Have you ever wondered if there’s a natural way to lower your high blood pressure, guard against Alzheimer's, lose weight, and feel better? Well as it turns out there is. Michael Greger, M.D. FACLM, founder of NutritionFacts.org, and author of the instant New York Times bestseller “How Not to Die” celebrates evidence-based nutrition to add years to our life and life to our years.

Intermittent Fasting Part 2

Today we feature part 2 of our series on the effects of intermittent fasting with the pros and cons of caloric restriction.

This episode features audio from The Benefits of Calorie Restriction for Longevity, Potential Pitfalls of Calorie Restriction, and Benefits of Fasting for Weight Loss Put to the Test. Visit the video pages for all sources and doctor’s notes related to this podcast.


Have you ever noticed that every month seems to bring a trendy new diet? And yet obesity rates continue to rise and with it a growing number of health problems. That’s why I wrote my new book How Not to Diet. Check it out at your local public library. Welcome to the Nutrition Facts Podcast. I’m your host Dr. Michael Greger. 

Today, we continue our series on intermittent fasting with a story on how a slower metabolism may actually be a good thing.

We’ve known for more than a century that calorie restriction can increase the lifespan of animals, and the metabolic slowdown may be the mechanism. That could be why the tortoise lives 10 times longer than the hare. Rabbits can live 10 to 20 years, whereas “Harriet,” a tortoise evidently collected from the Galapagos by none other than Charles Darwin himself in the 1830s, lived until 2006. Slow and steady may win the race.

One of the ways your body lowers your resting metabolic rate is by creating cleaner-burning, more efficient mitochondria, the power plants that fuel our cells. It’s like your body passes its own fuel-efficiency standards. These new mitochondria create the same energy with less oxygen, and produce less free radical “exhaust.” After all, your body is afraid famine is afoot, and so, it is trying to conserve as much energy as it can.

The largest caloric restriction trial to date indeed found both metabolic slowing and a reduction in free radical-induced oxidative stress, both of which may slow the rate of aging. The flame that burns twice as bright burns half as long. But whether this will result in greater human longevity is an unanswered question. Caloric restriction is often said to extend the lifespan of “every species studied.” But that isn’t even true of all strains within a single species. Some scientists don’t think calorie restriction will improve human longevity at all; others suggest a 20 percent calorie restriction starting at age 25 and sustained for 52 years could add 5 years onto your life. Either way, the reduced oxidative stress would be expected to improve our healthspan.

Members of the Calorie Restriction Society, self-styled CRONies (for Calorie-Restricted Optimal Nutrition), appear to be in excellent health, but they’re a rather unique self-selected bunch of individuals. You don’t really know until you put it to the test. Enter the CALERIE study, the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy, the first clinical trial to test the effects of caloric restriction.

Hundreds of non-obese men and women were randomized to two years of 25 percent calorie restriction. They only ended up achieving half that but lost about 18 pounds and three inches off their waists, wiping out more than half of their visceral abdominal fat. That translated into significant improvements in cholesterol levels, triglycerides, insulin sensitivity, and blood pressures. Eighty percent of those who were overweight when they started were normal weight by the end, compared to a 27-percent increase in those who became overweight in the control group.

In the famous Minnesota Starvation Study that used conscientious objectors as guinea pigs during World War II, the study subjects suffered both physically and psychologically, experiencing depression, irritability, and loss of libido. The subjects started out lean, though, and had their calorie intake cut in half. The CALERIE study ended up being four times less restrictive, only about 12 percent below baseline calorie intake, and enrolled normal-weight individuals, which in the U.S. these days means overweight, on average. As such, the CALERIE subjects experienced nothing but positive quality-of-life benefits, with significant improvements in mood, general health, sex drive, and sleep. They only ended up eating about 300 fewer calories than they were eating at baseline. So, they got all these benefits—the physiological benefits, the psychological benefits—all from only cutting about a snack-sized bag of chips worth of calories from their daily diets.

What happened at the end of the trial, though? In the Minnesota Starvation Study and calorie deprivation experiments done on Army Rangers, as soon as subjects were released from restriction, they tended to rapidly regain the weight, and sometimes even more. The leaner they started out, the more their bodies seemed to drive them to overeat to pack back on the extra body fat. In contrast, after the completion of the CALERIE study, even though their metabolism was slowed, they retained about 50 percent of the weight loss two years later. They must have acquired new eating attitudes and behaviors that allowed them to keep their weight down. After extended calorie restriction, for example, cravings for sugary, fatty, and junky foods may actually go down.

In our next story, we take a close look at how to preserve bone and muscle mass on a low calorie diet.

One of the most consistent benefits of calorie restriction is blood pressure improvements, in as short as one or two weeks. Blood pressures can sometimes even be normalized in a matter of weeks and blood pressure pills discontinued. Unfortunately, this can work a little too well, and cause something called orthostatic intolerance, which can manifest as lightheadedness or dizziness upon standing which in severe cases can cause fainting. Staying hydrated can help.

What about loss of muscle mass? In the CALERIE trial, 70 percent of the lost body weight was fat, and 30 percent lean body mass. So, they ended up with an improved body composition: 72 percent lean mass, compared to 66 percent in the control group. Though leg muscle mass and strength declined in absolute terms, relative to their new body size, they generally got stronger. Is there any way to preserve even more lean mass, though, particularly among older individuals who naturally tend to lose muscle mass with age?

Increased protein intakes are commonly suggested, but most studies fail to find a beneficial effect on preserving muscle strength or function whether you’re young or old, active or sedentary. For example, researchers randomized overweight older men and women to a normal-protein diet, 4 grams for every 10 pounds of body weight. Or a high-protein diet, about 8 grams per 10 pounds during a 25-percent calorie restriction. That doubling of protein intake had no discernible effect on lean body mass, muscle strength, or physical performance. Most such studies found the same lack of benefit. But, put them all together, and one can tease out a small advantage, about a pound or two difference over an average of six months. Unfortunately, high-protein intake during weight loss has been found to have profoundly negative metabolic effects; for example, undermining the benefits of weight loss on insulin sensitivity. Lose 20 pounds, and you can dramatically improve your body’s ability to handle blood sugars, compared to a control group that maintained their weight. Lose the exact same amount of weight, but on a high-protein diet, getting about an extra 30 grams a day, and it’s like you never lost any weight at all.

Though you can always bulk back up after weight loss, the best way to preserve muscle mass during weight loss is exercise. The CALERIE study had no structured exercise component and, like after bariatric surgery, about 30 percent of the weight loss was lean mass. In contrast, that proportion was only about 16 percent in The Biggest Loser contestants, chalked up to their vigorous exercise program. Resistance training even just three times a week can prevent more than 90 percent of lean body mass loss during calorie restriction.

The same may be true of bone loss. Lose weight through calorie restriction alone, and you experience a decline in bone mineral density in fracture risk sites, such as the hip and spine. In the same study, though, those randomized to lose weight with exercise didn’t suffer any bone loss. The researchers conclude: “Our results suggest that regular exercise should be included as part of a comprehensive weight loss program to offset the adverse effects of caloric restriction on the bone.“ You can never argue with calls for increased physical activity, but even without an exercise regimen, the “very small” drop in bone mineral density in the CALERIE trial might only increase a 10-year risk of osteoporotic fracture by only about 0.2 percent. The benefits of calorie restriction revealed by the study improved blood pressures, improved cholesterol, improved mood, improved libido, improved sleep would seem to far outweigh any potential risks. The fact that a reduction in calories seemed to have such wide-ranging benefits on quality of life led commentators in the AMA’s Internal Medicine journal to write: “The findings of this well-designed study suggest that intake of excess calories is not only a burden to our physical equilibrium, but also to our psychological well-being” as well.

Finally today, we look at the benefits of fasting for weight loss throughout medical history.

I talked about the benefits of calorie restriction. Well, the greatest caloric restriction is no calories at all. Fasting has been branded the “next big weight loss fad,” but has a long history throughout various spiritual traditions, practiced by Moses, Jesus, Muhammed, and Buddha. In 1732, a noted physician wrote, “He that eats till he is sick must fast till he is well.” Today, about one in seven American adults report taking that advice using some sort of fasting as a means to control body weight.

Case reports of the treatment of obesity through fasting date back more than a century in the medical literature. In 1915, two Harvard docs indelicately described “two extraordinarily fat women,” one of whom was a “veritable pork barrel.” Their success led them to conclude that “moderate periods of starvation constitute a perfectly safe, harmless, and effective method for reducing the weight of those suffering from obesity.”

The longest recorded fast, published in 1973, made it into the Guinness Book of World Records. To reach his ideal body weight, a 27-year-old man fasted for 382 days straight, losing 276 pounds and managed to keep nearly all of it off. He was given vitamin and mineral supplements, so he wouldn’t die, but no calories for more than a year. In their acknowledgements, the researchers thanked him for his “cheerful cooperation and steadfast application to the task of achieving a normal physique.”

A U.S. Air Force study of more than 20 individuals at least 100 pounds overweight, most unable to lose weight on previous diets, were fasted for as long as 84 days. Nine dropped out of the study, but the 16 who remained were unequivocally successful at losing between 40 and 100 pounds. In the first few days, subjects were noted losing as much as four pounds a day. This was mostly water weight as the body starts to adapt, but after a few weeks, they were steadily losing about a pound of mostly straight fat a day. The investigator described their starvation program as a “dramatic and exciting treatment for obesity.”

Of course, the single most successful diet for weight loss, namely no diet at all is also the single least sustainable. What other diet can cure morbid obesity in a matter of months, but practically be guaranteed to kill you within a year if you stick with it? The reason diets don’t work, almost by definition, is that people go on them, and then they go off of them. Permanent weight loss is only achieved through permanent lifestyle change. So, what’s the point of fasting if you’re just going to go back to your regular diet and gain it all right back?

Fasting proponents cite the psychological benefit of realigning people’s perceptions and motivation. Some individuals have resigned themselves to the belief that weight loss for them is somehow impossible. They may think they’re just “made differently” in some way, and no matter what they do, the pounds don’t come off. But the rapid unequivocal weight loss during fasting demonstrates to them that with a large enough change in eating habits, it’s not just possible, but inevitable. This morale boost may then embolden them to make better food choices once they resume eating.

The break from food may allow some an opportunity to pause and reflect on the role food is playing in their lives—not only the power it has over them, but the power they have over it. In a fasting study entitled “Correction and Control of Intractable Obesity,” a person’s personality was described as changing “from one of desperation, with abandonment of hope, to that of an eager extrovert full of plans for a promising future.” She realized that her weight was within her own power to control. They concluded: “This highly intellectual social worker has been returned to a full degree of exceptional usefulness.”

After a fast, newfound commitments to more healthful eating may be facilitated by a reduction in overall appetite reported post-fast, compared to pre-fast, at least temporarily. Even during a fast, hunger may start to dissipate within 36 hours. So, challenging people’s delusions about their exceptionality to the laws of physics with a period of total fasting may seem barbaric, but in reality, this method of weight reduction is remarkably well tolerated by obese patients. That seems to be a recurring theme in these published series of cases. In this influential paper, “Treatment of Obesity by Total Fasting for Up to 249 Days,” the researchers remarked that the most surprising aspect of the study was the ease with which the prolonged fast was tolerated.” All their patients evidently “spontaneously commented on their increased sense of well-being” throughout the process even, “in some cases frank euphoria.” Though it’s essential that “fasting should only be prescribed under close medical supervision,” they concluded that they were “convinced that it is the treatment of choice, certainly in cases of gross obesity.”

Fasting for a day can make people irritable, moody, and distracted, but a few days in, many report feeling clear, elated, and alert even euphoric. This may be in part due to the significant rise in endorphins that accompanies fasting. Mood enhancement during fasting is thought to perhaps represent an adaptive survival mechanism to motivate the search for food. This positive outlook towards the future may then facilitate the behavioral change necessary to lock in some of the weight loss benefits.

But is that what happens? Is it actually effective over the long term?

We would love it if you could share with us your stories about reinventing your health through evidence-based nutrition. Go to NutritionFacts.org/testimonials. We may share it on our social media to help inspire others.

To see any graphs charts, graphics, images or studies mentioned here, please go to the Nutrition Facts Podcast landing page. There you’ll find all the detailed information you need plus links to all of the sources we cite for each of these topics.

For recipes, check out my How Not to DietCookbook. It’s beautifully designed, with more than 100 recipes for delicious and nutritious meals. And all proceeds I receive from the sales of my books goes to charity.

NutritionFacts.org is a nonprofit, science-based public service, where you can sign up for free daily updates on the latest in nutrition research via bite-sized videos and articles.

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Thanks for listening to Nutrition Facts. I’m your host, Dr. Michael Greger.

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