For most of human history, eating was naturally interrupted by periods without food. We didn’t have refrigerators, stocked pantries, or the ability to eat from the moment we woke up until shortly before bed. Our bodies evolved to function through periods of both nourishment and scarcity.
Today, food is available almost continuously, and many of us move from breakfast to snacks to lunch to more snacks to dinner without giving our metabolism much of a pause. This is one reason fasting has become such an interesting area of research, and such a popular topic in the longevity conversation.
But fasting needs perspective. It isn’t magic, and longer isn’t automatically better. If we become so focused on how many hours we can go without eating that we neglect muscle, nutrition, sleep, or overall metabolic health, we may undermine some of the very things we’re trying to protect as we age.
For me, the more interesting question isn’t simply, “Should I fast?” It’s how metabolically flexible my body is, and what I can do to preserve that flexibility throughout my life.
What Happens When We Fast?
When we stop eating for a period of time, the body makes a series of coordinated metabolic adjustments. Insulin generally falls, stored glycogen begins to be used, fat mobilization increases, and, as carbohydrate availability decreases sufficiently, the liver begins producing ketone bodies, particularly beta-hydroxybutyrate, or BHB.
This allows the body to shift away from relying predominantly on incoming glucose and make greater use of stored energy.
Our metabolism was designed with options. We can use glucose, fatty acids and ketones. The ability to move efficiently between those different fuel sources is what we mean by metabolic flexibility, and I believe that flexibility is an important part of healthy aging. Fasting is one way of challenging that system, but fasting itself can mean different things. Time-restricted eating may simply involve consuming food within a consistent daily window. Intermittent fasting can include longer fasting periods on selected days, while periodic fasting may extend beyond 24 hours and requires considerably more caution. These approaches don’t produce identical effects, and they aren’t appropriate for everyone.
What Does the Research Tell Us?
There is meaningful human evidence supporting some cardiometabolic benefits associated with intermittent fasting. Research has found improvements in measures such as waist circumference, fat mass, fasting insulin, triglycerides, and cholesterol in adults with overweight or obesity.
However, fasting has not consistently been shown to outperform a well-designed continuous calorie-restriction approach. For some people, its greatest advantage may simply be practicality. A defined eating window can be easier to maintain than continually counting calories, and sustainability matters far more to me than an extreme protocol someone follows for a week.
We also need to be careful when metabolic benefits are translated into claims that fasting will make us live longer. Animal studies have shown that energy restriction and fasting-related pathways can influence lifespan, but we don’t currently have decades-long human trials proving that a particular fasting schedule extends human life.
Research such as the CALERIE trial, which studied sustained calorie restriction rather than intermittent fasting, has reported a modest slowing in one measure of the pace of biological aging. That’s encouraging, but it isn’t the same as proving longer lifespan.
As a clinician, I think we should be comfortable saying both things. The science is promising, and there is still much we don’t know.
Metabolic Flexibility Matters More Than Chasing Ketosis
One mistake I see in the wellness world is turning a normal physiological state into something we believe we need to pursue constantly. Ketosis is a good example. When carbohydrate availability becomes sufficiently low, the liver can produce ketones that provide energy to tissues including the brain, heart, and skeletal muscle. That’s an incredibly useful metabolic adaptation, but I don’t believe most people need to remain in ketosis every hour of every day.
I’m more interested in whether the body can adapt. Can it manage glucose effectively after a meal? Can it access stored fat when appropriate? Can it produce and utilize ketones when carbohydrate availability decreases? Can it respond appropriately to exercise, nourishment, and periods without food?
My husband, Dr. Mark Sherwood, and I approach health through this broader lens at the Functional Medical Institute. Nutrition affects metabolism. Metabolism affects hormones. Sleep influences glucose regulation and appetite. Stress can influence inflammation and metabolic health. Muscle plays an enormous role in glucose disposal and physical resilience. One laboratory value, supplement, or dietary strategy rarely tells the whole story.
Muscle Must Be Part of the Longevity Conversation
This is an area where I particularly want women to pay attention. The wellness world can sometimes present fasting as though more must always be better. But human physiology isn’t that simple. As we age, maintaining lean muscle becomes increasingly important. Muscle isn’t simply about appearance or athletic performance. It is metabolically active tissue, an important site of glucose disposal, and essential to strength, balance, mobility and our ability to remain physically independent. That makes muscle a longevity asset.
Repeated aggressive fasting without adequate nutrition, protein, and resistance training can work against that objective, particularly in older adults, lean individuals, and women vulnerable to low energy availability.
I don’t want a woman pursuing longevity while gradually sacrificing the muscle she will need to remain strong and independent later in life.
For many people, a consistent overnight fasting interval may be useful. But so is eating enough quality protein. So is resistance training. So is recovery, restorative sleep, and providing the body with the nutrients it needs.
Fasting has to be considered as part of a larger health strategy, not in isolation.
Mitochondria and Metabolic Resilience
Metabolic flexibility also connects to mitochondrial health, something Mark and I have become increasingly focused on through our clinical and educational work. Mitochondria produce much of the usable energy our cells require. Your brain needs energy to think. Your muscles need energy to contract. Your heart requires a continuous supply of energy. Repair, immune function, and recovery all depend on it.
This thinking is part of the Sherwood Mitochondrial Clinical Framework™, which considers interconnected factors involved in cellular energy, metabolism, and resilience rather than searching for one isolated answer to every health concern. From this perspective, I don’t see fasting as a trick for losing weight or achieving a particular ketone reading. I see it as one potential metabolic stimulus. Exercise is another. Nutrition provides the materials for repair. Sleep supports recovery. Stress management helps prevent the body from remaining under constant physiological demand. Longevity is built through the interaction of all of these things.
Why Fasting Should Be Personal
We also need to be careful about turning fasting protocols into universal recommendations, particularly for women. Nutritional and metabolic needs change throughout life. Age, body composition, activity level, hormonal status, pregnancy, breastfeeding, menopause, medications, and existing health conditions can all influence whether fasting is appropriate and how it should be approached.
Fasting may require medical supervision or may not be appropriate for people who are pregnant or breastfeeding, underweight or frail, have a current or previous eating disorder, have type 1 diabetes, use certain glucose-lowering medications, or have significant kidney, liver or cardiovascular disease.
This isn’t about making fasting frightening. It’s about remembering that personalized health should actually be personal. I don’t believe in giving every patient the same diet, fasting schedule, or supplement regimen. The question should be: What are we trying to accomplish for this person, and what is the safest, most sustainable way to get there?
The Fundamentals Still Win
Longevity has become an exciting field, but that excitement can make health seem more complicated than it needs to be. We hear about fasting, ketones, peptides, hormones, supplements, continuous glucose monitors, biological-age testing, and new longevity technologies. Some of these tools are fascinating and may have an appropriate place. But none replaces the foundation.
We still need movement, muscle, quality nutrition, restorative sleep, metabolic health, stress management and appropriate hydration. And we also need relationships, emotional wellbeing, and purpose. You can optimize every biomarker available to you, but longevity means very little if you don’t have the health, connection, or sense of purpose to enjoy the years you’re working so hard to preserve.
Ultimately, the most compelling thing about fasting isn’t how long we can go without food. It’s what fasting teaches us about adaptability. The human body can move between fuels, mobilize stored energy, and alter its metabolism in response to availability and demand. Healthy aging should be about preserving as much of that resilience as possible.
That philosophy is also at the heart of the new book Mark and I have written together, Authentic Longevity: Unlocking Lifelong Vitality, The Sherwood Way.
After decades of clinical experience and working with tens of thousands of patients, we wanted to create a practical roadmap that moves beyond the idea that longevity can be found in one diet, supplement, or breakthrough technology. We begin with what we can control: nutrition, movement, sleep, stress, and hydration, and then explore where we may need help, including genetics, peptides, supplements, hormones, and laboratory testing.
Importantly, we also address something many longevity conversations leave out: the relationship between body, mind and spirit. Healthy aging isn’t about winning a competition to see who can fast the longest or follow the most complicated biohacking routine. It’s about creating a body that remains metabolically capable, physically strong, and resilient enough to adapt as life changes. Adding years to life is valuable only if we’re also working to add life to those years. That’s what authentic longevity means to me.
About Dr. Michele Sherwood
Dr. Michele Sherwood, DO, is a physician, educator, author, and speaker, and co-founder of the Functional Medical Institute in Tulsa, Oklahoma. Together with her husband, Dr. Mark Sherwood, ND, a traditional naturopathic doctor and clinician, she focuses on personalized, whole-person health with an emphasis on prevention, metabolic health, healthy aging, performance, and longevity. The Sherwoods are also behind the Countdown Method, their health and performance brand offering products designed to support energy, nutrition, movement, recovery, and overall wellness. Through their clinical work, wellness brands, books, podcasts, speaking engagements and educational outreach, the Sherwoods have dedicated their careers to helping people better understand their health and become proactive participants in their long-term wellbeing.
Their new book, Authentic Longevity: Unlocking Lifelong Vitality, The Sherwood Way, combines decades of clinical experience, real patient stories, and actionable science to explore nutrition, movement, sleep, stress, hydration, genetics, peptides, supplements, hormones, and laboratory testing alongside the often-overlooked role of emotional and spiritual health.
Authentic Longevity: Unlocking Lifelong Vitality, The Sherwood Way is available HERE.
Disclaimer: This article is intended for general informational and educational purposes only and should not be considered medical, nutritional, fitness, or other professional healthcare advice. Fasting, dietary changes, exercise strategies, and approaches to metabolic health may affect individuals differently depending on factors such as age, health status, medications, activity level, pregnancy or breastfeeding status, and personal medical history. The information presented is not intended to diagnose, treat, cure, or prevent any disease or replace guidance from a qualified healthcare professional. Readers should consult an appropriately licensed medical provider before starting a fasting protocol, making significant dietary changes, using supplements, or modifying an existing health routine, especially if they have an underlying medical condition or specific health concerns. References to research, wellness approaches, or individual experiences do not guarantee specific health, longevity, or performance outcomes.
