Diabetes can be produced experimentally by certain poisons which kill the insulin-producing cells in the pancreas. Rabbits were experimentally made diabetic, and when treated with DHEA their diabetes was cured. It was found that the insulin-producing cells had regenerated. Many people with diabetes have used brewer's yeast and DHEA to improve their sugar metabolism. In diabetes, very little sugar enters the cells, so fatigue is a problem. DHEA stimulates cells to absorb sugar and to burn it, so it increases our general energy level and helps to prevent obesity.
Yes, calisthenics can be progressed to be quite difficult. Planches and handstand pushups require a ton of stabilizer muscle strength and a ton of balance. This makes them rather impressive (and fun) … but the very thing that makes them impressive also makes them rather poor at building up muscle mass. Even with advanced progressions of bodyweight workouts the limiting factor will very rarely be mechanical tension in the targeted muscle, but rather stabilizer muscle strength and/or balance. As a result, your balance and stabilizer muscle strength will improve while your muscle size will remain more or less the same.