Dear diapolical,
Yoga is not a system of beliefs. It is a process of self-realization. However, unlike science that confines to the observation and the observed, Yoga concerns itself with the experience and the experiencer.
It is not surprising that we find different individuals holding different views of the same world and its phenomena. We are naturally endowed with sensory perception that is central to our experiencing. Senses have a limited bandwidth. The resultant experience brings in limited data that we color and condition while cognizing. Yoga practices sharpen the senses and later enable sensing beyond senses.
One of our survival instincts is to take the personal view as the ‘whole truth’ without which we cannot live amidst objects. So, to opine that ‘food and oxygen IS prana’ is a perfect truth for the beholder. One would move from this position if and only if a realization occurs that there is always more to a thing if one tries to see. So, holding a limited view is not a problem in itself; however, not knowing its limitedness is. Yoga begins with a realization of the limits of human perception. Yoga encourages looking at other people’s views as possible hypotheses one could test and own up only if convinced .
Some data may help in the context of this thread:
Oxygen is not prana in yogic terminology, but prana-vayu. Pure prana abiding in the whole universe is so fine that it is inside and outside of every thing. But once inside, a part of it starts degenerating into grosser forms and becomes various things like, vayus and mind. Like proof of air is in the wind, proof of vayus and mind is in their manifest functions. Oxygen, much grosser than pure prana, is carried in and out by the breathed air. It clears impurities from the bodies. When we learn to control prana (pranayama) oxygen, the cleaning agent, is needed less and less; eventually not at all.
Similarly, food metabolism results in generating physical energy for sustaining the body functions. Movement of the vayus supports varios body functions. Samana vayu helps food churning. But, food metabolism is much cruder and wasteful energy source when compared with prana. So, pranayama rekindles the atomic plants at muladhara and swadhisthana chakras to obtain prana energy that makes food-based system redundant.
So, is food and oxygen, prana? in a very philosophic sense ? yes. Every object is a manifestation of prana. But, no; as deeper reality shows them in a different light. Is this important to know? yes, if you are doing pranayama or are a seeker of truth . ?No?, otherwise.