life.
Interventional studies, however, are limited by their relatively short follow-up that does not enable any long-term inferences to be made.
Mediterranean diet and asthma control
Asthma control implies fewer or no symptoms and exacerbations and elimination of risk factors for future poor outcomes. The Mediterranean diet has been shown to influence asthma control and evidence for this has been documented in several studies.
In particular, several cross-sectional studies in children support a beneficial effect of the Mediterranean diet on the reduction in asthma symptoms. However, some other studies have shown no significant association. Indeed, one case-control study demonstrated that vegetable consumption was inversely associated with allergic asthma as well as moderate too sensitive away hypersensitivity.
Nonetheless, other studies have supported the role of fruit intake at an early age as well as the role of long-term fruit consumption in the prevention of fewer asthma symptoms. A prospective study found the adherence to a Mediterranean diet was correlated with the limitation of asthma attacks and the need for short-acting beta 2 agonists and inhaled corticosteroids. Moreover, in pediatric patients, supplementation with vegetables, fruit, as well as fish oil supplements well positively correlated with asthma control.
In adults, some studies have similarly associated the Mediterranean diet with improvement in asthma control; however, two randomized control trials demonstrated no changes in asthma control and inflammatory markers or asthma outcomes with regards to uptake of Mediterranean dietary patterns.
Overall, there is a complex interrelation between nutritional factors and asthma and its control. This comment combined with the relatively few studies available that assess the role of diet and asthma results in several methodological limitations on challenges that need to be addressed in future analysis and comparison of results.
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