Fasting has become one of the most talked-about strategies in the longevity world. But according to surgeon and longevity physician Dr. Jon Schoeff, much of the conversation has become confused by one basic problem: people are using the word “fasting” to describe very different things.
On a recent episode of “A Healthy Point of View” podcast with Sam Tejada, CEO and Founder of Liquivida®, Schoeff challenged some of the popular assumptions surrounding fasting, ketosis, exercise, muscle, and metabolic health. His central argument was simple: fasting should not be judged by the number of hours on a clock. It should be understood as a biological state.
You’re Probably Fasting Wrong — Dr. Jon Schoeff Explains Why | Ep. 179
Fasting Is a Biological State, Not Just a Timer
Schoeff makes a sharp distinction between fasting and time-restricted eating. A person who eats within a certain daily window may be practicing time-restricted eating, but that does not necessarily mean they have entered the biological state Schoeff associates with fasting.
So how can someone know when that state has actually been reached?
For Schoeff, ketones provide an important piece of the answer.
When the body has reduced its reliance on readily available glucose and begins mobilizing stored fat, the liver can produce ketone bodies. Schoeff describes endogenous ketones, meaning ketones produced by the body itself, as both an energy source and a signaling molecule. He points to blood ketone measurements as a way of observing this process rather than simply assuming that a certain number of fasting hours guarantees it.
He describes nutritional ketosis as roughly 0.5 to 1.5 millimoles per liter and identifies approximately 2.5 millimoles per liter as a level he considers “target ketosis” in his practice. More importantly, he argues that individuals can reach these levels at very different speeds.
That is why, in his view, saying “everyone needs to fast for 24, 36, or 48 hours” misses the point.
Why Your Pre-Fast Workout May Matter
One of the more unexpected parts of the conversation was Schoeff’s approach to exercise before a fast.
He explains that his fasting protocols begin with glycogen depletion. Glycogen is stored carbohydrate that the body can readily draw on for fuel. His approach is therefore to use intentional physical activity to reduce those stores before the fasting period begins.
For Schoeff, that can mean making the pre-fast workout the hardest training session of the week, using resistance training and cardiometabolic work to consume glycogen. Once the fast begins, however, the strategy changes.
High-intensity training is not what he recommends during the fast itself. As the body relies more heavily on ketones and fat, Schoeff says peak power output can suffer. Crossing into higher-intensity, more anaerobic work also increases the body’s demand for rapidly available carbohydrate.
Instead, he favors lower-intensity movement during the fast, particularly the lower end of Zone 2. For resistance training, he suggests reducing both training time and intensity rather than trying to maintain a normal heavy lifting session while fasting.
The distinction is important because Schoeff does not view fasting and exercise as competing strategies. He sees them as tools that can be used differently depending on the metabolic state someone is trying to create.
Longevity Is Not Just About Building More Muscle
The conversation eventually moved beyond fasting into one of Schoeff’s bigger ideas about longevity: the body needs both growth and repair.
He describes mTOR as a major pathway associated with growth and AMPK as a pathway associated with repair and recovery. The mistake, he argues, is treating either pathway as something that should simply be switched permanently on or off.
Growth matters. Muscle matters. Progressive resistance training provides a powerful growth signal. But Schoeff argues that constantly pushing growth without giving the body adequate opportunities for repair is not a complete longevity strategy.
He believes the balance should change with age. Someone in their 30s may emphasize growth more heavily, while someone in their 50s or 70s may need a greater emphasis on recovery and repair.
Rather than thinking of biology as a collection of switches, Schoeff prefers the image of a set of dials. The goal is not to turn one pathway completely off and another completely on. It is to adjust them in relation to each other.
That same philosophy shapes his view of muscle.
Schoeff argues that muscle power, followed by muscle strength and then muscle mass, deserves far more attention in longevity discussions. He still considers cardiovascular fitness and VO2 max important, but he does not believe cardio should overshadow the role of maintaining functional muscle as people age.
Metabolic Health May Be the Bigger Conversation
For Schoeff, fasting ultimately leads back to metabolic health.
He describes metabolic flexibility as the ability to move between different fuel sources depending on the body’s needs. At rest, the body can rely heavily on fat. When sudden physical demand appears, carbohydrate becomes much more important. A metabolically flexible body should be capable of making that transition.
This is also why Schoeff pushes back against the idea that everyone needs to remain on a ketogenic diet indefinitely.
A ketogenic diet may be useful in specific circumstances, but he argues that constantly restricting carbohydrates can reduce the body’s ability to use them efficiently. His goal is not simply to make someone better at burning fat. It is to make the body capable of using both fat and carbohydrate appropriately.
That distinction changes the way he looks at glucose, too.
Rather than treating every glucose spike as an emergency, Schoeff focuses more heavily on overall glucose regulation and glycemic variability. He argues that carbohydrates are not inherently the enemy. They are a fuel source, and how effectively the body handles them matters more than simply eliminating them.
More Data, Fewer Assumptions
Schoeff repeatedly returns to one principle throughout the conversation: data should drive decisions.
In his practice, he discusses using measurements such as fasting insulin, fasting glucose, oral glucose tolerance testing, body composition analysis, and other forms of metabolic data to create a clearer picture.
He is particularly critical of relying on a single marker to declare someone metabolically healthy. A normal A1C, for example, may not tell the entire story. He argues that looking at A1C alongside fasting glucose and fasting insulin provides a more useful starting point for assessing metabolic health.
The same principle applies to body composition. Schoeff describes using multiple tools, including bioimpedance, DEXA, and ShapeScale, rather than depending on a single measurement.
The goal is not to collect data for the sake of collecting it. It is to make sure the different measurements are telling a consistent story.
The Foundation Still Comes First
Despite spending more than an hour discussing ketones, fasting protocols, metabolic pathways, diagnostics, GLP-1s, exercise technologies, and longevity interventions, the conversation ultimately landed somewhere surprisingly simple.
The foundation matters more than the latest biohack.
Schoeff argues that people can become so focused on expensive interventions and emerging technologies that they overlook the things that have been foundational all along: movement, exercise, muscle, sleep, nutrition, and strategically using tools such as fasting.
His approach to fasting itself reflects that philosophy. He says the duration and intensity should depend on a person’s starting health, including factors such as visceral fat, fasting insulin, and waist circumference. In his framework, a fast moves through distinct phases: glycogen depletion, metabolic transition, ketosis, and refeeding.
That does not mean everyone should immediately attempt a prolonged fast. In fact, Schoeff’s own framework argues against a one-size-fits-all approach.
The larger lesson is about understanding why a strategy is being used in the first place.
Fasting is not simply about enduring hunger for a predetermined number of hours. Exercise is not just about burning calories. Muscle is not only about appearance. And longevity is not necessarily about collecting the most expensive interventions available.
As Schoeff put it toward the end of the conversation, the place to start is the foundation. Move more, and move more intentionally.
For anyone overwhelmed by the constant stream of new longevity trends, that may be the most useful takeaway of all.