*Geek Box: Stable Isotope Tracers
So, what is a “stable isotope”? 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.
You could do this for leucine, if the substrate you intended to ‘trace’ was a protein, and you could do it for glucose if the substrate you intended to ‘trace’ was a carbohydrate. There are other methods, too, that depend on the type of measurement being undertaken. For example, in the present study, doubly-labelled water was used to determine the contribution of DNL to triglycerides.
The use of stable isotopes in nutrition research is a fascinating area and provides nutrition science with a highly accurate methodology to precisely trace the metabolic fate of nutrients through the body.