Hormones are the chemicals that play important role in human body. The changes in human body is mainly related to the hormonal changes. In case of a female, hormones play a very significant role. If the diet is unhealthy or laden with extra hormones or chemicals that mimic hormones, the female my experience and early puberty. In such case, the menarche can be reached earlier than expected.
Thus, diet laden with hormone mimicking chemicals are expected of contributing to girl's earlier experience of menarche.