@koos Meta study on caffeine safety came out a couple days ago. Makes some really straightfoward recommendations on how much is safe: "400 mg/day for adults, 300 mg/day for pregnant women, and 2.5 mg/kg/day for children and adolescents" http://www.sciencedirect.com/science/article/pii/S0278691517301709

Of importance maybe that that is less than the 600mg/day from the other study. But also tantalizingly not that far away. 400mg is like 4 cups.