Indeed, they are a lot of factors in growth, and I should have mentioned phosphorous and calcium (this is what happens when you catch up on the forum in-between other things). From my professional opinion protein is actually still an factor (I'm an animal scientist). It's also easier to give people protein guidelines as most foods with higher protein also have higher calcium and phosphorous as they are trying to prompt growth. This is good in small to medium size breeds. It's really even okay in large breeds. It's just a problem with the giants. That being said if he comes from smaller lines and will probably not be one of the mastiffs above 180, you can probably go higher on the protein. I went up to 28 per cent and teensy bit higher on phosphorous and calcium because Gummy Bear had such a slow growth as a puppy (he was still only around 45 lbs at six months--I'll have to post his growth chart, it's interesting but I only have weight points because the silly puppy moved too much to have reliable data in terms of height and length, margin of error was just too high).
Personally (and this isn't an professional endorsement) I like the Earthborn Holistic better than Blue Buffalo. But all in all, once you get into the higher quality brands, there's not much a difference between them. My biggest thing is how digestible it is for my puppy (and Gummy Bear doesn't do well with high protein, it's too rich for his stomach). However, here's the other thing about protein, vitamin, mineral, and carb content. The number they give you on the bag are formulated to give you the most favourable numbers and are often not reflective of how much is really in the food. There isn't a specific formula they have to use and therefore can manipulate the numbers to be favourable to what they believe the consumer is looking for. Therefore, food can have lower or higher content of protein, vitamins and minerals than what they say on the bag due to how it's calculated.
And honestly, due to how difficult it is to obtain funding (from neutral sources) to do research on animals--particularly companion animals, we still have limited data (which is better than none). I still find the Ns on must studies too low (and the number of research studies done too low) to feel that we can say anything strongly, but studies do indicate that it's important to regulate the growth of giants. On the other hand, I also exercise my giant more than a lot of people would for fear of injury, but there's very little research to indicate that it is indeed harmful. Although I did restrict Gummy's running during his most rapid growth phase because if they do hurt themselves at that age, it's a potentially difficult and costly recovery.
