Children with a form of asthma called severe therapy-resistant asthma (STRA) were more likely to have lower blood levels of vitamin compared with moderately asthmatic and non-asthmatic children, according to findings published in the American Journal of Respiratory and Critical Care Medicine.
"This study clearly demonstrates that low levels of vitamin D are associated with poorer lung function, increased use of medication, worse symptoms and an increase in the mass of airway smooth muscle in children with STRA," said lead author Atul Gupta, MD, from the National Heart and Lung Institute (NHLI) at Imperial College and King's College London.
"It is therefore plausible that the link between airway smooth muscle mass and lung function in severe asthma may be partly explained by low levels of vitamin D."
"Our results suggest that detecting vitamin D deficiency in children with STRA, and then treating that deficiency, may help prevent or reduce the structural changes that occur in the airway smooth muscle, which in turn may help reduce asthma-related symptoms and improve overall lung function," added Dr Gupta.
The sunshine vitamin and asthma
This is not the first study to link vitamin D status and asthma risk, but it is reportedly the first to investigate if levels of the sunshine vitamin were associated with SRTA.
SRTA refers to the 5 and 10% of asthmatic children who do not respond to low doses of corticosteroids. These children are known to experience more asthma episodes and asthma-related illnesses, and require more healthcare services, than their treatment-receptive peers.
Dr Gupta and his co-workers note that previous studies have established a connection between levels of vitamin D and the proliferation of airway smooth muscle.
"For our study, we hypothesized that children with STRA would have lower levels of vitamin D than moderate asthmatics, and that lower levels of vitamin D would be associated with worse lung function and changes in the airway muscle tissue,” he said.
Eighty-six children were recruited to participate in the study, including 36 children with STRA, 26 with moderate asthma and 24 non-asthmatic controls. Blood samples were taken from the children and correlated with lung function, medication usage and symptom exacerbations. Tissue samples from the airways of the STRA group were also evaluated to determine if structural changes were observed in the airway's smooth muscle.
Results showed that children with STRA had significantly lower levels of vitamin D, as well as greater numbers of exacerbations, increased use of asthma medications and poorer lung function, compared with the non-SRTA children
"The results of this study suggest that lower levels of vitamin D in children with STRA contribute to an increase in airway smooth muscle mass, which could make breathing more difficult and cause a worsening of asthma symptoms," said Dr Gupta.
"The determination of the exact mechanism between low vitamin D and airway changes that occur in STRA will require intervention studies," he added.
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive.
Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
While our bodies do manufacture vitamin D on exposure to sunshine (UV-B radiation with a wavelength between 290 and 315 nm), the levels in some northern countries are so weak during the winter months that our body makes no vitamin D at all, meaning that dietary supplements and fortified foods are seen by many as the best way to boost intakes of vitamin D.
The effects of deficiency
Vitamin D deficiency in adults is reported to precipitate or exacerbate osteopenia, osteoporosis, muscle weakness, fractures, common cancers, autoimmune diseases, infectious diseases and cardiovascular diseases. There is also some evidence that the vitamin may reduce the incidence of several types of cancer and type-1 diabetes.
Source: American Journal of Respiratory and Critical Care Medicine
Published online ahead of print, doi:10.1164/rccm.201107-1239OC
“Relationship Between Serum Vitamin D, Disease Severity and Airway Remodeling in Children with Asthma”
Authors: A. Gupta, A. Sjoukes, D. Richards, W. Banya, C. Hawrylowicz, A. Bush, S. Saglani