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How Fitness Affects Metabolism


Which comes first: the chicken or the egg? Good fitness or good metabolism? How does one affect the other? Does being fit automatically confer metabolic blessings? Before proposing answers to these questions, I need to clarify that metabolism doesn’t just refer to your rate of caloric burning - as in “fast” or “slow.” 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 that operate in a seemingly endless series of if-then statements. That said, there are genuine metabolic benefits to be had. Being fit is associated with better (and often faster) metabolism. But how?

Many people try to regulate their metabolism without much regard to developing fitness, while some try to manipulate their fitness without paying attention to the health of their metabolism. The best approach is to give each their due attention. We all know that being fit is healthier than being un-fit. What’s most important, however, is practicing fitness. That’s where your control of all those endless metabolic switches begins. Everyone has the ability to move their fitness (and metabolism) towards the optimal end of the biological spectrum. As I see it, fitness and metabolism are co-dependent, not independent. 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 (seemingly 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 energy sources. They’re more metabolically flexible, and to some degree “burn off whatever they eat.” It’s not simply because they’ve mastered calorie math more than unfit people, nor is it just because they expend more energy (a.k.a. burn more calories) becoming or staying fit either. It’s likely due to the fact that they “practice” fitness more often - meaning 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 (with great accuracy), but you can actually measure what types of calories one is burning – primarily glucose or fatty acids. Our mitochondria (the little power plants in our cells, particularly numerous in our muscle cells) have the ability to adjust their preferred fuel source, much like a hybrid vehicle. In fact, these little powerhouses 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? Guess which fuels you get better at burning by using every “gear” and “fuel tank” available on a regular basis? Those who practice fitness habitually tend to end up with the most flexibility in their food choices.

Yet, anyone can move consistently and significantly towards fitness (and, thus, toward metabolic flexibility). All it takes is habitual practice of fitness – through all available metabolic pathways – more regularly than before. Gaining metabolic flexibility starts with moving more often throughout the day, exploring and expanding physical activity limits, and adjusting the types of fuel fed into the system. This is a simplistic view of a highly individualized concept, which is why it’s so important to consider personal assessments to determine your own fastest path to fitness and metabolic flexibility.

Fitness Revs the Metabolic Engine

Turning yourself into more of a hybrid vehicle may not sound all that appealing if you purely desire a faster or more powerful metabolism. However, establishing a foundation of flexible metabolic functioning is a prerequisite to adding horsepower to your overall engine capacity. You see, once our bodies become adept at using all types of fuel more effectively, it becomes easier to bolt on those shiny new accessories (i.e. more muscle) that require extra investment on our part.

Practicing fitness and exploring new limits of physical abilities (e.g. think of adding another set of butt-to-stair squats or another interval on the rower this week) force our metabolism to temporarily ratchet up the rate at which we use energy and increase the demand for the building blocks 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 these periodic bouts of certain types of workouts, we may be able to adapt by building larger or more numerous muscle fibers altogether (known as hypertrophy). More muscle means more space to store energy (NOT as fat) and more energy burned just while sitting around between workouts.

A common mistake many fitness enthusiasts make when trying to “tone up” and add power to their metabolic engine is 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 seems like an excess of nutritional building blocks (protein intake) adequate to surplus energy (calories) as well as rather frequent muscle overload (a.k.a. lots of lifting and lowering of heavy stuff). In this way, practicing a very specific type of fitness can have enormous metabolic effects. That said, the method teems with variables that need to be personalized to each individual reading this. That’s where individual coaching and training can help fill in the picture.

Fitness enhances longevity and resilience.

Developing fitness with the aim of being metabolically flexible (tapping into a hybrid fuel flexibility) and having a large, powerful metabolic engine (maintaining a large proportion of lean muscle mass) is a good way to promote longevity and resilience. You see, being metabolically flexible and practicing fitness is associated with easier adaptation to any physical or environmental situation: it makes you more resilient in the face of any type of internal or external stress you encounter. You’d have the flexibility to utilize any type of fuel to more effectively and efficiently restore a state of homeostasis (balance) between your body and the demands of your environment. Having more muscle mass would allow you to encounter more severe stressors with less risk of becoming too weak to overcome the stress encounter itself.

Whether you’re practicing, developing, or maintaining fitness, know that gaining metabolic flexibility, stronger metabolic horsepower, or better longevity and resilience will put you in position for more optimal health. Regardless of whether your fitness and metabolism are in order today, what matters most here is that you’re pursuing your next level of fitness. While we often get caught up in what we haven’t yet achieved, the pursuit of fitness itself puts us on the continuum toward better metabolic health in this case and better wellbeing overall. Every effort counts!

Thanks for reading, everyone. How have you gotten to know more about your metabolic health in your weight loss journey? How has fitness figured into the picture for you? Share your questions and feedback.

Written by Paul Kriegler - Corporate Registered Dietitian

This article is not intended for the treatment or prevention of disease, nor as a substitute for medical treatment, nor as an alternative to medical advice. Use of recommendations in this and other articles is at the choice and risk of the reader.


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