Food science is the end-to-end study of food, including raw materials selection and harvesting, composition and development, production, preservation and safety, packaging and storage, through to end-user preparation, consumption and sensory analysis. Food science also includes food-involved scientific fields such as food microbiology, food chemistry and the physics of food. Food technology is the practical application of the food sciences to the development and production of food. Is is generally included as a part of food science (most related university courses are described as 'Food Science & Technology').
Nutrition is a area that deals with nutritional content (fats, carbs, protein, etc.) of food and the effect on the body. Depending what part of the world you are in, a nutritionist may or may not have had specialized training.
A food scientist may have performed the tests to determine the nutritional content of food. A nutritionist might be the one who puts it to practical use.
Food Nutrition & Dietetics is the study of difference categories and compositions of food and their health consequences in the make up of a diet is a related field.
Diet is the usual food and drink of a person or animal. It also refers to a regulated selection of foods, as for medical reasons or cosmetic weight loss. It is also defined as something used, enjoyed, or provided regularly: subsisted on a diet of detective novels during his vacation.
