Hi,

usually reduction in milk supply is complicated, and there are a whole lot of factors involved. I won't comment on Christy's situation, because I don't know enough about it. But, most of the world's population don't lead the somewhat cushy lives we lead today. For most of history and in most parts of the world women have to do hard physical work and they breastfeed just fine.
Milk supply reduces when the baby feeds less. It may be that when mothers go back to exercising a lot they don't feed their baby enough. Most often babies start to behave differently - could be a period of physical or emotional growth - and we and people close to us read that as "she must be hungry" Many people have fairly rigid views that a woman's milk supply will fail at X months - it can become a self fulfilling prophesy. It is a myth that "milk just dries up". It is usually shorthand for a long culmination of a whole lot of factors.

Barb