For nearly a century, nutrition scientists and dietitians have been estimating their patients' calorie needs based on the individual's sex, size, age, activity levels and even disease and injury states.

        Getting the numbers right can be a matter of life or death when cancer, diabetes, thyroid abnormalities, nervous system disorders and serious burns alter calorie requirements. (A severe infection, for example, increases body temperature and raises energy requirements.) Especially when patients have a reduced appetite, are on tube feedings or are unconscious, the calculations become essential for deciding how much to feed them.

        Early in the 20th century, scientists began studying the energy needs of healthy people. This research produced equations published in 1919 by Francis G. Benedict and J. Arthur Harris while they were working at a nutrition laboratory in Boston operated by the Carnegie Institution of Washington.

        These “Harris-Benedict equations” are still widely used and are considered highly accurate in establishing a person's resting metabolic rate, or RMR.

        In early studies, participants had to sleep with tubes up their noses and plaster strips sealing their mouths while Benedict and Harris measured the oxygen and carbon dioxide being inhaled and exhaled.

        In later years, participants wore helmets constructed from metal buckets to help measure the gases, according to a 1998 article in the Journal of the American Dietetic Association. Today, a plastic helmet with sheeting that covers the nose and mouth makes the procedure more comfortable, though it can still be intimidating.

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