The National Institute of Diabetes & Digestive & Kidney Diseases

Energy Metabolism

Introduction

For essential functions of our body such as maintaining temperature, breathing, and circulation, calories are expended from foods we eat and energy we store as fat. Metabolism is closely linked to body weight. The MCRU is equipped with gold-standard instruments to accurately measure energy expenditure non-invasively. We are also improving techniques to measure human metabolism with cutting-edge technology.

Techniques

• Whole-room respiratory suites

  Recognized as the most precise measurement of daily energy expenditure, this instrument is capable of determining energy expenditure during sleep, rest, meals, exercise, and other conditions. It continuously measures the oxygen you inhale and the carbon dioxide you exhale. Fresh air is continuously supplied to the room during the study, which can last up to 24 hours. There is no known risk to this measurement. In the MCRU, we have three suites each measuring 15x15 ft (30,000 L in air volume). Each suite is equipped with amenities similar to those found in a standard patient room.

• Metabolic carts

  For shorter energy expenditure measurements, such as during resting, after meals, and during certain exercises, a metabolic cart is used to measure the oxygen you inhale and the carbon dioxide you exhale. Unlike the calorimetry suites, where measurements are reported once every minute, metabolic carts can measure changes in oxgyen and carbon dioxide on a breath by breath basis allowing for measurements of rapidly changing phenomena. There is no known risk to this measurement.

• Doubly-labeled water

  In order to measure energy expenditure over several days to several weeks during normal life, we use this gold-standard and non-invasive method. The measurement is performed by giving participants a special water where some of the atoms of hydrogen and the oxygen are replaced with non-radioactive and non-toxic isotopes (deuterium and oxygen-18, which both occur naturally but rarely). The concentrations of the deuterium and O-18 over time can be measured by sampling saliva, urine, or blood. There is no known risk to this measurement.