Can teenagers fast?

Ask Dr. Jason Fung

Can intermittent fasting raise blood pressure? Does high-intensity exercise elevate blood sugar even in a fasted state? And can teenagers fast?

It’s time for this week’s Q&A about intermittent fasting and low carb with Dr. Jason Fung:

Can teenagers fast?

My daughter is 14 years old and is overweight. We try to have her eat LCHF diet plus intermittent fasting (skip breakfast, eat only lunch and dinner from 12:00 to 20:00 of 8 hours eating window) for almost 2 months. But she hardly lost any weight. Do you recommend teenagers going longer fasting to lose weight? Such as 20 hours per day eating one meal a day? Would longer fast affect their growing and normal physical development?


I don’t generally recommend extended fasting because children are still growing and there is a risk of malnutrition. I generally limit children to 16 hours fasting (8 hour eating window). However, the most important part of treating children is education. The main components of good nutrition are not fasting, but instead

1. There is no need to eat snacks
2. Eat real food
3. Decrease added sugars severely.

Dr. Jason Fung

Can intermittent fasting raise blood pressure?

I am a 54-year old woman who has lost 40 lbs (18 kg) with a combination 16:8 intermittent fasting and a low-carb diet. I was at 235 lbs (107 kg) when I started and on a low dose (10 mg twice a day) of Benicar to control mild HBP.

After losing 20 lbs (9 kg) and fasting regularly, my blood pressure dropped so much that my readings were regularly 117/72 or so. So, I took myself off the BP meds which my doctor later said was ok (after an exam).

Now, 5 months later and 40 lbs (18 kg) down… My blood pressure seems to be rising higher than ever. I’ve taken regular readings of 145/80 or 150/101 with headaches. I’m alarmed. I’ve read some accounts from other women of blood pressure rising on intermittent fasting protocols.

A week back I recently started to drink caffeine daily, I think this might be a factor. One latte and one Diet Coke daily. So, I will stop that and see how that affects my BP.

What is your advice or experience with IF and BP in your patients?


Yes, it can. Fasting can both raise and lower blood pressure. Let me explain. During fasting, the body releases counter-regulatory hormones, including activation of the sympathetic nervous system, nor-adrenalin growth hormone and cortisol. These hormones are so called because they counter the effects of insulin. So insulin goes down and these hormones go up during fasting. The main action of these hormones is to release stored food energy to keep blood glucose high. However, in some cases, if the body overshoots, then blood pressure may go higher. Heart rate may also go higher.

However, hyperinsulinemia is also associated with high blood pressure, so sometimes the weight loss often causes blood pressure to drop. The correlation there, though is not nearly as tight as that with blood sugar.

In both cases, you need to consult your physician if medication changes are needed.

Dr. Jason Fung

Does HIIT (high-intensity interval training) elevate blood sugar even in a fasted state?

Been nicely in ketosis (1.5-1.8 mmol/L) for several weeks with blood sugar at 71-92 mg/dl (3.9-5.1 mmol/L). I workout 3-5 times per week in a fasted state. 14-16/8-10 hours. For some reason I seem to get dizzy during my workouts. Today I was really feeling it and general felt yucky all the way home. Checked my blood for interest sake and this is what I got: ketones 2.3 mmol/L YAY!!! But curious that my blood sugar was 102 mg/dl (5.7 mmol/L)? How does that happen? I guess I expected it to be on the lower end and the “cause” of my dizziness. Does HIIT elevate blood sugar even in a fasted state?


Yes, HIIT may raise blood sugar even in the fasted state. During intense exercise, the body releases hormones to increase the availability of glucose. This is normal. Cortisol, sympathetic nervous system activation and nor-adrenalin are all examples. If the system overshoots, then glucose may be high in anticipation of demand.

Dr. Jason Fung



Intermittent Fasting for Beginners

How to Reverse Type 2 Diabetes – The Quick Start Guide

Earlier Q&A sessions with Dr. Fung:

Intermittent Fasting Q&A

More with Dr. Fung

Dr. Fung has his own blog at 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.


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