Could Diet Effect Be Related to Overweight?

As atopy, asthma, and obesity increased in the last decades, it was reasonable to speculate that maybe they are linked. When evaluating the effect of the Mediterranean diet, it was reported that obesity was a risk factor for current severe asthma in girls. A practical measurement of total body fat is the estimation of body mass index (BMI)–weight/height ratio.
However, controversies about the relationship of BMI with the presence of atopy and asthma is shown by a report from Australia, which states that increased BMI was a risk factor for cough, ever wheezing and atopy (predominantly in girls), but not for diagnosed asthma or bronchial hyper-responsiveness. Without these last
two conditions, it is difficult to be conclusive, as gastro-esophageal reflux, sleep disorders, being unfit, or altered mechanical ventilation could explain symptoms, and all are associated with overweight.

So some meta-analysis was required to elucidate the real impact of overweight in the incidence of asthma, and 1 with a sample larger than 300,000, evidenced a dose–response increasing odds for incident asthma: odds ratio (OR) 1.38 for normal versus overweight comparison, and OR 1.92 for obesity; none of them was
affected by sex. These odds have a huge impact on populations like the USA, where more than 60% of adults are overweight/obese, and in consequence at risk of developing asthma.

Also considering a meta-analysis in children, the same evidence was reported. The relative risk (RR) of high birth weight on developing asthma later was RR = 1.2 (95% confidence interval (CI) 1.1–1.3), while the effect of overweight in middle childhood was RR = 1.5 (95% CI 1.2–1.8) [31]. Misclassification, diagnostic
bias, and individual confounders are always doubts emerging from meta-analysis; however, the results from an enormous cohort study, from childhood to adulthood, are the only possibility to corroborate or contradict this evidence.

Let us analyze the risk factors that could help to explain these phenomena:

Sex
In childhood, male sex has been considered to be a risk factor for having atopic diseases and asthma. Some years ago, this predominance was partially explained by an increased sensitivity to inhalant allergens.
However, we mentioned earlier that the increasing prevalence among girls equalized the male to female ratio
recently, even being more prevalent when considering eczema.
By the age of 11, male sex is still stronger when considering current wheezing. As the age of the sample evaluated increases, the predominance reverses. In a cohort evaluation, male in childhood declined by adolescence and early adulthood, considering female sex as one of the major risk factors for having asthma. It was also a predictive factor for persistence of asthma symptoms from childhood, but this conclusion needs to be reinforced in larger populations because the odds obtained revealed evidence of a wide confidence
interval.
Not only the former but also allergic rhinitis shows similar transition from male in childhood to female in adolescence. Having those repeated observations reinforced by evaluations in large population samples, the fact that estrogen has pro-inflammatory and testosterone anti-inflammatory effects could explain this trend.

Diet
Recently, Garcia-Marcos  evaluated the relationship of the Mediterranean diet (vegetables, pulses, cereals, potatoes, pasta, and rice) with asthma and rhinoconjunctivitis in more than 20,000 children, adjusting for exercise and obesity, finding its protective effect against current severe asthma in girls. Also, seafood and
fruit were protective against having rhinoconjunctivitis.
In the same direction, Wickens  corroborated that fast food intake was related with asthma symptoms in a frequency-dependent manner. Takeaway consumption greater than once a week showed an increased (although not significant) bronchial hyper-responsiveness, but had no effect on atopy.

Not only animal fat consumption was implicated as a risk factor for atopic diseases expressions. Vegetable oils contain linoleic acid, an Omega 6 polyunsaturated fatty acid (PUFA) precursor of arachidonic acid and consequently of eicosanoid metabolites, promoting the Th2 imbalance while decreasing interferong (IFNg); omega 3 PUFA found in fish oil inhibits PGE2 formation, modulating the production of immunoglobulin E (IgE) indirectly. However, the clinical relevance of adding fish oil in pregnancy diet demonstrated just a decrease in the severity of eczema in infants at high risk of atopy.
Feeding habits in the UK over the last decades, where atopic expressions grew, evidenced diminished saturated fat consumption. This growing could then be attributed to a reduction of antioxidants in the diet, since only the fatty acids deregulations could oversimplify the frame. Anyway, more studies are needed in
this field as interventional strategies have been disappointing as of date.

What Is the Scenario of the Rest of Atopic Diseases?

Other than analyzing asthma, a European study (SCARPOL) that was conducted four times between 1992 and 2001, revealed evidence of stabilizing asthma and hay fever, but with a predominant increase in atopic eczema in girls that was stable in boys.
The same tendency was found in the Aberdeen evaluation when considered up to 2004. There, the three atopic illnesses demonstrated a stable prevalence that was a pattern in the past 10 years, with a continuous increase present in girls that makes no sex difference at the end. As in the former study, when evaluating
eczema, females were more prevalent.
However, an Italian evaluation demonstrated an increasing trend from 1994 to 2002 in wheezing, allergic rhinoconjunctivitis and atopic eczema in both 6- to 7-year-old and 13- to 14-year-old populations, except for wheezing in the last group.
A global time trend analysis of prevalence in rhinoconjunctivitis symptoms evidenced yet again a smooth increase, being more evident in LMIC and in the older age group, suggesting that environmental influences in the development of allergy may not be limited to early childhood.
Related to these asseverations, a recent evaluation in the tendency of aeroallergen sensitization for 25 years (from 1976–1977 to 1999–2001) evidenced a significant increase in the prevalence of sensitivity as well as in the mean age of allergic patients. Again, ISAAC is the option to have a global vision. A recent publication of a worldwide comparison of two phases in 6- to 7-year-old and 13- to 14-year-old populations, using the same methodology both times with a mean of 7 years of difference, allowed to evidence several projections of concern: (a) In 6- to 7-yearold, an incremental tendency in asthma, rhinoconjunctivitis, and eczema was observed in Asia-Pacific, India, North America, Eastern Mediterranean, and Western Europe.  In 13- to 14-year-old, this augmentation was evidenced in Africa, Asia-Pacific, India, Latin America, and Northern and Eastern Europe. (c) In asthma at 6- to 7-year-old, more centers reported increase of prevalence, while in the 13- to 14-year-old group, almost equal centers reported up and down tendency.

Those having larger prevalence in the first phase tend to have a decrease in the third phase and vice versa. (d) For allergic rhinoconjunctivitis, most centers at both ages. Changes (delta) and 95% confidence interval in prevalence of wheezing, atopic rhinoconjunctivitis, and atopic eczema in the past 12 months, reported by parents of children 6–7 years of age (left) and by adolescents 13–14 years of age (right) in six areas of Italy.

Is the Prevalence of Allergy Continuously Increasing?
For atopic eczema, the 6- to 7-year-old participants showed increased tendency in average, while in the 13- to 14-year-old samples, such tendency was not that evident. Taking all disorders together, the younger group had an increase from 0.8% to 1%, and the older one from 1.1% to 1.2%.
We can then preliminarily conclude that globally, there is still a growing prevalence of atopic disorders, predominantly in developing regions of the planet.