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Good fitness and good metabolism — how does one impact the other? Does being fit automatically translate to metabolic blessings?

Before we get into those questions, I want to first clarify that metabolism does not just refer to your rate of caloric burn — as in fast or slow — as we often hear. It’s much more complicated than that.

Fast or slow metabolisms are the result of literally thousands of normal or abnormal biologic processes in the body — the outcome of countless, intricate signals whose operations both depend on and change based on one another.

That said, being fit is associated with better (and often faster) metabolism. But how?

Many people try to regulate their metabolism without much regard to developing their fitness, while some try to manipulate their fitness without considering the health of their metabolism. The best approach is to give each the attention they deserve. As I see it, fitness and metabolism are codependent, not independent.

We all know that being fit is healthier than being unfit. What’s more important, however, is  exercising consistently and following a progressive fitness program. That’s where your ability to control of all those countless metabolic switches begins.

Let’s break this down a bit.

Fitness allows for metabolic flexibility.

Perhaps the most fascinating observation I’ve made in people who seem to be naturally fit (meaning they make little effort to maintain average or above average markers of physical health and ability) is that they appear to easily burn energy from a variety of dietary and body sources. They’re more metabolically flexible — meaning they can burn fat efficiently at rest and low intensities and are able to switch to glucose metabolism for high intensity efforts — and, to some degree, burn off whatever they eat.

It’s not because they’ve mastered calorie math better than unfit people. Nor is it because they expend more energy (AKA burn more calories) simply by exercising more often either.  

Their metabolic flexibility is likely due to the fact that they “practice” fitness more often. Fit people very likely use every possible metabolic pathway available to human metabolism on a regular basis, whereas unfit individuals spend far more of their time in a single metabolic “gear” close to sedentary.

You see, not only can you measure how many calories one burns at rest or during activities, but you can actually measure what types of calories one is burning, primarily glucose or fatty acids. At rest and low exercise intensities, you should be able to draw most of your energy from fatty acid oxidation, while at higher intensities your primary fuel source is glucose.  

Our mitochondria (the little power plants in our cells that are particularly numerous in our muscle cells) have the ability to adjust their preferred fuel source, much like a hybrid vehicle. In fact, our mitochondria tend to adapt over a period of days and weeks to burn more of whatever types of fuel we feed them.

You are not what you eat, but you will burn more of whatever you eat to a significant degree!

Guess what type of fuel you won’t burn very well if you eat primarily sugar? Fat. Guess what fuel you get better at burning by using every “gear” and “fuel tank” available on a regular basis? Fat.

Anyone can move significantly toward improved fitness and, thus, toward metabolic flexibility. All it takes is the habitual practice of fitness — through all available metabolic pathways. Of course, between bouts of exercise, you also need to nourish your body with a wholesome diet rich in produce and protein, as well as get adequate rest and recovery.

Gaining metabolic flexibility starts with moving often throughout the day, resistance training consistently, and adjusting the types of fuel fed into your system (e.g. if you eat mostly carbs, your body will burn mostly carbs).  

Fitness revs the metabolic engine.

Turning your body into more of a hybrid machine takes a larger effort than simply trying to temporarily boost your metabolic rate with an intense workout.

Establishing a foundation of flexible metabolic functioning is a prerequisite to adding horsepower to your overall metabolic capacity. Once our bodies become adept at using all types of fuel more effectively, it becomes easier to bolt on those shiny new accessories (e.g. more muscle) that require extra investment on our part.

Practicing fitness and exploring new limits of physical abilities (e.g. adding another set of deep back squats, or another interval on the rower) forces our metabolisms to temporarily ratchet up the rate at which we use energy and increases the demand for the protein building blocks that are used for repair.  

In the short term (during and for several hours after each workout), our rate of fuel burn goes up considerably. Over time (days, weeks, and months), with periodic bouts of certain types of workouts, our bodies may be able to adapt by building larger or more numerous muscle fibers altogether (known as hypertrophy). More muscle means more space to store carbohydrate energy (NOT as fat), as well as more energy burned during the time in between our workouts.

A common mistake many fitness enthusiasts make when trying to “tone up” and add power to their metabolic engine is that they under-eat protein and/or calories while not putting enough mechanical resistance on the muscles they’re trying to build.

Building a stronger metabolic engine — and a fitter-looking physique — often requires what may seem like an excess of nutritional building blocks (protein intake), adequate or surplus energy (calories), as well as rather frequent muscle overload (i.e. lots of lifting and lowering of heavy stuff).

That said, this method teems with variables that need to be personalized to each individual. That’s where coaching and training can help with tailoring a plan to fit your individual body and needs.

Fitness enhances longevity and resilience.

Developing fitness with the aim of being metabolically flexible (i.e. tapping into a hybrid of fuel-burning sources) and having a large, powerful metabolic engine (i.e. maintaining a high proportion of lean muscle mass) are good ways to promote longevity and resilience.

Metabolic flexibility and practicing fitness are associated with easier adaptation to any physical or environmental situation. They make you more resilient in the face of any type of internal or external stress you encounter.

If your body has the flexibility to utilize any type of fuel, it can more effectively and efficiently restore a state of homeostasis (balance) between your body and the demands of your environment. Having more muscle mass enables you to encounter more severe stressors with less risk of becoming too weak to overcome the stress encounter itself. As you become more resilient, you improve your odds of increasing longevity and health span — not just years of life, but life in your years.

Gaining metabolic flexibility and building stronger metabolic horsepower will improve your position for more optimal health — and that can’t happen without practicing fitness.

Regardless of whether or not your fitness and metabolism are in an ideal state today, what matters most is that you work toward your personal next level of fitness. While we often get caught up in what we haven’t yet achieved, the simple pursuit of fitness itself puts us on the path toward better metabolic health, not to mention improved overall wellbeing. Every effort counts!

Keep the conversation going.

Leave a comment, ask a question, or see what others are talking about in the Life Time Health Facebook group.

Paul Kriegler, RD, CPT

Paul Kriegler, RD, LD, CPT, CISSN, is the director of nutritional product development at Life Time. He’s also a USA track and field coach.

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