The profile constraint, resulting from the poloidal Ohm's law, is used to derive a relationship between the normalized energy content (toroidal beta) and the normalized plasma current (1/q) for tokamak discharges. This relationship depends on the plasma fuelling, (gas puffing and neutral beam injection are considered) and is independent of energy balance. Good agreement is found with a wide range of experiments, both gas fuelled and neutral beam fuelled, when use is made of the extended neoclassical theory (which assumes an anomalous electron-electron collision rate but a Coulomb electron-iron collision rate).