I've fed beef heart to my discus for 30+ years and consistently produced 6" adults. The largest discus I ever grew with beef heart reached 7.5" (it's my avatar). I've made my own beef heart mix in the past and tried, and discarded, all these recipes. People say that these additives provide additional nutrients that beef heart is lacking. When I asked them exactly what nutrients are missing with beef heart, no one has an answer. I've know local breeders that have produced 5 generations of discus raised solely on beef heart, so beef heart by itself provides all the ingredients discus will need to grow and breed.
My current batch of discus are fed pure beef heart with no additives and dried pellets as their entire diet, 1 - 2 feedings of dried food (I favor VibraBites) and 1 feeding of beef heart daily. When I go on vacation, they get dried food from the auto-feeder.
Every scientist will tell you that the term "natural" is incredibly non-scientific. Discus don't run into cows in the Amazon. Nor do they encounter brine shrimp or black worms. Their diet is primarily insects, large ones - not the blackfly larva available in commercial food preparations. But they're not swimming in the Amazon either. We grow discus in rather small glass boxes and they thrive on high protein diets.