The heat-producing power of foods is often measured in joules, as they quantify the amount of energy contained within a food item that can be converted to heat during metabolism. The joule is a standard unit of energy in the International System of Units (SI), making it a widely accepted measurement in scientific contexts.
Calories, while commonly used in the context of nutrition to express the energy content of food, are technically a different measurement based on the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. In nutritional contexts, calories can be converted to joules (1 calorie is approximately 4.184 joules), but the primary scientific unit is the joule.
Watts is a unit of power, which is the rate at which energy is transferred or converted, and liters measure volume, which is not related to energy content. Therefore, having verified that joules is the unit that accurately measures heat energy produced from foods confirms why this choice is correct.