Since body mass gains of about 1 kg can occur in a week's time, many studies suggest that the gain is simply due to greater water retention inside the muscle cells [REF].
In another study, researchers concluded that changes in substrate oxidation may influence the inhibition of fat mass loss associated with creatine after weight training when they discovered that fat mass did not change significantly with creatine but decreased after the placebo trial in a 12-week study on ten active men.