Personally, I believe it helps to increase neural growth and organization, my own research suggests that nano-gold reflects and amplifies the infrared frequency at which neural cells communicate as well as having a positive effect on re-establishing optimum cellular voltage.
The only way gold might be bad for the kidneys is if it is a gold chloride, i.e., ionic, and dangerous at a certain dose; nano gold that is non-ionic would be safe up to any reasonable dose.
Personal research has found that Gold in the 5-25 nm size is sufficient enough to be bioavailable.
Edgar Cayce claimed Colloidal Gold catalyzed all the other elements in the body to do their job right.