Tea drinking became a necessary addition to the diet of herding peoples living in the border areas. About the 5th century the tea had been bartered for horses with nomadic peoples on Chinese borders. Tea was much desired to aid digestion of their meat-and-milk diet, and in Tibet its stimulating properties made life in that thin-oxygen area more bearable. Soon tea was recognized for its importance to China’s national economy and as a source of revenue through taxation. The government made the sale of tea a monopoly since 835. The Qing Dynasty government, attempting to use the tea trade to control the border peoples, set strict limits on the amount that could be sold into Tibet. Anyone who exceeded it would be given the serious penalty for smuggling tea.