Monday, January 01, 2007

Deconstructing Nutrition - When you don’t eat, your metabolism slows down

When you don’t eat, your metabolism slows down.

This "nutrition fact" says that when your caloric intake is too low, your metabolism actually slows down. It has been referred to as “starvation mode” and is the basis for the current trend of advising people that they should eat every couple of hours, never skip meals, and never cut your calories too low while dieting.

But is it true?

To examine this truth, I decided to look into what happens to a person’s metabolism when they fast for a short period of time (between one and three days).

The first study I reviewed had six healthy male volunteers fast for 3 days. During this fast, the researchers measured the changes in the subject’s metabolism at 12, 18, 24, 20, 42, 54, and 72 hours of fasting. What they found was that there was no significant change in energy expenditure during any of the 7 time points. So from this study we can say that a 3 day fast in men does not cause a decrease in metabolism.

Already this evidence goes against the idea that if we don't eat every couple hours our metabolism will slow down.

The second study I reviewed had 8 men and 8 women fast every other day for 22 days straight. There was no significant change in the subject’s metabolism from the start to the end of the study. From this study, we can say that fasting every other day for a period of 22 days results in no significant changes to a person’s metabolism.

So we now know that even if we eat sporadically for 22 days, most likely our metabolism won't be affected.

The third study I reviewed had 8 men fast for either 12 hours or 72 hours and then eat a meal. After the meal their metabolisms were measured for 12 hours. This study found that regardless of whether or not the men fasted for 12 hours or 72 hours, their metabolism for 12 hours after eating a meal was not significantly different. So this study shows us that a short term fast doesn’t affect your metabolism after you eat.

So even if we don't eat for 3 days, there is absolutely no change in the way eating effects our metabolism.

So from the research I was able to review, it doesn’t seem like short periods of fasting cause any negative effects on our metabolism. Nor does it affect our metabolism once we start eating again.

Since having absolutely no calories for a short period of time does not effect our metabolism, I feel safe in saying that the idea that you need to eat every couple hours in order to keep your metabolism “revving” is actually a nutrition myth and not a fact at all. Also, the phenomena of a “starvation mode” does not happen until after at least 3 days of fasting, if it actually even exists.


PS- For those of you who are interested, I will post my references in the comments section.

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Marc said...

Hi Brad, we're compiling a list of the best Men's health blogs and would love to talk more about what you're doing here. Please shoot me an e-mail (marc[dot]zawel[at]revolution[dot]com) when you get a chance. Thanks, Marc

WP said...

So based on your research, eating a lower calorie diet for an extended period of time- cycling 30% deficit from maintenance for 3 days and increasing to a 10% defecit for a day before cycling again- would have no long term effect decreasing your metabolism because you are not really in a deficit state for more than 3 days? If so, that's great news!

Brad Pilon said...

Hi WP,

Based on what I've read, if a day of 0 calories (a true 100% deficit) does not change your metabolic rate in a significant manner, then I do not see how a smaller 30% deficit would become significant.

My next "Deconstructing Nutrition" post will be on the thermic effect of food, and should shed some more light on this whole metabolism thing.


PS- Just curious, you mentioned this being great news?

WP said...

Yes, great news because I can keep my clients at lower calories for longer periods of time without worrying about their metabolism shutting down!

Can't wait to read the next blog!

Mike Roussell said...


This is an interesting topic. There is an article that gives a counter opinion in the newest American Journal of Clinical Nutrition ( It was a study that involved calorie restriction for 3 weeks and the researchers found significant drops in metabolic rate (granted these people were sedentary).

It is a rather interesting study.

Craig Ballantyne, CSCS, MS said...

What aboot muscle loss?

Brad Pilon said...

Hi Mike,

Thanks for letting me know about the new AJCN study, I will read it today and post about it when I am done.


Brad Pilon said...

Hi Craig,

Muscle loss will be my next topic after thermic effect of food.

It will be a tricky one, since the idea of muscle loss is based on assumptions that any nitrogen loss is due to protein breakdown and that this breakdown is mostly skeletal muscle.


Mike Roussell said...

Craig - In the AJCN study only 28% of the weight loss was attributed to loss of fat mass. Brad - Looking forward to your comments.

Brad Pilon said...

Hi Mike,

I've had a chance to review the AJCN paper. It was a very interesting comparison of two different methods of measuring energy expenditure (EE).

I was unable to find any direct mention of a change in the subjects EE, accept for page 76 which notes a 167 Kcal decrease from baseline measurements.

The authors do not mention if this finding is significant, nor do they speculate whether or not it may be attributed to the 2.3 Kg weight loss experienced by the group of subjects.

I did notice that the graph on page 77 comparing EE to daily caloric intake could be misconstrued as suggesting that EE was lower with lower amounts of calorie intake.

It should be noted that each plot on this graph represents an individual subject, and that each subjects caloric intake was calculated as a percentage of their body weight.

So most likely, the graph would look almost identical if the y-axis was replaced with the subject’s weight, instead of their daily caloric intake.

With regards to the study in general, I was fascinated by the measurements and calculations that the authors made regarding "metabolic energy" used while dieting.

I found these measurements very creative and I was very impressed with how they affected their correlation measurements.

This is definitely something I will have to look into further.

Thanks for the great find!


Mike Roussell said...


This is definitely not my area of research (I'm a CVD/fish oil guy) and I admit my head was spinning a little while reading the article but I took the follow:
"Twenty-four–hour EE during CR during the 7-d chamber study decreased from 2073 ± 368 kcal/d at baseline to an average of 1906 ± 327 kcal/d, resulting in an energy deficit of 24.9 ± 4.7%"

to infer a reduction in metabolic rate as a result of the calorie restriction. Thoughts?

Brad Pilon said...

Hi Mike,

My head was spinning too; analyzing predicative equations in metabolism is definitely not my specialty.

With regards to your quote - I originally thought that as well.

What the authors are actually doing is back calculating to double check the difference between Caloric intake and Caloric expenditure in their subjects.

They were aiming for a 30% reduction, however based on their metabolic chamber measures, (the subject’s average caloric intake was 1432 KCals and their caloric expenditure was 1906 KCals) they were at a 24.9% caloric deficit.

So the references was to the subjects consuming 25% less Calories then what they were metabolically expending, and not a 25% reduction in their metabolic rate.