 Sedentary women who start briskly walking on a treadmill 45 minutes a day for a few months may cut their risk of upper respiratory tract infections in half. But how does exercise improve our immune system? Approximately 95% of all infections are initiated at the mucosal surfaces, the moist surfaces like our eyes, nose, and mouth that are protected by antibodies like IGA, which provides an immunological barrier by neutralizing and preventing viral pathogens from penetrating the body. The IGA in our saliva, for example, is the first line of defense against respiratory tract infections such as pneumonia and influenza. And moderate aerobic exercise, even just 30 minutes in the gym three times a week, may be all it takes to significantly boost IGA levels, and significantly decrease the risk of coming down with flu-like symptoms. But we've known for a long time that prolonged heavy exercise may reduce resistance to infectious disease, manifesting by an apparent 2-6-fold increase in upper respiratory tract infection symptoms for several weeks following marathon running. Even just a single bout of overstrenuous exercise may drop IGA levels. Within a day of starting an international competition, for example, elite soccer players suffered a significant drop in IGA secretion. Yacht racing athletes, training for America's Cup, who got upper respiratory tract infections during training, had significantly lower IGA concentrations. Those with higher levels had fewer infections, and if you measure over time, you can see dropping levels precede the infection. Furthermore, a simple fatigue rating appears to reflect changes in salivary immunity. If you just ask them, you know, how rested do you feel? Those reported feeling worse than normal had significantly lower IGA levels. Sport coaches are advised to monitor immune function since illness could ultimately lead to a decrease in performance. Therefore, it may be necessary to take protective actions to, for example, minimize contact with cold viruses. But the reason athletes can't get away with just washing their hands and wearing a mask is because upper respiratory tract infections are often triggered by reactivations of latent viruses already inside our bodies, like Epstein-Barr virus, EBV, and as soon as our immune function dips, the virus becomes reactivated. IGA levels drop the day before EBV comes out, hiding and causes a spike in symptoms. These results suggest that the appearance of upper respiratory symptoms is associated with reactivation of EBV and reduction of salivary IGA during training. So how are we going to preserve immunity in athletes? Well, I talked about the efficacy of using a one-celled fungi, a nutritional yeast, to boost the immune system of athletes. What about a one-celled plant? Researchers out of Japan found IGA concentrations in breast milk could be increased by giving mothers chlorella, a unicellular freshwater green algae sold as powder or compressed into tablets. What about other parts of the body? About 30 tablets of chlorella a day for a month increased IGA secretion in the mouth as well. But does that actually help in a clinically meaningful way? Researchers in Canada tried to see if they could boost the efficacy of flu shots, but a chlorella-derived dietary supplement did not appear to have any effect. They were using some purified extract of chlorella, though not the real thing. What about giving chlorella to athletes during training camp? My intensity of physical activity grew up living ripe for infection, and indeed the training was so intense, IGA levels significantly dropped, but not in those given chlorella each day. So chlorella intake may attenuate the reduced IGA secretion during athletic training.