Role of Nonexercise Activity Thermogenesis in Resistance to Fat Gain in Humans – Research Review

Levine JA et. al. Role of nonexercise activity thermogenesis in resistance to fat gain in humans.  Science. (1999) Jan 8;283(5399):212-4.

Humans show considerable interindividual variation in susceptibility to weight gain in response to overeating. The physiological basis of this variation was investigated by measuring changes in energy storage and expenditure in 16 nonobese volunteers who were fed 1000 kilocalories per day in excess of weight-maintenance requirements for 8 weeks. Two-thirds of the increases in total daily energy expenditure was due to increased nonexercise activity thermogenesis (NEAT), which is associated with fidgeting, maintenance of posture, and other physical activities of daily life. Changes in NEAT accounted for the 10-fold differences in fat storage that occurred and directly predicted resistance to fat gain with overfeeding (correlation coefficient = 0.77, probability < 0.001). These results suggest that as humans overeat, activation of NEAT dissipates excess energy to preserve leanness and that failure to activate NEAT may result in ready fat gain.


There has long been a question of why some people seem to be able to ‘eat anything they want’ and remain thin while others can do no such thing; in fact this is often used as an argument that The Energy Balance Equation is wrong.

More in fact, the paper I’m going to talk about today was once trotted out by several individuals as ‘proof’ that The Energy Balance Equation was incorrect.  Unfortunately all their discussion really ended up proving was that, as I suggest in The Energy Balance Equation, the issue was not the equation, but that they had no clue what they were talking about.  But I’m getting ahead of myself.

Certainly we all have seen, known (or in lucky situations been) that person who seems to ‘eat anything they want’ without gaining appreciable weight.  This is in contrast to those people who seem to be able to simply look at food and get fat. What’s going on?

At least part of what’s going on, and this is outside of the paper I’m going to discuss today, is that these folks in question often don’t eat as much as you think they are.  Certainly you may see them gorging on food acutely (at a single meal, perhaps out with friends) but what you often don’t see is what they are doing the rest of the day, or the day before, or the day after.

So while you may see the single enormous meal, what you don’t see is the smaller or non-existent meals that they are eating at other times of the day.  Or the compensations that occur a day or two later to drastically reduce their food intake and keep them in energy balance in the long-term.  So while you may assume that they eat like that all day every day, you don’t know that for sure.

But as it turns out, that’s not all that’s going on.  As I discussed in The Energy Balance Equation one mistake people often make is assuming that the output side of the equation is static; that the energy output of a given individual is invariant over time.  Thus if you plug in X calories and the person doesn’t gain exactly Y weight, the equation must be invalid.  This is wrong for a bunch of reasons discussed in that article not the least of which being that the out side of the equation changes in response to cahnges in food intake, activity and obesity.

For example, in response to both increases and decreases in food intake (as well as body weight), we know that basal or resting metabolic rate (BMR/RMR) can go up and down.  Similarly, the thermic effect of food (TEF) is related to the amount (and type) of food being eaten and will adjust upwards or downwards as well.

Activity of varying sort can be affected by energy intake as well as body weight (e.g. larger bodies burn more calories in movement).  Clearly the idea that the out side of The Energy Balance Equation is unchanging is wrong.  Yet people keep pretending that it is when they simply look at calories in or out and what they think should happen to body weight without accounting for those changes.

But as it turns out, changes in the above three factors don’t seem sufficient to explain some of what is seen when people are overfed with studies finding a huge individual variance in how much fat is gained with identical amounts of overfeeding and that brings me in a very roundabout way to today’s paper; while over 10 years old, this was a seminal study that goes a long way towards explaining the odd observation that some people are seemingly able to ‘eat’ whatever they want and not get fat.   The researchers wanted to try to determine mechanistically what might be causing that to occur.


The Paper

It’s been known for quite some time that people show a rather large amount of variability in terms of actual fat/weight gain in response to overfeeding and the researchers wanted to try to figure out some of the mechanistic reasons why this might be the case.

Towards this goal, the study recruited 16 people (12 males and 4 female) who underwent body composition measurement (via DEXA) and total energy expenditure (measured by doubly labeled water) who were then overfed by 1000 calories per day for 8 straight weeks.

I’d note that both basal metabolic rate and TEF were measured via indirect calorimetry, mainly to see if changes there could explain anything about the measured results.  As a control, subjects were required to maintain their exercise type activity at very low levels; this was done to prevent folks from trying to compensate for the increased caloric intake by simply exercising more.  While slightly artificial in terms of how people often work in the real world, this was simply a way of controlling the study to see what else might be going on.

Over the course of the study, an average of 432 cal/day was stored and 531 was dissipated through increased energy expenditure: this accounted for 97% of the total (note: this means that the energy equation was essentially balanced in that all calories were accounted for, either being stored or burned; none magically went anywhere else).  However, looking at the averages obscure what was really happening.

Moving to individual results, fat gain varied from a low of 0.36 kg (0.79 lbs) to 4.23 kg (9.3 lbs) a 10 fold variance despite the same 1000 calorie/day increase in energy intake.  Changes in BMR and TEF were unable to explain this difference.  BMR went up only 5% accounting for 8% of the extra energy while TEF went up 14%, simply in response to the increased food intake; none of those changes showed any correlation with changes in fat mass.   As I noted above, exercise type activity was clamped at low levels so changes there can’t explain the difference either.

And that brings us to NEAT, an acronym referring to Non-Exercise Activity Thermogenesis.  As the researchers define it:

NEAT is the thermogenesis that accompanies physical activities other than volitional exercise, such as the activities of daily living, fidgeting, spontaneous muscle contraction, and maintaining posture when not recumbent.

Basically, think of NEAT as the calorie burn associated with all activities that aren’t formal exercise.  And that’s where the researchers saw the massive difference between subjects; while the average increase in NEAT across all subjects was 336 cal/day, the individual changes in NEAT varied from -98 (that is it actually went down in at least one person) to +692 cal/day.

That is, in at least one subject, approximately 700 calories of the 1000 extra was burned off via NEAT.  That’s in addition to the increase in BMR and TEF which would have burned off even more of the total calories.  The researchers calculated that the increase in NEAT in the greatest responder would be the equivalent of strolling for 15 minutes per hour during waking hours.

In this vein, in the review of the Bodybugg/GoWearFit I mentioned that even small increases in activity over the course of the day can end up having a massive impact on overall energy balance because of how it can really add up.  The subjects with the increase in NEAT effectively had that happen without trying.

Of more importance, changes in NEAT directly predicted fat gain (or the lack thereof): people who showed the greatest increase in NEAT showed the smallest fat gain and vice versa.   I’d note in finishing out the paper that the four worst responders in terms of NEAT were the 4 female subjects; this really isn’t news inasmuch as we’ve also known for decades that women get the short end of the stick in terms of both weight gain and loss.

I’d also mention that this paper did nothing to determine the mechanisms behind NEAT (later studies have tried, and done poorly, at determining what is the actual cause of the increase in NEAT) only mentioning that NEAT seems to be a familial trait (suggesting a genetic basis).  Other later studies have shown that NEAT is essentially subconscious, people either do it or don’t.


My comments:

There’s really not a huge amount to say about this paper; it’s a point of interest without a lot of practical application.  I only bring it up to make the point that many people’s assumptions about what does or does not disprove The Energy Balance Equation tend to stem with their misunderstanding of things; especially their failure to realize that the out side of the equation is not static.  And this goes especially for the NEAT component of energy expenditure, with individual increases in NEAT varying massively from one person to another.

Of more relevance, not only is the out side of the equation not static, there appears to be quite a bit of variability involved.  While some people get effectively no (or a negative) increase in NEAT with overfeeding, which makes their gaining of fat quite easy, others have essentially won the genetic lottery: in response to overfeeding, they subconsciously ramp up small calorie burning activities that add up over the course of the day to burn off the excess.

To beat that dead horse, the equation isn’t wrong, the out side of the equation in terms of NEAT simply differs massively between people especially in terms of the NEAT response.  The people who can apparently ‘eat like gluttons’ and not gain weight appear to have a physiological mechanism by which they burn off the excess, essentially protecting them from fat gain.

In that vein, I’d mention at least one other study that compared the response to overfeeding and dieting in terms of metabolic rate adjustment.  It found that those individuals who showed the greatest increase in metabolic rate to overfeeding showed the least drop in response to dieting; by contrast those people who showed the least increase to overfeeding showed the biggest drop with dieting.

That study posited the existence of spendthrift (big increase with overfeeding/small decrease with dieting) and thrifty (small increase with overfeeding/big decrease with dieting) physiologies.  Clearly the first has as huge benefit in terms of both avoiding weight gain as well as losing it if necessary; the second group will have a much larger problem.

As I mentioned above, follow up work to this seminal paper has done little to determine the mechanisms behind it (which might lead to some way of increasing NEAT in those not disposed to it).  It appear to be genetic and more or less subconscious.  Of course, that doesn’t stop people from consciously trying to do things to increase their activity levels and energy expenditure outside of formal exercise.  All of the old behavioral strategies such as taking the stairs instead of the elevator, parking further away, etc. all end up adding up over time.

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