Are you Metabolically Flexible?


Metabolic flexibility is the body’s ability to preferentially use fat or carbohydrate as fuel under specific conditions. It is an Essential Physiological process needed for health, weight loss or weight gain and Performance.

Metabolic inflexibility occurs when the body has a limited capacity to switch from one fuel source to another (i.e. carbohydrates and fats). It has  been linked to type II diabetes, insulin sensitivity, obesity and metabolic syndrome.

Ideally, the body adapts to varying conditions based on the metabolic flexibility of our cells. These adaptations are crucial when it comes to weight loss as well as getting the appropriate energy source for activities, such as lifting weights, doing aerobic exercise, sprinting or playing sports.

Metabolically flexible individuals are able to adapt their metabolism to burn fat most of the time and then switch to carbohydrates as the intensity of exercise increases, e.g sprinting and weightlifting.

Insulin: The “Fuel Switch”

Insulin is a key hormone that can effectively switch the body from fat metabolism to carbohydrate metabolism, and vice versa.

Metabolically Flexible individuals are able to switch to fat burning under fasting conditions and when insulin levels are low.

In addition, under fed conditions where insulin is elevated, metabolically flexible individuals are able to switch to carbohydrate metabolism and fat storage.

Diabetes and obesity: A “bad switch”

Individuals with type 2 diabetes as well as those who are obese, tend towards being metabolically inflexibility. Consequently, they derive less energy from fat burning during resting and fasting conditions and have poor glycemic control.

Lean body mass: A “good switch”

Lean individuals and those with more lean body mass tend to have a greater ability to suppress fat burning during insulin-stimulated conditions (after eating) and therefore shift to carbohydrate metabolism. This is favorable for performance and exercising as well as preventing ingested carbohydrates from being stored as fat.

How do we measure Fat and Carbohydrate Burning?

Respiratory Exchange Ratio (RER)

The ratio between the amount of CO2 produced and O2 consumed in one breath is referred to as the respiratory exchange ratio (RER).

Measuring this ratio can be used for estimating the respiratory quotient (RQ), an indicator of which fuel (carbohydrate or fat) is being metabolized to supply the body with energy. It is also indicative of the intensity of an activity.

A RER of 0.70 indicates that fat is the predominant fuel source, a RER of 0.85 suggests a mix of fat and carbohydrates, and a value of 1.00 or above is indicative of carbohydrate being the predominant fuel source.

RER and metabolic flexibility:

  • When we ingest carbohydrates, metabolic flexibility helps us control blood glucose, by burning glucose instead of fat (The RER should be high).
  • When we ingest fat without carbohydrate, metabolic flexibility helps us burn the fat instead of storing it. (The RER should be low).
  • When we fast, metabolic flexibility helps us burn rather than store fat. In addition, it reduces sugar cravings and catabolism (The RER should be low).
  • During exercise, metabolically flexible individuals are able to mobilize and  burn more stored fat and  perform an activity longer before fatiguing.

Metabolic inflexibility often starts with an inability to oxidize fats in response to abnormal weight gain, improper fasting, poor diet, and/or chronic cardio along with calorie restriction. It adversely affects one’s ability to switch between fuel sources. Consequently, the RER, or RQ, is stuck in the middle. This can cause:

  • Poor glycemic control
  • Inefficient fat burning, especially after consuming a fatty meal
  • Decreased post-prandial thermogenesis  (the rate at which food is broken down after a meal and used by your body)
  • Carbohydrate cravings
  • A reduction in one’s metabolic rate under fasting conditions
  • Diminished fat burning during exercise and a greater demand for glucose
  • Blood sugar spikes and crashes, making us more dependent on stimulants and crave carbohydrates


How can we achieve optimum Metabolic Flexibility?

  • Moderate aerobic exercise, especially when combined with a high fat (e.g. ketogenic)  diet can help restore your ability to oxidize fat as well as support optimum insulin levels. Eating more fat leads to burning more fat
  • Increasing lean body mass and reducing fat through resistance training and High Intensity Interval Training. Fat loss can restore your ability to absorb and oxidize glucose
  • Cold induced thermogenesis increases Brown Adipose Tissue and causes significant insulin sensitivity, nutrient partitioning, and mitochondrial biogenesis (a defect in the mitochondria has been linked to metabolic inflexibility)
  • Moderate protein consumption, e.g. 0.7-0.8 g/lb
  • Fast one day a week. Work up to 24 hours by extending your fast after waking
  • Increase omega 3 fatty acids
  • Drink Green Tea


For more information on this topic, please see references below:


Battaglia, Gina M.; Zheng, Donghai; Hickner, Robert C.; Houmard, Joseph A. Effect of exercise training on metabolic flexibility in response to a high-fat diet in obese individuals. American Journal of Physiology – Endocrinology and Metabolism. Vol. 303 (12). DEC 2012. E1440-E1445.


Corpeleijn, E., Saris, W. H. M., & Blaak, E. E. (2009). Metabolic flexibility in the development of insulin resistance and type 2 diabetes: effects of lifestyle.obesity reviews, 10(2), 178-193.

Most, J., Goossens, G. H., Jocken, J. W. E., & Blaak, E. E. (2014). Short-term supplementation with a specific combination of dietary polyphenols increases energy expenditure and alters substrate metabolism in overweight subjects.International Journal of Obesity, 38(5), 698-706.


Storlien, L., Oakes, N. D., & Kelley, D. E. (2004). Metabolic flexibility.Proceedings of the Nutrition Society, 63(02), 363-368.


Stump, C. S., Henriksen, E. J., Wei, Y., & Sowers, J. R. (2006). The metabolic syndrome: role of skeletal muscle metabolism. Annals of medicine, 38(6), 389-402.


I would like to give credit to Mike T. Nelson, PhD,  who has a applied a lot of the work on Metabolic Flexibility to the field of sports nutrition, for weight loss, weight gain and performance enhancement.

About the Author

Dr. Geoff LecovinNaturopathic Physician/Chiropractor/Acupuncturist/Certified Strength and Conditioning Specialist/Corrective Exercise Specialist/Performance Enhancement Specialist/Certified Sports Nutritionist/View all posts by Dr. Geoff Lecovin