If you’ve ever felt like your body is “stuck”, you may be dealing with poor metabolic flexibility. Maybe you’re always hungry, energy crashes, struggle to lose fat, need sugar/caffeine just to function. These are all classic symtoms of poor metabolic flexibility.
Metabolic flexibility is one of those concepts that sounds technical but is actually very practical.
It affects:
- your energy levels
- your appetite and cravings
- your blood sugar control
- your ability to use fat and carbs appropriately
- your exercise performance
- your long-term risk of metabolic disease
It’s a concept that overlaps with what many doctors, researchers, and health commentators (including Jason Fung, Robert Lustig, Richard Johnson, Gary Taubes, and Zoë Harcombe) have been discussing for years from different angles: insulin resistance, sugar/fructose load, metabolic syndrome, and the importance of giving the body opportunities to use stored energy.
What is Metabolic Flexibility?
In the research, metabolic flexibility is generally described as the body’s ability to adapt fuel use to changing conditions — for example, switching between using more carbohydrate or more fat depending on:
- whether you’re fed or fasted
- rest vs exercise
- exercise intensity
- energy availability
- hormone signals (especially insulin)
A metabolically flexible person can usually:
- use carbohydrate well when it’s available/needed (e.g., after meals, high-intensity work)
- use fat well when carbohydrate is not immediately available (e.g., between meals, lower-intensity work, overnight fasting)
A metabolically inflexible person tends to struggle with that switching. This is commonly seen in obesity, insulin resistance, and type 2 diabetes, where fuel handling becomes less adaptive.
Why This Matters
Think of it like a hybrid car. A well-functioning hybrid can switch efficiently between battery and fuel depending on demand. A poorly functioning system burns the wrong thing at the wrong time, wastes energy, and performs badly. Your body is similar.
If you can’t access stored fuel well, you may feel:
- hungry again too soon
- “flat” between meals
- dependent on snacks
- sleepy after meals
- low energy during long sessions
- prone to crashes and cravings
That doesn’t automatically mean carbs are “bad.” It means your system may be struggling with fuel selection and regulation.
How Metabolic Health Becomes Distrupted
Metabolic dysfunction results in more than just a change in your scale weight. It can lead to a whole host of chronic diseases.
1) Insulin resistance and hyperinsulinemia matter
Jason Fung and Gary Taubes frequently emphasize that metabolic disease is not just about calories in/calories out in a simplistic sense, but about hormonal regulation (especially insulin), fuel partitioning, and the progression of insulin resistance/metabolic syndrome.
2) Sugar/fructose and processed food load can worsen metabolic dysfunction
Robert Lustig and Richard Johnson have strongly argued that fructose (especially in high amounts and in processed forms) plays a key role in fatty liver, insulin resistance, and metabolic syndrome. Johnson’s work also explores uric acid and a “survival switch” hypothesis.
3) Fasting and meal spacing can help restore fuel switching
Fung often discusses intermittent fasting and reducing meal frequency/snacking as ways to lower insulin exposure and improve access to stored energy. The fasting literature also describes a “metabolic switch” (glucose/glycogen toward fatty acid and ketone use) that emerges during fasting.
4) Low-carb approaches may help some people regain control
Gary Taubes and Zoë Harcombe commonly promote carbohydrate restriction (especially refined carbs) as a strategy to improve insulin control, appetite regulation, and metabolic health — particularly in people who are already insulin resistant.
Important note on nuance
Some of these experts advocate models that are still debated (for example, the carbohydrate-insulin model as the explanation for obesity). This is a legitimate scientific debate but it’s important to take into account that every human reacts differently when it comes to nutrition. But there is broad agreement that processed food environments, overconsumption, inactivity, poor sleep, and metabolic dysfunction/insulin resistance are major issues.
How Metabolic Flexibility Benefits Health
1) Better blood sugar regulation
When your body can handle fed/fasted transitions better, blood glucose and insulin responses are often better regulated, especially alongside exercise and body composition improvement. Poor flexibility is linked with insulin resistance and metabolic disease.
2) Improved energy and fewer crashes
People often report more stable energy when they stop constantly topping up with ultra-processed snacks and improve their ability to use stored fuel between meals. (This is a practical coaching observation that aligns with the fuel-switching concept.)
3) Better body composition outcomes
Metabolic flexibility supports better fuel usage and may help with appetite regulation and fat oxidation, especially when paired with resistance training, daily movement, and nutrition quality.
4) Lower risk profile for metabolic syndrome-related disease
Metabolic syndrome (abdominal obesity, elevated triglycerides, low HDL, high blood pressure, elevated glucose/insulin resistance) is tied to cardiovascular and diabetes risk. Improving metabolic health behaviors can improve those markers.
How metabolic flexibility benefits performance
This is a huge one for your gym audience.
1) Better endurance and work capacity at lower/moderate intensities
If you’re better at using fat during lower-intensity work, you can spare glycogen and sustain effort more efficiently. This matters for longer sessions, Hyrox-style work, circuits, and general conditioning.
2) Better use of carbs when intensity rises
Metabolic flexibility is not “never use carbs.” It’s the opposite: it means you can use the right fuel at the right time. High-intensity efforts still rely heavily on carbohydrate metabolism.
3) More stable training energy
Clients who improve sleep, nutrition quality, protein intake, strength training consistency, and daily activity often stop feeling like every session depends on caffeine/sugar. That’s a practical sign their system is becoming more resilient.
4) Recovery and resilience
Exercise training itself improves mitochondrial function, insulin sensitivity, and substrate handling — which means training is both a performance tool and a metabolic health intervention.
Barriers to Metabolic Flexibility
Here are the biggest obstacles most people run into:
1) Constant grazing/snacking
If someone is eating all day (especially refined carbs + processed snacks), they rarely spend time in a low-insulin state and may struggle to access stored energy efficiently. This is one reason Fung emphasizes meal spacing and fasting windows.
2) Ultra-processed food and high sugar/fructose intake
Lustig and Johnson heavily emphasize sugar/fructose load, liver fat, insulin resistance, and metabolic syndrome. Even where details are debated, reducing ultra-processed sugary intake is a sensible and widely supported move.
3) Low muscle mass / lack of resistance training
Skeletal muscle is a major site for glucose disposal and metabolic regulation. More muscle and better muscle function generally improve metabolic outcomes.
4) Sedentary lifestyle
Being inactive reduces insulin sensitivity and adaptive fuel use. Training helps, but so does reducing prolonged sitting and increasing daily movement.
5) Poor sleep and stress
Chronic stress and poor sleep can worsen appetite regulation, insulin sensitivity, and recovery, making consistent metabolic improvement harder. (This is strongly supported across metabolic health literature, though not unique to metabolic flexibility papers.)
6) Going too extreme too fast
People often fail because they jump from chaos to perfection:
- zero carbs overnight
- daily long fasts
- huge training volume
- no recovery
That usually backfires. Metabolic flexibility is built with consistency, not heroics. (Coaching principle; also aligns with long-term adherence concerns in nutrition debates.)
How to Become More Metabolically Flexible (practical steps)
This is where coaching matters most.
1) Build muscle and train consistently
Resistance training improves insulin sensitivity, mitochondrial function, and glucose handling. It also increases your “metabolic engine” by preserving or building lean muscle mass.
Practical target: 2–4 strength sessions/week.
2) Move daily (not just workouts)
Walk. Stand. Do chores. Carry things. Break up sitting time. Structured exercise is great, but metabolic health also depends on what you do in the other 23 hours.
3) Improve food quality first
Before obsessing over macros, fix the obvious problems:
- Reduce sugary drinks
- Reduce ultra-processed snacks
- Focus meals around protein
- Add whole-food carbs strategically
- Include healthy fats
- Eat enough micronutrient-dense foods
This fits with both mainstream metabolic health advice and the anti-processed-food emphasis from Lustig/Johnson.
4) Space meals (if appropriate)
Not everyone needs aggressive fasting, but many people benefit from:
- fewer eating occasions
- less random snacking
- clearer meal structure
This creates opportunities for the body to transition between fed and fasted states.
5) Use carbohydrates strategically
For many active people, carbs are useful — especially around training and for higher-intensity work.
The goal is not “fear carbs.”
The goal is earn your carbs and use them well:
- train hard
- build muscle
- improve insulin sensitivity
- match intake to activity
That’s metabolic flexibility.
6) Prioritize sleep
Sleep deprivation impairs insulin sensitivity and recovery. If someone is sleeping 5 hours and wondering why they crave sugar all day, that’s not a willpower issue — that’s biology.
7) Be patient
Metabolic inflexibility often develops over years. Rebuilding flexibility takes time:
- better training
- better recovery
- better food quality
- body composition improvement
- consistency
How It Looks In Real Life
Metabolic flexibility = the ability to use the right fuel at the right time.
- At rest / between meals → use more fat
- During hard training → use carbs well
- After meals → handle glucose effectively
- Overnight fasting → access stored energy without crashing
That’s what “metabolically healthy” often looks like in real life.
Final takeaway
You don’t need to become a keto zealot, a fasting zealot, or a macronutrient Myfitnesspal robot to improve your metabolic flexibility.
You do need to:
- train
- move
- sleep
- reduce or remove ultra-processed foods
- eat sufficient protein
- structure meals
- stop constantly topping up energy you haven’t even used
For many people, the biggest success is not just fat loss. It’s this: you stop feeling like your body is fighting against you.
If you’d like help improving your metabolic health, get in contact with us.