Abstract
Deficiency of vitamin C causes a specific deficiency disease, scurvy, which led to the deaths of numerous seamen in the Age of Sail because they had no access to fruit or vegetables during long voyages. Nowadays scurvy is very rare in the Western countries. Vitamin E is found in such a great variety of foods and is stored in the body for such long periods that deficiency of this vitamin hardly ever results from poor diet.
Since a large number of studies found that vitamins C and E affect the immune system, it has been suggested that these vitamins might affect infectious diseases. Numerous animal studies found that vitamins C and E reduce the incidence and severity of various viral and bacterial infections. These findings justify research into whether these two vitamins affect the incidence and severity of respiratory infections in humans.
The earlier medical literature contains suggestions that vitamin C may be beneficial against respiratory infections, but this topic achieved wider popular interest only after 1970 when Linus Pauling, a dual Nobel Laureate, wrote a book in which he concluded that large doses of vitamin C prevent and alleviate colds. Pauling’s proposal led to a large series of new placebo-controlled trials. It is shown in this thesis that these new trials consistently found that regular vitamin C supplementation shortens the duration and alleviates the symptoms of the common cold, partly confirming Pauling’s hypothesis. The new trials did not find reduction of common cold incidence in the ordinary Western population with vitamin C supplementation. Nevertheless, in this thesis it is shown that there may be sub-populations, such as people undergoing heavy acute physical stress and young males with low dietary vitamin C intakes, in which regular vitamin C supplementation may reduce the incidence of the common cold.
While the trials carried out since 1970 have shown that vitamin C alleviates common cold symptoms, it is strange that major medical textbooks state that this vitamin has no effect on colds. In this thesis it is shown that the most influential reviews on vitamin C and the common cold cited in the major textbooks contain numerous erroneous statements, and that they even present data that are inconsistent with the original study reports. Consequently, the negative statements in the medical textbooks are based on biased reviews. It is also shown that the most influential vitamin C common cold trial carried out at the National Institutes of Health in the USA and published in JAMA in 1975 was erroneously analyzed.
Overall, the analyses of the vitamin C common cold trials indicate that further therapeutic trials examining the effect of vitamin C on the common cold are warranted, in particular among children. Proper understanding of the restricted subgroups or particular living conditions in which daily vitamin C supplementation might be beneficial also requires further study.
The second part of this thesis consists of analyses of the ATBC (Alpha-Tocopherol, Beta-Carotene Cancer Prevention) Study with 29,133 male smokers. The current analysis focuses on the question of whether vitamin E might affect respiratory infections.
In the ATBC Study, vitamin E supplementation had no overall effect on the incidence of the common cold or pneumonia, but in both cases there was substantial modification of the vitamin E effect, suggesting benefit in restricted sub-populations. Common cold incidence was reduced by vitamin E in a small group of elderly city-dwellers who smoked 5-14 cigarettes per day. Although the effect was statistically highly significant, it was small and the assessment of its practical importance requires further study. The incidence of pneumonia was reduced among participants who started cigarette smoking at later ages. This effect was also statistically highly significant, but further work is needed to evaluate its practical significance. Vitamin E supplementation had no effect on the risk of tuberculosis.
The intake level of vitamin C in diet was estimated at the baseline of the ATBC Study. It was found that the level of vitamin C in diet had no association with the risk of the common cold or tuberculosis.
A major finding in this thesis is heterogeneity in the effects of daily vitamin C and vitamin E supplementation. While there is no evidence that ordinary people might benefit from supplementation of these two vitamins, the heterogeneity in effects indicates that further study should be carried out to identify and characterize the population groups or living conditions in which these vitamins might be beneficial.
Since a large number of studies found that vitamins C and E affect the immune system, it has been suggested that these vitamins might affect infectious diseases. Numerous animal studies found that vitamins C and E reduce the incidence and severity of various viral and bacterial infections. These findings justify research into whether these two vitamins affect the incidence and severity of respiratory infections in humans.
The earlier medical literature contains suggestions that vitamin C may be beneficial against respiratory infections, but this topic achieved wider popular interest only after 1970 when Linus Pauling, a dual Nobel Laureate, wrote a book in which he concluded that large doses of vitamin C prevent and alleviate colds. Pauling’s proposal led to a large series of new placebo-controlled trials. It is shown in this thesis that these new trials consistently found that regular vitamin C supplementation shortens the duration and alleviates the symptoms of the common cold, partly confirming Pauling’s hypothesis. The new trials did not find reduction of common cold incidence in the ordinary Western population with vitamin C supplementation. Nevertheless, in this thesis it is shown that there may be sub-populations, such as people undergoing heavy acute physical stress and young males with low dietary vitamin C intakes, in which regular vitamin C supplementation may reduce the incidence of the common cold.
While the trials carried out since 1970 have shown that vitamin C alleviates common cold symptoms, it is strange that major medical textbooks state that this vitamin has no effect on colds. In this thesis it is shown that the most influential reviews on vitamin C and the common cold cited in the major textbooks contain numerous erroneous statements, and that they even present data that are inconsistent with the original study reports. Consequently, the negative statements in the medical textbooks are based on biased reviews. It is also shown that the most influential vitamin C common cold trial carried out at the National Institutes of Health in the USA and published in JAMA in 1975 was erroneously analyzed.
Overall, the analyses of the vitamin C common cold trials indicate that further therapeutic trials examining the effect of vitamin C on the common cold are warranted, in particular among children. Proper understanding of the restricted subgroups or particular living conditions in which daily vitamin C supplementation might be beneficial also requires further study.
The second part of this thesis consists of analyses of the ATBC (Alpha-Tocopherol, Beta-Carotene Cancer Prevention) Study with 29,133 male smokers. The current analysis focuses on the question of whether vitamin E might affect respiratory infections.
In the ATBC Study, vitamin E supplementation had no overall effect on the incidence of the common cold or pneumonia, but in both cases there was substantial modification of the vitamin E effect, suggesting benefit in restricted sub-populations. Common cold incidence was reduced by vitamin E in a small group of elderly city-dwellers who smoked 5-14 cigarettes per day. Although the effect was statistically highly significant, it was small and the assessment of its practical importance requires further study. The incidence of pneumonia was reduced among participants who started cigarette smoking at later ages. This effect was also statistically highly significant, but further work is needed to evaluate its practical significance. Vitamin E supplementation had no effect on the risk of tuberculosis.
The intake level of vitamin C in diet was estimated at the baseline of the ATBC Study. It was found that the level of vitamin C in diet had no association with the risk of the common cold or tuberculosis.
A major finding in this thesis is heterogeneity in the effects of daily vitamin C and vitamin E supplementation. While there is no evidence that ordinary people might benefit from supplementation of these two vitamins, the heterogeneity in effects indicates that further study should be carried out to identify and characterize the population groups or living conditions in which these vitamins might be beneficial.
Original language | English |
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Place of Publication | Helsinki |
Publisher | |
Print ISBNs | 952-91-9700-4 |
Electronic ISBNs | 952-10-2837-8 |
Publication status | Published - 2006 |
MoE publication type | G5 Doctoral dissertation (article) |