This is part five of a series of blogs from attendees at ACSM's Conference on Integrative Physiology of Exercise. The following blog is a reflection on the symposium "Metabolic Flexibility in Health and Disease" by Ayland Letsinger. 

Carbohydrate and fat are the dominant fuel sources for energy production in our bodies. Everyone needs energy – from the most sedentary individual to the most elite athletes. On the one hand we often have trouble getting athletes to eat enough food given their high daily energy needs, while on the other hand many sedentary people who do not move much are consuming more energy than they need. In a world where individuals crave the best diet for their needs, Lawrence Spriet, Ph.D., FACSM, of the University of Guelph led a symposium on metabolic flexibility at ACSM’s Conference on Integrated Physiology of Exercise. The discussion provided science-based advice for elite athletes, sedentary people and individuals with obesity and type II diabetes. 

Dr. Spriet opened with a complex image outlining the major pathways that allow the tissues in the human body (i.e. skeletal muscle) to use fat and carbohydrate to produce energy. He commented that, “My students hate when I show this diagram, but it is important to see the whole picture of fat and carbohydrate metabolism in order to make informed choices. They call it the ‘Dreaded Metabolic Pathway Diagram.’” The key point was clear: the impressive and tremendous “flexibility” to alternate between carbohydrate or fat utilization to meet the energy demands of the body is critical for optimal performance and health. And this is most impressive when responding to single and repeated bouts of exercise. The following impressive cast of scientists supported this point with elegantly performed studies.   

Carbs: Friends or Foe? 

The high fat/low carbohydrate diet is being touted as a game changer for improving endurance performance and has become common among non-athletes. Louise Burke, Ph.D., FACSM, presented very convincing evidence showing that low carb/high fat diets had no clear benefit for low intensity exercise and simultaneously ruined high intensity performance in endurance athletes. She let the audience know her studies were followed by a barrage of angry Twitter warriors calling her work poor and claiming the studies were not long enough for the athletes to adapt to the diet. However, Dr. Burke and others have shown humans on a low carb/high fat diet DO in fact adapt and have increased ability to utilize energy from fat. Although, primarily using fat as fuel is less metabolically efficient than using glucose (the total energy yield is lower), glycogen (stored glucose) utilization is impaired, and certain high intensity points in endurance races (think of uphill portions or the final sprint to the finish line) requires carbohydrate usage for optimal performance.  In other words, the improved utilization of fat for fuel means losing vital efficiency in glucose utilization. 

The work of Bret Goodpaster, Ph.D., in studying individuals with type 2 diabetes, has revealed a decreased ability to store fuel (fat and carbohydrate) after a meal and severely impaired metabolic flexibility in utilization of fat and glycogen.  Simple exercise and weight loss programs, individually and together, restored much of the lost ability to store fuel and the flexibility to use it in skeletal muscle and the whole body. The messages were clear: you have to move more, and don’t overeat.  

Deb Muoio, Ph.D., closed the symposium by giving a great illustration of how important the mitochondria are for maintaining metabolic flexibility and health. These are the organelles in all of our cells where the majority of our energy is produced. The mitochondrial volume in cells increases when you are physically active and shrink when you are not. She showed that metabolic inflexibiliy is like a traffic jam in the common carbohydrate and fat utilization pathways that exist in the mitochondria. In simpler terms, a high fat/low carb diet is not the secret to health improvement, as exercise is proven to improve optimal metabolic function.

Finding the “perfect diet” for everyone is likely fleeting, but all four presenters agreed on a similar theme: the body’s ability to utilize both carbohydrates AND fats is critical for optimal performance and health. If you decide to remove either substrate, your body will miss that critical fuel.


Read part 1 of this series: "Can Exercise Fill the Reductionist Gap? Reflections on Dr. Michael Joyner's Keynote."
Read part 2 of this series: "Are Exercise 'Mimetics' a Realistic Substitute for Exercise Training? Reflections on the Debate." 
Read part 3 of this series: "Exercise and Energy Restriction to Improve Health: Recent Research." 
Read part 4 of this series: "Molecular Transducers of Physical Activity (MoTrPac) Update." 


Ayland Letsinger is a Doctoral Student at Texas A&M University. Ayland was presented with an award for poster presentation at ACSM's Conference on Integrative Physiology of Exercise in 2018 for the abstract titled: A High Fat/High Sugar Diet Alters the Gastrointestinal Metabolome in a Sex Dependent Manner.