The use of 24-h urine samples and energy expenditure to validate dietary assessments.

The doubly labeled-water technique can be used as a noninvasive measure of energy expenditure, and, with no gains or losses in weight, expenditure should equal intake. Provided sufficiently complete urine collections are obtained, urine nitrogen is an accurate measure of protein intake in individuals in nitrogen balance. Validations of dietary surveys by using estimates of energy expenditure and the 24-h urine nitrogen technique show that inaccurate reports of food intake occur in a proportion of individuals within a group, leading to bias in overall average intakes for some nutrients, if only one method of dietary assessment is used. Errors between methods however are not usually correlated, so that improved estimates can be obtained from a combination of two or more methods of dietary assessment. All future dietary surveys should require independent evidence of their validity.