Originally Posted by Raul_SJ
If you go running uphill, and do it relatively slow (in the "fat burning zone"), wouldn't you be losing fat and building up some leg muscle at the same time?
You would think yes but for most adults past the age ~25 the answer is no as Raul_SJ said.
I'm a med student with a keen interest in sports medicine and fitness and from what I understand the principle issue preventing simultaneous fat loss and muscle building is a metabolic and hormonal one.
The so-called fat burning zone is controversial and likely a myth. The reason is because if you're in a caloric deficit, your body enters a catabolic/starved state that burns BOTH fat and muscle. When you are in a starved-state, your body releases hormones such as glucagon and adrenaline which bind to liver cell receptors and stimulate gluconeogenesis, the formation of glucose from protein and fat substrates. Unfortunately, both protein and fat feed into the gluconeogenesis pathway and you cannot selectively decide which does and doesn't feed into it.
In fact, studies have shown that high intensity training, which was previously though to cause more muscle breakdown than fat loss, is in fact equally as effective as low intensity ("fat burning zone") in weight loss without significant difference in loss of lean muscle mass at the same measured caloric deficit. The only difference was that high intensity training takes a shorter amount of time to meet the target caloric expenditure.
The reason why young athletes are much more successful in simultaneous fat loss and muscle building is because they are blessed with an increased level of growth hormone which has the magical ability to promote muscle growth and block fat storage. Unfortunately the release of growth hormone tapers off as we age but some people cheat through doping and injecting it.