Short Term Fasting Increases Metabolic Rate

Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Zauner C, Schneeweiss B, Kranz A, Madl C, Ratheiser K, Kramer L, Roth E, Schneider B, Lenz K. Am J Clin Nutr. 2000 Jun;71(6):1511-5. [free full text]

The effects of food restriction on energy metabolism have been under investigation for more than a century. Data obtained are conflicting and research has failed to provide conclusive results.

The objective of this study was to test the hypothesis that in lean subjects under normal living conditions, short-term starvation leads to an increase in serum concentrations of catecholamines and thus to an increase in resting energy expenditure.

Resting energy expenditure, measured by indirect calorimetry, and hormone and substrate concentrations were measured in 11 healthy, lean subjects on days 1, 2, 3, and 4 of an 84-h starvation period.

Resting energy expenditure increased significantly from 3.97 +/- 0.9 kJ/min on day 1 to 4.53 +/- 0.9 kJ/min on day 3 (P < 0.05). The increase in resting energy expenditure was associated with an increase in the norepinephrine concentration from 1716. +/- 574 pmol/L on day 1 to 3728 +/- 1636 pmol/L on day 4 (P < 0.05). Serum glucose decreased from 4.9 +/- 0.5 to 3.5 +/- 0.5 mmol/L (P < 0.05), whereas insulin did not change significantly.

Resting energy expenditure increases in early starvation, accompanied by an increase in plasma norepinephrine. This increase in norepinephrine seems to be due to a decline in serum glucose and may be the initial signal for metabolic changes in early starvation.

My comments:
Since I started experimenting and talking about intermittent fasting, the big thing I hear is “won’t that decrease your metabolic rate?” EVERYBODY asks that and it seems to be entrenched in the culture that if you miss meals your body will go into ‘starvation mode’ and the metabolism will drop such that when you return to eating ‘normally’ you will gain all your weight back and then some. At this point I think of intermittent fasting as a lifestyle rather than a diet so a normal ‘Merican’ diet isn’t something I ever intend to return to, however how fasting affects metabolism is still a good question.

I think there is a general consensus that weight loss in general does slow the metabolism because carrying around a lot of fat and/or muscle throughout the day, burns a fair amount of calories that you otherwise wouldn’t. However there is a tradeoff when excess fat mass leads to arthritis and joint pain, as well as general effort to move, making the person more sedentary, just decreasing calories burned per day even if the resting metabolic rate is higher. So prolonged dieting and weight loss I would expect to decrease resting metabolic rate, though perhaps not total calories burned per day.

As for the short term effects of fasting, this study shows the opposite is true. Healthy, lean subjects in this study were fasted for 4 days with resting energy expenditure increasing each day, from 3.97 kJ per minute on the first day of the fast (12 hours in), 4.37 kJ on day 2, 4.56 kJ and 4.43 kJ on day 4. That works out to a 10.08% from day 1-2, 14.11% by day 3 and 11.59% at day 4.

At day four the researchers noted that the resting metabolic rate was starting to drop, but it was still higher than day one and it was increasing each day prior to that. With the various versions of intermittent fasting requiring no more than a day of fasting, and my favorite version being 5-6 hours of eating and 18-19 hours of fasting every day. This study would tend to indicate that if anything, resting metabolic rate is increased rather than decreased, and ‘starvation mode’ mode isn’t a risk at all.

The researchers attributed the rise in resting metabolic rate to norepinephrine, which according to wiki is the neurotransmitter most associated with concentration, vigilance and alertness. The idea apparently being that paleolithic man, and probably Jurassic dinosaurs, wouldn’t survive long if they slowed down and became less alert when they were hungry. Which would probably go along way towards explaining why I, and everyone I have heard who tries it, reports feeling more energy when fasting. Which if you ask me, is one of the best parts.

Thanks for reading my blog. If you have any questions or comments (even hostile ones) please don’t hesitate to ask/share. If you’re reading one of my older blogs, perhaps unrelated to neck or back pain, and it helps you, please remember Spinal Flow Yoga for you or someone you know in the future.

Chad Reilly is a Physical Therapist, obtaining his Master’s in Physical Therapy from Northern Arizona University. He graduated Summa Cum Laude with a B.S. Exercise Science also from NAU. He is a Certified Strength and Conditioning Specialist, and holds a USA Weightlifting Club Coach Certification as well as a NASM Personal Training Certificate. Chad completed Yoga Teacher Training at Sampoorna Yoga in Goa, India.

3 thoughts on “Short Term Fasting Increases Metabolic Rate”

  1. great read chad…..i am finding that the 5 hr eating protocol works well for me….i started at 205 with hyper tension and my latest readings are 122/72 avg……and my body wt is around 175 or so… it is working for me….kidney function is back to normal… i am a happy camper. i do want to say that i have bullet proof coffee in the am with raw, no salt kerry gold butter blended in with mct oil…..and that keeps me engaged and focused throughout the day. this has had no effect on my blood panels as well. all normal. there are times when i am traveling that i will push up my 5 hr window….say 2:30 or so, but i will try to get back to my 4-9 pm eating window when i get back…….we are still working on this……my wife is now on the 5 fast and is having success……she wants to push the time back to 2:30-3:00 to 7:30-8:00……i might agree with that………..what are your thoughts?? i find that if i start my eating time at 4-5…..i am done at 7-8 anyway because i am full. again…thoughts?


    mike burgener

    • Hi Mike, great to hear! You’ll have to tell me what your wife thinks after she has been on it for a while.

      From what I have read with all the various intermittent fasting regimens, and people’s comments regarding their experiences, I don’t think that it physiologically matters when your window of eating is. I tried to find research on circadian rhythms but didn’t find anything particularly relevant. This being said, I think you just want to pick what works with your lifestyle/personality best. I get off work at 5-6 so it works for me to start eating then, and a full stomach makes me fall asleep so I don’t have any qualms eating right up until I go to bed. That tired feeling after eating is also why I don’t want to eat a lunch in my window, as I get groggy afterwards and I don’t want to blow my day. I know there is research that suggests it healthful to wrap up your eating earlier in the evening so you don’t go to bed on a full stomach, but that was before intermittent fasting became ‘a thing.’ I think the benefit from earlier dinners was that it was another way to increase your interval of not eating, much the same as you get by skipping breakfast and/or lunch. If I come across anything that changes my mind, I’ll be sure to post it.

      I do like the psychological aspect of being able to look forward to a big meal and ending the day satisfied. So for instance, even if it might physiologically be just as good to eat all your calories before noon, I don’t think I would like it very much and I would probably be tempted to cheat at night., even if I didn’t I think I would be less happy with the schedule. However, in your case, if you are getting done by 7-8 anyway then you might as well push up when you start eating. Also, I don’t think it’s a big deal to move your interval around by a couple hours if you are traveling. I think big changes (more than a few hours) might start to cause problems with hunger as ghrelin levels, which are associated with hunger, are supposed to rise when you get close to when you are used to eating. It’s thought that ghrelin levels getting used to the new program is why it seems to take 2 weeks for a lot of people to get used to fasting in general, so a big change in the timing of your eating interval might take 2 weeks to get used to.

  2. I started it 18 days ago and weighted right over 215 on my docs scale. Today I weighted 206.4, and other then one mild PT visit I have done NO exercise. I am recovering from a concussion and I did not want to put on weight, and this has got me the lightest I have been in over 8 years. My blood sugars are not on a 200 to 100 dip and seem to be leveling off in the high 100’s. Which the ups and downs of diabetes is just as bad or worse. I am sure that once I get under 200, Chad, my doctor and I will see what meds or eating time changes will do, since the meds did nothing.


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