Metabolic Flexibility: How To Use Carbs AND Fat As Fuel
Setting The Dogmatic Diet Record Straight Once And For All

By Mike T Nelson, PhD

metabolic flexibility

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Introducing Functional Strength Training: 
The Monthly Membership Training Solution For People Who Want To Look, Feel And Function Their Very Best, Forever.

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The Grand Debate: Carbs vs. Fat for FUEL…

Stop me if you have heard this one…

“Fat is the best fuel and you need to stop being a sugar burner for better body composition.” Or, “Carbs are evil and should not be eaten.”

And then the inverse, “Carbs are a requirement for the high intensity exercise like interval work and weight training.”

No wonder you are confused.

This debate about what fuel is the best source to use is a very stupid debate in fitness because it contains the built-in assumption that you must pick one fuel. You don’t.

I sat down with Dr. John Rusin to discuss metabolic flexibility and more at the HQ last month, and man was this a fun conversation full of debunking common dietary myths with science, and common sense:

So What Fuel Does YOUR Body Need To Perform?

The best fuel is the one that fits the task at hand. It would be like arguing what is better – a screwdriver or a hammer? Both can be great, but it depends on the task you want to accomplish. If you want to get a screw into a board, a hammer is a crappy tool.

The same concept applies to fuel you want your body to use; what you are doing is important.

If you are going to perform some high-intensity exercise or some awesome Functional Hypertrophy Training with Dr. Rusin, then carbohydrates are going to be your preferred fuel of choice as you can generate ATP (the cellular energy currency) at a much faster rate.

When you are resting in-between training days and hanging out doing some light reading or even doing a light walk, then fat is a much better fuel. You want to use the right fuel at the right time; employ the concept of metabolic flexibility. Teach your body to use both fat and carbs – just at the right time and to the highest degree.

Aerobic vs. Anaerobic Metabolism

If we take a dive down the nerd chute, the 2 basic processes that create energy in the body are:

  1. Aerobic – the use of oxygen to create ATP and
  2. Anaerobic – oxygen is not required to produce ATP.

While it is an oversimplification, aerobic metabolism uses fat as a fuel for lower intensity work (hanging out at rest, a light walk, low intensity run / bike, etc.).

Anaerobic metabolism is best for the creation of ATP at a faster rate such as higher intensity work (intervals, weightlifting, etc.) which is fueled by carbohydrates, mainly from glycogen (stored carbs in the liver and muscles).

Both processes and fuels have their benefits depending upon what you are doing. Anaerobic use of carbohydrates is able to generate a fair amount of ATP in a very fast rate AKA a very short period of time. This is to your advantage when you are lifting and trying to do more rep work. The downside is that it creates less overall ATP.

metabolic flexibility

Fatty acid oxidation (techy word for burning fat as a fuel) can create almost 3 times as much fuel from the same amount of starting substrate, however it is a very slow process.  Trying to use only fat to fuel your intense weight training sessions is like expecting a Prius to outrun a Ferrari. The Prius may go much longer on less fuel, but it is not nearly as fast. For the record, I do NOT drive a Prius, or a Ferrari.

I can see the PubMed ninjas and their dormant glute max fibers getting ready to pounce as they yell at me, “Hey, dimwit, your body can use carbs via both aerobic and anaerobic pathways. Go back to school.”  While it is true that carbohydrates can be used both in aerobic and anaerobic processes, the goal here is to keep it simple yet honor the concept of context. That is also why I did not discuss other intermediate fuels like ATP-PC, lactate, pyruvate, ketones, etc.

Metabolic Flexibility Training Template

Based on this concept of metabolic flexibility, below is a typical good starting template that is right from the Flex Diet Certification.

Training + Nutrition: Monday – Wednesday – Friday

  • Weight Training / HIIT work
  • Higher carbohydrate intake
  • Push insulin up around training
  • Teach the body to use carbs as a fuel
  • Bracket some carbs pre / post training
  • The goal is to fuel high intensity work
  • Molecular target is primarily mTOR1
  • More of a sympathetic day

Training + Nutrition: Tuesday – Thursday – Saturday – Sunday

  • Lower intensity aerobic work
  • Heart rate around 110 -140 bpm
  • Ideally done fasted in the AM
  • Lower insulin
  • Teach the body to use fat as a fuel
  • Lower carbs overall (min is about 120 grams/ day)
  • Molecular target is primarily AMPK
  • More of a parasympathetic day

This gives you a nice blend of training for performance in a fueled state and health / body comp benefits of using fat as a fuel.   Plus the aerobic days serve as recovery days from the more intense weight training work.

The ideal is to use both fats and carbs as a fuel. Neither one on its own is superior since it depends on the exercise you are going to perform.  The adaptability of using both types of fuels can help you maximize in the gym along with body composition and health.

About The Author

mike t nelson

Dr. Mike T. Nelson CSCS has spent 20 years of his life learning how the human body works, specifically focusing on how to properly condition it to burn fat and become stronger, more flexible, and healthier.  He has a PhD in Exercise Physiology, and a MS in Mechanical Engineering (biomechanics). His main research interests are human performance, heart rate variability, and metabolic flexibility.  In his free time he enjoys spending time with his wife, lifting odd objects, and kiteboarding. Find out more at

Selected References

Apostolopoulou, M., Strassburger, K., Herder, C., Knebel, B., Kotzka, J., Szendroedi, J., & Roden, M. (2016). Metabolic flexibility and oxidative capacity independently associate with insulin sensitivity in individuals with newly diagnosed type 2 diabetes. Diabetologia, 59(10), 2203-2207. doi:10.1007/s00125-016-4038-9

Boyle, K. E., Friedman, J. E., Janssen, R. C., Underkofler, C., Houmard, J. A., & Rasouli, N. (2017). Metabolic Inflexibility with Obesity and the Effects of Fenofibrate on Skeletal Muscle Fatty Acid Oxidation. Horm Metab Res, 49(1), 50-57. doi:10.1055/s-0042-111517

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Gribok, A., Leger, J. L., Stevens, M., Hoyt, R., Buller, M., & Rumpler, W. (2016). Measuring the short-term substrate utilization response to high-carbohydrate and high-fat meals in the whole-body indirect calorimeter. Physiol Rep, 4(12). doi:10.14814/phy2.12835

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Lund, J., D, S. T., Wiig, H., Stadheim, H. K., Helle, S. A., J, B. B., . . . Jensen, J. (2018). Glucose metabolism and metabolic flexibility in cultured skeletal muscle cells is related to exercise status in young male subjects. Arch Physiol Biochem, 124(2), 119-130. doi:10.1080/13813455.2017.1369547

Parker, S. J., Svensson, R. U., Divakaruni, A. S., Lefebvre, A. E., Murphy, A. N., Shaw, R. J., & Metallo, C. M. (2017). LKB1 promotes metabolic flexibility in response to energy stress. Metab Eng, 43(Pt B), 208-217. doi:10.1016/j.ymben.2016.12.010

Pascual, F., & Coleman, R. A. (2016). Fuel availability and fate in cardiac metabolism: A tale of two substrates. Biochim Biophys Acta, 1861(10), 1425-1433. doi:10.1016/j.bbalip.2016.03.014

Pedersen, M. H., Svart, M. V., Lebeck, J., Bidlingmaier, M., Stodkilde-Jorgensen, H., Pedersen, S. B., . . . Jorgensen, J. O. L. (2017). Substrate Metabolism and Insulin Sensitivity During Fasting in Obese Human Subjects: Impact of GH Blockade. J Clin Endocrinol Metab, 102(4), 1340-1349. doi:10.1210/jc.2016-3835

Peterson, C. M., Zhang, B., Johannsen, D. L., & Ravussin, E. (2017). Eight weeks of overfeeding alters substrate partitioning without affecting metabolic flexibility in men. Int J Obes (Lond), 41(6), 887-893. doi:10.1038/ijo.2017.58

Peterzan, M. A., Lygate, C. A., Neubauer, S., & Rider, O. J. (2017). Metabolic remodeling in hypertrophied and failing myocardium: a review. Am J Physiol Heart Circ Physiol, 313(3), H597-h616. doi:10.1152/ajpheart.00731.2016

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Stellingwerff, T., Spriet, L. L., Watt, M. J., Kimber, N. E., Hargreaves, M., Hawley, J. A., & Burke, L. M. (2006). Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration. American Journal of Physiology-Endocrinology and Metabolism, 290(2), E380-E388. doi:10.1152/ajpendo.00268.2005

Storlien, L., Oakes, N. D., & Kelley, D. E. (2004). Metabolic flexibility. Proc Nutr Soc, 63(2), 363-368. doi:10.1079/pns2004349

Trexler, E. T., Smith-Ryan, A. E., Wingfield, H. L., Blue, M. N., Roelofs, E. J., & Hirsch, K. R. (2017). Dietary macronutrient distribution influences postexercise substrate utilization in women: a cross-sectional evaluation of metabolic flexibility. J Sports Med Phys Fitness, 57(5), 580-588. doi:10.23736/s0022-4707.16.06284-8

Trico, D., Baldi, S., Frascerra, S., Venturi, E., Marraccini, P., Neglia, D., & Natali, A. (2016). Abnormal Glucose Tolerance Is Associated with a Reduced Myocardial Metabolic Flexibility in Patients with Dilated Cardiomyopathy. J Diabetes Res, 2016, 3906425. doi:10.1155/2016/3906425

Zhang, S., Hulver, M. W., McMillan, R. P., Cline, M. A., & Gilbert, E. R. (2014). The pivotal role of pyruvate dehydrogenase kinases in metabolic flexibility. Nutr Metab (Lond), 11(1), 10. doi:10.1186/1743-7075-11-10

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One Comment

  1. Lewis Siwel Walters-Girout September 20, 2018 at 3:15 am - Reply

    Thanks Dr John Rusin
    Im studying for my Sports science diploma and this was a good read I always read your mails.hopefully when I’m done studying I can invest in your programme.Thanks once again.
    President SAFF
    Lewis Walters-Girout.

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