Confessions of a Former Snack King: Rethinking Metabolic Flexibility

Bottom Line

• Metabolic flexibility is the body’s ability to switch between using fats (lipids) and carbohydrates as fuel, depending on activity and availability.*  I use the analogy of our body as a hybrid car to explain how we want to train our energetic systems to behave like an efficient, adaptive, and reliable vehicle.

• In adults, strong evidence shows that maintaining metabolic flexibility reduces the risk of chronic diseases.
• For children, research is less robust (often animal or cross-sectional studies), but the available evidence suggests metabolic flexibility likely helps lower future disease risk, too.
• The 80/20 approach for you and your kids: Don’t snack constantly and commit to regular physical activity. That’s the simple formula.

Confessions of a Former Snack King: Rethinking Metabolic Flexibility

I have a confession.  Until I was around 30 years old, maybe older, I was the king of snacking. As a skinny, active guy, I didn’t see any negative impact—at least on the surface. It was only because of my complex medical history and fascination with how the body works that I began to realize that being the ‘snack king’ came with a cost.  I was hampering my body’s ability to maintain metabolic flexibility.

I’m currently working on building a course on metabolic health (as well as courses on inflammation and lipids) for busy, normal people.  I want these courses to be actionable and impactful, and make the absolute best use of your time because ain’t no parents, healthcare workers or the other people I’m gearing this course towards have the time or bandwidth for anything but the meat.  Because I’m working so hard to make it actionable and practical with minimal frills, I’m not sure I’m going to be able to cover the concept of metabolic flexibility in much depth if at all in the course.  So this blog is a great forum to introduce the topic and share a couple thoughts on how you might be able to build awareness regarding your own degree of metabolic flexibility and set your kids on a healthy path towards maintaining their own metabolic flexibility. 

What Is Metabolic Flexibility (In a Nutshell)?

Metabolic flexibility = the ability of your body to flip the switch between using lipids (fats) and carbohydrates as a fuel source.  This capacity is fundamental for maintaining energy balance and for adapting to differing fuel availability.  When we take in ‘fuel’ (i.e. eat) continuously, we are doing our bodies a disservice above and beyond the potential weight gain.  Non-stop eating doesn’t allow us to utilize all the biochemical pathways that make us function efficiently**.  And things work better when they are utilized to their full potential and run efficiently.  

The Hybrid Car Analogy

Just like a hybrid car switches between gas (quick acceleration = carbs) and electric (steady cruising = fat), you want your body to use all its available fuel lines. Neither fuel is truly superior; what matters is the seamless switch for optimal performance across a range of activities.

My Story: The Carb-Centric Years

Now back to me and my obsession with Goldfish and cookies (Milano, Oreo, whatever, I loved them all) for decades upon decades.  But even if we don’t realize it, we are always “training” our bodies.  And by only gassing up my own take with carbs, I was steadily losing the knack for burning fat efficiently.  This leads to long-term health issues as well as energy crashes and cravings when quick-acting carb fuel isn’t available.

Why does it so often come back to balance!  It’s so much easier just to eat a lot of carbs or be hardcore and go on a ketogenic diet.  I’m not saying either of these options are in any way wrong for any one individual depending on their unique physiology and how they might be incorporating physical training into their lives.  But for the majority of us normal people, maintaining some semblance of metabolic flexibility is a good thing.  Even in the world of peak fitness and conditioning, well-known trainers are beginning to see metabolic flexibility as a grounding concept when it comes to fueling their athletes***.

Building Self-Awareness:

Here’s how you can get a sense of your own metabolic flexibility—these are DIY assessments and basic lab markers:

Can You Burn Fat?

• 4pm test: Skip eating until 4pm. How do you feel—shaky, tired, fine?
• Bonk test: Try low-intensity workouts without carb refueling. How long until you hit a wall?
• Fasted exercise: Can you train in the morning before eating?

Can You Burn Carbs?

• Fasting glucose: Under 90 mg/dL is good.
• A1C: Below 5.6%.
• Insulin-30 < 60 IU/mL (this is a shortened oral glucose tolerance test and means your measured insulin level 30 minutes after consuming 75g of carbohydrates is <60.  This requires working with a medical provider to get the test.  I haven’t done this yet myself but am very interested in making it happen as it is a functional test instead of a static test like the first two).
• Continuous Glucose Monitor (CGM): See your blood sugar response to foods. Dexcom now offers CGMs without prescriptions****.

What About Kids?

I set out to write this post about children, but high-quality research is sparse. Still, here’s what we know so far:

• Animal studies: Continuous eating leads to weight gain, more abdominal fat, disruption of hormones like leptin, and insulin resistance—even without higher calorie intake.
• Human studies: Obese children burn less dietary carb during exercise than their healthy peers, indicating lower metabolic efficiency.  This does not demonstrate that metabolic inflexibility causes obesity, just that they’re linked.
• Review article: Early obesity is associated with signs of metabolic inflexibility in kids (similar conclusion to the preceding study).

Takeaway: Helping kids keep metabolic flexibility likely reduces future disease risk.

How to Build (or Keep) Metabolic Flexibility

Lastly, the important part.  What can we do?  Basically, it boils down to moving and not eating all the time.  Getting a little more technical about it might help you decide on what tactical decisions to make for yourself.  So here is how I would break it down…

• First and foremost, work on improving your insulin sensitivity (more to come on insulin sensitivity in the future and especially in the metabolic health course I’m creating).  There are lots of ways to improve insulin sensitivity.  But the big levers I’ll mention here include the following…
• Any safe exercise pattern—whatever you’ll keep up
• Better body composition (more muscle, less visceral fat)
• Prioritize sleep
• Exercise in different nutritional states: Mix up fueled and fasted workouts.
• Be thoughtful with carbs if you’re training hard: Old wisdom = “replace every burned calorie with carbs.” New wisdom = use fats and carb timing to keep your engine adaptable.

Conclusion: Embrace the Experiment and Be Kind to Yourself

After years of trial and error, I’ve learned:

1. There’s no perfect answer when it comes to anything with our health. Cut yourself some slack, try new things, and enjoy the process.  Man do I struggle to remember this when it comes to fitness and nutrition.
2. What works for you will change as your circumstances and life changes.
1. When I first started to gain some awareness of how bad of a shape I was in after residency, fellowship, having young kids, and starting a new job with lots of in-house call, my body needed a real re-set.  I needed to teach my body how to utilize fat as a fuel source.  So I utilized some time-restricted feeding techniques that included skipping breakfast most days and the occasional longer fast while on call.  As I got in better shape with more consistent cardio training, my sleep improved a bit as I gave up more call, and I trained harder, my body could handle more carbohydrates and I really needed to get some food on board in the morning or else my training coupled with long work days had me consistently losing more weight than was healthy.  That has improved, and I think I’m on a decent path these days.  But who knows where it will go! 
3. Lastly, don’t let yourself stress about metabolic flexibility.  It’s just a cool ability your body has.  Your body is incredible and will do what you ask of it.  I recommend you treat it and yourself with some kindness, whatever that looks like today and might evolve into tomorrow.

Resources & Sidenotes

*The technical definition: “Metabolic flexibility is the capacity of the organism to adapt fuel oxidation to fuel availability.”

**Summary Table: Major Biochemical Pathways and Their Roles

***Training concepts inspired by the 8 Weeks Out Conditioning Course—highly recommended for trainers or anyone wanting to peek into top-level thinking in fitness.

****Stelo/Dexcom now offers at-home CGM: see manufacturer sites for more info.

• https://pubmed.ncbi.nlm.nih.gov/32520873/
• https://onlinelibrary.wiley.com/doi/10.1111/j.1467-789X.2010.00812.x
• https://pmc.ncbi.nlm.nih.gov/articles/PMC6093334/
• https://pmc.ncbi.nlm.nih.gov/articles/PMC5513193/
• https://www.iomcworld.com/open-access/the-role-of-metabolic-flexibility-in-energy-regulation-and-disease-prevention-130981.html
• https://d-nb.info/1167156129/34
• https://www.nature.com/articles/s41392-025-02141-x