A master student of nutritional science can learn the relationships between diet and human health. There are many factors that can contribute to nutrition, and they may include how food interacts with the body, physical differences in diet and epidemiological factors.

The PhD is a doctoral degree, specifically called a "doctor of philosophy" degree. This is misleading because PhD holders are not necessarily philosophers (unless they earned their degree in philosophy!). That said, PhD recipients are able to engage in thought experiments, reason about problems, and solve problems in sophisticated ways.