To quote Sparks et al. 2009, metabolic flexibility is the capacity for skeletal muscle to acutely shift its reliance between lipids and glucose during fasting or in response to insulin, such as in postprandial conditions.
Why is this important? When you are metabolically flexible you can access all fuels, preserving glucose for when it is required and using fat as your primary fuel source (rather than carbohydrate). This is the preferred metabolic state of the human.
It is estimated that only about 15% of the Western population is metabolically flexible. In today’s society there are many factors that disrupt metabolic flexibility, especially a modern diet and the associated metabolic disorders including insulin resistance and diabetes. These metabolic conditions are linked to varying degrees of metabolic inflexibility.
Are you part of the 15%? Five signs that you’re metabolically flexible are:
1. You have stable blood sugar levels
Burning fat for energy will break the vicious cycle of the blood sugar rollercoaster, or the severe highs and lows that accompany a sugar-burning metabolism. This is often one of the first benefits observed with metabolic flexibility, and a sign you are keeping your insulin levels in check. The metabolic benefits of low insulin are many, including a body that is primed to burn, rather than store, fat.
2. You have excellent satiety between meals
Can you go 5 hours without needing your next meal or snack? Appetite control is one of the many amazing benefits of metabolic flexibility. On the other hand, if you are always carrying a muesli bar or piece of fruit “just in case you get hungry”, then it is likely that you have poor blood sugar control, a sugar-burning metabolism and a body that is geared to store fat. We only need to look to our ancestors to see that periods without food are extremely healthy, but unfortunately our current constant supply of food, and incorrect education and media messaging around meal times and frequency, have us bound by our appetites and resorting to purely calorie restriction in a futile attempt to shift body fat.
3. Elimination of the ‘hangries’ (hungry + angry)
The stable blood sugar created by metabolic flexibility can be quite life changing as you are no longer dependent on your appetite, or counting down the minutes until your next meal (or sugar fix). Too many of us experience the hangries on a daily basis and accept that it’s normal, when it’s a complete by-product of our previous meal choice/s. Shifting towards burning fat for fuel not only keeps blood sugar levels in check, but it prolongs energy and doesn’t leave room for mood swings and the hungry/angry feeling that comes with a sugar-burning state and the urgent need for your next fix.
4. You can extend your overnight fast
The word breakfast can be defined as “the first meal of the day especially when taken in the morning”, however it really means “breaking the fast” and does not need to be at 7am (or any defined time of the day). Extending your overnight fast, and comfortably eating your first meal mid-morning (for example), is an excellent example of the capacity of a fat-burning metabolism. If you have always eaten breakfast at 7am you can expect to feel hungry at this time, but often it is ghost hunger as your body has been programmed this way over many years. Over time you can reverse this habit and the more you practice fasting, the easier it will become. Please note: extended fasting is not suitable for everyone, including children, breast-feeding or pregnant women, and those with medical conditions if unsupervised.
5. You have the ability to exercise in a fasted state for 2+ hours
If you can exercise in a fasted state for over 2 hours, you are definitely fat adapted. A sugar-burner will use up their glycogen stores very quickly and therefore require an exogenous source to continue exercising beyond the 2-hour mark. When your body relies on fat as its main source of energy during exercise, there is more than double the amount of calories available (carbohydrates contain 4 calories per gram, while fat provides 9 calories per gram). This fat-burning ability also has a glycogen sparing effect, so that the carbohydrate stored in muscle is available to support high intensity activity (where it is most required). This is one of the major benefits of metabolic flexibility for endurance athletes, as they often exercise for greater than 2 hours however still need to maintain their speed or “top end”, which required glucose as the substrate.
How to get there
The process of developing metabolic flexibility starts with real food and usually, lowering your intake of carbohydrates. The initial phase is 8–12 weeks, and full metabolic efficiency can take up to two years to completely develop. Please keep in mind that this is highly dependent on many factors including the individual, their metabolic health and the appropriate degree of carbohydrate restriction
Want to learn more? As always you can book your complimentary 15-minute virtual consultation here as we would love to guide your journey to becoming metabolically flexible.
Kelley DE, Mandarino LJ. Fuel selection in human skeletal muscle in insulin resistance: a reexamination. Diabetes. 2000 May;49(5):677-83.
Sparks LM, Ukropcova B, Smith J, Pasarica M, Hymel D, Xie H, Bray GA, Miles JM, Smith SR. Relation of adipose tissue to metabolic flexibility. Diabetes Res Clin Pract. 2009 Jan;83(1):32-43.