Vitamin C may affect lung infections.

Abstract

Vitamin C was identified in the early 1900s in the search for a deficient substance responsible for scurvy, which was a serious disease of sailors in the Age of Sail.1,2 In the early literature, scurvy was directly linked to pneumonia. The American paediatrician Alfred Hess carried out extensive studies on scurvy and summarized autopsy findings as follows: ‘pneumonia, lobular or lobar, is one of the most frequent complications [of scurvy] and causes of death’ and ‘secondary pneumonias, usually broncho-pneumonic in type, are of common occurrence, and in many [scurvy] epidemics constitute the prevailing cause of death’.3 Starting in the 1930s, some German and US physicians proposed that vitamin C would be beneficial in the treatment of pneumonia.4 Although the burden of pneumonia has decreased dramatically in developed countries during the past century, lung infections are still a leading cause of mortality and morbidity globally.5 Therefore, the question of whether vitamin C might affect pneumonia is an issue worthy of systematic consideration. The best known physiological function of vitamin C is its participation in collagen hydroxylation, and the symptoms of scurvy are largely explained by changes in collagen metabolism. The biochemistry of vitamin C is, however, much more complex. For example, it also participates in the enzymatic synthesis of dopamine, carnitine and a number of neuroendocrine peptides.2,6,7 In the immune system, the major role of vitamin C seems to be as a physiological antioxidant, protecting host cells against oxidative stress caused by infections. Its concentration in phagocytes and lymphocytes is very high compared with the level in plasma, indicating that vitamin C may have functional roles in these immune system cells. In various experimental settings, vitamin C increased the functioning of phagocytes, the proliferation of T-lymphocytes and the production of interferon, and decreased the replication of viruses.2 According to animal studies, vitamin C increases resistance to various viral and bacterial infections. Moreover, many infections, including pneumonia, lead to reduced vitamin C levels in plasma, leukocytes and urine.2 Because of these changes in metabolism, vitamin C might have a therapeutic effect on pneumonia patients. Thus there is a biological rationale to examine the effect of vitamin C on infections in humans. We carried out a systematic review, published in the Cochrane Library, in which we examined the role of vitamin C supplementation on preventing and treating pneumonia.4 We identified three controlled trials reporting on the preventive effect and two on the therapeutic effect of vitamin C against pneumonia.8-12 Each of the trials found a statistically significant benefit of vitamin C supplementation on at least one clinically relevant outcome. In the therapeutic trials, vitamin C was tested over and above the normal medication, which included antibiotics.11,12 Two of the five trials were randomized, placebo-controlled double-blind trials.10,12 The other three trials were technically deficient to varying degrees, but their short-comings do not cause biases that would give evident explanations to the reported differences in favour of the vitamin C groups.4