Nutritional strategy is a hot topic in endurance sports these days with debate raging between whether a fat-centric or carb-centric diet is better. But maybe there is a middle ground. Maybe athletes can develop a nutritional strategy designed around fueling for the energy system they are going to use on a given day. This nutritional strategy is called metabolic flexibility.
In simple terms, Metabolic Flexibility is your body’s ability to switch between fat fueling and carb-fueling. It may not be a surprise to say that a healthy metabolism is flexible. For our purposes here, you can think about metabolism as functioning on a spectrum ranging from fat fueling on one end to carb fueling on the other end. As one consistently trends toward either extreme of the spectrum, their metabolism becomes relatively inflexible.
For example, someone who relies on a carbohydrate-rich diet becomes more insulin resistant which undermines their ability to consume fat and even carbohydrates (which leads to overeating and more weight gain). While this specific metabolic inflexibility is seen most prominently in obese people and type-2 diabetics, it is also common for seemingly healthy people to have a high degree of metabolic inflexibility.
Many endurance athletes, for example, can develop metabolic inflexibility and, eventually, insulin resistance from consistently eating carb-rich diets and fueling on high sugar supplements like gels and sugary electrolyte drinks. The insulin resistance can be masked by high levels of exercise which often helps to curb excess weight gain but, eventually, the symptoms of insulin resistance will manifest in other ways.
Why is flexibility important?
Most importantly, the more metabolically flexible you are, the easier it is for you to eat a diet that is balanced in macronutrients (fat, protein, and carbohydrates). The more balanced your macronutrient composition, the more likely you are to receive the necessary micronutrient (vitamins, minerals, and polyphenols) intake your body needs to thrive and recover from regular physical activity.
When you are metabolically flexible, you can eat the right things at the right time because you can utilize different fuel sources to power different energy systems (i.e. Aerobic = higher proportion fat and Anaerobic = higher proportion glycogen).
It also means you can optimize how you feel from day-to-day by eating in a way that aligns with your energy demands in order to improve sleep, curb hunger cravings, and stabilize your mood and energy levels throughout the day.
How does metabolic inflexibility develop?
Lifestyle is the most common factor. The most common trend in our society today is a sedentary lifestyle combined with the Standard American Diet - characterized by high consumption of processed and refined foods (mostly carbohydrates) and overeating. Both of these factors contribute to insulin resistance and, eventually, overweight and/or obesity and potentially type-2 diabetes.
Another important contributor is mitochondrial dysfunction and low mitochondria levels, both of which can often stem from a poor lifestyle. Too few and dysfunctional mitochondria make it more difficult for your body to produce and utilize energy, especially stored fat. Because your body can’t utilize all of your ingested or stored energy, you experience an increased desire to eat even in the face of excess energy. This creates a negative feedback loop that is detrimental to your health.
How do you become more metabolically flexible?
Eat Paleo + Portion Control
Specifically, more easy running + more weight training
Specifically, more easy running, but also less life stress
The Paleo nutrition template is an ideal template for ensuring your diet balances macronutrients and maximizes micronutrients. There are endless resources to help you understand and learn the Paleo Principles. A good starting place is to eliminate all processed and refined foods and sugars from your diet (except high-quality dark chocolate!). It’s okay to slip up from time-to-time but it is imperative you commit to the life change to succeed and sustain metabolic flexibility and; thereby, a healthier and happier version of yourself!
Portion control is your ticket to controlling energy excess which is a critical contributing factor in developing insulin resistance. If you’re worried about your inability to intuitively settle on reasonable portion control guidelines for yourself, you can test your metabolic rate (i.e. respiratory quotient) and then track your daily exercise and caloric intake for a couple of weeks to get a good sense of how to balance your caloric intake to avoid overeating. And keep in mind that caloric excess should be viewed in a window of about 3 days versus day-to-day. Some days, you may need to eat more and some days you may need to eat less. Be, well, flexible!
Exercise - aerobic activity and strength training - can help in two key ways. First, it improves insulin sensitivity which, in turn, helps to restore your fat burning capability. Second, exercise helps to both repair dysfunctional mitochondria and produce more mitochondria through mitochondrial biogensis.
For runners, both high-intensity interval sessions and long, slow aerobic sessions support mitochondrial biogenesis. For stress management purposes, it is important to strike a balance between intensity and easy aerobic/recovery running. This facilitates a variety of external stressors to produce differentiated adaptation and it helps to minimize stress on the central nervous system that accumulates from high-intensity exercise - specifically the sympathetic branch of the autonomic nervous system.
Aside from recovery running, cold Stress is the primary recovery modality that can aid mitochondrial biogenesis and, thereby, help improve your metabolic flexibility. Cold stress is optimal 30 minutes after exercise (specifically cold tubs). Less than 30 minutes undermines the pro-inflammatory effects that support exercise-induced physiological adaptations. After 30 minutes, you will still realize benefits for mitochondrial biogenesis. You can also incorporate cold showers.
Cortisol is a catabolic hormone; especially, when regularly present in excess. It breaks down healthy muscle tissue which undermines gains in physiological adaptations from exercise. The answer: More recovery running and less life stress!
Running below your Maximum Aerobic Function (MAF) facilitates recovery and does not activate the sympathetic nervous system and corresponding stress hormone response. Mitigating stress in the rest of your life will also help prevent cortisol production (and other stress hormones).
How to use metabolic flexibility to improve performance?
You want the right fuel at the right time. In other words, you want to burn fat when it makes the most sense and carbohydrates when it makes the most sense. A Paleo nutrition approach is inherently lower in carbohydrates and supports your body fueling more from fat. Aerobic training (training at or under MAF effort) also favors burning fat over glycogen. Thus on days when you’re not training or you’re training easy, you need fewer grams of carbohydrate in your diet.
Conversely, on days when you plan to go hard. Your body needs more carbohydrates. In this context, it’s valuable and appropriate to consume a higher concentration of carbohydrates both prior to and during exercise.
You can improve your body’s metabolic flexibility - especially fat adaptation - by focusing at times on extended periods of MAF training and lower carbohydrate eating. This is important because the human body (on average) more easily and preferentially consumes carbohydrates (glycogen) while being slower to adapt to fat consumption.
So, what does periodization look like? During aerobic base building phases, eat a Paleo diet that includes fewer starchy tubers like potatoes and yams. When your training starts to add intensity, start adding more carbohydrate to your diet from sources like fruit and starchy vegetables such as sweet potatoes, yams, and even red or white potatoes if you can tolerate them.
For inter-workout fueling, look for super starch fueling products that are low in sugar. Super starch products won’t spike your insulin or blood sugar and will allow you to cintinue accessing fat as a fule source during extended periods of exercise while also benefiting from extended access to glycogen.