Why metabolic flexibility is one of the strongest predictors of long-term health, and how Zone 2, muscle, and GLP-1s help rebuild it.
As we move through these late spring days and Memorial Day—the unofficial start of summer—I have been thinking about how many of you are stepping back outside, walking more, training for races, or simply trying to feel a little bit better in your own body. It feels like the right moment to talk about the kind of metabolic health that lets you enjoy these longer, brighter days with more ease and more resilience.
This month, I want to focus on one of the core themes that keeps coming up for so many of you: metabolic flexibility, Zone 2, and why I am weaving GLP‑1 medications into some of your plans not as quick fixes, but as tools to help your cells, your brain, and your heart work differently over the long term.
I sure hope that by now you have heard Rachel or myself talk about metabolic flexibility—your body’s ability to shift seamlessly between burning glucose and burning fat, depending on what you ask it to do. This kind of physiology is now rare. In one large analysis, only about 7% of American adults met criteria for good cardiometabolic health, and nearly 40% of adults already met criteria for metabolic syndrome. That means the vast majority of people in this country are living in some degree of metabolic inflexibility, even if their annual labs still say “normal.”
Loss of metabolic flexibility is the on-ramp to almost every chronic disease we work so hard to prevent. When the body can no longer smoothly shift between burning glucose and burning fat, several things begin to happen in the background. Insulin levels stay chronically elevated, fat accumulates in the liver and around the organs, and cells stop responding properly to insulin’s signal. Over time, this pattern drives insulin resistance, metabolic syndrome, and type 2 diabetes, and it markedly increases the risk of heart attack, stroke, and heart failure. In the liver specifically, poor metabolic flexibility feeds metabolic dysfunction-associated steatotic liver disease (MASLD), which is now one of the leading causes of cirrhosis, liver cancer, and need for liver transplant.
Metabolic inflexibility changes how we age. Chronic exposure to high glucose and insulin, plus the inflammation that comes with them, accelerates vascular damage, kidney injury, and loss of muscle mass, and it is increasingly linked to cognitive decline and dementia risk. The mitochondria in your muscles, brain, and immune cells become less efficient, so you tire more easily, recover more slowly, and live with a constant sense of being “run down” even when labs still look acceptable. In other words, losing metabolic flexibility is not only about weight or cravings; it is the underlying shift that sets the stage for cardiometabolic disease, fatty liver, neurodegeneration, and the slow erosion of energy and resilience over years.
Zone 2 exercise, typically referred to as Z2, is a non-negotiable part of rebuilding true metabolic health. When metabolic flexibility is lost, no supplement, lab, or medication can fully substitute for the signal your mitochondria get from steady, sub-maximal movement. Zone 2 work—your “conversational pace” walks, rides, or easy jogs—asks your body to generate energy for a sustained period without the drama of all-out effort, which forces it to rebuild the machinery for burning fat efficiently instead of relying on quick glucose bursts. Over time, that specific intensity increases mitochondrial number and efficiency, improves how your muscles handle fuel, lowers baseline inflammation, and directly counteracts the insulin resistance, fatty liver, and cardiometabolic risk that come with metabolic inflexibility.
It is not entirely true that a medication can never touch metabolic flexibility—this is where GLP-1 medications like Ozempic (semaglutide) and Zepbound (tirzepatide) really shine. They essentially override parts of the system and do some of what Zone 2 does: they lower glucose exposure, calm insulin signaling, reduce appetite, and, in many people, make the mitochondria behave more efficiently. But they do not truly correct the underlying capacity. Once the medication is stopped, the mitochondrial signal is gone; if nothing else has changed—no Zone 2, no muscle, no protein—the system tends to drift back to its old settings, and the weight (and risk) slowly, or sometimes not so slowly, returns.
This is why Z2 exercise is essential: medications can help shift the terrain, but it is the consistent, steady work of Zone 2 that teaches your mitochondria, muscles, and liver how to operate differently for the long term.
Zone 2 only helps if you actually stay in Zone 2. If you push harder and into a higher heart-rate zone, your body shifts toward burning glucose and you lose the continuous, fat-focused training effect we are trying to give your mitochondria. The win is not “harder and sweatier”; the win is longer time at the right easy pace, where your muscles are quietly practicing burning fat as their main fuel.
PNOĒ testing becomes especially helpful here. Rather than guessing from how breathless you are or a calculated formula, breath analysis shows exactly how your body is using oxygen and fuel at different workloads, including where you are primarily burning fat and where you start shifting towards using carbohydrates. PNOĒ testing gives us a precise fat-burning heart-rate zone, which makes your training time far more efficient.
The beauty of GLP-1 medications really starts to come into view when we look at the body as a whole. These medications are not simply helping people lose weight. They improve the metabolic environment throughout the whole body. Supporting healthier mitochondrial function across all systems improves hormone production and signaling, calms inflammation, reverses early kidney decline, reverses symptoms of cognitive decline and liver damage from metabolic disease. When used properly and thoughtfully, these are not band-aids, but rather very powerful and specific tools that can help move the body back toward a healthier physiologic state.
New GLP Research
The research is finally catching up to what many cellular doctors have been experiencing using the GLPs for years.
GLP‑1 research in cardiovascular medicine has become more concrete. These medications are showing meaningful benefit in heart failure and broader cardiometabolic risk, particularly in people with obesity, diabetes, and metabolic liver disease. In one recent study of patients with heart failure with preserved ejection fraction (HFpEF)—a condition in which the heart muscle becomes stiff and cannot fill properly—people started on semaglutide or tirzepatide had more than a 40% lower risk of heart-failure hospitalization or death compared with those on sitagliptin. That is a striking degree of benefit, and it reinforces the idea that GLP‑1s are supporting the mitochondria, improving cellular function and making muscles stronger.
The liver story is just as important, and in many ways it sits at the center of the whole metabolic conversation. Metabolic liver disease, known as Metabolic dysfunction-associated steatotic liver disease (MASLD), is recognized as the major driver of cirrhosis, liver failure, and transplant need. It is tightly linked with diabetes, cardiovascular disease and heart failure risk. Newer data show that in patients with MASLD, GLPs are associated with lower risk of progression to cirrhosis and lower mortality, especially when they are started earlier in the course of disease rather than after cirrhosis is already established.
Other studies suggest GLP‑1 use is linked with lower risks of cardiovascular events, decompensated cirrhosis, hepatic encephalopathy, and liver failure in people with diabetes and compensated liver disease. In other words, when we improve the liver’s metabolic burden, we are often helping far more than the liver alone—we are improving the entire system that sits upstream of heart disease, inflammation, and long-term decline.
One of the most interesting areas of GLP-1 research is happening in the brain. Microglia and astrocytes—your brain’s resident immune cells and caretakers, helping regulate blood flow, clean up extra neurotransmitters, and support neurons—carry GLP-1 receptors. This means these medications can directly influence how inflamed or calm the brain’s internal environment feels. GLP-1 signaling has been shown to reduce pro-inflammatory cytokines (signaling proteins the immune system uses to communicate), and shift microglia away from a more neurotoxic, inflammatory state toward a more reparative, neuroprotective one. This may help explain why some people notice improvements in migraines, brain fog, and pain amplification that seem to go beyond what weight loss alone would predict.
To Try This Month
One simple way to experience your own metabolic flexibility is to try a carefully planned 24-hour fast. Start with an early, balanced dinner the night before, then choose the following day as a lower-stress day and have only water, mineral water, and, if you tolerate it, black coffee or tea until dinner that evening. No intense exercise on the fasting day—just normal daily movement or light walking.
As you move through the day, notice what happens in your body and mind. At what time do you start to feel hungry, and how does that hunger show up—mild and passing, or sharp, shaky, irritable, “I can’t think” hunger? Do symptoms come in waves and ease with distraction, or do they build and feel alarming? If making it anywhere close to 24 hours feels impossible, that is not a failure; it is information. It is one sign that your system may be highly dependent on frequent glucose hits and has more work to do in rebuilding true metabolic flexibility.
If this experiment feels very hard, or if it brings up strong symptoms, that is worth paying attention to. It may be the beginning of a very important conversation about what your body is asking for and which tools—nutrition, Zone 2, muscle, sleep, and sometimes medication—make the most sense for your biology right now.
With gratitude,
Dr. Lechner