In this post, I will discuss the Respiratory Quotient: what is it; how do you go about testing for it; what do the test results mean; and how do you then use it.
The respiratory quotient is a number unique to your lifestyle and body composition that shows up primarily in the world of fitness and nutritional health. Used to gain a better insight into the inner workings of your metabolism, this number can help you gain insight into how your body is processing your diet and harvesting energy.
Your body uses energy simply to stay alive. How much energy to stay alive, is called your basal metabolic rate. But most of us move around throughout the day, adding extra caloric expenditure into the mix.
You need to get this energy from somewhere.
Usually, this energy comes from food. When you haven’t eaten in awhile, your body uses stores of energy from stored glycogen and body fat.
Your body oxidizes these various substrates to harvest energy for your cells to reproduce, muscles to grow, brains to think, and so on.
Your respiratory quotient is the direct measure of carbon dioxide production against oxygen utilization. What this information provides is a measure of where your body is primarily getting its fuel from.
In other words, which substrate is being oxidized more for your body’s energy needs: carbohydrates or fats?
Many fitness centers and wellness clinics will perform respiratory quotient tests, because unfortunately this is not something you can easily do at home. It requires a bit of equipment and a trained professional.
You typically need to fast for some amount of time. For my test, I fasted for at least 12 hours before the test. This is because recent intake of any foods can significantly alter the results.
It’s also important not to do much physical activity beforehand. This, too, can impact the test results.
The test itself is rather simple. You sit still. While you sit, recline, or lay, you wear a mask that covers your mouth and nose. You’ve likely seen these kinds of masks on people performing a VO2 max test. The mask connects to a hose, which is connected to a computer and a machine. You also wear a heart rate monitor.
And that’s it! You sit and breathe for around 20 minutes, because it is useful to have some time to get relaxed (usually only around 5 minutes of continuously clean data is needed).
This test is also the same conditions required to assess your basal metabolic rate (or, resting metabolic rate). So, make sure both numbers are calculated and you can kill two birds with one stone!
Your respiratory quotient is a decimal between 0.7 and 1.0. A normal range is typically around 0.8 - 0.85.
The results may also be broken down by macronutrients to represent how much of each substrate your body is deriving its energy from. Here is a pie chart of my recent test results showing the breakdown.
A respiratory quotient of 1.0 essentially means that all of your energy is derived from the metabolism of carbohydrates.
The reason for this number takes one back to college chemistry. Remember, the respiratory quotient is the ratio of oxygen consumption to carbon dioxide expulsion. Glucose metabolism breaks down in a 1-to-1 equivalent of oxygen to carbon dioxide. The equation below shows that 6 oxygen molecules are used to produce 6 carbon dioxide molecules.
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
A respiratory quotient of 0.7 is the opposite, meaning that all of your energy is derived from the metabolism of fat. While different fatty acids have different ratios, palmitic acid gives an example of producing a respiratory quotient 16 CO2 / 23 O2 = 0.696.
Complete protein metabolism has a respiratory quotient of around 0.81. Similar to fat, this is an average because protein as a term embodies all of your amino acids. The body doesn’t contain large stores of protein for energy metabolism. However, some protein is metabolized internally. And the rate of protein turnover does not differ significantly between undernourished and well-nourished individuals.
These numbers for each macronutrient constitute energy being derived solely from these categories. But because we almost never exist in this state, the number falls somewhere between them and exists as a combination of these macronutrient substrates.
Some studies have investigated the relationship between various aspects of body composition, like body weight, fat mass, and fat-free mass to the respiratory quotient and found significant correlations to all. Generally, the higher the body weight, fat mass, or fat-free mass, the lower the respiratory quotient; lower body weight and fat mass led to higher respiratory quotients.
The logic here follows that fat oxidation is directly related to the amount of free circulating free fatty acids in your body. The less you have, the less your body can metabolize and derive energy from them. Thus, carbohydrates and glycogen stores occupy a greater percentage of your energy source.
The food quotient is a number that is similar to the respiratory quotient. It is designed to represent the macronutrient breakdown of your diet. Some researchers have hypothesized that the food quotient should equal the respiratory quotient for those with stable weight and in energy balance. Correlations have been found, but this relationship is not always clear.
One stark example comes from Jewish physicians investigating metabolism in the Warsaw ghetto of the 70s. Chronically undernourished individuals had respiratory quotients over 0.95. Later, other researchers also showed that individuals at stable weight but that are chronically energy deficient have higher respiratory quotients, typically over 0.90 (whole well-nourished individuals were closer to 0.83).
However, in all cases, increasing carbohydrate consumption has a percentage of one’s diet increased the respiratory quotient. So, a high respiratory quotient also reflects recent dietary patterns alongside body composition.
One study investigated the relationship of the respiratory quotient to the food quotient, to determine whether the respiratory quotient correlates well to measures of recent dietary patterns. They did not find a good correlation during their 10-week study.
Rather, they found a strong correlation between the respiratory quotient and a recent change in body mass. Having lost body mass in the past 5-7 days was correlated with a lower respiratory quotient (closer to 0.8 and below), while increasing body mass was associated with a higher respiratory quotient (closer to 0.9).
Another study showed similar results in obese women. This, in part, explains why it is difficult to keep off fat after weight loss. As your body loses weight, your degree of fat oxidation decreases and thus your propensity to store the fat you ingest rises.
However, some of your respiratory quotient may be genetic. Your body’s tendency to oxidize fat has been shown to follow familial lines to some degree. And other studies have identified particular genes that are associated with high and low fat-oxidation. Thus, some variability is to be expected, despite similar body compositions and dietary patterns.
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Your respiratory quotient gives you insight into your dietary patterns, your body composition, and your metabolism. Currently, one of the best ways to use this information is to help inform future personalized dietary patterns that may be more beneficial for your dietary goals.
Most well-nourished individuals will exhibit similar respiratory quotients and basal metabolic rates. This is, usually, a good thing.
But sometimes individuals may be outside of the norm. This is useful to identify and may indicate either skewed dietary patterns, rapidly changing body composition, or potentially chronic undernutrition.
For example, studies have shown that low fat oxidation (high respiratory quotient) is associated with future weight gain. The theory being that those who metabolize carbohydrates better than fat will naturally store more fat over time, leading to weight gain.
And another review article corroborates this research, connecting genetics and recent body composition changes to your respiratory quotient as a measure of past and predictor of future weight maintenance.
Particularly in obese individuals, their metabolic “flexibility” (ability to switch metabolic oxidation easily from carbohydrate to fat) is diminished and makes losing weight and keeping it off more difficult. This may imply certain diets or fat oxidation stimulants may be beneficial for these individuals.
Due to the inherent variability among individuals and their respiratory quotient, it is sometimes difficult to identify the precise mechanisms behind skewed, but not extreme, values. What can be useful, however, is monitoring your own value over time.
By tracking your respiratory quotient against possible changes in weight and diet, you may be able to verify healthy and stable metabolic activity, or potentially identify issues that are a cause for concern or help better understand the results you are seeing with weight management.
Ultimately, your respiratory quotient can help you understand how your body is oxidizing various fuel sources. It can be difficult to parse out exactly what is the source, but trends exist.
For example, high body weight, high fat mass, and high fat-free mass tend to associate with a lower respiratory quotient; recent weight gain, high carbohydrate consumption and chronic undernutrition tend to associate with a higher respiratory quotient.
The test is fairly simple if you have means to a facility that does this testing. After a short fast, you simply need to sit quietly with a mask and heart rate monitor for less than half an hour.
Once you have an understanding of your dietary patterns and metabolic substrate, you can begin to personalize your fitness regimen and track your dietary intake to meet your dietary and fitness goals.
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