Long Fasts: Worth the Risk?

Inline_Long-Term_FastingIntermittent fasting, schmittermittent schmasting. The hot new trend is the extended fast—eating nothing and drinking only non-caloric beverages for no less than three days and often as many as 30-40 days. A mere compressed eating window this isn’t.

If fasting for more than three days sounds riskier than just skipping breakfast, you’re right. Long fasts can get you into trouble. They’re a big commitment. You shouldn’t just stumble into one because it sounds interesting or some guy on your Twitter feed wrote about it.

Skipping a meal or even an entire day of food makes evolutionary sense. We weren’t always successful on the hunt or with foraging. We couldn’t head down to the Trader Joe’s for shrink-wrapped steak, sacks of apples, and jars of honey. Reaching the fed state wasn’t a sure thing. Intermittent fasting—going out of your way to not eat, even though food is available—is a modern contrivance meant to replicate the ancestral metabolic environment.

But long fasts seem more evolutionarily aberrant. The evidence from extant hunter-gatherers, many of whom live on land far more impoverished and limited than our hunter-gatherer ancestors, indicates that outright famine is rare. The Hadza may not eat honey and wildebeest every day, but there’s usually plenty of something to eat.

Are there benefits to the longer fast, though? What’s the purported reasoning behind not eating for days on end?

The Logic of Long-Term Fasting

Weight loss

Back in the 1960s, obesity researchers were quite open to the notion that not eating for long periods of time could combat the results of overeating for long periods of time. The most famous case was of the Scotsman, an obese 27-year-old man clocking in at 456 pounds who, upon asking his doctor for help losing weight, was told to stop eating for a few days. He did it for a week, lost five pounds, and decided to continue the experiment for a total of 382 days. He didn’t do this willy-nilly. He took potassium, sodium, and various vitamins each day. He was under medical supervision for the duration, getting checkups each week.

It worked. After 382 days, he was 180 pounds, having lost 276 pounds. At the five-year checkup, he’d only regained 16. He might have been a bit stocky for the times, but by all accounts this long fast was a huge success. Most dieters nowadays eventually regain most or all of their lost weight.

In another study from the 1960s, 46 obese adults fasted for two weeks. No food, just water and vitamins.

On the good side, they all lost weight—an average of 17.2 pounds (from 7.7 to 31.9 pounds). At the two-year followup, half of them had either kept it all off or regained some of the weight they’d lost. The patients with diabetes enjoyed normal glucose levels throughout the fast and continued to have better glucose control after it had ended.

The bad news is that the other half regained every pound they’d lost or were so embarrassed at their progress that they failed to respond to the followup calls at the two-year mark.


Cancer patients typically lose their appetite, and oncologists often prescribe anti-nausea meds to restore it. What if low appetite is adaptive?According to Valter Longo, a cancer researcher from USC, “normal cells” go into survival mode during starvation and display “extreme resistance to stresses” like chemotherapy. If this is the case, extended fasting could improve normal cells’ resistance to harsh cancer treatments. 

In one of Longo’s more recent studies, fasting for 3 days improved cancer patients’ resistance to chemotherapy. Leukocytes in those fasting for 48 hours before chemotherapy followed by another 24 after had less DNA damage than those who fasted for just 24 hours. saw less evidence of breaks in their leukocyte DNA. The result is preliminary but promising.

In an older case study (also authored by Longo), a woman with breast cancer underwent four rounds of chemo. The first round came during a six day fast. Other than dry mouth, fatigue, and hiccups, she felt well enough to continue working. For the second and third rounds of chemo, she didn’t fast. She felt awful the entire time, couldn’t work, and complained of severe nausea, fatigue, and pain. She decided to fast for the fourth and final round, which went as well as the first round. Fasting also improved her biomarkers, including white blood cell, platelet, and neutrophil counts.

There’s even a recent case study suggesting that fasting itself might combat cancer directly. In a woman with stage IIIa low-grade follicular lymphoma (non-Hodgkin’s), a 21-day water fast greatly reduced lymph size. She followed a plant-based whole foods diet immediately after, and by month 9 her lymph nodes were still of normal size.

If you’ve got cancer and are interested in long fasts, clear everything with your doctor first.


In 2014, researchers used a 3-day fast to protect against damage to the immune system and induce total systemic regeneration of the immune system in mice. Fasting actually triggered the mice’s stem cells to begin production of new blood and immune cells.

A recent report on 6 autoimmune case studies sounds quite promising.

Case 1, rheumatoid arthritis: Symptoms included constant pain in all extremities, extreme fatigue, headaches, and occasional autoimmune conjunctivitis (pink eye). A month after stopping RA meds, the patient fasted for 17 days. Two days in, joint pain had subsided. A week in, all pain was gone and mobility was restored. Electrolytes were stable, and he maintained his progress at follow-up visits.

Case 2, mixed connective tissue disease: Symptoms included severe joint pain, chills, facial edema, weakness, fatigue, myalgia, photosensitivity, and tachycardia. She weaned herself off meds before fasting for 21 days. The first week was rough, but by day 10 she felt better. By 21 days, she had no complaints and remained off her meds. Electrolytes remained stable.

Case 3, fibromyalgia: Symptoms were pain, poor sleep, inability to sustain activity for more than an hour. A 24-day fast cleared them up. Electrolytes were stable.

Case 4, systemic lupus erythematosis: Symptoms were joint pain and skin rashes. Two weeks before the fast, she had weaned completely off her meds. On day 3, she was sleeping poorly and feeling nauseated, but on day 4 she began improving. Joint pain was gone. She cut the fast short after 7 days due to weakness and mild tachycardia, but that was enough—she remained symptom free at one year post-fast.

Cases 5 and 6, rheumatoid arthritis: 12- and 24-day respective fasts fixed symptoms for two patients with RA.


A 2001 study involving 174 patients with hypertension found that a 10-11 day water-only fast led to an average blood pressure reduction of 37 mm HG systolic and 13 mm HG diastolic. Those with severe hypertension (180+ mm HG/110+ mm HG) saw even bigger improvements—a 60/17 mm HG reduction on average.

You might have noticed that many of the cited studies were case studies of single individuals. While it’d be great to have RCTs with placebos and control groups and double-blinding, it’s hard and expensive to get a huge group of people together to fast for 21 days, monitor their vital signs, keep them honest, and ensure their safety. You couldn’t conduct a free-living long fasting study because you’d lose too many to McDonald’s-based attrition. You have to keep people in the facility. That takes a lot of money and manpower.

What Are the Risks?

Loss of lean mass

Any weight loss diet will lead to the loss of lean mass in addition to fat mass. The goal is to minimize the former and maximize the latter. Remember: when most people talk about weight loss, they really mean “fat loss.”

When a slightly overweight, otherwise healthy man drank only water for 44 days, he lost 25.5% of his body mass. A quarter to a third of the loss was body fat, the rest lean mass—mostly muscle.

Nutrient deficiencies

 Nothing’s coming in. You’re going to run out of stuff.

  • In the 44-day fasting study, the subject also developed deficiencies in thiamine, riboflavin, and vitamin K.
  • A man with non-Hodgkin’s lymphoma fasted for 53 days (he intended 60 days, but couldn’t make it) and ended up with a dangerous neurological condition called Wernicke encephalopathy caused by severe thiamine deficiency. 
  • A woman admitted herself to the hospital after a 40-day water-only fast. She had severe sodium deficiency upon admittance and developed severe deficiencies in magnesium, phosphorus, and potassium upon eating.

Refeeding syndrome

Nutrient requirements drop during a fast. Your body isn’t doing nearly as much as it does when you’re fed, so you can get away with less. Serum levels of basic minerals like magnesium, potassium, and phosphorus remain normal, while intracellular levels drop. But upon refeeding, the situation reverses. Your insulin spikes in response to incoming food. You’re suddenly having to store fat, make and store glycogen, and conduct various other metabolic processes that increase intracellular nutrient requirements. To meet the need, electrolytes move from serum to cells, creating a deficiency on the serum level that can be quite dangerous.

Increased susceptibility to infections

A study in famine victims found that starvation increased susceptibility to infections, particularly malaria. Sometimes the infections were suppressed during the fast and only manifested upon refeeding. Fasting isn’t famine, but it’s similar enough that we should heed the story.                 

Tips for doing it safely…

Drink green tea during the fast. Purists will scoff at you for ingesting anything but water. Forget them. A 2003 rat study found that green tea protects against the fasting-induced damage to the intestinal lining during a 3-day fast. Remember: these were rat days. In human days, those 3 days are more like 90.

Take MCTs. Tim Ferriss recommends taking medium chain triglyceride oil in the first couple days as a tool to ease your way into a long fast. If you’re already on a ketogenic diet or count yourself as a fat-burning beast with robust fat-burning mitochondria, you can probably skip this.

Take magnesium, calcium, potassium, and sodium. Long fasts seriously perturb electrolyte homeostasis. I vastly prefer getting my electrolytes through a tall glass of Gerolsteiner mineral water (magnesium and calcium) spiked with sea salt (sodium) and lime juice (potassium).

Take thiamine/B-complex. Many studies indicate that fasting depletes thiamine and other vitamins, so stay on top of that. The Scotsman I discussed earlier took a nutritional yeast tab each day, probably for the B vitamin content.

Take vitamin K. A week of fasting depletes vitamin K, which is incredibly problematic if you’re fasting before major surgery. It’s also not great for general health.

Make mineral bone broth. With all the fat strained out, bone broth is quite low in calories and will have a negligible impact on your fast. Throw in a big handful of leafy greens with the stems. I like collards and beet greens, personally. You can either remove the veggies (all the minerals will have gone into the broth) or eat them.

Refeed with a light, low-carb meal. Don’t come off a week-long fast and immediately tuck into a platter of ribs. Don’t refeed with high-carbs. Large meals are difficult to handle after a long fast, and high-carb meals may lead to dangerous levels of fluid retention.

Take two or three days to ease yourself back into your normal routine. Eat smaller, lighter meals. Don’t train too hard. Refeeding syndrome is a real threat.

Have a good reason for doing it. Long fasts are serious, and you should have a serious reason for embarking on one.

Obese? Sure, a long fast with medical supervision and electrolyte and vitamin supplements can work.

Got non-Hodgkin’s lymphoma? If your doctor is on board, a 10-15 day fast might really improve survivability.

Got rheumatoid arthritis or some other autoimmune disease that just doesn’t respond to anything you’ve tried? Maybe a week-long fast will help.

Got invited to a silent meditation/fasting retreat? Go for it.

Does your spiritual practice or religion call for a four day fast? If your faith is important to you and completing this fast is integral to it, you should fast.


If you’re trying to reveal the bottom half of the six-pack you just know is lurking beneath your gut, long fasting is not the answer.

If you’re hoping to shed the last of your baby weight, don’t try a long fast.

If you’re sleeping five hours a night, working twelve hour days, and walk around a frazzled ball of stress, don’t not eat for a week straight.

If you’re fairly healthy and happy and everything’s going well, I’m not sure. Don’t make it a habit. Treat it like a marathon, maybe. A once in a blue moon event you pursue purely for the novelty.

By now, you should have a better grasp of the potential benefits and drawbacks of long fasts. They’re not for everyone or every situation—and I think shorter fasts or compressed eating windows make more sense for most people—but the long fast is an intriguing option that can be safely done if you take the right precautions.

Do you think you’ll try one? Have you tried one? How’d it go?

Let’s hear from you down below. Thanks for reading, everyone.


About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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