Heal Your Body Through Fasting:
An Interview with Dr. Jason Fung


Fasting is not about starving yourself. So says Dr. Jason Fung, who just wrote a new book, The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting, about the therapeutic benefits of short-term fasting.

Fasting allows the body to shed weight, says Dr. Fung, because it can help prevent the development of insulin resistance. In fact, Dr. Fung has put more than 1,000 of his own patients on a fasting protocol to deal with health issues, including Type 2 diabetes and obesity.

“In most cases, fasting has allowed us to reverse these patients with Type 2 diabetes,” Fung tells us. “We have taken hundreds of people off diabetes medications, insulin, and high blood pressure medications.

“More than that, we allow patients to take control of their own health. Rather than simply going to the doctor to get yet another medication, we take them off and show people how to manage their metabolic problems with diet and lifestyle.”

In this interview with Be Well, Dr. Fung breaks down the health benefits of short-term fasting and offers some tips to get started. Most importantly, he reminds us not to fear fasting: “Our bodies are equipped to handle it. The lions do it. The tigers do it. The bears do it. And the humans should do it, too.”

Why is when you eat as important as what you eat?

Most conventional diets only consider the total caloric value of their foods. However, weight gain is not the result of excess calories, but hormonal effects of the food that instruct our bodies to gain weight. The primary hormone involved is insulin.

One of the mechanisms by which insulin stays elevated is the phenomenon of insulin resistance. Persistent high levels of insulin causes insulin resistance, which is important because this leads in turn to higher insulin levels, which then drive obesity.

Periods of very low insulin, as can be achieved with fasting, will prevent the development of insulin resistance and help with weight loss efforts in the long term.

Studies directly comparing daily caloric restriction with intermittent fasting show similar weight loss, but much improved insulin levels and insulin sensitivity with intermittent fasting despite equal weekly caloric intakes.

You discuss various fasting protocols in your book, including intermittent fasting, alternate-day fasting, and extended fasting. Can you describe the difference between them?

The main difference is the length of the fast. Intermittent fasting can be of any duration. There are fasts for 16 hours, 20 hours, 24 hours or 36 hours.  36-hour fasts are often called alternate-day fasting since it generally means eating every other day. Shorter fasts are generally done more frequently.

Once past 36 hours, these fasts are classified as extended fasts. They are more powerful, but are sometimes more difficult and generally done less frequently. If you have health issues, or are taking medications, you should consult your physician.

Most of us grew up on the advice to either eat three square meals a day, plus a couple of snacks, or constantly graze throughout the day. What is going on with our metabolism when we are constantly eating versus intermittently fasting?

The NHANES survey in the United States showed that in 1977, the average American ate 3 times per day — breakfast, lunch and dinner. By 2005, Americans were eating closer to 6 times per day — breakfast, snack, lunch, snack, dinner, snack. In essence, we are eating constantly throughout the day, from the time we wake up until the time we go to sleep.

Essentially, the body can only exist in two states — the fed state (insulin high, storing energy) or the fasted state (insulin low, burning energy). It cannot do both at the same time. So, instead of 1977, where we balanced the fed and fasted state, we now spend 80% of our time in the fed state, telling our bodies to store energy as fat. No wonder we have an obesity epidemic.

All fasting does, at its core, is allow our body to use some of the food energy we have stored (body fat). That’s all. And there’s nothing wrong with that. It’s natural. It’s normal.

What effect does short-term fasting have on our blood-sugar and insulin levels and how our body stores fat?

Fasting is simply the most efficient, quickest way to lower blood sugar and insulin levels. Since insulin is the main driver of obesity, there is no surer way to lose weight. After all, if you don’t eat, you’ll lose weight. Nobody can tell you otherwise.

You write about how fasting can help you lose weight without slowing down your metabolism — unlike cutting calories. How does that work?

Intermittent fasting is not simply about cutting calories, although it does often lower caloric intake. Fasting is about creating periods of low insulin, and alternating periods of high food intake and no food intake. This is more physiological.

Think about the weather. In 1 week, we get 7 inches of rain. In one situation, every single day is grey and drizzles 1 inch per day. In the second situation, we get 6 sunny days and 1 day of thunderstorms. Are these two situations equivalent? No.

It’s the same with intermittent fasting. We alternate feasting and fasting because it keeps insulin levels and insulin resistance lower. This leads to easier weight management.

Can you explain what ketosis is, and its relationship to fasting and health?

Ketosis occurs when the body has little sugar to burn for fuel. The body produces ketones from fat to power the brain. This can happen in very low carbohydrate diets as well as intermittently with fasting.

Ketosis is not necessary to derive benefits, but many people find that they have better mental clarity and less hunger with ketosis.

Does intermittent fasting work if it’s not being paired with a low-carb, high-fat, ketogenic lifestyle?

It works with any diet. Fasting has been performed throughout human history and has been paired with every diet known to mankind. In the end, if you don’t eat, you will lose weight.

Any tips for getting started? What should we expect?

You should expect that fasting, if you are not used to it, will be difficult in the first few weeks, so be prepared. Also weight loss only averages ½ pound per fasting day. However, you will likely lose more than that, typically 1 pound per day. Much of that is water weight and will return once you start eating again.

It is important to understand this so that you do not get discouraged when half your weight loss is regained. That is normal and expected. That is, if you fast 4 days of the week, you can expect to lose 4 pounds, but then regain 2 of them. This does not mean the fasting is not working.

Are there any downsides to short-term fasting? What are some common mistakes people make? Also, is there anyone who should not try fasting?

Fasting should not be done by pregnant or breastfeeding women, children under 18, the malnourished or underweight (BMI<20). If you are taking medication, you should consult your physician.

There are many nuisance side effects, such as constipation, cramps, headaches that can occur during fasting. There are often simple tips than can help. Also many of these problems go away once your body gets used to it.

The most common mistake is to change your daily schedule too fast, thinking that you will have no energy and should stay at home. Your body will have the same amount of energy, but we want it to burn body fat, not food for energy. Stay busy, as it will make fasting much easier to your mind off food.

the Be Well Team
We are a team of wellness pros who support, guide, educate and advise on living a healthy life through nutrition, exercise, meditation, sleep and happiness.