Is Fasting for Weight Loss Safe?

Michael Greger M.D. FACLM · November 4, 2019 · Volume 48

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The reasons why fasting longer than 24 hours, and particularly three or more days, should only be done under the supervision of a health professional and preferably in a live-in clinic.

Discuss

Transcript

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Although fasting for a week or two can actually interfere with the loss of body fat, eventually, after the third week of fasting, fat loss starts to overtake the loss of lean body mass in obese individuals. But is it safe to go that long without food?

Proponents speak of fasting as a cleansing process, but some of what they are purging from their body are essential vitamins and minerals. Heavy-enough people can go up to 382 days without calories, but no one can go even a fraction of that long without vitamins. Scurvy, for example, is diagnosable within as few as four weeks without any vitamin C. Beriberi, thiamine (vitamin B1) deficiency, may start out even earlier in fasting patients, and once it manifests can result in brain damage within days—which can eventually become irreversible.

Even though fasting patients report problems such as nausea and indigestion taking supplements, all of the months-long fasting cases I’ve mentioned in the last few videos were given daily multivitamins and mineral supplementation as necessary. Without supplementation, hunger strikers, and those undergoing prolonged fasts for therapeutic or religious purposes (like the Baptist pastor hoping to “enhance his spiritual powers for exorcism”) have ended up paralyzed, comatose, or worse.

Nutrient deficiencies aren’t the only risk. Reading about all the successful reports of massive weight loss from prolonged fasting in the medical literature, one doctor decided to give it a try with his patients. Of the first dozen he tried it on, though, two died. Now, in retrospect, both the two patients that died started out with heart failure and were on diuretics. Fasting itself produces a pronounced diuresis, meaning loss of water and electrolytes through the urine. And so, it was the combination of fasting on top of the water pills that likely depleted their potassium and triggered their fatal heart rhythms. The doctor went out of his way to point out that both of those who died started out in severe heart failure, and “both had improved greatly whilst undergoing starvation therapy.” Small consolation, since they were both dead within a matter of weeks.

Not all therapeutic fasting fatalities were complicated by concurrent medication use. “At first he did very well and experienced the usual euphoria…[his] electrolytes remained [fine], but in the middle of the third week…he suddenly collapsed and died. This line of treatment is certainly tempting because it does produce weight-loss, and the patient feels so much better, but the [whole killing people thing] must make it a very suspect line of management.”

Contrary to the popular notion that the heart muscle is specially spared during fasting, the heart appears to experience similar muscle wasting. This was noted in the victims of the Warsaw ghetto during World War II in a remarkable series of detailed studies carried out by the ghetto physicians, before they themselves succumbed. In a case entitled “Gross Fragmentation Of Cardiac Fiber After Therapeutic Starvation For Obesity,” a 20-year-old woman successfully achieved her ideal body weight after losing 128 pounds fasting for 30 weeks. After a breakfast of one egg, she had a heart attack and died. On autopsy, the muscle fibers in her heart showed evidence of widespread disintegration. The pathologists suggested that fasting regimens “should no longer be recommended as a safe means of weight reduction.”

Breaking the fast appears to be the most dangerous part. After World War II, as many as one out of five starved Japanese prisoners of war tragically died following liberation. Now known as “refeeding syndrome,” multiorgan system failure can result from resuming a regular diet too quickly. See, there are critical nutrients, such as thiamine and phosphorus, that are used to metabolize food. So, in the critical refeeding window, if too much food is taken before these nutrients can be replenished, demand may exceed supply. And whatever residual stores you still carry can be driven down even further, with potentially fatal consequences. That’s why rescue workers are taught to always give thiamine before food to victims who had been trapped or otherwise unable to eat. Thiamine is responsible for the yellow color of “banana bags,” a term you might have heard used on medical dramas, used to describe an IV fluid concoction often given to malnourished alcoholics to prevent a similar reaction. Anyone “with negligible food intake for more than five days” may be at risk of developing refeeding problems.

Medically-supervised fasting has gotten much safer now that there are proper refeeding protocols, we know what warning signs to look for, and we now know who shouldn’t be fasting in the first place (such as those with advanced liver or kidney failure, porphyria, uncontrolled hyperthyroidism, and pregnant and breastfeeding women). The most comprehensive safety analysis of medically-supervised, water-only fasting was recently published out of the True North Health Center in California. Out of 768 visits to their facility for fasts up to 41 days, were there any adverse events? Yes, 5,961 of them! But most were mild known reactions to fasting, such as fatigue, nausea, insomnia, headache, dizziness, upset stomach, and back pain. They report only two serious events, and no fatalities.

Fasting longer than 24 hours, and particularly three or more days, should only be done under the supervision of a physician and preferably in a live-in clinic. In other words, don’t try this at home. This is not just legalistic mumbo-jumbo. For example, normally your kidneys dive into sodium conservation mode during fasting, but should that response break down, you could rapidly develop an electrolyte abnormality that may only manifest with non-specific symptoms like fatigue or dizziness, which could easily be dismissed until it’s too late.

The risks of any therapy must be premised on the severity of the disease. The consequences of obesity are considered so serious that effective therapies could have “considerable acceptable toxicity.” For example, many consider major surgery for obesity to be a justifiable risk. But the key word is effective.

Therapeutic fasting for obesity has been largely abandoned by the medical community not only because of its uncertain safety profile, but its questionable short- and long-term efficacy. Remember, for a fast that only lasts a week or two, you might be able to lose as much body fat or even more on a low-calorie diet, than a no-calorie diet.

Please consider volunteering to help out on the site.

Sources

Acknowledgements

Image credit: Jean Fortunet via wikimedia. Image has been modified.

Motion graphics by Avocado Video

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Although fasting for a week or two can actually interfere with the loss of body fat, eventually, after the third week of fasting, fat loss starts to overtake the loss of lean body mass in obese individuals. But is it safe to go that long without food?

Proponents speak of fasting as a cleansing process, but some of what they are purging from their body are essential vitamins and minerals. Heavy-enough people can go up to 382 days without calories, but no one can go even a fraction of that long without vitamins. Scurvy, for example, is diagnosable within as few as four weeks without any vitamin C. Beriberi, thiamine (vitamin B1) deficiency, may start out even earlier in fasting patients, and once it manifests can result in brain damage within days—which can eventually become irreversible.

Even though fasting patients report problems such as nausea and indigestion taking supplements, all of the months-long fasting cases I’ve mentioned in the last few videos were given daily multivitamins and mineral supplementation as necessary. Without supplementation, hunger strikers, and those undergoing prolonged fasts for therapeutic or religious purposes (like the Baptist pastor hoping to “enhance his spiritual powers for exorcism”) have ended up paralyzed, comatose, or worse.

Nutrient deficiencies aren’t the only risk. Reading about all the successful reports of massive weight loss from prolonged fasting in the medical literature, one doctor decided to give it a try with his patients. Of the first dozen he tried it on, though, two died. Now, in retrospect, both the two patients that died started out with heart failure and were on diuretics. Fasting itself produces a pronounced diuresis, meaning loss of water and electrolytes through the urine. And so, it was the combination of fasting on top of the water pills that likely depleted their potassium and triggered their fatal heart rhythms. The doctor went out of his way to point out that both of those who died started out in severe heart failure, and “both had improved greatly whilst undergoing starvation therapy.” Small consolation, since they were both dead within a matter of weeks.

Not all therapeutic fasting fatalities were complicated by concurrent medication use. “At first he did very well and experienced the usual euphoria…[his] electrolytes remained [fine], but in the middle of the third week…he suddenly collapsed and died. This line of treatment is certainly tempting because it does produce weight-loss, and the patient feels so much better, but the [whole killing people thing] must make it a very suspect line of management.”

Contrary to the popular notion that the heart muscle is specially spared during fasting, the heart appears to experience similar muscle wasting. This was noted in the victims of the Warsaw ghetto during World War II in a remarkable series of detailed studies carried out by the ghetto physicians, before they themselves succumbed. In a case entitled “Gross Fragmentation Of Cardiac Fiber After Therapeutic Starvation For Obesity,” a 20-year-old woman successfully achieved her ideal body weight after losing 128 pounds fasting for 30 weeks. After a breakfast of one egg, she had a heart attack and died. On autopsy, the muscle fibers in her heart showed evidence of widespread disintegration. The pathologists suggested that fasting regimens “should no longer be recommended as a safe means of weight reduction.”

Breaking the fast appears to be the most dangerous part. After World War II, as many as one out of five starved Japanese prisoners of war tragically died following liberation. Now known as “refeeding syndrome,” multiorgan system failure can result from resuming a regular diet too quickly. See, there are critical nutrients, such as thiamine and phosphorus, that are used to metabolize food. So, in the critical refeeding window, if too much food is taken before these nutrients can be replenished, demand may exceed supply. And whatever residual stores you still carry can be driven down even further, with potentially fatal consequences. That’s why rescue workers are taught to always give thiamine before food to victims who had been trapped or otherwise unable to eat. Thiamine is responsible for the yellow color of “banana bags,” a term you might have heard used on medical dramas, used to describe an IV fluid concoction often given to malnourished alcoholics to prevent a similar reaction. Anyone “with negligible food intake for more than five days” may be at risk of developing refeeding problems.

Medically-supervised fasting has gotten much safer now that there are proper refeeding protocols, we know what warning signs to look for, and we now know who shouldn’t be fasting in the first place (such as those with advanced liver or kidney failure, porphyria, uncontrolled hyperthyroidism, and pregnant and breastfeeding women). The most comprehensive safety analysis of medically-supervised, water-only fasting was recently published out of the True North Health Center in California. Out of 768 visits to their facility for fasts up to 41 days, were there any adverse events? Yes, 5,961 of them! But most were mild known reactions to fasting, such as fatigue, nausea, insomnia, headache, dizziness, upset stomach, and back pain. They report only two serious events, and no fatalities.

Fasting longer than 24 hours, and particularly three or more days, should only be done under the supervision of a physician and preferably in a live-in clinic. In other words, don’t try this at home. This is not just legalistic mumbo-jumbo. For example, normally your kidneys dive into sodium conservation mode during fasting, but should that response break down, you could rapidly develop an electrolyte abnormality that may only manifest with non-specific symptoms like fatigue or dizziness, which could easily be dismissed until it’s too late.

The risks of any therapy must be premised on the severity of the disease. The consequences of obesity are considered so serious that effective therapies could have “considerable acceptable toxicity.” For example, many consider major surgery for obesity to be a justifiable risk. But the key word is effective.

Therapeutic fasting for obesity has been largely abandoned by the medical community not only because of its uncertain safety profile, but its questionable short- and long-term efficacy. Remember, for a fast that only lasts a week or two, you might be able to lose as much body fat or even more on a low-calorie diet, than a no-calorie diet.

Please consider volunteering to help out on the site.

Image credit: Jean Fortunet via wikimedia. Image has been modified.

Motion graphics by Avocado Video

Doctor's Note

Fasting for a week or two can actually interfere with the loss of body fat? If you missed it, check out my last video Is Fasting Beneficial for Weight Loss? and for more background, Benefits of Fasting for Weight Loss Put to the Test.

What’s the best way to lose weight? I wrote a whole book about it – How Not to Diet.

Next I turn my focus to intermittent fasting—stay tuned:

All of my fasting videos can be found on the topic page.

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