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Units of Measure

Phil B. Fontanarosa

and Stacy Christiansen

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The calorie is the unit of measure often used in chemistry and biochemistry for reporting heat energy. A value of 1 calorie is the amount of energy (heat) required to raise the temperature of 1 g of pure water by 1°C. The joule is the preferred SI unit for energy, and calories and kilocalories may be converted to joules (J) and kilojoules (kJ) by using the following formulas:

1 calorie = 4.186 J

1 kilocalorie = 4.186 kJ

JAMA and the Archives Journals prefer to report heat energy in calories or kilocalories.

Formerly a distinction was made between this “small calorie” (with a lowercase c) and the “large calorie,” designated as Calorie (with a capital C and abbreviated Cal)2 and equivalent to 1000 calories or 1 kilocalorie (kcal). In metabolic studies, the Calorie is the amount of heat energy required to raise or lower 1 kg of pure liquid water by 1°C.2 The Calorie also is used in nutrition to express the energy content of food.3 By convention, the use of capitalized C in dietary Calories indicates kilocalories (ie, 1 Cal is equivalent to 1 kcal or 1000 cal). For example, if the label on a food package indicates that a serving contains 300 Cal, that serving would yield 300 kcal (not 300 cal) of heat energy when subjected to complete combustion. JAMA and the Archives Journals prefer Calories or kilocalories for expressing the energy content of food.

Energy expenditure also is reported as Calories (or kilocalories) to reflect the amount of energy required for the work done. The values for Calorie expenditure are based on the metabolic cost, expressed as METs, or metabolic equivalents. One MET represents the metabolic rate for an adult at rest (ie, set at 3.5 mL of oxygen consumed per kilogram of body mass per minute) or approximately 1 kcal/kg/h.3 Activities with MET values near 1 are sedentary activities (eg, sitting quietly), whereas activities with higher MET values involve higher levels of energy expenditure (eg, brisk walking has a MET value of 3, or 3 times the resting metabolic rate).

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