What’s the difference between a ketogenic and a low-carb diet?

Ask Dr. Jason Fung

It’s time for this week’s Q&A about intermittent fasting and low carb:

  • What to do about constipation after fasting?
  • What’s the difference between a ketogenic and a low-carb diet?
  • Is drinking Bulletproof Coffee a good way to lose weight?

Dr. Jason Fung is one of the world’s leading experts on fasting for weight loss and diabetes reversal. Here are a his answers to those questions and more:

Constipation after fasting?

How to avoid this? I have drunk much water and included vegetables in my meals.

Wai Yin

This is very common to have less bowel movements. Less going in, less coming out. During the world record fast of 382 days, the fellow only had a bowel movement (BM) every 30-45 days! It is only a problem if you have abdominal discomfort. If your bowels are empty, and that is the reason you do not have a BM, that’s normal. But if you are having discomfort, you can try standard laxatives – milk of magnesia is often available over the counter.

Dr. Jason Fung

What’s the difference between a ketogenic and a low-carb diet?

What is the difference between ketogenic diet and low-carb diet? Who has to follow a ketogenic diet and who needs to follow a low-carb diet?

Please let me know the main difference between the two diets above?


Ketogenic is an ultra low-carb diet, which causes production of ketones. Ketones are produced when the body runs out of sugars (carbs). The blood glucose is saved for the brain, and the body converts fatty acids into ketones. These can cross the blood brain barrier to fuel the brain. The muscles do not use ketones – instead they can directly metabolize fatty acids.

So you can still follow a low carb diet but not be ketogenic. You do not necessarily need to be ketotic to lose weight, but it is something that people can measure through breath testing or urine testing to ‘confirm’ they are following a low carb diet.

Dr. Jason Fung

Bulletproof Coffee and fasting for 5 days?

Hello Dr. Fung,

I just want to start by saying thank you for your information about intermittent fasting. It wasn’t until I incorporated that into my low-carb lifestyle that I was able to lose as much weight as I did. Now I’m about 10 pounds (5 kg) away from my goal and I want to lose them quickly because we’re going on vacation and I was wondering is it possible to fast for 5 to 6 days of having nothing but Bulletproof Coffee and then have a normal low-carb day on day 6 or 7 and then go back again 5-6 days of nothing but Bulletproof Coffee and maybe some other fat to starve off hunger to lose the weight quickly. Thank you for your information.

Kind regards, Helen

BPC works for some and not others. Because it contains lots of calories and fat, it is not a true fast, but can often still work well. This is because pure fats do not stimulate insulin and the point of fasting is to lower insulin.

However, not everybody can do it. There are definitely people that find that the extra fat slows weight loss completely. You need to try it and see for yourself. The reason is a little complex, but I will cover in a future post.

Dr. Jason Fung



Intermittent Fasting for Beginners

How to Reverse Type 2 Diabetes – The Quick Start Guide

Earlier Q&A sessions with Dr. Fung:

Many more questions and answers:

Intermittent Fasting Q&A

Q&A videos

  • Is low carb bad for exercise?
  • Is low carb bad for the kidneys?
  • Everything in moderation?

Top Dr. Fung videos

  • Diet Doctor Podcast #64 with Jason Fung
  • Part 8 of Dr. Jason Fung's diabetes course
    The essential problem with type 2 diabetes

More with Dr. Fung

Dr. Fung has his own blog at intensivedietarymanagement.com. He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code

His new book, The Complete Guide to Fasting is also available on Amazon.



  1. Judy Davidson
    Is this Ketogenics diet good for someone with kidney failure? My Nephrogist said I can't do the low carb because of all of the proteins.
  2. Devendra vyas
    I read this book. My question is...We will have to adopt this diet for rest of life..... or after getting right level of insuline and overcome with resistance....,will we eat nomal diet as others

Leave a reply

Reply to comment #0 by

Older posts