This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
Environmental factors such as type of food and exercise in puppyhood
have been shown to affect the displayed symptoms within the same
litter. However, subsequent generations from both groups showed the
same rates of dysplasia meaning that while the phenotype may be
affected, the genotype is what determines whether a dog has the
potential for being affected with HD.
In general, low protien diets and low activity levels through
puppyhood reduced the symptoms of HD markedly. However, the degree of
diet reduction and no activity may or may not be practical for the
average dog-owner to attempt. (See Willis.)
It's best to keep your puppy from any kind of jumping for the first
year or so in life. It's also best to keep from sustained exercise
until at least a year old. Sustained exercise includes: jogging with
owner, pulling weights, mushing, running with owner on bike, etc. Even
for dogs not at risk from HD, it's wise not to exercise too strongly
too early as such exercise may interfere with proper growth of joints,
leading to similar problems such as arthritis on the joint or OCD.