Association between serum 25-hydroxyvitamin D concentration and symptoms of respiratory tract infection in a Norwegian population: the Tromsø Study

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HOT TOPIC - Public health nutrition aspects of vitamin D

Symptoms of respiratory tract infection (RTI) are a frequent reason for visiting a doctor^(1,2), although most people with symptoms of a common cold do not seek health care⁽³⁾. RTI represent a major economic burden because of the need for hospitalization in severe cases and because of disruption of work caused by RTI symptoms^(4,5).

Vitamin D plays an important role in bone mineralization and skeletal health⁽⁶⁾. Vitamin D is formed in the skin from 7-dehydrocholesterol after activation by UV-B radiation (290-315 nm)⁽⁷⁾. Dietary products are the most important source of vitamin D at northern latitudes during the wintertime because sunlight exposure during this time is inadequate for inducing sufficient endogenous production of vitamin D^(8,9). Foods that contribute the most to dietary intake of vitamin D vary between countries and habitual dietary patterns. In Norway, fish and cod-liver oil are the predominant food sources^(9,10)and intake of fresh cod liver is important in northern Norway⁽¹¹⁾. The presence of the vitamin D receptor and enzymes needed to hydroxylate 25-hydroxyvitamin D (25(OH)D) to its active form 1,25-dihydroxyvitamin D is found in the intestines and in tissues throughout the body⁽¹²⁾; this widespread distribution suggests that vitamin D is important for more than skeletal health. Low circulating levels of 25(OH)D have been associated with a number of extraskeletal diseases such as cancer, diabetes and CVD^(13-16), and 25(OH)D plays a key role in immunity⁽¹⁷⁾.

Previous studies have reported associations between low serum 25(OH)D concentration and increased occurrence of RTI^(2,18,19). However, other studies performed in selected populations have not found a significant association^(20,21). The evidence for a favourable effect of vitamin D on resistance to RTI is inconsistent and is not based on randomized controlled trials⁽¹⁷⁾. The main aim of the present study was to determine...