Marathoners are at an increased risk of developing upper respiratory tract infections (URTIs) following races and periods of hard training, which are associated with temporary changes in the immune system. The majority of the reported changes are decreases in function or concentration of certain immune cells. During this period of immune suppression, by some referred to as an 'open window' in immune function, it has been hypothesised that viruses and bacteria might gain a foothold, which would increase the risk of infections. In light of this, nutritional interventions that can enhance immune function and reduce the risk of URTIs have been sought. This paper focuses on the effect of glutamine, vitamin C, bovine colostrum and glucose. Although, some of these supplements can affect the physiological and immune changes associated with marathon racing, none of the supplements discussed have consistently been shown to reduce the risk of URTIs and therefore cannot be recommended for use as enhancers of immune function in marathon runners.