Nutritional epidemiology is the study of human health in relation to nutrition. What started as a small subdiscipline of epidemiology some decades ago has grown into a branch with major public health importance. Now that nutritional deficiencies have been dramatically reduced in most developed countries, the purpose of nutritional recommendations has been the prevention of diseases.nutritional epidemiology examines the role of nutrition in the etiology of disease, monitors the nutritional status of populations, develops and evaluates interventions to achieve and maintain healthful eating patterns among populations, and examines the relationship and synergy between nutrition and physical activity in health and disease.Nutritional epidemiology is one of the younger disciplines in epidemiology. This may be partially due to the difficulties in measuring diet as an exposure. Diet and physical activity are arguably the most difficult exposures to assess in observational research and are plagued by considerable measurement error. We all eat, we all eat many different foods, we tend to forget rather quickly what we ate, and we often do not know the ingredients of the dishes we consume. Hence we are all exposed, and the variation may be more subtle than with other, more distinct exposures such as smoking or use of hormone replacement therapy. Few people maintain extreme diets; thus assessing diet within fairly homogeneous populations makes it difficult to detect associations between dietary patterns (or particular foods and nutrients) and health or disease due to the lack of sufficient variation.