Kumis is a drink that is made by fermented mare milk and it was originated in central Asia. Warriors such as Atilla the Hun and Genghis Khan drank this growing up and their soldiers relied on it for strength and endurance. Kumis is a part of the regular diet at this day in Mongolia, Kazakhstan, Turkmenistan, Kyrgyzstan. It is even quite popular in Japan.
Kumis is basically fermented horse milk. Since horse milk has more sugar compared to cow's milk, it produces more alcohol when it is fermented. Kumis has about 2.5 percent of alcohol which is not that much.
The Mongol Empire ruled Asia 1206-1368 and had the most fierce warriors. The Mongol warriors believed that Kumis kept them healthy and provided them strength but they also believed that it made them fearless.
Mongolian Judo athletes are known to be one of the most physical Judo athletes in the international scene.. They are known to have rigorous strength and endurance.
You can find Kumis at Russian markets. Just make sure that is made out of mare milk. Sometimes companies make Kumis from cow's milk since it is easier.
Below is an video which Mongolian Judo players discuss of importance of Kumis for strength and endurance.