One thing you can do is contact the breeder's at the national breed club and ask them what they feed and why. Age is often a factor. Avoid overworking and accelerated growth (high protein) as well (this helps avoid expensive hip replacements) during the growth period in particular which is long with giant breed's. Here's the link to the analysis site most people into nutrition use (often referring to our food by the star rating, lol). Find one that's 5 or 6 star rated that fit's the information your getting. :-).
www.dogfoodanalysis.com www.mastiff.org www.bornfreeusa.org www.thedogsbone.com
www.dogfoodanalysis.com www.mastiff.org www.bornfreeusa.org www.thedogsbone.com
