I think much of the creatine research, which consistently shows positive health outcomes like enhanced physical and mental performance, is simply telling us — eat more meat.
Almost everyone is "creatine deficient" if we define it as not having optimal stores. Like "creatine deficiency" is a
proxy measure for how much we undereat meat.
The body only makes about 1g/day, the rest we must get from our diet (3-5 g/day to optimize creatine stores).
Since it only comes from animal foods (notably red meat), the only way to optimize creatine stores without supplementation is to eat a meat heavy diet.
As a baseline ballpark: 1+
pounds for women and 1.5+ pounds for men.