It's not that nutrition is harder than physics or chemistry, instead it is far less ideal, and more importantly, purely empirical (as opposed to experimental).

It's not that nutrition is harder than physics or chemistry, instead it is far less ideal, and more importantly, purely empirical (as opposed to experimental). This is they key distinction between hard and soft sciences and, incidentally, why it is dangerous to treat soft sciences like psychology and sociology (and nutrition) with the same certainty as hard science like physics.