Mastiff, being a large breed as well as the Great Dane are often treated and fed the same as other large dog breeds.

The fact is that the Mastiff need special dietary considerations to stay healthy.

Puppy food should never be fed to a mastiff puppy. Not even puppy food designed for large breed dogs.
 
Puppy food contain calcium and fat levels which could be detrimental to your mastiff puppy.
Mastiffs being prone to growth disorders involving their bones, such as hip dysplasia, panosteitis, and wobblers syndrome are believed to do better on a diet not exceeding 20-25% protein and 12% fat.

 The danger with feeding puppy foods high in protein is that it allows the bones to grow at a faster rate than their muscles.  A slow growth rate, with the proper amount of food and no over feeding will allow your mastiff puppy to become a healthy happy adult.

Find a premium, high quality natural commercial food for your mastiff puppy, or, prepare your own raw diet.  Some better brands are Wellness, Innova, Blue and Wysong.  Pet food sold in the grocery store should be avoided as it has very low quality.

A raw diet prepared a thome, or purchased already prepared, is the absolute best mastiff puppy food.
Fresh, raw meat and bones are nutritious and what dogs have evolved on for the past million years.

Raw food will keep your mastiffs teeth and gums sparkling clean, avoiding the health risks associated with dental issues, and also avoiding the cost of dentals. Your dogs coat will be shiny and sweet smelling, the eyes clear, your dog will be vibrantly healthy.

Chewing on bones have wonderful psycological benefits as well.  Feeding raw may seem like something very cumbersome, but once the details have been worked out, it is very easy, and so rewarding.