Indirect Calorimetry

What is indirect calorimetry?

Have you ever wondered how many calories you would burn if you just stayed in bed all day? For people who are critically ill or bedbound, this is a key question. If you’re not moving each day, it’s important to know how many calories you should eat and drink in order to stay healthy — especially if you’re already in the hospital with a serious illness. 

One way for doctors and nutritionists to calculate this metabolic rate is called indirect calorimetry. Indirect calorimetry is a test that measures the gases in the air you exhale as a way to determine how many calories your body burns at rest. Even when you’re not moving, your body breathes in oxygen and breathes out carbon dioxide, which burns calories.

Testing involves lying down and spending a few minutes breathing into a collection container. Common containers include:

• A facemask that fits over your nose and mouth
• An airtight plastic bag called a Douglas bag
• A small plastic canopy hood
• An attachment to a breathing ventilator

During the test, you breathe into the collection container. The collection container is attached to a small machine that analyzes the volume of oxygen and carbon dioxide in each breath and uses those numbers to calculate your metabolic rate at rest.

Using the baseline number of calories calculated by the analyzer, a nutritionist or registered dietitian can figure out exactly how many calories you need to consume while you’re in the hospital.