R. J. Scott Lacombe, Nutritional Sciences
Supervisor: Richard P. Bazinet, Associate Professor and CIHR Canada Research Chair in Brain Lipid Metabolism
PhD Thesis Title: The Application of Natural Abundance Carbon Isotope Ratio Analysis for the Study of Brain Fatty Acid Metabolism
Fat is ubiquitous throughout our diet, but the fatty acid composition of our diets is varied and certain fats in our diets prove to be important beyond basic nourishment. Research from both animal and large human observational studies have linked higher consumption of the long chain omega-3 fat found in fish, docosahexaenoic acid (DHA), to protection against certain brain diseases. Similarly, studies of brain fatty acid metabolism have found disruptions of DHA metabolism to be associated with Alzheimer’s disease. This highlights the importance of studying the influence of diet on the metabolism of specific fatty acids within the brain. Unfortunately, the tools available to investigate this connection are limited, and routine techniques often rely on the use of expensive custom synthesized compounds called tracers. My PhD research focused on developing a new method adapting techniques from forensic analysis and ecological studies, called natural abundance carbon isotope ratio analysis, to study brain fatty acid metabolism.
Isotopes are two atoms of the same element that share the same number or electrons and protons, but differ in their number of neutrons. With regards to carbon, there are 2 stable isotopes, carbon-12 with 6 neutrons and carbon-13 with 7 neutrons. Although typically thought to be static at approximately 1%, the abundance of carbon-13 does vary subtly throughout the environment. Forensic scientists routinely use very highly precise mass spectrometers to measure the carbon-13 to carbon-12 ratio and apply it as a carbon “fingerprint” to provide information on the material origin of their analyte of interest. Because the carbon isotope ratio of a dietary component remains relatively unchanged after consumption, I developed a method to determine the dietary origin of brain fatty acids leveraging this approach.
In my research I used the differences in carbon isotope ratios of marine and terrestrially sourced oils to track the incorporation of dietary DHA into the brains of mice. Furthermore, I was able to distinguish between DHA that was incorporated preformed from the diet and that which had been synthesized endogenously from other precursor fatty acids. Following validation of my methods, I adapted a diet switch experiment commonly used in ecology studies to characterize the metabolism of brain DHA. By manipulating the naturally occurring carbon isotope ratio of the omega-3 fatty acids in the rodent diets, I was able to track the corresponding changes in brain DHA carbon isotope ratios overtime. Modeling these changes yielded an estimation of brain DHA turnover and incorporation from the diet. Importantly, my values matched those determined from conventional techniques that used expensive tracers and more invasive procedures.
In addition to my research on brain DHA, I adapted natural abundance isotope ratio analysis to determine the dietary contributions to an important saturated fatty acid in the brain, palmitate. Palmitate can be synthesized de novo using carbon from dietary sugars or it can be incorporated intact from the diet. Notably, the major sources of added dietary sugars originate from corn and sugar cane, to plants that have unique carbon isotope ratio. Because of the unique carbon finger print of dietary sugar, I was able to use my approach and estimate the contribution of synthesis from dietary sugars to brain palmitate levels, in the process ending a decades old debate regarding the origin of brain palmitate. This method has now reached beyond the study of fatty acid metabolism and is now being investigated as a potential biomarker for sugar intake in an ongoing human clinical trial.
Over the course of my PhD I was able to publish 17 articles in peer reviewed journals including 5 as first author, as well as technical report commissioned by the World Health Organization on which I was the lead author. In addition to publications I was able to present my research on the national and international stage. Among other accolade, I was awarded a Top New Investigator Award from the International Society for the Study of Fatty Acids and Lipids at their biennial congress for my research on carbon isotope ratio analysis. My academic and research achievements throughout my graduate career contributed to multiple awards from the Peterborough K.M. Hunter Foundation and funding support through the Ontario Graduate Scholarship program.
Following my graduate research, I was awarded a fellowship from the Partnership for Clean Competition to help develop improved methods for the detection of anabolic steroid use in high performance athletes. Currently, I am building off of my graduate experience and research on carbon isotope ratio analysis at the Dell Medical School at the University of Texas at Austin. Going forward I plan on continuing research in the area of carbon isotope ratio analysis, both on the technical method development and applications side for anti-doping and nutritional analysis.