Asthma Affected Differently by Subcutaneous and Visceral Abdominal Fat

Abdominal fat is associated with asthma, and its effects depend on the location of the fat accumulation, according to the results of a study published in Allergy, Asthma & Immunology Research.

Obese individuals have a higher incidence and prevalence of asthma, and the presence of obesity is associated with greater asthma severity and poorer patient response to treatment. Abdominal obesity may be implicated in mechanical alterations in the airways and lung parenchyma, as well as systemic and airway inflammation that can adversely affect lung function and therapeutic response.

Researchers recruited patients with asthma ≥40 years and assessed airways and fat deposits by computed tomography (CT). They measured bronchial wall thickness (WT), lumen diameter (LD), lumen area (LA), wall area (WA), total area (TA), and WA/TA percentage at the apical segmental bronchus in the right upper lobe of the lung. Fat accumulations were determined by measuring visceral (VFA), subcutaneous (SFA), and total (TFA) fat areas (cm²). Elevated visceral to subcutaneous fat area ratio (EV) was defined as VFA/SFA ≥0.4.

A total of 50 patients underwent CT scanning and were included in the analysis (52% women; mean age, 62.9 years). More than half of the patients had a body mass index (BMI) ≥25 kg/m², 12% were current smokers, 14% had atopy, 60% had chronic rhinitis symptoms, and 38% had severe asthma.

The correlation analyses demonstrated significant negative correlations between VFA and bronchial LD and LA (r= –0.35, P =.01; r= –0.34; P =.01, respectively) and significant positive correlations between SFA and bronchial WA and TA (r=0.38, P =.007; r=0.34, P =.01). In contrast, the analyses found no significant correlations for TFA. Although BMI showed significant correlations with LD and LA (r = –0.34; P =.02 for both), emphysema and air-trapping percentage did not show a significant association with abdominal fat area. Additionally, 92% of patients demonstrated EV. These patients were older, had been diagnosed with asthma at an older age, and had smaller airway LDs and thinner airway WTs.

The study authors noted that the inclusion of patients with nonsevere asthma limited the ability to compare the results with studies that include only patients with severe asthma. The study is further limited by the small sample size, the inclusion of current smokers, and the use of only the apical segmental bronchus of the right upper lobe for airway parameters.

Nonetheless, the researchers contended that these results suggest that the relationship between abdominal fat and asthma varies according to the distribution of fat tissues. Whereas visceral fat appears to contribute to bronchial luminal narrowing, subcutaneous fat may be implicated in the thickening of the bronchial wall in patients with asthma.

Reference

Yang MS, Choi S, Choi Y, Jin KN. Association between airway parameters and abdominal fat measured via computed tomography in asthmatic patients. Allergy Asthma Immunol Res. 2018;10(5):503-515.