Fasting (and Breaking Fasts) in 2026 — The Safest, Easiest and Most Beneficial Ways to Fast

By Nils Osmar

The information in this article is not intended as, and should not be taken as, medical advice. Interventions like exercising, dietary changes, and taking medications and supplements can have both benefits and side effects, Please consult with a medical professional about any medical concerns.  See full Medical Disclaimer

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I used to do several 5 day fasts every year. I don’t fast as much these days, in part because I’m 73 and some recent studies suggest that there could be an age after which the possible problems caused by fasting might begin to outweigh the benefits. So I’m trying not to overdo it.

I decided to fast a day and a half ago, starting at 7 PM on Saturday night; this is Monday afternoon. It just felt like a good time to fast again. This means that I didn’t eat yesterday, and I have not had breakfast or lunch today. I may end the fast around 7 PM, or I may continue fasting until tomorrow. I’ll check in with my body to see how I’m feeling in a couple of hours. (Later note: I ended up fasting 44 hours this time.)

Combining Fasting With Ketosis…

I had actually forgotten how easy it is to start fasting when you’re in a ketogenic state. I’ve been eating a ketogenic diet for the last few days. I went ketogenic a few days ago in order to reset my metabolic state. This was motivated by my wanting to lose a few extra pounds. 

Since I was in ketosis when I started fasting, I have had virtually no hunger. This means that my decision about whether to go ahead and eat dinner tonight, or wait and break my fast sometime tomorrow, is more of a logic decision than one based in hunger. 42 hours in, I’m really not hungry at all. 

This isn’t surprising. When I move into fasting after a few days in ketosis, I generally don’t experience hunger. I may eventually experience other symptoms such as lightheadedness. If I do, I will probably break the fast. 

In the old days, I would set a goal of fasting for four or five days, and I noticed that I was judging myself if I decided to stop the fast early. Kind of silly, but I think a lot of people who fast do that. Three day fast have particular benefits for autophagy, and for a stem cell renewal. So I’m motivated in that sense to continue. But I’m sure that the fast will have had had some benefits regardless of how long it continues.

When I’m not fasting, I eat a high-nutrient diet, which includes about 180 grams of protein a day, eating a largely animal-based diet with lots of salmon and sardines and beef and poultry. I also eat foods such as kefir, butter, wild blueberries, fermented vegetables, onions, garlic, and a few root vegetables, and take a good multivitamin/mineral supplement and a range of anti-aging supplements. My nutrient consumption is so high that I don’t think there’s a big danger in a day or two of fasting.

There could in theory be a greater dangers related to fasting for people who are eating low protein or low fat diet, or avoiding animal proteins, unless they are very careful to combine foods to get a full array of essential amino acids when they are not fasting.

…. and with Red Light Therapy

I’ve been doing a lot of red light therapy lately. I did 15 minutes of RLT today during my fast and 15 minutes after. I felt unusually energized.

My Experience

I love how my mood improves, my energy increases, and my mental clarity sharpens when I’m fasting. It’s a water fast, but I’m also drinking coffee with some C8 MCT oil in it—the type of MCT which turns rapidly into ketones. So I’m sure my brain is bathed in a steady supply of ketones.

Post-fast note: My body feels unusually good, energized. I’m not sure how else to describe it without a metaphor. So here’s one: It feels almost like it’s “glowing from inside,” like a sun radiating light and heat. My tendonitis feels quite a bit better, though it hasn’t been fully healed. It may be that fasting reduced general inflammation for more healing to take place.

Supplements During Fasting…

I have found that if I tried to be a purist and avoid taking supplements, I can develop problems, particularly on a long fast. So I do take several supplements.

They include lutein, which I take along with zeaxanthin; astaxanthin; a high-quality fish oil supplement; krill, oil and electrolytes. If I skip the electrolytes, I can get leg cramps. If I skip the other supplements, I notice that my vision seems a little ragged and it’s harder to read fine print.

To get additional nutrients that are beneficial for the eyes, I will sometimes also eat an egg yolk or two, sometimes hard-boiled, sometimes raw. (If this bothers you and you think I’m not doing “real” fasting, feel free to cuss me out in the comments below; it’s what works for me.) I don’t eat any protein because I don’t want to turn off autophagy. I might sometimes also take a high-quality multivitamin/mineral.

I don’t take any other supplements currently when I’m fasting.

… and Supplements After Fasting

After fasting, I took some more fish oil, and also some sea buckthorn oil, CoQ10 and PQQ, compounds associated with mitochondrial biogenesis and with the growth and differentiation of new stem cells. I had some whey powder in water after my meal and took some NAC and cysteine to support the production of glutathione.

Breaking A Fast

I’ve tried easing back into eating at the end of a fast in a number of different ways. For a while, I would break my fast by drinking some homemade kefir. But it has carbohydrates, so I won’t be doing so because my goal is to stay in ketosis for at least another week or two.

My plan this time is to eat some sardines or wild Pacific sockeye salmon, both of which are rich in nutrients including Omega 3 fatty acids and low in contaminants. If I go with the sardines, I’ve found a really good brand packed in glass rather than little aluminum/plastic-lined tins (MW Polar Naturally Alder Wood Smoked Brisling Sardines in Olive Oil). If I go with sardines, I might have some salmon sauteed over low heat in a little MCT or coconut oil, with a few wild blueberries on the side. I’ll eat them along with some sauerkraut, kimchi, or radish kimchi.

Whatever your diet, it’s important, in my estimation, to choose highly nutritious foods when fasting. Back in my vegan days I tried a few fasts, but I often felt sick after I added foods back in, possibly because of food sensitivities, but possibly because of a reaction to plant defense chemicals. Another issue may be if that when I was vegan, I was typically not getting enough of the essential amino acids that my body needed to start the rebuilding process following autophagy. When I combined foods to make sure I was getting all of them, it raised the calories and increased the carbohydrates, which raised my blood sugar and defeated the whole purpose of fasting. Whatever your diet, it’s important in my opinion to make sure that you are getting enough protein and other nutrients to start the rebuilding process.

It may be that some people can do fasting then break their fast with a totally vegan meal and do fine; I’m just reporting in this section on my personal results. I actually. experienced many health problems when I was vegan, including memory problems, mood problems, aching teeth, aching joints, bone loss, poor energy, weight loss, followed by weight gain until I actually ended up obese; the poor recovery after fasting was just one problem. But others’ results may vary.

Fasting and Anti-Aging: Sorting Through the Research

When we stop eating, the body begins an powerful metabolic transition. For the first 12 or so hours, it draws on stored glycogen—glucose packed away in the liver and muscles. Once those stores start running low, the liver shifts to breaking down fatty acids and converting them into ketone bodies: beta-hydroxybutyrate (BHB), acetoacetate, and acetone. These ketones cross the blood-brain barrier and serve as a remarkably efficient fuel source for the brain, capable of supplying up to 75% of its energy needs.

This metabolic switch—from glucose-based to ketone-based energy—appears to be one of the key mechanisms underlying the health benefits of fasting. A review published in the New England Journal of Medicine noted that this transition triggers a cascade of beneficial changes: increased stress resistance, reduced inflammation, and activation of cellular repair pathways. 

If you’re already eating a ketogenic diet when you begin a fast—which I was before I began the fast—this switch happens much faster, since the body is already primed for fat oxidation.

Autophagy: The Anti-Cancer, Anti-Aging Cellular Cleanup Process

One of the key benefits of fasting as it relates to longevity and general health is the activation of autophagy—from the Greek for “self-eating.” Autophagy is the body’s way of cleaning house at the cellular level. During this process, cells identify and break down damaged proteins, dysfunctional organelles, and round up any random viruses and bacteria that have gotten into our cells along with other molecular debris, recycling the components into raw materials for building new cellular structures. Think of it as our cells running a deeply thorough housecleaning and garage sale simultaneously.

Researchers have found that autophagy markers begin rising noticeably after about 14–16 hours of fasting, with significant upregulation around 24 hours. In animal studies, autophagy continues to intensify through 48 hours of fasting, after which it rolls along at a more steady speed. 

The process is driven in part by the activation of AMP-activated protein kinase (AMPK)—an enzyme that acts as a cellular fuel gauge—and the inhibition of mTOR (mechanistic target of rapamycin), more specifically mTOR complex 1, the growth-and-build signalling pathway.

When mTOR complex 1 is dialed down and AMPK is cranked up, the cell shifts from “growth mode” into “repair and recycle mode.”

The importance of autophagy for health was underscored by the 2016 Nobel Prize in Physiology or Medicine, awarded to Yoshinori Ohsumi for his discoveries of the mechanisms underlying this process. Dysfunctional autophagy has been implicated in a wide range of age-related diseases, including neurodegeneration, cancer, and cardiovascular disease. The ability of fasting to robustly activate autophagy is one of the strongest arguments for its role as a longevity intervention.

Brain Benefits: Ketones, Clarity, and Neuroprotection

Many people who fast report enhanced mental clarity and improved mood. I experience this myself. These experiences are not simply a placebo effect. There’s real science to support them.

The key thing to understand is that ketone bodies aren’t just an alternative fuel; they appear to be a signaling molecule with potent neuroprotective properties. BHB, the most abundant ketone body during fasting, inhibits histone deacetylases, which in turn increases the secretion of brain-derived neurotrophic factor (BDNF)—a protein essential for learning, memory, and the growth and survival of neurons.

Research on ketogenic diets and fasting has shown neuroprotective effects in animal models of Alzheimer’s disease, Parkinson’s disease, traumatic brain injury, and stroke. A randomized crossover trial in older adults at risk for Alzheimer’s found that a modified Mediterranean-ketogenic diet that raised fasting ketone levels was associated with cognitive benefits and a reduction in Alzheimer’s disease biomarkers. Additionally, ketones appear to reduce neuroinflammation by suppressing the NLRP3 inflammasome, a key driver of inflammatory responses in the brain.

The addition of C8 MCT oil during a water fast provides even more fuel fuel for the creation of ketones. C8 (caprylic acid) is rapidly converted to ketones by the liver, providing a direct supply of brain fuel without significantly disrupting the fasted metabolic state.

Stem Cell Regeneration and Biological Age Reduction

One of the more interesting findings in fasting research comes from MIT, where researchers demonstrated that a 24-hour fast dramatically improves the regenerative capacity of intestinal stem cells—in both young and aged mice. This is significant because intestinal stem cell function naturally declines with age, making it harder to recover from infections and injuries. The researchers found that fasting causes stem cells to switch from burning carbohydrates to burning fatty acids, and this metabolic shift activates transcription factors (PPARs) that drive regeneration.

Meanwhile, research led by Valter Longo at the University of Southern California on the fasting-mimicking diet (FMD)—a 5-day, low-calorie, low-protein, plant-based protocol designed to produce fasting-like metabolic effects while still providing nutrients—has shown remarkable results in human clinical trials. Participants who completed cycles of the FMD reduced their biological age by an average of 2.5 years, as measured by DNA methylation clocks, and showed improvements in insulin resistance, liver fat, and immune system markers. Longo has suggested that healthy people between 20 and 70 consider the FMD two to three times per year.

To my mind, Longo’s research in this area is brilliant and key to understand. I don’t care for his “longevity diet”; when I tried following it, I started losing muscle and experiencing fatigue. I seem to need a higher protein, diet myself. But his fasting mimicking diet is a great alternative, in my opinion, for people who want to experience many of the benefits of fasting without going into a full-blown prolonged water fast.

Cardiovascular and Metabolic Benefits

Multiple studies have documented improvements in cardiometabolic markers with various fasting regimens. Time-restricted eating and intermittent fasting have been associated with reductions in blood pressure, improvements in lipid profiles, better insulin sensitivity, and lower levels of inflammatory markers.

A particularly interesting finding from lipid metabolomics research is that even a single day of fasting triggers a shift in triglyceride composition—toward a profile that, in epidemiological studies, predicts reduced risk of diabetes and age-related frailty. Intriguingly, this shift persists even after refeeding, suggesting that fasting may actually reprogram lipid metabolism in lasting ways.

Research on long-term caloric restriction practitioners from Washington University in St. Louis has found that severe CR completely protects against high blood pressure, atherosclerosis, and diabetes, with participants appearing 20–30 years younger than their chronological ages in terms of cardiovascular markers. While strict caloric restriction isn’t realistic for most people, periodic fasting may capture some of these benefits with a much lower day-to-day burden.

What About the Risks?

Fasting is an important intervention. But it does have some possible downsides. Certain populations need to exercise particular caution.

Muscle Loss and Sarcopenia: The Top Concern for Older Adults

Sarcopenia—the age-related loss of skeletal muscle mass and function—is one of the most important risk to consider, particularly for people over 65. Skeletal muscle loss begins around age 30 and accelerates after 70, with the fastest-twitch (Type II) muscle fibers declining most rapidly. Fasting, by its nature, is a catabolic state. When the body runs out of easy fuel, it can turn to muscle protein as a source of amino acids. Even doing very short, fests such as skipping breakfast, which is common in time-restricted eating, can lead to problems for some people in some situations.

A 2025 review in Frontiers in Nutrition reported that a 12-week time-restricted eating protocol led to lean mass losses accounting for roughly 65% of total weight lost—significantly higher than the typical 20–30% seen with standard calorie-restricted diets. For older adults already fighting sarcopenia, this is a serious concern. The mitigation strategies are clear: maintain high protein intake on non-fasting days (the 180 grams per day mentioned above is well above recommended thresholds), combine fasting with resistance training, and keep fasts relatively short.

I had a friend who was just a couple of years older than me who was eating the standard American diet and decided to try a three day water fast; he experienced dizziness. I encouraged him to stop his fast and consult with his doctor. People our age can be in great health or they can already be experiencing frailty, as he was. In his situation, there were serious dangers; a fall caused by dizziness could have led to a bone breaking. He later did Dr. Valter Longo’s fasting mimicking diet with his doctors approval, and did not experience any problems.

The Refeeding Window: A Time of Special Concern

A 2024 study from MIT, published in Nature, revealed a fascinating and somewhat unsettling finding. While fasting suppresses intestinal stem cell proliferation, the refeeding period after a fast triggers a powerful surge in stem cell regeneration—driven by the mTOR pathway and polyamine synthesis. This regeneration is the mechanism behind fasting’s gut-healing benefits. 

However, the researchers found that when cancer-causing mutations were introduced during this highly regenerative refeeding state, the mice were significantly more likely to develop precancerous polyps.

The lead researcher, Omer Yilmaz, noted that this was done in mice with specific cancer mutations, and the situation in humans is far more complex. But the finding does suggest a practical consideration: one approach which strikes me as sensible would be to choose clean, whole foods, avoid all processed foods, and avoid known mutagens (like charred meats or other foods high in acrylamides) during the initial refeeding window. 

A Warning from the AHA – and My Take On It

In March 2024, a preliminary study presented at the American Heart Association’s scientific sessions generated disturbing headlines: an analysis of over 20,000 U.S. adults found that those who ate within an 8-hour window had a 91% higher risk of cardiovascular death compared to those eating across 12–16 hours. The finding was widely covered in media and alarmed many of the people practicing time-restricted eating.

It’s important, though, to realize the limitations of the study. It was observational, based on only two 24-hour dietary recalls per participant—a snapshot that may not reflect habitual eating patterns. 

The AHA study could not establish causation, and many researchers pushed back hard. Former AHA president Donald Lloyd-Jones noted publicly that the abstract was “not ready for prime-time consumption.” It’s possible that the people eating in compressed windows were doing so for reasons correlated with poorer health (shift work, irregular schedules, underlying conditions). There’s a massive difference between eating a high nutrient, high protein, high healthy fat diet on your eating days, then deliberately taking one or more days off for fasting, and eating a poor diet and skipping breakfast because of scheduling issues. The AHA study remains unpublished in a peer-reviewed journal as of early 2026. I’m personally not worried about it.

Fasting After 70: Is It Safe? Really?

Most fasting studies have been conducted on young, healthy volunteers or people with obesity or even morbid obesity. Very few clinical trials have specifically enrolled non-obese adults over 70. A 2024 review in the Canadian Journal of Cardiology emphasized that the safety of fasting in elderly individuals remains understudied and called for “prudent consideration” when implementing intermittent fasting protocols in this population.

Specific concerns for older fasters include: the sarcopenia risk discussed above; potential blood pressure fluctuations that increase fall risk; electrolyte imbalances that could predispose to dehydration or cardiac arrhythmias; and the possibility that chronic mTOR suppression could impair immune function. 

These risks have not established that older adults should never fast—but they do suggest that shorter, less frequent fasts, combined with high-quality nutrition and resistance training, may be safer than long water fasts, in particular for people who eat poor diets or have health conditions that might contraindicate fasting.

The Bottom Line

The evidence for fasting as a longevity intervention is substantial and growing. The metabolic switch to ketones, the activation of autophagy, the boost to stem cell regeneration, the reduction in inflammatory markers, and the demonstrated reversal of biological age in clinical trials—all of these represent real and meaningful mechanisms through which periodic fasting may extend both healthspan and lifespan.

But again, context matters enormously. A 24–48 hour fast undertaken by someone who eats a nutrient-dense, high-protein, high-healthy fat diet, maintains muscle through resistance training, is already fat-adapted through a ketogenic approach, and breaks their fast with clean whole foods is one thing; a fast of similar length by someone who is underweight, sick or nutrient-depleted is another. I can’t recommend what others should do, but I can say for myself that if I were in the latter situation, I would start with a couple of days a week of time, restricted eating, see how that goes for a while, make sure I’m improving my nutrition during the periods when I am eating, before trying a longer fast. Or I might just move into a fasting mimicking diet.

As with so many interventions in the longevity space, the dose makes the medicine. The bulk of the research suggests to me that periodic well-planned fasts offer a favorable risk-to-benefit ratio for most people. The key is to fast intelligently: know your body, maintain your muscle, feed yourself exceptionally well between fasts, and—especially as you get older—err on the side of moderation.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. The fact that I do fasting and have experienced benefits from it does not mean that it is necessarily safe or advisable for everyone.Consult a qualified healthcare professional before beginning any fasting regimen, particularly if you have existing health conditions, take medications, or are over 65.

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More from Nils Osmar

• Rekindle School classes in Seattle: Rekindl.e School
• Anti-Aging Youtube channel Pathways to Longevity
• Anti-Aging Facebook group: Anti-Aging and Life Extension
• Another interesting topic: AI Consciousness
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