In Genghis Khan and the Making of the Modern World (2004) by Jack Weatherford, I read this (p. 87):
Compared to the Jurched [Chinese] soldiers, the Mongols were much healthier and stronger. The Mongols consumed a steady diet of meat, milk, yogurt, and other dairy products, and they fought men who lived on gruel made from various grains. The grain diet of the peasant warriors stunted their bones, rotted their teeth, and left them weak and prone to disease. In contrast, the poorest Mongol soldier ate mostly protein, thereby giving him strong teeth and bones.
To tenderize meat a Mongol would put it under his saddle while riding. I was pleased to read this because I eat a lot of meat and yogurt (but not milk). The source of this information is unclear but it’s a surprisingly modern comparison. Good Calories Bad Calories (2007) by Gary Taubes says much the same thing (minus the yogurt — the part that most interests me). Weston Price wrote many similar passages comparing people eating traditional food (= Mongols) with people eating modern food circa 1930 (= Chinese). Long ago, grain was modern food.
Thanks to Tucker Max.