You don’t need a faster metabolism. You need a more flexible one.

Many of my patients in their 40s and 50s tell me the same thing: they’re gaining weight, workouts feel harder, and they’re just...tired. They assume their metabolism is slowing down.

But what’s really happening is a loss of metabolic flexibility: your body’s ability to switch between burning carbs and fat for fuel. 

🚦Here’s how it should work:

• At rest, sleeping, or light movement: You burn fat (a slow, steady fuel source).

• During moderate activity: You burn fat + carbs for a balanced energy mix.

• In intense exercise: You burn mostly carbs for quick power.

When you're metabolically inflexible (often from insulin resistance or overfeeding) your body gets "stuck" in carb-burning mode. You lose the ability to tap into stored fat.

Here’s how to know if you’re stuck and how to get more flexible.

🤓 What to know: Metabolic flexibility is the engine of longevity.

Metabolic inflexibility drives nearly every chronic condition we see today:

🧠 Alzheimer’s

🫀 Heart disease

🍩 Type 2 diabetes

🍷 Fatty liver

🌀 PCOS + hormone dysfunction

🎯 Cancer

Over 1 in 3 Americans already have prediabetes—many without knowing it. (This is one of the many reasons I recommend functional labs 2x per year starting at age 25!) 

🛑 Signs your metabolism is stuck 

If these feel familiar, your body may be struggling to switch fuels (even if your labs look “normal”):

• You crash after meals or need caffeine/sugar to get through the afternoon

• You feel dizzy or irritable if you skip a meal

• You gain weight easily—despite eating “clean”

• You’re always hungry (especially for carbs)

• Your endurance is tanking

• Skin flare-ups (acne, rosacea, eczema) won’t quit

📈 How to measure metabolic flexibility

🧪The gold standard is Indirect Calorimetry, which measures oxygen and CO2 exchange to show your current fuel mix.

A flexible metabolism has:

• Low RER at rest (burning fat)

• High RER during effort (burning carbs)

You can get this done at sports performance labs. Some wearables like Lumen offer consumer versions.

🧬 Easier-to-track clues:

• Fasting insulin + glucose → Elevated levels suggest insulin resistance

• CGM or post-meal glucose → >140 mg/dL after eating = early dysfunction

• Triglycerides >100 or low HDL (<50 mg/dL for women, <40 mg/dL for men) → Poor fat metabolism

• Waist circumference >35 (for women or >40 for men) → Strong predictor of metabolic disease

• Low VO2 max → Sign of poor mitochondrial function poor fat oxidation

💪 What to do: Muscle building, restorative sleep, magnesium supplements and hormone-therapy reverse metabolic rigidity.

The beauty of metabolic flexibility? It’s trainable.

With the right mix of movement, food, and recovery, you can rewire it in months.

🏋️ 1. Build muscle

Muscle is your largest glucose sink; it soaks up ~80% of post-meal glucose (Comprehensive Physiology, 2021).

• Strength train 2–3x/week → Even a single weight training session improves insulin sensitivity for 24–48 hours (Diabetes Care, 2013).

• Eat 1.2–1.6 g protein/kg/day spread across meals (Journal of Cachexia, Sarcopenia and Muscle, 2022).

🔥 2. Train your fat-burning engine

• Do Zone 2 cardio (150–300 min/week)

• Go at a pace where you can still hold a conversation

• Builds mitochondrial density + fat oxidation (Translational Sports Medicine, 2025)

⏰ 3. Time your meals

• Try early time-restricted eating → 8–10 hour window ending by late afternoon (Cell Metabolism, 2018)

• Avoid grazing

• Limit refined carbs

• Aim for 30g+ fiber/day

😴 4. Prioritize sleep

• Getting <5 hours of sleep can reduce insulin sensitivity by 24% (Sleep Medicine Reviews, 2019). 

• Aim for 7–9 hours/night.

💊 5. Smart supplementation

• Magnesium: Improves fasting glucose + insulin resistance (Pharmacological Research, 2016)

• Omega-3s: Supports mitochondrial function + lowers inflammation (Nutrients, 2018)

• Berberine: Lowers glucose + insulin resistance (Biology, 2023)

🌸 6. Consider HRT (if applicable)

• Estrogen makes your mitochondria more efficient, spares glycogen, and improves fatty acid transport (Acta Psychologica, 2024).

• If you're in perimenopause or menopause, work with an HRT-trained provider to assess if hormone therapy could support your metabolic health.

⚡️ Quick Hits

🌙 Melatonin misuse = wired + tired. 

If melatonin has ever left you wired, groggy, or awake at 3 a.m., here’s why: most people take too much, too late, or use the wrong type. Melatonin is a signal, not a sedative. 

Try 1–3mg time-release melatonin 1–2 hours before bed.

⏱ "Exercise snacks" boost VO2 max fast. 

Per a new study: short bursts of movement (≤5 minutes, done a few times per day) significantly improve cardiovascular fitness in inactive adults—in just 4 weeks.

My 3 go-to moves when I’m short on time.

🎧 On the mic

I joined WHOOP's first-ever Medical Advisory Board! Listen to my latest podcast with them on advanced lab testing and performance health.

💛 The Momgevity Files

This week, I finally got to share something I’ve been quietly working on for months: I joined WHOOP—a top wearables company with millions of users—as a member of their new medical advisory board, alongside some truly incredible doctors.

Ten years ago, when wearables first came out, I honestly thought they were overly simplistic at best (yay, steps!) and borderline creepy at worst. Do you really need to track yourself 24/7? Isn’t it just going to confirm what you already know?

Lately, now that I’m a full-time WHOOP user, I’ve become a convert. It’s partly because the tech has evolved. We are way beyond steps. Now, I get real-time data on stress, sleep, heart-rate variability, VO₂ max, heart rate zones, and even healthspan.

But the real reason I’m sold? The wake-up call.

Since having kids, exercise has been my Achilles’ heel. I eat an anti-inflammatory, high-fiber, low-refined-carb, near-zero UPF diet. I meditate daily. My supplement stack is dialed. But exercise? Honestly, my “good weeks” used to mean one yoga class…maybe.

Seeing that my heart rate barely gets out of zone one (ha!) has been eye-opening and, more importantly, motivating.

I’m actually lifting weights now (mostly in my basement, but every other week with a trainer—pricey, but they push me farther than I’d go solo). I’m carving out time for Japanese interval walks. I’m doing YouTube HIIT sessions in the evenings. And I love seeing that I actually hit zone 2—sometimes even zone 3! Like all of us, I’m wired to want the gold star.

Longevity, at its core, is about time and I used to say I didn’t have it. But time is plastic—it stretches and contracts around what we prioritize.

The other day, a friend said it finally hit her: she has to find those 20 minutes for weights despite the two kids, the demanding family, and the big job, because if she wants to be dancing at her daughter’s wedding, longevity isn’t something she can just get to later. 

Later is now.

In my case, I’m deeply proud that I—a mom of three, a non-athlete, a nerd, in fact!—am now a face of longevity for a performance health company like WHOOP. We longevity moms may not have started the movement, but the biohacker bros have nothing on us. We’re more motivated than anyone.

We have lifetimes to live, in the blink of an eye.

Stay strong, stay curious, and breathe, 

Robin

As always, this newsletter is for informational and educational purposes only and is not intended as medical advice. Always consult your healthcare provider before making any health decisions or changes to your treatment plan.