Fasting and Metabolism with Dr. Jason Fung: Top 5 Takeaways

Written By Tina O

Does fasting decrease your metabolic rate?

In these two enlightening videos Dr. Jason Fung explains what happens to the body and its metabolism when we implement fasting and explains the advantages of fasting over calorie restriction. 

Who is Dr Fung?

Dr. Jason Fung is a Canadian nephrologist (nephrologists specifically treat kidney diseases). He’s a world-leading expert on intermittent fasting and low carb, especially for treating people with type 2 diabetes. He has written three best-selling health books and co-founded the Intensive Dietary Management program.


  • Studies show that if we don’t eat for 4 days straight there is actually an INCREASE  in our metabolic rate by 10 percent!
  • If we try to loose weight by calories alone metabolism decreases. Interestingly, when we fast our body responds by burning more calories and increasing  the basal metabolic rate 
  • Growth hormone increases 3-5 fold on a 5 day fast
  • “The body (during fasting) is not shutting itself  down… it is actually ramping itself up 


What happens to metabolism when we don’t eat?



  • Insulin levels start falling ……body switches from burning food to burning fat …(yay)
  • Now there is a counter regulatory hormone surge 
  • The body increases the sympathetic hormones …Adrenalin, noradrenaline, growth hormone, and cortisol (these hormones are trying to get the glucose out from the liver and into the blood ).  These same hormones actually increase metabolism. 
  • As glycogen levels go down  Growth hormone goes up (24-36hrs).
  • The increased growth hormone preserves lean body mass.
  • At 24-36 hours we are burning protein but NOT muscle. The protein we are using up is the protein that needs to be burned . Example: excess skin and tissue.
  • However the body cannot indefinitely keep burning protein. So in an effort to preserve… (and this is where the magic happens) the body looks for an alternate fuel source… We utilize our own body fat for fuel.
  • Now the Growth Hormone keeps increasing and goes up 3-5 fold on a 5 day fast.
  • Now when we do eat… we rebuild the protein that we lost… but this time it is the protein that we do need.  Dr Fung gives the example of remodeling bone.  We must  shed the old damaged in order to lay new muscle / bone.

  Additional Points of interest and visual explanations 

  • Dr Fung defines metabolic rate as the amount of calories the body uses in a day, not including exercise.
  • This chart shows the resting energy expenditure over a 4 day period . Here we can see the calories burned at day 3 and 4 was higher than the beginning of them fast by 10 percent!
  • Norepinephrine increased by 50% by day 4! This hormone is responsible for energizing the body.
  • Another chart showing the results of a study on what happens during a 72 hour fast. This is my favorite chart  It shows: lowered body weightincreased resting energy expenditure and decreased arterial insulation concentration!  Wow!
  • Calorie restriction does not give these same benefits as using your own body fat for fuel.
  • As we switch over to  using our own body fat, our body gives us more energy. This is survival response which allowed us to forage and hunt.
  • This chart shows the enhanced thermogenic response during a 48 hour fast.
  • Using our own body fast during fasting is a huge advantage for weight loss.

 Tips for successful fasting for Dr.Megan Ramos

  • Flood social media with fasting support groups… before and after are motivating.
  • Participate in fasting supports.
  • Hold your self accountable.
  • Set goals with no time limit.
  • This lifestyle is a marathon not a sprint.
  • Megan has had setbacks but still has achieved remarkable results.

 To summarize as succinctly as possible: Fasting=insulin down, Noradrenaline up, growth hormone up… Increased energy…. calorie restriction = (meh)

Watch Jason Fung’s full video on Fasting and Metabolic Rate and how to avoid starvation here! 

Watch Jason Fung’s full video on Fasting and your Metabolism here! 

Here are some other fasting resources you might be interested in:

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