QuestionI am very thin and my periods don't come regularly. Why is it so important to eat right to have regular periods? What will happen if you don't?
A woman's body is designed to be able to bear children. However, there is a danger of being pregnant when not enough food is available. Since the growing child need nutrients, the body would be forced to "steal" from the mother's body in order to support the child. God designed the woman's body to hinder this situation. When you don't have the minimum reserves to support a developing child, the body stops the menstrual cycles -- no menstrual cycles, no baby will be produced.
There have been studies on women athletes that show that a few months in a row with no periods begins a loss in bone strength. Whether this extends to non-athletes without periods is currently not known. But loss in bone strength can be permanent, so it is not worth the risk.
Periods during your teenage years will be irregular anyway. But if you are not eating enough to maintain a regular period, then you are probably not getting enough nutrition for other vital parts of your body. Especially during your years of growth, a lack of proper nutrition will impact your growth.