*Geek Box: Measuring Gastric Emptying in Humans
Perhaps you may have read that sentence above and thought, “how do you measure food leaving the stomach through the breath?!” The answer lies in yet another excellent use of stable isotopes in nutrition research.
When talking about chemical elements, like nitrogen, carbon, or hydrogen, these elements exist in a form that is abundant in nature. For example, about 99% of the carbon is 12C, which reflects the fact that it has 6 protons and 6 neutrons [adding the protons and neutrons give the element its ‘atomic mass’, in this carbon has an atomic mass of 12, thus ’12C’. However, around 1% of the carbon on Earth has an extra neutron, i.e., with 7 neutrons and 6 protons it has an atomic mass of 13, and is written as 13C.
Now, what does this have to do with nutrition research? Well, recall that carbon is an element in each macronutrient; fats, carbohydrates, and proteins. As such, it is possible to chemically enrich nutrients with less abundant stable isotopes. For example, you could take a fatty acid, and substitute the 12C for a 13C isotope [this would all be done in the lab]. Substituting the more abundant 12C for the less abundant 13C in the fatty acid would then create a ‘tracer’, meaning that it has the same chemical properties of the original compound, but the appearance of the 13C in the body is much more readily identifiable because of its scarcity.
This is helpful to then measure the 13C in different bodily compartments. For gastric emptying, certain fatty acids labelled with 13C are added to foods, often eggs due to their binding affinity, and consumed in a test meal. These stable isotope tracers are not digested in the stomach and pass into the small intestine, where they are rapidly broken down and absorbed. In the process of metabolism of the 13C stable isotope tracer fatty acid, 13CO2 is created [a 13-carbon dioxide], which is released in the breath. The rate at which the 13CO2 appears in the breath reflects the rate at which the food it was bound to left the stomach into the small intestine.
Thus, the production and expiration of the 13CO2 provides a means of measuring the rate of gastric emptying, in a way that is non-invasive for participants. The participants breathe into a test tube, which is sealed. The levels of of 13CO2 in each breath sample are then analysed using mass spectrometry, an advance laboratory technique. Thus, by labelling a nutrient with a less abundant stable isotope, it is possible to ‘trace’ the metabolic fate of that nutrient through the body, depending on the exact measure of interest.