The energy content of foods, or the energy requirements of physical activities, can be measured in a number of ways. Basically there are three categories of measures that are used:
1. Direct calorimetry. Direct calorimetry can also be used to measure energy expenditure in the form of heat loss from the human body. This requires placing a person in a small chamber in which all the heat released is measured. Unfortunately, such chambers are very expensive and are only present in well-established research laboratories. Direct calorimetry can also be used to measure the energy content of food through ‘bomb calorimetry’, as previously discussed.
2. Indirect calorimetry. This calculates energy use directly from measurements of the amount of oxygen (O2) consumed, carbon dioxide (CO2) and nitrogen produced (the latter to eliminate protein metabolism in the equation). This can also be done in a chamber (metabolic or respiratory chamber) over periods of 24 hours or more, or it can be done with a hood system over a period of minutes or hours.
From indirect calorimetry equations, the amount of each type of energy substrate being used can also be calculated. This is expressed as the respiratory exchange ratio (RER), or respiratory quotient (RQ).
The RER, which is based on the ratio of oxygen (02) consumed to carbon dioxide (CO2) produced, can provide information about whether the predominant energy source being used is carbohydrate or fat. The theory behind this is that more oxygen is required to bum fat relative to the amount of carbon dioxide produced. In fact, the ratio of 02 to C02 is around 0.7 when fat is the total fuel being oxidised. On the other hand, when carbohydrate is used as a fuel, equal amounts of 02 and C02 are involved. Where carbohydrate is the sole fuel therefore the ratio of 02 to C02 is 1.0.
At rest, the terms respiratory exchange ratio (RER) and respiratory quotient (RQ) can be used interchangeably, although strictly speaking there is a difference. RER refers to the exchange ratio of gases expired, whereas RQ is a measure of the ratio of fuel use at the tissue level. RER, which is more relevant for our purposes here, is derived from the chemical calculations.
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