To help solve this problem, Conway, ARS colleague James L. Seale, and university collaborators conducted a study to test how well activity records and questionnaires estimate daily energy expenditures. They found that it's possible to use activity records to estimate energy expenditures in groups, but they are not reliable for individuals. Questionnaires were less reliable than activity records. Poor questionnaire design, overestimating time spent moving and using up energy, and a too-small sample size can cause errors, even in group estimates. There is a more accurate method, but it's high-tech.
Scientists use a test with doubly labeled water to determine energy expenditure in people.
The water contains heavy forms of hydrogen and oxygen called isotopes, which can be traced in the urine or blood to determine how much of the isotope is still present in the body. From this measurement, scientists can determine how much energy a person used during the study and then calculate average daily energy expenditure. But this test is expensive, so it's not practical to use with large groups. Tara Weaver-Missick, Agricultural Research Service Information Staff.