Glycogen is the primary fuel for short-term fasting. That's anything from 30 minutes onwards. Glycogen will run out very quickly, and your body will start to metabolise fat stores. Fat takes some effort to mobilise and process, and so your body supplements it with protein, which is mostly taken from the muscle. To start with it's taken from muscle protein stores, which are there as an energy reserve, but rapidly from break-down of muscle itself.
But more likely to hunt? I can hardly imagine a crowd of mice with arrows and spears.Humans are also opportunistic scavengers.
I never said most. Most would still come from fat, but there is significant loss of muscle mass after this point.
I doubt that scientists would want to mimic all the effects of frugal eating. Whereas the only problems with exercise are wear on muscles and joints, frugal eating can lead to this and more.
If frugal eating has its effects on stress, as appears likely, we would do better to find ways to reduce stress specifically.
Heart rate and O2 consumption are not that well-linked, and nor is rapid breathing and O2 consumption. Your resting O2 levels are high enough that rapid breathing makes pretty much no difference. High cardiac output (which isn't the same as HR) and ventilation may be symptoms of stress, but are not causes.
Barring, of course, the solid but small body of evidence that says that yogic breathing and similar slow-breathing techniques can help to reduce stress levels and control things such as hypertension. But that's an artificial imposition on an otherwise normal system.
Resting HR isn't very important as a measure, as long as it's vaguely normal. Things like recovery time of resting HR after exercise are better measures.
