Metabolism is a series of chemical reactions that extract energy trapped in food to fuel bodily functions. Wilbur Olin Atwater and Francis Gano Benedict, pioneers in the area of human nutrition and metabolism, built an apparatus that measured heat and gaseous exchange with the outside environment to quantify and describe human metabolism. This apparatus, known as Atwater & Benedict calorimeter, was the first research tool to study human nutrition.
Calorimetry
Calorimetry is an area of science that accounts for conservation of energy and the laws of chemical transformation in metabolic breakdown to study metabolism. Gas exchange directly relates to chemical reactions in the human body, which needs oxygen to burn macronutrients and make energy. Carbon dioxide is a byproduct of this process.
Metabolism
The three types of macronutrients -- proteins, fats and carbohydrates -- contain carbon and oxygen, but in a slightly different proportion. Depending on the carbon-oxygen ratio in the chemical composition of a nutrient, the amount of oxygen consumed and carbon dioxide produced can be different. The ratio of carbon dioxide eliminated to oxygen consumed is the Respiratory Quotient. Respiratory Quotient is one of the key criteria used to describe metabolism.
Respiratory Quotient
In simple sugars like glucose, RQ is equal to 1. Fat is the most oxidized macronutrient and will yield RQ around 0.7 -- the amount of oxygen consumed to metabolize fats is less than the amount of CO2 eliminated in the process. Protein RQ calculations are more complex since it is the only macronutrient that contains nitrogen. The value of RQ for proteins is slightly higher than that of fat. An individual on a mixed diet composed of protein, fat and carbohydrate would yield an RQ value of 0.85.
Basal Metabolic Rate
RQ calculates another important characteristic of metabolism, Basal Metabolic Rate, or BMR. BMR is the amount of calories burned at rest. BMR values vary depending on height, weight and age. Knowing BMR helps to establish a calorie budget and come up with the most efficient workout plan. Fit individuals have a higher BMR since muscle requires more energy to build and maintain.
Applications
The U.S. Department of Agriculture Human Nutrition Research Center uses calorimeters to formulate caloric requirements for healthy weight. Researchers also use calorimeters to study the metabolism of individual nutrients and assess how people with various body compositions respond to different diets. Devices built based on the principles of Wilbur Olin Atwater and Francis Gano Benedict are common in the fitness industry, as they allow nutritionists to measure BMR and make individualized diet and fitness recommendations.
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