If your programme has suggested you avoid diet drinks, it's because Limbo's trying to get you to consume less sweet food and drink.
Evidence suggests diet drinks with sweeteners such as aspartame may drive us to crave more sweet things and look for more energy when we don't need it.Artificial sweeteners have been shown to activate areas of the brain linked to reward-seeking behaviour (which is about seeking energy). This affects cravings and perceptions of hunger.