Cazzzzz is correct in that it is the chromosome in the sperm that will determine the sex of the baby, depending on whether the individual sperm that makes the egg is carrying X or Y.
Thats not to say that there cant be influence about which you get. The research indicating that what you eat can affect the success rate of whether the X or Y sperm is stronger and gets to the egg first. "Male" sperm are actually weaker than female, so, in theory the condidtions inside the mother might be able to influence it, for example if the ovum is thicker, the "male" sperm is less able to penetrate and the "female" will be first in. The food research suggests the glucose levels caused by eating more in the morning, my therefore have an effect.