Nobody likes to waste precious training time at the gym. That is where having an understanding of the concepts of metabolic flexibility comes into play. I had the pleasure of having a discussion with Doctor Mike T. Nelson, a metabolic flexibility expert who has a Ph.D. in exercise physiology and over a decade of experience as a trainer. He authored the e-book Metabolic Flexibility for Fat Loss, which thoroughly covers the concepts of metabolic flexibility and performance. Dr. Nelson helped break down the concepts of metabolic flexibility for Olympic weightlifting and other sports and activities.

What exactly is metabolic flexibility?

Metabolic flexibility simply refers to how efficiently the body switches between using carbohydrates (carbs) or fats for fuel. Insulin is the hormone that "chooses" whether fats or carbohydrates will be used for fuel and can be influenced by diet and eating patterns. Consistently high insulin levels, such as those that may occur with diabetes, tend to cause fat storage and inefficient use of fat for fuel. Improvements in diet and increasing exercise intensity can lower insulin levels and increase insulin sensitivity, which in turn makes the body more adaptable to available fuel sources. Less insulin is needed as the body begins metabolize glucose more efficiently and tap fat stores for fuel when needed (i.e. increased metabolic flexibility). Being metabolically flexible means having enough lean body mass to utilizes fuel sources efficiently, which increases lifting performance. Increasing muscle mass increases insulin sensitivity and keeps insulin levels even. Greater metabolic flexibility also increases strength training performance by increasing the ability to lift repeatedly and with greater power.

How does metabolic flexibility improve lifting performance (and what should people do differently)?

All sports require some degree of alternating between using carbs and fats for fuels, so having an appropriate aerobic base will improve performance. Having a significant aerobic base is not as important for pure Olympic weightlifting; however, a more aerobically trained athlete can output strength and power REPEATEDLY in a shorter amount of time. Most strength training athletes do benefit from incorporating short cardio sessions such as high-intensity interval training (HIIT) and higher-rep complexes with lighter weight into their training plans. Any aerobic training should be complementary to strength goals (i.e. avoid long “endurance” cardio training).

You don't want to train overusing carbs to the point where it is a detriment to the ability to use fat, so prioritize training sessions to the most appropriate fuel source. Pair higher volume training/HIIT with more carbs (and thus increased insulin levels). You want lower insulin levels (fewer carbs the rest of the day) for lower-intensity work to use fat for fuel.

Principles of training to increase metabolic flexibility:

• Set a specific training goal for each day. Nelson recommends doing cardio on non-lifting days (for example, do lift one day and perform low-intensity cardio for recovery the next).
• Incorporating some higher-rep complexes with lighter weight and short HIIT circuits will assist in building an aerobic base.
• Athletes with better aerobic capacity tend to recover faster and feel better, and can handle more high-intensity work for longer.
• For Olympic weightlifting with long rest periods, you want lots of carbs to fuel the muscles. On the recovery day, aim for more fasted/low-insulin state to increase fat burning.
• Switching faster between fuel sources also helps with recovery DURING the workout as well as between training sessions.

How does metabolic flexibility fit into a Paleo diet?

Eating to optimize metabolic flexibility is not about avoiding any particular food group. Rather, timing is key in helping the body optimize fuel sources from foods. Insulin release is stimulated after eating any foods, but to varying degrees depending on the food. Carb sources such as fresh fruit, sweet potatoes, and jicama cause an insulin spike that allows glucose to enter muscle cells and be efficiently utilized for fuel during high-intensity exercise. Lifting requires a large amount of carbs (maximum muscle overload will be compromised by carb restriction), so concentrate higher-carb foods around training sessions.

During intense training sessions, carbs are utilized during the lifts. While taking a break between sets to "refuel," the body is utilizing fat. This is the reason we have to make sure we are eating correct proportions of carbs, fats, and proteins to support our training goals. Training with metabolic flexibility in mind is more concerned with the type of fuel being utilized and the effectiveness of switching between those fuel sources. Those who are metabolically inflexible have difficulty shifting between using carbs and fat for fuel AND have more difficulty metabolizing both of them. That means that the body may not produce enough energy even when there are fuel sources available.

THE BREAKDOWN:

• Insulin is released when any food is eaten and levels can be manipulated with diet. Carbs cause more of an insulin spike than fats or proteins.
• For high-volume and intense training sessions, you will want to have more carbs that day to meet the energy demands.
• To increase fat used, you will want more of a fasting, low-insulin state and lower-intensity activity.
• You will compromise maximum muscle power by eating a diet that severely restricts carbs. Don't do it!

Metabolic flexibility is a massive and complex topic, but it is important for trainers and athletes to understand the basics so that training and nutrition can be optimized. This will save time and frustration at the gym, and improve your health. Try incorporating some of these principles into your (or your clients’) training plans.

OTHER SOURCES

Corpelejin, E., Saris W. H. M., and Blaak, E.E. (2009). Metabolic flexibility in the development of insulin resistant and type 2 diabetes: effects of lifestyle. Obesity Reviews, 10.

Storlien, L., Oakes, N. D., and Kelley, D. E. (2004). Symposium 6: Adipose tissue-liver-muscle interactions leading to insulin resistance. Proceedings of the Nutrition Society, 63.