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.
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.