What organ burns the most calories

There are a lot of myths that still float around the training world and these are often used to make suggestions about how to train or approach the goal of improving body composition.  Perhaps one of the most prevalent is that adding muscle mass will significantly increase daily energy expenditure, helping both to lose fat and keep it off.  Unfortunately, as I have stated on the website, this isn’t really true.  To examine why I want to look at the following paper:

McClave SA, Snider HL. Dissecting the energy needs of the body. Curr Opin Clin Nutr Metab Care. (2001) 4(2):143-7.

Incorrect Claims About Energy Expenditure

As I stated above, one of the longest held myths is that gaining muscle will significantly increase daily energy expenditure.  This is essentially based on some very old research that concluded that one pound of muscle burned 50 calories per day.  Unfortunately, this is utterly wrong.  I mean, it’d be great if it were true but it’s simply not.  I’ll give you the real value below.

There are other claims that the paper I want to examine today addresses.  One of those is the idea that fat cells burn no calories and are metabolically inert.  This is another old and out of date idea.  It’s well known that body fat is incredibly metabolically active and relevant in the body, producing endless hormone and compounds.  But it also burns calories.

Now, the paper I want to look at wasn’t really geared at looking at those questions specifically.   Rather it was a more general paper examining a number of interrelated topics, only some of which I will detail.  The primary one of relevance to this article is the actual daily calorie expenditure of different tissues in the body.  It also examined issue such as disease, growth/development, undernutrition and others in terms of how they impact on daily energy expenditure.

A Precis on Metabolic Rate

There are four components that determine the body’s daily energy expenditure.  They are Resting Metabolic Rate (RMR), the Thermic Effect of Activity (TEA), the Thermic Effect of Food (TEF) and Non-Exercise Activity Thermogenesis (NEAT).  The sum of these four is the Total Daily Energy Expenditure or TDEE.

RMR represents the number of calories that the body burns at rest.   And unless massive amounts of activity are being done, RMR makes up roughly 65-70% of total daily energy expenditure.  For that reason, looking at how different tissues in the body impact RMR give a pretty decent picture of what’s going on.

Energy Expenditure and Body Composition

The paper begins by pointing out that estimating RMR in people of different body sizes has been classically difficult.  While body weight per se tends to be a decent indicator/determinant of RMR, it’s not ideal.  Rather, RMR tends to scale better with body surface area.  But even this is only a way of estimating daily RMR, it gives no indication of which tissues in the body (or in what proportion) are contributing to overall RMR.

I suspect most readers have seen the statement that “The largest predictor of RMR is lean body mass.”  If you haven’t, well you have now.  And there is definitely much truth to this.  However lean body mass (also called fat free mass) only predicts 53-88% of the variability in RMR.  That’s a pretty big range.  And while there are a number of reasons for this, one of them is that all LBM/FFM is not the same.

Rather, lean body mass represents organs such as brain, liver and kidneys, skeletal muscle, bone, skin and basically everything in the body that isn’t fat mass.      Now, in the fitness industry, many tend to equate LBM with muscle mass.  But this is drastically incorrect.   On average, muscle mass only makes up about 40-45% of total LBM although this may achieve 50% in highly trained lean athletes.  That means that 50% or more of the body’s LBM is not muscle.

And you’ll see momentarily, those different tissues burn very different numbers of calories on a day-to-day basis.  Which means that variability in the amounts and proportions of those tissues will impact on overall resting energy expenditure.

Methodologies of Measuring RMR

While I’m going to skip the details, the paper does detail the different methodologies that are used to estimate the resting energy expenditure of the different tissues in the body.  Sufficed to say that newer technologies have allowed for more and more accurate methods of estimating calorie expenditure for those tissues.

While they are still not error-free (nothing in science ever is), some of the newer methods of measurement may explain why some of the oft-held beliefs about caloric expenditure and values that are often thrown out are turning out to be wrong.   Of course that also means that future developments may render current values incorrect.

The Normal Human

The next part of the paper is really the key one.  Because here it provides a chart showing the different calorie burn of all of the tissues in the body along with how much they contribute to overall RMR.

.Other refers to bone, skin, intestines and glands.
Note: the lungs have not been measured for methodological reasons but have been estimated at 200 kcal/kg similar to the liver.

Ok, so looking at adipose tissue, we see that it burns 4.5 cal/kg/dy or 2 cal/lb/day.   This represents 4% of the body’s total RMR although this is based on a “relatively” low 33 lbs of body fat or 21.4% of total body weight.  So it’s certainly true that fat doesn’t burn many calories. Despite being 21.4% of total weight, it’s only 4% of metabolic rate.

But now look at muscle.  Here one pound burns a whopping 13 cal/kg or 6 cal/lb, representing 22% of RMR despite making up nearly 40% of total body weight.   Certainly muscle is more metabolically active than fat but not by much.  Importantly it only burns 6 calories per day at rest.

Skipping over other, next look at the different organs.  The liver burns a whopping 200 cal/kg or 91 cal/lb per day.  Despite making up 2.6% of total body weight, it contributes nearly as much to metabolic rate as muscle at 21%.   The brain is similar, at 240 calories and 22% of RMR despite being only 2% of body weight.  The heart and kidneys burn an enormous number of calories per kg or per pound.   But they are also smaller than the liver or brain so their total contribution isn’t as high.

If you add the organs up you find that despite making up about 7% of total body weight, they are responsible for 70-80% of RMR.  Despite their small weight, they are just massively metabolically active on a day-to-day basis.

In contrast, while skeletal muscle may contribute roughly 40% of total weight (and a little bit less in women), it only contributes 28% of total resting energy expenditure.   The only reason muscle mass burns a significant number of calories in absolute terms is because there is so much of it.  But it’s relative contribute is quite small on a per pound basis.

Adding Muscle to Raise Energy Expenditure

Which brings me back to the introduction of this paper, the claim that adding muscle can significantly impact on daily energy expenditure.  Because clearly that isn’t really the case.

Gaining 10 lbs of muscle, which might take a beginner male half a year has the potential to increase RMR by a whopping 60 calories.   This is about half an apple or maybe 10 minutes of moderate aerobic activity per day.  A woman gaining half that might see an increase of 30 calories.  In the big picture this is irrelevant.

A male gaining 20 lbs of muscle, which would take a beginner about a year would increase energy expenditure by 120 calories per day.  This is approaching significance and might add up to one pound worth of fat per month.  It’s still only equal to a big apple or 15 minutes of moderate aerobic activity.  The woman gaining 10 lbs over that same year is only burning 60 calories extra per day.

And yes, fine, if you are looking at someone who has maximized their genetic potential, a male who gained 40 lbs of muscle over a career, that gets significant.  He’s burning 240 extra calories per day.  So now it’s 3 apples or 30 minutes of moderate cardio.

A woman who tops out at 20 lbs of muscle gained if she’s lucky will burn an additional 120 calories/day or 1800 calories per month.  That’s about 1/2 lb of fat.  And it will take her several years to achieve it if she does so at all.

The reality is that, in the short-term, adding muscle has an essentially insignificant impact on RMR.  Over very long time periods of continuous training, it can add up.  But that’s not the claim that’s usually being made.  Rather, people will state that weight training should be done to raise metabolic rate to help burn fat.  And the math doesn’t work out that way.

Frankly, any impact of building muscle mass in terms of fat loss or the energy balance will come through the calories burned during training (which also isn’t very high) and the calories needed to synthesize new muscle.  Even that isn’t that big simply because muscle growth is so slow.  But once that muscle has been gained, it’s resting energy expenditure is quite low.

I’d finish by noting this.  Consider that gaining 3 lbs of fat would burn the same number of calories as gaining 1 lb of muscle. And that’s a lot easier and more fun to do.   To that I’d add that carrying around those extra 3 pounds of weight will increase the energy expenditure during exercise or NEAT.  So the effect on TDEE might be even higher.  No I’m absolutely not saying to do this.  Rather, I’m just making two points.  The first is that fat does burn calories.  Not much but it does.   And it’s only 1/3rd as much as muscle to begin with.

Organs Win the RMR Game

The big calorie burn ultimately comes from non-muscle LBM and non-fat tissue.  It’s the organs that do the heavy lifting in this regard, burning a staggering number of calories per day despite making up a tiny proportion of total body weight.  If you could gain one pound of kidney you’d burn nearly 200 calories per day extra, the equivalent of gaining 33 lbs of muscle.

But I’m not aware of any way that it can be done. You could ostensibly do the same by gaining one pound of heart muscle.  But the only training program I know that can accomplish that is currently copyrighted by the Grinch.

Factors Affecting the Energy Needs of the Body

Having examined the average contribution of different tissues to the body, the researchers then look at a host of other topics, only a few of which I’m going to really look at in any detail.

Growth and development is covered first, examining how the ratios of energy expenditure to body weight changes over the lifespan.  Since most reading this are full grown adults, the changes that occur from childhood to maturity don’t seem that relevant.

One issue of some importance is covered next and that’s the effect of differences in body size between individuals.  In general, if you look at two people of different body sizes, larger folks tend to have lower resting energy expenditures relative to their body mass.  This is most likely related to differences in the proportion of organ weight (recall from above that the organs contribute the most to overall resting energy expenditure) to total body weight.

Meaning this: on average, organ weight won’t vary much between individuals.  So if one person is larger than another, that difference in size is likely to occur through changes in either muscle mass or fat tissue, neither of which makes massive contributions to resting energy expenditure (and differences in body composition won’t have nearly the impact that most think given the relatively small difference in caloric expenditure between muscle mass and fat mass).

Practically, this means that equations that estimate resting energy expenditure based solely on body weight will tend to overestimate larger individuals to some degree.   This is important to keep in mind in a real world sense.  At the same time, all estimates of maintenance calories are just that and the diet will always have to be adjusted based on real-world results.

A Question About a Common Quick Estimate

I should probably address a question that I imagine will come up in the comments, given the enormous variability in energy expenditure per pound of tissue, where does the quick estimate of 10-11 calories/pound (22-24 cal/kg) come from?  And the answer is that it’s basically a weighted average of the above values.

That is, if you took the values for caloric expenditure/unit weight times their contribution to overall weight and worked it out, you’d get a value that was pretty close to the quick estimate value.  Again, this will tend to vary based on actual body size due to differences in the relative contribution of each tissue to the body’s total weight.

This is part of why estimation equations tend to fall apart at the extremes.  If someone is carrying a tremendous amount of body fat, their relative ration of LBM to fat will be lower.  Hence so will their RMR relative to their body weight.

Undernutrition and Refeeding

Next the researchers looked at the impact of both undernutrition and refeeding on energy expenditure at rest.  During underfeeding, they point out that skeletal muscle and fat are generally the major tissue lost while organs are spared.

This tends to have the impact of raising the relative proportion of energy expenditure to body weight (because the low energy expenditure tissues are being lost).  Of course, with extended dieting, there is also an adaptive component of metabolic rate reduction as all tissues in the body tend to slow their overall energy expenditure.

In contrast, during refeeding, there is often a hypermetabolic state that occurs, possibly due to increases in protein synthesis, core temperature and the thermic effect of food.  As well, there are a number of hormonal effects that occur when calories are raised.  This is why various intermittent dieting strategies such as The Full Diet Break can be so beneficial during a fat loss diet.

Finally the researchers examine the impact of disease and injury on energy expenditure.  I’m not going to discuss disease conditions.  However, injury is interesting since recovery from that can induced a hypermetabolic state where RMR is raised by 20-50% depending on the extremity of the injury.  Calorie intakes often have to go up when someone is injured and this is especially critical for optimal injury recovery and healing.

Summing Up

The main point that I wanted to make with today’s research review was to clear up some of the oft-held (and unfortunately incorrect) ideas regarding the impact of things like skeletal muscle mass and fat mass on resting energy expenditure.  Based on current data, the idea that skeletal muscle burns massive numbers of calories at rest would appear to be 100% incorrect.

Rather, skeletal muscle actually burns fairly few calories on a per pound basis.  It primarily has a major impact on resting energy expenditure because there is a good bit of it.  But adding even moderate amounts of muscle are unlikely to massively impact on energy expenditure.

As noted above, I expect the major effect to be from the effort of stimulating muscle mass gains along with the energy needed to synthesize that muscle tissue.  But once it’s there it doesn’t burn many calories.  Does this make weight training irrelevant while dieting?  Of course not.  But expecting gains in muscle mass to drastically impact on RMR or reduce bodyfat percentage is flatly incorrect.

Rather, the majority of resting energy expenditure is generated by the organs which, despite their small size, burn a massive number of calories per unit weight.   This leads to the obvious question which is “How do I hypertrophy my liver or kidneys?”  If only it were that easy.

Similar Posts:

• Training the Obese Beginner: Part 2
• A Guide to Body Composition
• The 3500 Calorie Rule
• Body Composition – Calculations
• The Energy Balance Equation