Intermittent fasting: Dr. Jason Fung answers your questions

Dr. Jason Fung is the author with Jimmy Moore of Complete guide to fastinga book that is a reference in the field because it is based on the careful examination of scientific data and the experience of its author with several thousand patients. After completing his medical studies at the University of Toronto (Canada), Dr. Jason Fung continued his specialization in nephrology (kidney medicine) at the University of California (Los Angeles) School of Medicine. Returning to settle in Toronto, he treated his obese and diabetic patients for a long time with medications, even bariatric surgery, before looking for a way to make them lose weight and lower their blood sugar levels sustainably. This is how he studied the biological effects of fasting, before starting to prescribe it.

Read an extract from Complete guide to fasting

What type of fast when you have a lot of weight to lose: 24 hours or 16:8?

Dr. Jason Fung : We use both. My own feeling is that 24-hour fasting, 2-3 times a week is more effective than the 16:8 diet, where you eat in an eight-hour window per day, but you have to experiment, and learn the type of fast that suits you best.

Read: Dr. Èvelyne Bourdua-Roy and Dr. Jason Fung, pioneers of metabolic health (subscribers)

If we follow a 16:8 fast, isn’t it better to eat in the morning and abstain at the end of the day, rather than fasting in the morning and eating in the evening?

The time of day probably matters, but only a little. Some advise eating in the morning, others in the evening, there is little consensus. I think you should try both and see what works best for you, both from a lifestyle and weight change perspective.

I have never fasted. Should I start with a short fast and then progress to longer fasts?

You can definitely try skipping breakfast 2-3 times a week and progress from there.

As a Muslim, I am required to fast in Ramadan. What do you recommend?

Some people tend to overeat before sunrise and after sunset. I recommend not eating before sunrise and eating as normally as possible after sunset.

Do you suggest drinks other than water during a 16-18 hour fast?

Tea, coffee, soups and broths are acceptable. I allow a small amount of cream or coconut oil in the coffee, although technically this isn’t really fasting anymore.

Can a child who needs to lose weight be made to fast?

Fasting is not for children. We must consider other approaches, start by examining the way we shop, and if necessary call on a dietician-nutritionist or a child psychiatrist.

Is Intermittent Fasting Appropriate for Teenagers?

No it is not appropriate. Short (less than 24 hours) and very occasional fasts are possible but you should not go further. At this age, the body needs all the nutrients to grow and develop.

Can you fast when you are athletic or is it not recommended?

Fasting is safe when active, and training on an empty stomach or with low glycogen stores has several benefits that many top athletes take advantage of. So, yes, it is possible and even recommended.

Can women fast during pregnancy and while breastfeeding?

I do not recommend fasting during pregnancy or breastfeeding. There is a nutrient deficit problem that I believe far outweighs any potential benefit.

Should I be concerned if fasting blood sugar rises when fasting?

This is quite common, due to the breakdown of glycogen or glucose genesis in response to some of the hormonal changes of fasting. It’s neither good nor bad. Think of it this way: your body is simply moving sugar from its stores (glycogen and fat) and into the bloodstream where it has a chance to burn it.

Is it necessary to take a multivitamin when following a ketogenic diet?

If you eat mostly whole, minimally processed foods, then no, it’s not necessary.

To compensate for the drop in protein intake, do you suggest using whey protein powders?

Protein loss with intermittent fasting is minimal. Do you believe, while we are made to store excess energy in the form of fat, that when we need it, we primarily burn proteins rather than fats? No, protein is retained and fat used as an energy source during a fast.
I advise against using whey for the main reason that it is a highly processed chemical, not a real food. Whey proteins strongly stimulate insulin.

Can we eat rice or potatoes outside of fasting periods?

As for rice and potatoes, they are not essential in the diet. Personally, I eat them because I enjoy them from time to time. However, if I was trying to lose weight, I would limit rice and potatoes. But it’s not carbohydrates per se that cause obesity, but insulin. While starches stimulate insulin, there are ways to contain it. You can add fiber and vinegar.

How much weight can you expect to lose in one day of fasting?

The initial rapid weight loss observed is often water. After that, it is estimated that we lose on average 500 g per day of fasting.

What diets do you recommend to a type 2 diabetic who follows an intermittent diet?

There is no ideal or better diet than another, there is what works for you. But in general, I advise in this case to follow a ketogenic diet along with intermittent fasting.

What are the side effects of fasting?

The most common are constipation, headaches and abdominal pain.

Does fasting cause hypothyroidism?

Fasting does not cause hypothyroidism unless you develop an iodine deficiency.

Does cortisol increase when you fast?

There are actually very few studies on cortisol and fasting and they are mostly contradictory. I do not measure cortisol in patients in our clinic. Cortisol is normally elevated in the morning, this is normal and found in everyone.

Is there a risk of kidney stones when fasting?

There is no connection between fasting and stones unless you are not drinking enough. It is important to consume enough water, as well as salt and other electrolytes during a fast.

What blood tests do you recommend if insulin resistance or metabolic syndrome is suspected?

The tests I focus on are fasting blood sugar and glycated hemoglobin HbA1C. I also measure liver enzymes to find out if there is a risk of fatty liver and triglycerides, which when high are a sign of a diet very high in carbohydrates.

Read also: An extract from Fasting and an extract from How to fast