Nutrition is a science which studies the relationship between diet and states of health and disease. Dieticians are Health professionals who are specialized in this area of expertise. They are also the only highly trained health professionals able to provide safe, evidence-based and accurate dietary advice and interventions.

Between extremes of optimal health and death from starvation or malnutrition, there is an array of disease states that can be caused or alleviated by changes in diet. Deficiencies, excesses and imbalances in diet can produce negative impacts on health, which may lead to diseases such as scurvy, obesity or osteoporosis, as well as psychological and behavioral problems. Moreover, excessive ingestion of elements that have no apparent role in health, (e.g. lead, mercury, PCBs, dioxins), may incur toxic and potentially lethal effects, depending on the dose. The science of nutrition attempts to understand how and why specific dietary aspects influence health.

A dietitian (sometimes spelled dietician) is an expert in food and nutrition. Dietitians help promote good health through proper eating. They also supervise the preparation and service of food, develop modified diets, participate in research, and educate individuals and groups on good nutritional habits. The goals of the dietary department are to obtain, prepare, and serve flavorsome, attractive, and nutritious food to patients, family members, and health care providers.

In the US nutrition professionals include the registered dietitian (RD) and the dietetic technician, registered (DTR). Some RDs or DTRs call themselves nutritionists. However, some people who may call themselves a nutritionist are not registered dietitians. Dietetic technicians are not the same as dietitians in terms of responsibilities and qualifications. Different professional terms are used in other countries.