Metabolic Medical Newsletter #9 | March 2026 | Fasting: The Physician Within

Fasting is something humans evolved to handle. We are the land mammals born with the highest percent body fat for a reason. Babies tap into fat burning to tolerate minimal intake while awaiting mother’s milk to flow: ketosis provides consistent fuel for our huge, energy hungry brains. Since time immemorial, humans have alternated between cycles of feasting or fasting. Fasting and starvation are commonly mistaken. Starvation is not by choice. The starving person wants to eat, but lacks access either by poverty, famine or war. However, fasting is the temporary, voluntary abstinence of food to fulfill a health, spiritual, religious or political goal. Fasting allows us to use stored energy rather than food energy. Every major religion incorporates fasting as a spiritual discipline for purification, atonement, humility and clarity. Should you use fasting for health optimization?

Medicine has had a long relationship with fasting. The ancient Greeks considered fasting a natural remedy. They observed a “fasting instinct” among animals when ill.¹ Over 2500 years ago, the “Father of Medicine,” Hippocrates used fasting for patients, stating “to eat when you are sick is to feed your sickness.” Resting the digestive system frees the body to focus energy on healing. Hippocrates used fasting therapeutically to treat epilepsy, infections and obesity. His prescription for weight loss still works today: strenuous work before eating and limiting meals to once per day. Many in the carnivore community faithfully abide by OMAD (one meal a day). It is my opinion that In the books of Matthew and Mark, The Bible describes controlling epilepsy through fasting.² Paracelsus, a 16th century Swiss physician and founder of toxicology said “fasting is the greatest remedy- the physician within,” reminding us of the healing power of our own bodies.

More recently, in 1921, Dr Russell Wilder at the Mayo Clinic controlled paediatric epilepsy by mimicking the well-known seizure reducing effects of fasting, using a therapeutic ketogenic diet.³ This high fat, moderate protein and low carbohydrate diet became a standard of care, but was later over-shadowed by the promotion of drugs such as phenobarbital. This ketogenic approach to epilepsy has had a resurgence in the last 30 years through the Charlie Foundation.⁴ This was established by Jim Abrahams when his son, Charlie, developed treatment-refractory epilepsy. His family uncovered this ketogenic approach from decades earlier. Now, ketogenic diets are an evidence-based treatment for epilepsy. Ketosis not only controlled Charlie seizures, but cured him of his disease within a few years. Being a Hollywood director, Abrahams created a movie called “First Do No Harm” starring Meryl Streep. I would highly recommend watching it for the powerful story and many of the actors in the movie were children with paediatric epilepsy, cured through a ketogenic diet. My specialty of anaesthesia has long kept patients fasted with nothing per os (NPO) to avoid fatal complications from inhaling gastric contents after unconsciousness. Interestingly, these aspiration events from the early days of anaesthesia are haunting the specialty again, as GLP1 medications are paralysing gastric emptying, allowing food to linger in the stomach for days despite fasting.⁵

However, as a society, we have shifted away from this ancient knowledge. In 1977, when obesity was less than 10%, most ate only three times per day, no snacks.6 Commonly, this created a 10 hour eating window (8am-6pm) followed by a 14 hour overnight fast. As food guides shifted people away from satisfying meat to less substantial carbohydrates, hunger rose. Industry responded with an explosion of addictive, packaged snack options. Snacking was encouraged in the food guides even though it was never scientifically demonstrated to reduce weight or consumption. Unfortunately, in the last 45 years, we have increased our eating events to six per day; obesity has quadrupled.6 Every time we eat, even low carbohydrate foods, we stimulate insulin secretion, our fat storage hormone. Snacking has shifted from rare to universal. Over 90% of us eat more than three times daily, with 10% eating ten times per day, grazing all day from waking until bedtime.⁶ Our hunger hormone, ghrelin, rises through the day; now most of our calories are consumed in the evening, when we are the most insulin resistant. This means more insulin is needed to metabolize food at night, creating more fat stores, more obesity.

After struggling for millions of years with food availability, we have evolved biochemical adaptations to fast. Fasting opens access to our stored energy for fuel. Our stored energy sources are either carbohydrate or fat; we have no protein stores. We meet our energy needs while fasting through a predictable sequence of burning. In the first six hours, as blood glucose and insulin levels decrease, we tap into our carbohydrate stores first: glycogen. Our ability to store carbohydrates as glycogen is limited by our liver and muscle size. Unless someone is built like The Hulk, it’s a pretty small tank, empty after 24h without food. The math is simple and helpful- stay with me!

The average adult has approximately 100 grams of glycogen in the liver and 400 grams in the muscles. Given that carbohydrates produce energy at a constant of 4 calories per gram, this 500 grams of stored glycogen carbohydrate will give: 

500 grams glycogen x 4 calories/gram= 2000 calories of energy. A day’s worth of calories.

After 24 hours, once the glucose, glycogen and insulin are low enough, we open up the fat stores. Even lean people have a relatively high amount of fat stores compared to mammals in the wild. Deer have body fat from 2-20% depending upon the season. I am a small human and lean, with 15% body fat.  This gives me approximately 8kg, 8000g, of body fat stored mostly subcutaneously and a small amount in my visceral fat stores. Given fat provides 9 calories/gram:

8000g fat x 9 cal/g= 72000 calories. If most adults use approximately 2000 calories/ day, this would fuel me for over a month, 36 days! 

The low insulin state of fasting unlocks the fat stores, allowing us to mobilize our biggest energy stores. Our clever body has evolved ways to use fat before dipping into precious muscle, by producing counter regulatory hormones.  This group of hormones- epinephrine, nor epinephrine, glucagon, cortisol, and growth hormone- stimulate our sympathetic nervous system. These hormones not only turn on fat burning, but they also increase our metabolic rate.⁷  This is unlike a low calorie diet which slows metabolic rate. Although small amounts of amino acids are used from muscle to create blood glucose (gluconeogenesis), lean muscle and proteins are preserved through the secretion of growth hormone and epinephrine.⁷ The low insulin levels of fasting turns on fat burning. Fat enters the blood as a fatty acid. The liver takes these fatty acids and turns them into ketones. Although fatty acids can’t cross the blood brain barrier, ketones easily go across. When given the choice between glucose and ketones, brain cells (neurons), preferentially use ketones. The brain benefits of fasting include ketone production, autophagy (removal of damaged cells) and the generation of BDNF, brain derived neurotrophic factor- “miracle gro” for brain cells! Ketones improve mental clarity and function. Many trials are now exploring the role of ketones for people with cognitive decline, dementia and neurodegenerative disorders.⁸ I would highly encourage exploring the extensive and free resources on Metabolic Minds, a charitable organization trying to heal mental illness through a ketogenic approach.⁹

Although we can fast, not everyone should. Those who are pregnant, breastfeeding, children, malnourished or underweight should avoid fasting. Caution should be employed to fasting among those with a history of eating disorders, gallstones, diabetes requiring medication, and gout.¹

Fasting has multiple benefits. It is free and even saves you money! It works with any food restrictions, saves time, is transportable, effective and simple. There are also a multitude of options, altering what can be consumed and how long to fast. Some drink non-caloric fluids, like water, coffee, tea, pickle juice or broth. Some dry fast, not consuming any food or drink. Some fast by exclusively eating specific foods for a set time, such as a 72 hour sardine, egg or fat fasts. Fasts lasting more than 24 hours are called extended fasts, with the longest medically supervised fast lasting 382 days (look up Angus Barbieri!).  Even short, intermittent fasts (IF) with time restricted eating (TRE) create real benefits. This time restricted eating is a means of putting intentional time boundaries on consumption. In overweight and obese middle aged and older adults, extended overnight fasting by eating the last meal more than three hours before bed (fasting 13-16 hours overnight) results in improved cardiometabolic health biomarkers such as fasting glucose and blood pressure.¹⁰

I don’t do extended fasts, but I harness the power of TRE everyday. When I wake, I work out fasted as I am fat adapted, burning ketones on a regular basis. I eat a hearty brunch and “lupper” late in the afternoon. This gives me a 16-18 hour overnight fast and a beautiful sleep. Even if we could return to the 1970’s when food was so satisfying we didn’t need snacks and every night we fasted more than 12 hours. We could be satiated, healthier, better rested. Repair requires rest. In relationships and food, we all do better when we have the safety of good boundaries.

References:

1. Nutrition Network. Ketogenic: The Science of Therapeutic Carbohydrate Restriction in Human Health. 1st ed. Nutrition Network; 2023.

   
   

2. The Holy Bible. Matthew 17:21; Mark 9:29.

   
   

3. Höhn S, Dozières-Puyravel B, Auvin S. History of dietary treatment from Wilder’s hypothesis to the first open studies in the 1920s. Epilepsy Behav. 2019;101.

   
   

4. Charlie Foundation. Accessed February 24, 2026. https://charliefoundation.org/

   
   

5. Van Zuylen ML, Siegelaar SE, Plummer M, Deane AM, Hermanides J, Hulst AH. Perioperative management of long-acting glucagon-like peptide-1 (GLP-1) receptor agonists: concerns for delayed gastric emptying and pulmonary aspiration. Br J Anaesth. 2024;132(4):644-648. doi:10.1016/j.bja.2024.01.001

   
   

6. Popkin BM, Duffey KJ. Does hunger and satiety drive eating anymore? Increasing eating occasions and decreasing time between eating occasions in the United States. Am J Clin Nutr. 2010;91(5):1342-1347.

   
   

7. Zauner C, Schneeweiss B, Kranz A, Madl C, Ratheiser K, Kramer L, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000;71(6):1511-1515. doi:10.1093/ajcn/71.6.1511

   
   

8. Rong L, Peng Y, Shen Q, Chen K, Fang B, Li W. Effects of ketogenic diet on cognitive function of patients with Alzheimer’s disease: a systematic review and meta-analysis. J Nutr Health Aging. 2024;28(8):100306. doi:10.1016/j.jnha.2024.100306

   
   

9. Metabolic Mind. Accessed February 24, 2026. https://www.metabolicmind.org/

   
   

10. Grimaldi D, Reid KJ, Abbott SM, Knutson KL, Zee PC. Sleep-aligned extended overnight fasting improves nighttime and daytime cardiometabolic function. Arterioscler Thromb Vasc Biol. Published online February 12, 2026. doi:10.1161/ATVBAHA.125.323355