The Diet Struggle You Can’t See (But Can Fix)

Metabolic Flexibility – The Hidden Key to Fat Loss and Energy

Take a moment and think about how your body feels when you skip a meal or push through a long workout. Do you feel sluggish, lightheaded, or irritable? Or do you power through, steady and energized?

That difference may come down to something many people haven’t heard of—metabolic flexibility. It’s a trait that could help explain why some people seem to handle fasting, dieting, and training effortlessly, while others feel like they’re constantly hitting a wall. And it might just be one of the most underrated drivers of fat loss, energy, and overall wellness.

At its core, metabolic flexibility is your body’s ability to switch between fuel sources—mainly carbohydrates and fat—based on what’s available and what you need at the moment [Goodpaster & Sparks, 2017]. Ideally, your metabolism should function like a hybrid car engine. When glucose is available, it burns glucose. When food is scarce or you’re in a fasted state, it shifts to burning stored fat. But in many people—especially those with insulin resistance or a sedentary lifestyle—this ability becomes compromised. Their metabolism gets “stuck” in carb-burning mode, unable to efficiently use fat for fuel.

This lack of adaptability isn’t just an inconvenience—it can impact everything from your ability to lose weight to your stamina in a workout. As NIH scientist Dr. Paolo Piaggi explains, people vary dramatically in how they process fuels, and this affects hunger, calorie needs, and how well a diet works for them [Piaggi et al., 2024].

In one key study, participants fasted for 24 hours while researchers measured their fuel usage. Some transitioned smoothly to fat-burning mode. Others didn’t. The next day, everyone was allowed to eat freely—and those who struggled to switch to fat-burning during the fast consumed significantly more calories than their flexible counterparts [Piaggi et al., 2024]. In short, if your metabolism can’t burn fat efficiently, your body will push you to keep eating carbohydrates. This may help explain why some people feel ravenous after skipping meals or struggle to lose weight despite “doing everything right.”

That insight is powerful. It suggests that for some individuals, diet struggles may have less to do with discipline and more to do with metabolic programming. Dr. Piaggi puts it bluntly: “For people with low metabolic flexibility, it's not about willpower—it’s biology” [Piaggi et al., 2024].

But here’s the encouraging part: metabolic flexibility is trainable.

Fat Loss Through Fuel Adaptability
When your body becomes more metabolically flexible, it can dip into fat stores more readily—whether you’re fasting, exercising, or simply between meals. Over time, this adds up to better body composition and energy balance.

For people trying to lose fat, this is a game-changer. A flexible metabolism means your body won’t panic when carbs are scarce—it will just burn fat. That can lead to fewer cravings, more consistent energy, and better fat loss over time [Levy, 2024].

Interestingly, this also challenges the common assumption that low-carb diets work best for everyone. The NIH study suggested that some metabolically inflexible individuals may actually benefit from including moderate carbs in their diets, because their bodies handle glucose better than fat [Piaggi et al., 2024]. For others, who already burn fat efficiently, a low-carb or time-restricted eating pattern may be a great fit.

In the future, nutritionists may assess a person’s metabolic flexibility before recommending any particular eating plan. Until then, we can look at how the body responds to different fuel sources and build from there.

Performance and Energy on Demand
Metabolic flexibility doesn’t just affect fat loss—it’s also a key component of athletic performance and energy regulation. Your body has a limited supply of stored carbohydrates (glycogen), but it carries tens of thousands of calories of fat, even in lean individuals. The ability to access that fat means longer endurance, more stable energy, and less dependency on constant fueling.

Athletes with strong metabolic flexibility can burn more fat at moderate intensities and save glycogen for explosive movements. This gives them an edge in endurance sports and even in mixed-modal training [Cadman, 2021].

But this isn’t just about elite athletes. Even for the average person, a flexible metabolism means fewer mid-day energy crashes, less “hanger,” and better workout recovery. Think of it as building metabolic resilience: your body doesn’t panic when food isn’t immediately available—it adapts, and you stay steady.

Training Your Metabolism: How to Improve Flexibility
Here’s the best part: just like muscles, your metabolism can be trained. Here’s how:

• Aerobic exercise, especially moderate-intensity “Zone 2” training, increases your body’s ability to burn fat by boosting mitochondrial function—the cellular sites where fat oxidation happens [Shoemaker et al., 2023].

• Strength training helps by increasing muscle mass, which improves insulin sensitivity and gives your body more tissue to store and utilize fuel.

• Time-restricted eating or intermittent fasting can gently encourage the body to become more fat-adapted, although results vary.

• Balanced macronutrient cycling—alternating higher-carb and lower-carb meals—may also train the body to switch between fuels more smoothly [Cadman, 2021].

Together, these strategies can “remind” your metabolism how to adapt again. With time, you’ll feel steadier, stronger, and better equipped to manage energy needs as they come.

A Flexible Future
The emerging science of metabolic flexibility is shifting how we approach fat loss and energy. Instead of obsessing over one fuel source or diet plan, we’re beginning to appreciate adaptability—the ability to handle multiple scenarios, adjust to different demands, and stay balanced in a changing environment.

That doesn’t just sound like smart metabolism. It sounds like smart living.

And in the world of fitness, it’s a quality we’re learning to value more and more—not just in how we eat or move, but in how we recover, stay resilient, and show up for what’s next.

Speaking of flexibility, there’s more to it than metabolism. In the next article, we’ll look at physical flexibility—and the often-overlooked connective tissue that makes it possible: fascia. If metabolic flexibility is about internal efficiency, fascial flexibility is about structural resilience. Both matter. Both are trainable. And together, they could be the foundation of how we thrive in our bodies for years to come.

🔬 Sources

• Goodpaster BH, Sparks LM. “Metabolic Flexibility in Health and Disease.” Cell Metabolism. 2017.

• Piaggi P, et al. “Impaired metabolic flexibility to fasting is associated with increased ad libitum energy intake.” Obesity. 2024;32(5):949-958.

• Levy B. “Metabolic Inflexibility Drives Increased Appetite.” NIH IRP Blog. July 2024.

• Cadman P. “Understanding Metabolic Flexibility for Performance.” MitoQ Blog. 2021.

• Shoemaker ME, et al. “Metabolic Flexibility and Inflexibility.” Journal of Clinical Medicine. 2023;12(13):4453.

^{Written by: L.R. Marshall}