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 1

Hundreds of calories can be cut from one’s daily diet.  What is the long-term effect of doing this?  Today we launch a four-part series on intermittent fasting.

This episode features audio from The 3,500 Calorie per Pound Rule Is Wrong, The Reason Weight Loss Plateaus When You Diet, and The New Calories per Pound of Weight Loss Rule?. 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 start a four part series on intermittent fasting. Fasting is the practice of abstaining from food for a period of time. Anyone who’s seen “the Biggest Loser” tv-shows knows that hundreds of pounds can be lost with enough caloric restriction and exercise. But what is the long-term effect of doing this?

Our first story is about calories. How many fewer calories do you have to eat every day to lose one pound of body fat?

The first surgical attempt at body sculpting was in 1921, with a dancer wanting to improve the shape of her ankles. The surgeon apparently scraped away too much tissue and tied the stitches too tight, resulting in necrosis, amputation, and the first recorded malpractice suit in the history of plastic surgery. Today’s liposuction is much safer only killing about 1 in 5,000 patients, mostly from unknown causes; throwing a clot off into your lung, or perforations of your internal organs.

Liposuction currently reigns as the most popular cosmetic surgery in the world, and its effects are, indeed, only cosmetic. A study published in the New England Journal of Medicine assessed obese women before and after having about 20 pounds of fat sucked out of their bodies, resulting in nearly a 20 percent drop in their total body fat. Normally, lose even just 5 to10 percent of your body weight in fat, and you get significant improvements in blood pressure, blood sugars, inflammation, cholesterol, and triglycerides but, liposuction sucks. None of those benefits materialized even after the massive liposuction. This suggests subcutaneous fat—the fat under our skin—is not the problem. The metabolic insults of obesity arise from the visceral fat: the fat surrounding or even infiltrating our internal organs, like the fat marbling our muscles and liver. The way you lose that fat, the dangerous fat, is to take in fewer calories than you burn.

Anyone who’s seen The Biggest Loser shows knows with enough calorie restriction and exercise hundreds of pounds can be lost. Similarly, there are cases in the medical literature of what some refer to as “super obesity,” in which individuals lost up to 374 pounds largely on their own, without professional help, and kept it off for years. The guy lost about 20 pounds a month cycling two hours a day and reducing intake to 800 calories a day, which is down around what some prisoners were getting at concentration camps in World War II.

Perhaps America’s most celebrated TV weight loss was when Oprah pulled out a wagon full of fat, representing the 67 pounds she lost on a very low-calorie diet. How many calories did she have to cut to achieve that within four months? Consult leading nutrition textbooks, or refer to trusted authorities like the Mayo Clinic, and you’ll learn the simple weight loss rule: one pound of fat is equal to 3,500 calories. Quoting from the Journal of the American Medical Association, “A total of 3500 calories equals 1 pound of body weight. This means if you decrease your intake by 500 calories daily, you will lose 1 pound per week. (500 calories per day × 7 days = 3500 calories.)” So, one pound of body fat. The simple weight loss rule that’s simply not true.

The 3,500-calorie rule can be traced back to a paper in 1958 that just noted that since fatty tissue on the human body is 87 percent fat, a pound of body fat would have about 395 grams of pure fat. Multiply that by nine calories per gram of fat gives you that “3,500 calories per pound” approximation. The fatal flaw that leads to “dramatically exaggerated” weight-loss predictions is that the 3,500 rule fails to take into account the fact that changes in the calories-in side of the energy-balance equation automatically lead to changes in calories out. For example, the slowing of metabolic rate that accompanies weight loss, known as metabolic adaptation. That’s one of the reasons weight loss plateaus.

For example, imagine a 30-year-old sedentary woman of average height who weighs 150 pounds. According to the 3,500-calorie rule, if she cuts 500 calories out of her daily diet, she’d lose a pound a week, or 52 pounds a year. In three years, then, she would vanish. She’d go from 150 pounds to negative 6. Obviously, that doesn’t happen. What would happen is that in the first year, instead of losing 52 pounds she’d likely only lose 32 pounds, and then, after a total of three years, stabilize at about 100 pounds. This is because it takes fewer calories to exist as a thin person.

Part of it is simple mechanics. In the same way a Hummer requires more fuel than a compact car. Think how much more effort it would take to just get out of a chair, walk across the room, or climb a few stairs carrying a 50-pound backpack. That’s no lighter than carrying 50 pounds in the front. Even when you’re lying at rest sound asleep, there’s simply less of your body to maintain as we lose weight. Every pound of fat tissue lost may mean one less mile of blood vessels your body has to pump blood through every minute. So, the basic upkeep and movement of thinner bodies takes fewer calories. So, as you lose weight by eating less, you end up needing less. That’s what the 3,500-calorie rule doesn’t take into account.

Or imagine it the other way. A two-hundred-pound-man starts eating 500 more calories a day. That’s like a large soda or two doughnuts. According to the 3,500-calorie rule, in ten-years he’d weigh more than 700 pounds! That doesn’t happen, because the heavier he is, the more calories he burns just existing. If you’re a hundred pounds overweight, that’s like the skinny person inside you trying to walk around balancing 13 gallons of oil at all times, or lugging around a sack containing four hundred sticks of butter wherever you go. It takes about two doughnuts worth of extra energy just to live at 250 pounds, and so that’s where he’d plateau out if he kept it up. So, weight gain or weight loss, given a certain calorie excess or deficit, is a curve that flattens out over time, rather than a straight line up or down.

Nevertheless, the 3,500-calorie rule continues to crop up, even in obesity journals. Public health researchers used it to calculate how many pounds children might lose every year if, for example, fast food kids’ meals swapped in apple slices instead of French fries. They figured two meals a week could add up to about four pounds a year. The actual difference, National Restaurant Association-funded researchers were no doubt delighted to point out, would probably add less than half a pound—ten times less than the 3,500-calorie rule would predict. The original article was subsequently retracted.

In our next story, we try and understand the metabolic and behavioral adaptations that slow weight loss.

We have millions of years of evolution hard-wiring us to survive scarcity—compensatory survival mechanisms our body uses to defend against weight loss. So, when we start losing weight, we may unconsciously start moving less as a “behavioral adaptation” to conserve energy. There are metabolic adaptations as well. Our metabolism slows down. Every pound of weight loss may reduce our resting metabolic rate by seven calories a day. This may only translate to a few percent differences for most, but can rapidly snowball for those who achieve massive weight loss.

During one season, some of The Biggest Loser contestants famously had their metabolic rates tracked. Above and beyond the hundreds of calories less it takes to just exist 100+ pounds lighter, by the end of the season their metabolic rates slowed by an extra 500 calories a day. The mindblower was that six years later, they were retested and still had the 500-calorie-a-day handicap. So, the contestants had to cut 500 calories more than anyone else their size to maintain the same weight loss. No wonder the bulk of their weight was regained. Most remained at least 10 percent lower than their starting weight, though, and even a 7 percent drop has been shown to cut diabetes rates about in half. Still, the metabolic slowing means you have to work that much harder than everyone else just to stay in place.

Analyzing four seasons of The Biggest Loser, minute-by-minute, researchers noted that 85 percent of the focus was on exercise rather than diet, though the exercise component accounted for less than half of the weight loss. Even six years after their season ended, the contestants had been maintaining an hour of daily, vigorous exercise, yet still regained most of the weight. Why? Because they started eating more. They could have cut their exercise to just 20 minutes a day and still maintained 100 percent of their initial weight loss if they would have just been able to keep their intake to under 3,000 calories a day. That may not sound like much of a challenge, but weight loss doesn’t just slow your metabolism; it boosts your appetite.

If it were just a matter of your weight settling at the point at which your reduced calorie intake matches your reduced calorie output, it would take years for your weight loss to plateau. Instead, it often happens within six to eight months. You may know the drill: start the diet, stick to the diet, and then weight loss stalls six months later. What happened? Don’t blame your metabolism. That just plays a small part. What happened is that you likely actually stopped sticking to your diet, because your appetite went on a rampage.

Let’s break it down. If you cut 800 calories out of your daily diet, 2,600 calories a day down to 1,800 and your weight loss stalls after six months, then what happened is that at the end of the first month, you think you’re still cutting 800 calories, but you may actually only be down about 600 calories a day. By month two, you’re only down about 500; month three, 300; and by month six, you’re only eating 200 calories less than before you went on the diet.

In other words, you inadvertently suffered an exponential increase in calorie intake over those six months. Yet you may not even realize it, because by that time your body may have ramped your appetite up 600 calories. So, it still feels as though you are eating 800 calories less, but it’s actually only 200. Since an 800 calorie drop in intake may slow your metabolism and physical activity about 200 calories a day, with no difference between calories in and calories out at six months, no wonder your weight loss grinds to a complete halt.

The slow upward drift in calorie intake on a new diet is not because you got lazy. Once your appetite is boosted by 600 calories after dieting for a while, eating 200 calories less at the end is as hard as eating 800 calories less at the beginning. So, you can maintain the same disciplined level of willpower and self-control, and still end up stagnating. To prevent this from happening, you need to maintain the calorie deficit. How is that possible in the face of a ravenous appetite?

Hunger is a biological drive. Asking someone to eat smaller portions is like asking someone to take fewer breaths. You can white-knuckle it for a bit, but eventually nature wins out. That’s why I wrote How Not to Diet. There are foods that can counter the slowing of our metabolism and suppress our appetite; ways of eating to counter the behavioral adaptation and even eat more food, yet still lose weight.

Due to the metabolic slowing and increased appetite that accompanies weight loss, sustained weight loss requires a persistent calorie deficit of 300 to 500 calories a day. This can be accomplished without reducing portion sizes, simply by lowering the calorie density of meals. This can result in the rare combination of weight loss with both an increase in quality and even quantity of food consumed. The bottom line is that sustainable weight loss is not about eating less food; it’s about eating better food.

In our final story today, we look at how losing a pound of fat can take as few as 10 calories a day or as many as 55, depending on whether you’re improving food quality or just restricting food quantity.

If the “3500 calories per pound” weight loss rule is bunk, what’s the alternative? How many fewer calories do you have to eat, or how many more do you have to burn, to lose a pound of fat?

There are validated mathematical models that take into account the dynamic changes that occur when you cut calories, such as the metabolic slowdown, and they’ve been turned into free online calculators that you can use to make personalized estimates. There’s the Body Weight Planner from the National Institutes of Health, and the Pennington Biomedical Research Center’s Weight Loss Predictor out of Louisiana State University.

The NIH Body Weight Planner has been found to be more accurate, since the LSU model appears to overestimate the drop in physical activity, but they both have their own pluses and minuses. The Body Weight Planner tells you how many calories you need to restrict and/or how much more exercise you need to do to achieve a specific weight loss goal by a specific date. If you click on the “Switch to Expert Mode” button you can get a graph and exportable chart showing your day-to-day weight loss trajectory. So, if you’re a middle-aged, sedentary average height woman obese at 175 pounds who wants to be closer to her ideal weight within a year, 2,000 calories a day would prevent future weight gain. About 1,400 calories a day would bring your weight and then you could maintain that lower weight at 1,700 calories a day. If, in addition, you walked a mile a day, it would give you a little more calorie leeway.

The LSU Weight Loss Predictor, on the other hand, doesn’t allow you to tweak physical activity, but the advantage is that you don’t have to choose a goal or time frame. Just put in different calorie changes, and it graphs out your expected course.

Is there any easy rule of thumb you can use? Yes. Every permanent 10-calorie drop in daily intake will eventually lead to about one pound of weight loss, though it takes about a year to achieve half the total weight change, and about three years to completely settle into the new weight. So, cutting 500 calories a day can cause the 50-pound weight loss predicted by the 3,500 calorie rule, but that’s the total weight loss at which you plateau, not an annual drop, and it takes about three years to get there. A 500-calorie deficit would be expected to cause about a 25-pound weight loss the first year, and then an additional 25 over years two and three. That’s only if you can maintain the 500-calorie deficit, though.

If you’re eating the same diet that led to the original weight problem, but just in smaller servings, you should expect your appetite to rev up about 45 calories per pound lost. So, if you were cutting 500 calories a day through portion control alone, before you were down even a dozen pounds, you’d feel so famished that you’d be driven to eat 500 calories more a day, and your weight loss could vanish. That’s why if you’re dead set on eating the same diet, the same foods, but just in smaller quantities, you have to cut down an additional 45 calories per pound of desired weight loss to offset your hunger drive.

So, to get that one pound off, instead of just 10 calories less per day using the 10 calories per pound rule, you’d have to eat 10 calories less on top of the 45 less to account for the revving of your appetite, so, 45 +10 = 55. So, just changing diet quantity and not quality, it takes 55 calories less per day to lose a pound. So, that 500-calorie daily deficit would only net you about a 9-pound weight loss over time, instead of 50. That’s why portion control methods can be such a frustrating failure for so many people.

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 Die Cookbook. 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.

Everything on the website is free. There’s no ads, no corporate sponsorship. It’s strictly non-commercial. I’m not selling anything. I just put it up as a public service, as a labor of love, as a tribute to my grandmother whose own life was saved with evidence-based nutrition.

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

6 responses to “Intermittent Fasting Part 1

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  1. Great podcast! Unfortunately, the transcript doesn’t cover the whole episode, it only goes as far as minute 15:00. Other than that, can’t wait to hear the rest of the series!

  2. I’ve downloaded the fasting infographic, which gives a nice overview of the types of fasting. However, this does mention intermittent fasting on the bottom whereas this is not explained on the upper half. Which fasting is this? I see different definitions being used…

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