Nutritional genomics. It is the study of how nutrition and genetic makeup work together. This incredibly interesting field covers many types of interactions: single gene mutations where a nutrient cannot be metabolized; genetic complexes that put a person at risk for increased chances of heart disease; and food substances that can cause cancer. These are just a few examples of this emerging field.
New field, beware. Studying how different people's genetic structure influences how their dietary choices affect their health is a very new field. It's interesting, but if you are being asked to pay money as an ordinary person to have a diet tailored to your specific genes, put your hand over you wallet and watch for other signs of nutritional charlatanism.