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Metabolic Flexibility

What is metabolic flexibility?

It’s a longevity buzzword that gets tossed around a lot, but do you actually know what it means? 

Or how to improve it?

I didn’t.

It felt like I required roughly one Ph.D. of knowledge to grasp the concept of metabolic flexibility.

But I dug down and did some research.

Here’s my shot at making metabolic flexibility simple, practical, and actionable…

Metabolic flexibility demystified 

Are you metabolically flexible?

The answer depends on your ability to tap into your body’s fuel sources:

Fat and glucose.

(Glucose comes from the carbs we consume and is stored as glycogen in the muscles and liver.)

The ability to use both fuels (fat and glucose), means you are metabolically flexible. 

Someone who is metabolically inflexible cannot tap into their fat stores. That means they’re left using only one source of fuel, glucose.

Your level of fitness determines the type of fuel you tap into depending on the intensity of your output (walking, jogging, sprinting).

Let me explain…

Fitness and metabolic flexibility

Broadly speaking, we have two types of muscle fibers (Type 1 and Type 2) and two energy sources (fatty acids (fat) and glucose).

At lower intensities (e.g. walking, running long distances) a metabolically healthy person would engage Type 1 muscle fibers and burn fat for energy.

At higher intensities (e.g. sprinting 100 meters) that same person would engage Type 2 muscle fibers and burn glucose for energy.

The fitter you are, the greater your ability to tap into fat stores for energy as the intensity increases.

The greater your ability to utilize fat — the body’s most efficient and abundant fuel source — at higher intensities, the healthier and more efficient your mitochondria. 

Mitochondria: Tiny organelles in our cells that generate much of our energy. They can burn both fat and glucose as energy and are thus critical to our metabolic health.

For example, a Tour de France competitor or Olympic marathoner would be able to utilize fat at an intensity that would make most of us switch over to glucose.

Similarly, an obese person may not be able to tap into their fat stores at all and have to burn glucose just to walk down the street whereas a moderately fit person would utilize fat.

People with obesity or other metabolic problems often rely solely on glucose for energy because they’re metabolically inflexible (and their mitochondria aren’t functioning optimally) and are unable to tap into their fat stores.

Bottom line: How we utilize different fuels (fatty acids and glucose) is paramount to our metabolic and overall health and fitness.

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Benefits of metabolic flexibility 

Zone 2 endurance training, the primary driver to develop metabolic flexibility, is essential for two reasons.

Reason #1: It builds a base of endurance for everything else you do.

Having a solid baseline of endurance is essential for everything we do in life.

Endurance enables us to sustain energy all day without crashing in the afternoon, go on a hike with our family without falling behind, and keep up with our friends on the basketball court or soccer pitch.

If you want to be prepared for any situation life throws at you and enjoy a high quality of life both now and in the decades to come, endurance is essential.

Reason #2: It plays a crucial role in preventing chronic disease by improving the health of your mitochondria.

A decline in the number and quality of mitochondria is one of the most significant hallmarks of aging. 

By engaging in Zone 2 aerobic exercise, we kickstart a process called mitochondrial biogenesis whereby we create new and more efficient mitochondria while eliminating dysfunctional ones through a process called mitophagy.

So next time you’re dreading another run, bike, row, or swim, just remember you’re improving your mitochondria with every stride, stroke, or rotation.

But if you don’t use 'em, you lose 'em.

Your metabolic health also plays a key role in disease risk management.

For example, insulin resistance, a hallmark of poor metabolic health, is associated with massive increases in disease risk:

• Cancer risk increases up to twelvefold.

• Alzheimer’s disease risk increases fivefold.

• Death from cardiovascular disease increases almost sixfold.

Engaging in regular Zone 2 training improves your metabolic health and your ability to utilize glucose and fat as fuel.

But what is Zone 2 and how do you train for it?

Training for metabolic flexibility

Lactate is a by-product of energy production.

Zone 2 training is the max effort we can sustain without accumulating lactate. We’re still producing it, but we’re able to clear it before it builds up.

If you “feel the burn” while exercising, you’re accumulating lactate and are exercising too hard for Zone 2.

The fitter you get, the more quickly you can clear lactate which means you’ll be able to sustain a higher pace while remaining in Zone 2.

Ways to determine Zone 2

The surefire way to determine if you’re in Zone 2 is by using a portable lactate meter that measures lactate concentrations in your blood.

For most people, the Zone 2 threshold is 1.7-2 mmol of lactate.

However, while this method is the most accurate, it’s also the most inconvenient.

Instead, use the talk test as a proxy of your exertion.

During a Zone 2 workout, you should be going slow enough so that you could maintain a conversation but fast enough that the conversation would be a little strained.

The intensity should be somewhere between easy and moderate. 

On good days, you’ll be tempted to pick up the pace because Zone 2 will feel easy.

Zone 2 Training Protocol

Strive for three hours of Zone 2 per week spread out over four 45-minute sessions.

If you’ve never done Zone 2 training before, start with two 30-minute sessions per week and build up from there.

“I’m too busy… I don’t have time for this!”

Most runners agree they get their best ideas on runs.

Other than acting as a potentially life-extending activity, Zone 2 also increases cerebral blood flow and stimulates the production of BDNF (brain derived neurotrophic factor).

Simply, it boosts your cognition and provides the perfect time to think through challenges in your life.

Take the time for Zone 2 training.

It can help you problem solve and work smarter, not harder.

I hope this cleared the fog on what metabolic flexibility is and how to develop it.

Leave a comment

If you have any feedback or questions, let me know by hitting reply to this email or leaving a comment on the post.

Much love,

Jack 

Related Reading:

• Zone 2 Deep Dive: Part One – Understand: Metabolic respiration, lactate thresholds, and classifying training zones by muscle fiber and energy substrate.

• Zone 2 Deep Dive: Part Two – Implement: How to implement the most effective form of lifespan extending exercise.

• Mitigating Disease Risk through Metabolic Health Management: The role of metabolism in disease risk mitigation, 5 markers of metabolic syndrome, and subcutaneous vs. visceral fat.