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A asma, a obesidade e a hormona vitamina D - A hipótese do sol

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A conhecida ā€œhipótese da higieneā€, segundo a qual a redução das doenƧas infecciosas leva ao aumento das doenƧas atópicas, foi explicada pelo desequilĆ­brio da actividade das linhas TH1 e TH2 a favor da Ćŗltima, uma vez que a activação da primeira se relaciona com a defesa Ć  infecção e a auto-imunidade, e a segunda com doenƧas atópicas. Mas o aumento simultĆ¢neo da prevalĆŖncia de doenƧas auto-imunes e atópicas obrigou a encontrar outra explicação, admitindo-se a existĆŖncia de cĆ©lulas T reguladoras (Tregs) da actividade das duas linhas, activadas por componentes infecciosos como enterotoxinas. Mas as Tregs tĆŖm receptores para a vitamina D, capazes de serem estimulados. E como se tem vindo a demonstrar uma carĆŖncia planetĆ”ria em vitamina D, pode aventar-se outra hipótese para explicar a maior prevalĆŖncia das doenƧas atópicas e auto-imunes, a hipótese do sol. Simultaneamente, a carĆŖncia de sol (isto Ć©, vitamina D) pode explicar o aumento e /ou gravidade de outras patologias, pois hĆ” receptores para a vitamina D em variadas cĆ©lulas, que nĆ£o enterócitos, cĆ©lulas tubulares renais e osteoblastos, e de correlaƧƵes patológicas, como asma e obesidade. ABSTRACT The hygiene hypothesis was developed to explain the increased prevalence of atopic diseases in the last decades, specialy in industrialized/developed countries. The central core of the hypothesis is that reduction in exposure to infectious disease and microbial products (through the introduction of vaccines, antibiotics, and improved sanitation) has resulted in deficient immune regulation resulting in hypersensitivity in both the T-helper type 1 (Th1) and T-helper type 2 (Th2) compartments (The Th1 pathway is characterized by the cell mediated proinflamatory responses and is also involved in certain autoimmune diseases; the Th2 pathway is envolved in production of immunoglobulin E, eosinophilia, atopy and airway hyperresponsiveness). However, the simultaneous increasing prevalence of autoimmune diseases led to the review of the scientific support of the hypothesis, introducing the role of Tregulatory cells (Tregs), up regulated by infectious components like enterotoxins. This up regulation can also be effectuated by vitamin D, as Tregs have receptors for this vitamin. Since now we assist to an epidemic deficiency of vitamin D, a new theory has been presented to explain the increased prevalence of atopic and autoimmune diseases: the lack of sunshine or vitamin D deficiency - the sunshine hypothesis. This hypothesis may contribute to explain the rising prevalence or the severity of other conditions, as there are now evidence of vitamin D receptors in many cells, besides enterocytes namely the kidney tubular cells and osteoblasts, and the association with obesity and asthma.

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Keywords

Vitamina D células Tregs doenças atópicas doenças auto-imunes obesidade asma Vitamin D Tregs cells atopic diseases autoimunne diseases obesity asthma

Citation

Nascer e Crescer 2008; 17(3): 125-128

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Nascer e Crescer

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