The study revealed that alcohol intake frequency, not its amount, was associated with the retrospective prevalence of common cold. Moderate alcohol intake has been recognized as a behavior that effectively reduces health risks, such as the development of myocardial infarction  and all-cause mortality . One of the previous studies showed that red wine intake reduced the risk of common cold, while intake of other beverages, such as beer and spirits, did not . Give that the beneficial effect of red wine may be because of its higher antioxidant  and the anti-inflammatory capacity  of polyphenols, the difference seemed to be explained by significantly lower content of polyphenols in beer . In the present study, however, the percentage of the subjects who drank wine was considerably lower than that of the subjects who drank beer or Japanese spirits. Because nearly two thirds of the drinkers consumed more than 2 types of beverages, mainly beer and Japanese spirits, and the numbers of subjects drinking each beverage alone were too small to analyze the independent effect of each type of alcoholic intake. Hence, although a possible effect of other components of beer and Japanese spirits cannot be excluded, alcohol itself may have a beneficial effect on reducing the incidence of common cold.
The associations between the retrospective prevalence and the amount and frequency of alcohol intake were stronger in subjects who had 2 or more episodes of common cold than those who experienced 1 or more episodes. Frequent incidence of common cold may be a reflection of higher susceptibility to common cold viruses, which, in turn, may be influenced by alcohol intake. Rhinovirus (RV) is one of the most common causative pathogens of common cold . The replication of some types of RV, such as RV14 and RV16, is optimal at lower temperatures (33°C) and attenuated at higher temperatures (37°C) . The average temperature in the upper respiratory tract is 32°C, as compared with 35°C in the sub-segmental bronchi . Thus, airway temperature may be one of the key factors determining the susceptibility to RV infections, as well as the attacks of other temperature-sensitive viruses .
Raising airstream temperature is positively related with increasing blood flow of the mid-trachea . Alcohol intake improves skin , cerebral  and coronary blood flow . Hence, alcohol might increase airway blood flow and temperature. One of the mechanisms by which alcohol intake may increase airway temperature could be associated with nitric oxide (NO), a potent vasodilator  stimulated by ethanol [40–43]. Other factor which may be associated with the increased airway temperature is blood acetaldehyde level, which indirectly causes vasodilation  and increases after alcohol intake, resulting in increase blood flow . The temporary effect of alcohol intake on increasing blood NO concentration is reported to last more than 1 hour and up to several hours [40, 41], and blood acetaldehyde level returns to normal within 4 hours after alcohol intake . Given that increased local blood flow in the upper respiratory tract contributes to protection against viruses, daily drinkers of small amounts of alcohol may have an advantage on viral protection over “weekend” or “occasional” drinkers.
There are possibilities that those who had history of stroke, cardiac disease, angina, or chronic renal failure may avoid or reduce alcohol drinking, and that those who have poorer health conditions may have higher incidence of common cold. In this study, however, only 1.7% of the participants had history of these diseases, and there were no skewed distribution of them across either the amount or the frequency of alcohol intake categories (data not shown). Moreover, in this study, there were no significant associations between the amount and frequency of alcohol intake and general health conditions such as physical activity level, depressive symptoms, and metabolic syndrome. Statistical adjustment for these variables did not affect the association between the retrospective prevalence of common cold and the frequency and amount of alcohol intake. Therefore, it is not likely that poorer health conditions have affected our results. Another possibility is that those who suffer common cold may have avoided drinking. Average incidence of common cold episodes per person in northern Japan is reported to be approximately 4 times per year among Japanese adults . Sick days in a common cold episode commonly last for several days. It is therefore likely that the reduction in the amount or the frequency of alcohol during the sick days may not affect our results.
In an elderly population aged 66 to 79 years, those with depressive symptoms had higher incidence of common cold during a 1-year follow-up period . In addition, previous studies showed that psychological stress was related with the risk of common cold . However, in the current study, examining a slightly younger population, no association between depressive symptoms and retrospective common cold incidence was observed (data not shown); further, no association was found between alcohol intake and depressive symptoms.
A previous experimental study showed that those who sleep over 8 hours a day have lower risk of developing common cold . A higher amount of alcohol intake per day has been associated with longer sleep duration . Although the causality between drinking patterns and sleep duration cannot be completely explained, intake of alcohol as a sleep aid by the Japanese  may be one of its reasons. The association between sleep duration and common cold, however, was not observed in the current study. This may be due to the low number of subjects who slept 8 or more hours per day.
The present study has several limitations. First, the incidence of common cold was self-reported, without its specific diagnostic criteria and identification of viruses. Accordingly, there is a possibility that non-clinically diagnosed common cold was included. Second, recall bias may be involved, as the information was collected retrospectively. Patients with asthma  and COPD  are likely to develop common cold; consequently, those patients may tend to remember their common cold episodes better. However, these data are not available in the current study. Finally, obviously, excessive alcohol intake has negative effects on health and increases mortality [49, 50]. Therefore, the results presented here should be interpreted with caution.