Energy expenditure of humans can be measured by which of the following methods?

Oxygen consumption is the correct method for measuring the energy expenditure of humans because it reflects the amount of energy the body is using during various activities. When the body metabolizes food, it uses oxygen to convert macronutrients (carbohydrates, fats, and proteins) into energy. This process is part of cellular respiration, and measuring the volume of oxygen consumed provides a direct indication of metabolic activity and energy expenditure.

This method is particularly reliable because it can be accurately measured through various techniques, such as indirect calorimetry, where the volume of oxygen inhaled and the volume of carbon dioxide exhaled are analyzed. A higher level of oxygen consumption typically correlates with higher energy expenditure, making it a practical choice for assessing an individual's metabolic rate during rest and physical activity.

Other options do not directly measure energy expenditure. For instance, heat intake refers to the energy consumed in the form of heat, while food intake represents the caloric input rather than the output or expenditure of energy. Nitrogen output is related to protein metabolism and does not provide an overall measure of energy expenditure, as it focuses on the balance of nitrogen in the body, indicating protein synthesis and degradation rather than total energy use.