But is there any research to back this all up? Well, yes; for example, this small study suggests that swimming in cold water may "toughen" the body. Ten healthy people who regularly participate in winter swimming were found to have higher levels of glutathione, the "master" antioxidant, than usual. This may be because of mild, acute oxidative stress caused by the cold water exposure. In another study on winter swimmers, the swimmers had higher levels of glutathione, and lower levels of oxidized glutathione compared to healthy controls. This time, there were 36 winter swimmers and 40 "normal people" (who wants to be normal anyway?) The winter swimmers also had much higher levels of the antioxidant enzymes catalase and superoxide dismutase. Interestingly, their levels of non-oxidized glutathione, vitamin C and uric acid, which are all antioxidants, dropped significantly during swimming. All of this could mean that the stress caused by swimming in cold water triggers the body to produce more of its own antioxidants, in order to protect itself from damage. In fact, previous research found that winter swimmers suffered 40% fewer upper respiratory infections, validating Olesya's observations. On the other hand, if left alone, levels of antioxidant enzymes fall with age. Overall, perhaps a teaspoon of cement to harden up and a cold shower or swim could be the best "flu shot" for you, but if you have any cardiovascular or other chronic health issues, it is best to see your doctor first.