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Spring 2022 - Safety

Weight Loss with Intermittent Fasting?

A growing trend for shedding unwanted pounds, this weight-loss technique may help many achieve their goals.

MANY HEALTHCARE professionals have no doubt heard of intermittent fasting (IF) and its current popularity for weight loss, among other health benefits. Although fasting has been used for religious and health purposes throughout history, it has only recently become a “fad” of sorts, suddenly trendy and much-discussed. Fasting is attracting everyone from elite athletes to supermodels. These days, however, it’s also of interest to physicians and other healthcare providers who see beyond the temptation to discount it as a dangerous and passing weight-loss craze. While there has been much controversy over this practice, many healthcare providers are beginning to have a new, more positive perspective on this ancient health practice and its past proponents. Hippocrates, for example, said, “To eat when you are sick is to feed your illness.” Plutarch agreed, saying, “Instead of using medicine, better fast today.” Beyond these two highly regarded historical figures, Plato and Aristotle were also enthusiastic devotees of fasting.1 Ancient Greeks believed in fasting largely because medical treatment could be observed through nature. For instance, most animals do not eat while ill, and humans typically do the same. These observations have led some to consider fasting the “physician within.” For example, when ill with the flu, most people do not feel like eating. In fact, fasting is a universal mammalian instinct in response to many types of illnesses.1

Yet, apart from fasting for illness, one question remains: Does IF truly work for safe weight loss? Or, are athletes, models, ancients and millions of others wrong?

What Is Intermittent Fasting?

Before delving into the research and anecdotal evidence, it’s helpful to understand what IF is.

First, for most who practice it, IF is a lifestyle and not a diet. Many who practice IF want to make this distinction clear since more than a few see this lifestyle as a means to long-term good health, rather than merely weight loss. The benefits of IF, which include autophagy (cleaning out damaged cells), managing or reversing type 2 diabetes, reversing fatty liver, lowering blood pressure, and improving overall quality and length of life, are all reasons many practice it.

Simply put, IF is a regimen that focuses on periods of fasting, with either abstinence or significant calorie reduction, as well as periods of eating. Increasingly, researchers and healthcare providers view IF as alternating body composition through loss of fat mass and weight, without lowering metabolism that prevents fat loss. This is accomplished by cycling between reduced calories and normal eating during “feeding windows.”2

Individuals adopt IF according to various fasting plans (see 7 Types of Intermittent Fasting), but following are three of the main fasting schedules.

Alternate-day fasting: As the name implies, individuals fast every other day and, on alternate days, there is no food restriction. An example might be fasting Monday, Wednesday and Friday, with no food restrictions on Tuesday, Thursday, Saturday and Sunday.

Whole-day fasting: This plan involves complete fasting or eating only approximately 25 percent of daily caloric needs one or two days per week, with no food restrictions on alternate days. An example is the 5:2 diet approach, which involves windows of feeding times five days of the week, cycled with a zero or 400- to 500-calorie diet the other two days of the week.

Time-restricted feeding: With this plan, individuals follow a meal plan each day with a designated time frame for fasting. An example would be meals eaten between 8 a.m. and 3 p.m., with fasting during the remaining hours of the day.2 Some fast for longer periods, but the most beneficial fasting time is generally three days.3

“[IF] makes intuitive sense,” states Harvard University physician and writer Monique Tello, MD, MPH. “The food we eat is broken down by enzymes in our gut and eventually ends up as molecules in our bloodstream. Carbohydrates, particularly sugars and refined grains (think white flours and rice), are quickly broken down into sugar, which our cells use for energy. If our cells don’t use it all, we store it in our fat cells as, well, fat. But sugar can only enter our cells with insulin, a hormone made in the pancreas. Insulin brings sugar into the fat cells and keeps it there.

“Between meals, as long as we don’t snack, our insulin levels will go down and our fat cells can then release their stored sugar to be used as energy. We lose weight if we let our insulin levels go down. The entire idea of IF is to allow the insulin levels to go down far enough and for long enough that we burn off our fat.”3

All of these regimens may sound drastic, and they could be for those who have not slowly prepared their bodies for such a lifestyle (e.g., fat adaptation). But, countless people who have prepared and tried this way of life have lost hundreds of pounds and benefited from a host of other health benefits.2

Why the Calories-In, Calories-Out Model Does Not Work

Physician, author and researcher Jason Fung, MD, a nephrologist at the University of Toronto, is integrally responsible for the current interest in IF. Years ago in his practice, he learned the importance and efficacy of utilizing fasting and a low-carb lifestyle while treating kidney patients with type 2 diabetes. In his books The Obesity Code and The Complete Guide to Fasting, among others, Dr. Fung discusses the fact that obesity is not related to consuming too many calories or by not exercising enough. “Fundamentally, obesity is a hormonal problem,” Dr. Fung writes in The Complete Guide to Fasting. “The underlying cause of obesity turns out to be hormonal, rather than a caloric imbalance. Insulin is fat-storage hormones. When we eat, insulin increases, signaling our body to store some of this food energy as fat for later use. It’s a natural and essential process that has helped humans survive famine for thousands of years, but excessively and persistently high insulin levels result inexorably in obesity. Understanding this leads naturally to a solution: If excessive insulin is causing obesity, then clearly the answer lies in reducing insulin.” Dr. Fung emphasizes that one way to reduce insulin is through IF. A simple illustration is to consider how insulin levels are elevated throughout the day when a person eats three meals and snacks throughout at least a 14-hour day. Insulin remains high. If a person has a time-restricted window for eating, however, insulin levels are not as elevated, causing the body to more efficiently use the body fat in storage.2

Anecdotal evidence, a glance at IF social media groups (including before-and-after photos of remarkable weight loss and improved health), YouTube videos, podcasts with IF experts, websites (e.g.,; and books reveal IF is much more than a passing fad. The scientific evidence and good results make IF worth a healthcare professional’s time, let alone a patient’s. The New England Journal of Medicine’s 2019 study on IF’s health benefits, including weight loss, also reveals an intriguing and effective “new” way that healthcare providers can help patients and themselves.4

In addition to his books and research, Dr. Fung cofounded the Intensive Dietary Management program and the health coaching program called The Fasting Method. BioSupply Trends Quarterly had a recent opportunity to interview Dr. Fung regarding his apparently controversial yet effective treatment methods.

10 Benefits of Intermittent Fasting

  • Protects against neurodegenerative disease
  • Decreases insulin levels and increases human growth hormone
  • Reduces insulin resistance and lowers blood sugar levels
  • Reduces risk of heart disease
  • Reduces blood pressure and cholesterol levels
  • Boosts metabolism for fat loss
  • Extends lifespan, helping people live longer
  • Reduces oxidative damage and inflammation in the body
  • Removes waste material from cells
  • Reduces leptin levels, increasing testosterone

An Interview with Dr. Fung

BSTQ: You’re a nephrologist and not a nutritionist. So, how did you learn about IF?

Dr. Fung: I work with kidney patients who often had type 2 diabetes. A main key to remedying that is to lose weight. So, I got really interested in weight loss and various weight-loss methods. Yet, a lot of what I initially read on weight loss was not very helpful or correct in the long run. The calories-in calories-out approach did not look like a good strategy to me. As I started to research fasting — because, honestly, you aren’t taught much about fasting in medical school — I saw that there’s quite a lot of good science surrounding it. So, after looking at the results, I began recommending fasting to patients. Fasting was a much better strategy. At least it was another weight-loss approach patients could use, and if it worked for them, great. If it didn’t, they could try another method. But, they got some really fantastic results, including putting type 2 diabetes into remission. That’s how it all began, and that’s why I’ve wanted to talk and write about IF — to highlight some of the misconceptions surrounding it. So many people think it’s unhelpful or dangerous such as that it will cause muscle loss, and there are other misunderstandings regarding what it is and how it works. I’ve tried to demystify it a bit. IF does not equate to starvation.

BSTQ: Beyond type 2 diabetes and obesity, what are the other common conditions that can be reversed if not cured with IF?

Dr. Fung: Fatty liver and polycystic ovarian syndrome, to name just two. Fatty liver is one of the leading causes of liver failure today, and it came out of nowhere. In the 1980s, for example, the disease barely existed. It went from being a rarity to becoming one of the most important liver diseases today, even while hepatitis B and C have been decreasing for decades. Fatty liver is reversible with fasting. Its origin has much to do with eating processed foods and frequent meals throughout the day. Those practices are two of the reasons there is metabolic disease today.

BSTQ: Can you tell us about the other benefits of fasting that are so exciting?

Dr. Fung: I think one is from a weight-loss perspective, naturally. Weight loss was the goal for many of my patients. One of the things people have always talked about is calories in calories out, but then I looked at what that underlying process was and started asking questions. What is it that causes people to have problems such as hunger and a lower metabolic rate? What is causing people to eat more than they should in the first place? Why aren’t more and more people losing weight and getting healthier using the food pyramid?

It’s the same thing with metabolic rate. One of the things that was fairly obvious initially was that people’s metabolic rate would go down as they started to lose weight on the calories-in calories-out approach and as they burned fewer calories. It would make their own weight loss less effective. That’s a huge problem. But when people start fasting, which results in hormonal changes, their metabolic rate typically goes up. For obvious reasons, that’s very beneficial.

BSTQ: If fasting is so healthy, why are so many healthcare professionals unaware of or opposed to it?

Dr. Fung: I think it’s because they never learned about it. The strange part is fasting has been around for thousands of years, but no one at all has been talking about it, thinking about it or using it. It’s actually the most obvious thing: If you don’t eat, you’ll probably lose weight. Fasting was the oldest thing, the most obvious thing, and nobody was using it. Also, when you introduce something new, there’s this suspicion that it’s faddish, or there’s some sort of trick, or if you didn’t learn it in medical or dietary school, there must be some hidden danger. But fasting is not new. Fasting was something that was forgotten over time and was lost in the notion that eating six times a day was the way to lose weight. There’s actually no science that shows eating six times a day is good for you.

BSTQ: Does funding have anything to do with the lack of general fasting knowledge?

Dr. Fung: Drug companies tend to have a lot of money and great marketing, and their general advice is to eat less and move more. But these general platitudes don’t really help much. Then, there are weight-loss drugs. A lot of the excitement within the medical community is driven by the hype of drug companies. But there’s no one saying, “We’ve got to push fasting.” Those who believe in IF are not going to invite these doctors to a free dinner, set up conferences or create educational materials. So it’s comparable to a big Hollywood movie that has a huge marketing budget versus the independent movie that has no marketing budget. It’s not that doctors are looking to make money; it’s just what is marketed to them on a much larger scale. It’s a public relations problem.

BSTQ: Even with good information, why do you think so many people are still skeptics?

Dr. Fung: It’s great to see IF featured in more mainstream publications such as The New England Journal of Medicine, but if there’s only a handful of scholarly articles each year about IF, versus hundreds of articles about medications for weight loss, it’s still an uphill battle. At least, however, the information is now out there.

BSTQ: Do you have any other thoughts for healthcare providers regarding efficacy and general information?

Dr. Fung: Two things: The first is that fasting is not some weird thing. It’s actually part of a balance. You have to balance feeding and fasting. Feeding means your body is going to store calories, and fasting means your body is going to use calories. That’s the way it works. If the body didn’t have a way to store calories, people would die in their sleep every single night. But obviously, we do store calories, and if we store them, we have to give our bodies time to use them. If we don’t, we gain weight. The thing is it’s not as if no one ever fasts. Everybody is supposed to fast every single day. That’s why we have the word “breakfast.” If you are balancing feeding and fasting, then you’re going to succeed, and if you want to lose weight, you’re going to need to increase the fast. So fasting is not unnatural. It’s just part of a natural process and balance. Second, a lot of the science does support fasting. It’s not like there’s no science behind it.

7 Types of Intermittent Fasting

  1. 5:2 Fasting: One of the most popular IF methods made mainstream by the book The FastDiet, this approach allows individuals to eat normally for five days (without counting calories) and then eat 500 or 600 calories a day for women and men, respectively, on the other two days. The idea is that short bouts of fasting keep people compliant; if they are hungry on a fast day, they just have to look forward to tomorrow when they can “feast” again.
  2. Time-Restricted Fasting: Using this approach, individuals choose an eating window every day, which should ideally leave a 14- to 16-hour fast. (Due to hormonal concerns, it is recommended women fast for no more than 14 hours daily.) For instance, the eating window might be from 9 a.m. to 5 p.m. Much of the time spent fasting is time spent sleeping.
  3. Overnight Fasting: This approach is the simplest and involves fasting for a 12-hour period every day. For example, individuals can choose to stop eating after dinner by 7 p.m. and resume eating at 7 a.m. with breakfast the next morning. The benefit of this method is that it’s easy to implement.
  4. Eat Stop Eat: With this approach, individuals complete one or two 24-hour fasts per week and commit to a resistance-training program. This allows people to eat a slightly higher amount of calories on the other five or six nonfasting days.
  5. Whole-Day Fasting: This approach allows individuals to eat once a day, so the fasting period is 24 hours (dinner to dinner or lunch to lunch). The advantage is that, if done for weight loss, it’s really tough (though not impossible) to eat an entire day’s worth of calories in one sitting. The disadvantage is that it’s hard to obtain all the nutrients the body needs to function optimally with just one meal.
  6. Alternate-Day Fasting: Using this approach, people might fast every other day, with a fast consisting of 25 percent of their calorie needs (about 500 calories) and nonfasting days being normal eating days.
  7. Choose-Your-Day Fasting: With this approach, individuals might do the time-restricted fasting (fast for 16 hours, eat for eight, for instance) every other day or once or twice a week. For example, if Sunday is a normal day of eating, a person would stop eating by 8 p.m. and then resume eating again on Monday at noon. Essentially, it’s like skipping breakfast a few days a week.

    Source: Migala J. The 7 Types of Intermittent Fasting, and What to Know About Them. Everyday Health, April 20, 2020. Accessed at

To Sum It Up

Dr. Fung and other scholars are great proponents of fasting. For example, Philip Paracelsus, the founder of toxicology and one of three fathers of modern Western medicine (along with Hippocrates and Galen) wrote, “Fasting is the greatest remedy — the physician within.” With more research surrounding IF, its excellent results and the increasing scientific support of the practice, IF could very well be something to begin incorporating into healthcare professionals’ general toolkits. At the very least, they can offer the information to patients to let them determine whether it’s a lifestyle they would like to try. After all, there is little to lose beyond weight.


1. Fung J. Fasting — A History, Part I. The Fasting Method. Accessed at

2. Fung J. The Complete Guide to Fasting. Victory Belt Publishing, 2016.

3. Harvard T. Chan School of Public Health. Diet Review: Intermittent Fasting for Weight Loss. Accessed at

4. de Cabo R and Mattson MP. Effects of Intermittent Fasting on Health, Aging and Disease. The New England Journal of Medicine, 2019; 381:2541-51. Accessed at

Meredith Whitmore
Meredith Whitmore is a freelance writer and clinical mental health professional based in the Pacific Northwest.