You might be thinking about “tsst” which was the episode with Cesar Milan. Another early one was when he went to fat camp and sold other kids candy bars, but I forget the episode name. In the episode where the KFC becomes a weed dispensary, Cartman has withdrawl from not eating the chicken in a while. Other than that, I don’t think there’s any episode where he eats healthier than usual. You may be thinking about another show