Mitchell is a Type 1 diabetic, so I very much doubt there's any way he's taking in 5,000+ calories a day. There is no endocrinologist that would sign off on that sort of diet, as the amount of insulin he'd need to take in daily to counteract that caloric intake to keep his blood sugar manageable would be extremely unhealthy. My guess is he's hitting the weights hard, and combining it with a high-protein, low carb diet. A portion of the muscle mass you accrue going this route is going to be water weight, which will account for a visible change in physique, but not really add much else athleticism-wise. One thing he will need to account for while he bulks up is a potential decrease in flexibility and joint issues, as diabetes, even if it is well managed (as I'm sure it is with a professional athlete), can have negative effects on those body systems. He's always going to be a guy who needs recovery days, as the diabetic body simply doesn't recover from strenuous activity as quickly as a normal, healthy body. That isn't to say that he cannot be a very productive MLB player, though.