According to Pat Hastings, it is actually the calories that are the main issue.
http://roverexposure.com/wordpress/?p=7
"Pat pointed out that in a “natural” situation, a puppy would eat milk, then would get regurgitated food (i.e. food which has already lost some nutrients), then would get the leavings from a kill (i.e. the lower-calorie less-choice parts of the animal), and would only get the high-calorie/all-you-can-eat food when it was old enough to expend the energy actually hunting the food itself. So feeding a high-calorie premium puppy food is exactly the opposite of what we should be doing. She says that almost all puppies thrive and grow properly on a high-quality adult diet."
Which makes sense, because when we're looking at morbidly obese babies and toddlers, these children are significantly taller than their normal counterparts.
Numbers and data sound very convincing, but when you listen to someone who has evaluated over 30,000 litters and studied the nutritional effects on growth and structure more than any other person (with no original ties to any food companies) and has come to the very obvious conclusion that puppy food can cause detrimental issues, and you see the pictures of these issues, that is alot more convincing. I wouldn't feed Pat's choice food, myself, based on ingredients; However I would never feed the top worst foods on her list either, which are consistently puppy foods.
Also, I wouldn't see why anyone would feed more adult food to make up for the lost calories in the puppy food. Maybe I am talking in a free-feeder perspective, but my dogs will only eat as much as they want.
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