For literally decades it was stated that daily energy expenditure/metabolic rate was made of up three components: Resting Metabolic Rate (RMR), the Thermic Effect of Activity (TEA) and the Thermic Effect of Food (TEF). It had long been known humans showed drastically different responses to overfeeding in terms of weight and fat gain. So you might imagine how much of a shock it was when, in 1999, a discovery was made that not only identified a fourth component to metabolic rate but also explained the huge variance in weight gain. That component would come to be called Non-Exercise Activity Thermogenesis or NEAT. Today I want to examine the paper that did both.
Variation in Weight Gain with Overfeeding
As I stated above, it’s long been known that two individuals may gain staggeringly different amounts of weight and fat when they overeat. We all seem to know someone who can “eat anything they want” without gaining fat while others can do no such things. Incorrectly, this observation is even used to try to deny the importance of energy balance to describe the changes in human body composition.
After it came out, the specific paper I’m going to talk about today was used as further “proof” that energy balance doesn’t hold. But as so often is the case, all that was demonstrated was that they don’t actually understand what the equation is or what it means. Simply, the energy balance equation states that
Energy In = Energy Out + Change in Body’s Energy stores
Energy in is from food and I’d note that this is corrected for digestive efficiency. Food or calories lost in the poop don’t count. Energy out represents the day’s energy expenditure. As I said above, for decades this was conceptualized as having three components: RMR, TEF and TEA.
The difference between those two is the change in the body’s energy stores. I’ve explained this in detail elsewhere but you can roughly take it to mean change in bodyweight or body fat. It’s not that simple but the details aren’t important here.
So their logic goes, if energy intake goes up identically for two people, the change in weight should be the same. And this would be essentially true if the energy out part of the equation didn’t change.
Note: technically it would not be true due to other issues I don’t want to get into here.
And that’s where they went wrong: assuming that the energy out part of the equation didn’t change. Or that it didn’t change differently for the two individuals.
Because as above, some people are demonstrably more resistant to weight and fat gain than others. Researcher usually say those people have a spendthrift metabolism. This is in contrast to the thrifty metabolism that makes weight gain easy.
Now one aspect of this is that some people have appetite regulation systems that seem to turn on very rapidly when they overeat. You may see such individuals eating a lot at one sitting but what you don’t see is that they eat very little the rest of the day or for the day or two afterwards. Even if those people say that they eat a lot, invariably it’s simply not true.
But even when you force overfeed those people, as happens in many studies, you still don’t see the expected weight or fat gains. Something else must be going on.
And that something is that the energy out side of the energy balance equation is adapting to the increased calorie intake. It’s been known for years that RMR can go up a little bit with increased food intake. So can TEF. TEA is less sensitive to this. But even those changes can’t really explain what’s going on.
Which brings me to the paper I want to look at today which wanted to examine mechanistically why some people gain more weight and fat than others when they are overfed.
Weight Gain and Overfeeding
Towards this goal, the study recruited 16 subjects, 4 females and we males. First the subjects underwent body composition measurement via DEXA along with having their total daily energy expenditure (TDEE) measured with doubly labelled water. I’m not going to try to detail this but, essentially, subjects are given radioactive water and its metabolism is used to measure energy expenditure.
Then they were overfed by 1000 calories over maintenance for 8 straight weeks. Sign me up.
The researchers also measured Resting Metabolic Rate (RMR) and the Thermic Effect of Food (TEF) via indirect calorimetry. This was done to see if changes in those components of energy expenditure could explain any results. To keep the study more controlled, subjects maintained their exercise type activity at a low level. This was to prevent anyone from just exercising more to offset the calorie surplus.
Now we might question how relevant this is to the real world. However, this had to be done to minimize any major confounds or variables.
Over the 8 weeks of the study, an average of 432 calories/day was stored while 531 were dissipated through increased energy expenditure. This accounted for 97% of the total calorie surplus.
Note: this alone shows that energy balance held as the entirety of the 1000 calorie per day surplus could be accounted for. 531 of the extra calories were burned off and 432 were stored. The 3% unaccounted for is assuredly just slop in the measurement.
However, looking at the average results obscures the individual results along with what was really happening.
Because looking at individual results, the net fat gain varied from a low of 0.79 lbs (0.36 kg) to 9.3 lbs (4.23 kb), a 10-fold variance. That is, despite the fact that all subjects received the same 1000 calorie per day surplus, their actual fat gains ranged from less than one to nearly 10 lbs.
But this doesn’t mean that energy balance doesn’t hold. Bear with me.
Now, changed in RMR and TEF could not explain the difference. RMR only went up by 5% while TEF went up by about 14%. Neither of those changes showed any relationship with the variable changes in body fat. I already mentioned that exercise activity was kept very low so differences there can’t explain the difference either.
What’s going on?
The Impact of NEAT on Energy Balance
Well that brings us to the idea of Non-Exercise Activity Thermogenesis or NEAT.
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.
Essentially you can think of NEAT as any calories that are burned in activities that aren’t formal exercise. So it is separate from the Thermic Effect of Activity (TEA) which, recall, was kept very low.
Because what the researchers found was that the average increase in NEAt was 336 calorie per day. But once again the average obscures the individual differences. When those were examined NEAT varied from -98 cal/day (that is NEAT actually went down) to +692 calories/day.
So in at least one subject, NEAT increased enough to burn off nearly 700 calories of the 1000 calorie surplus. And this was in addition to the small changes in RMR and TEF which burned off even more. The researchers calculated that the increase in NEAT in this individual would be the equivalent of strolling for 15 minutes per hour during their waking hours.
Incredibly this occurred without the person consciously trying to increase energy expenditure. It was simply an unconscious response to the increased calorie intake. Because it wasn’t due to the person actively walking around. Rather NEAT increased through a variety of fairly unconscious movements such as moving from sitting to standing, fidgeting and others.
If you think back to high school, that one kid who always stayed super skinny could never stand still. They were always tapping their fingers or bouncing their legs. This is NEAT and it turns out that it can burn a tremendous number of calories when it is maintained all day every day.
Of far more importance, the changes in NEAT directly predict fat gain or the lack thereof. That is, the people who showed the largest increase in NEAT had the smallest fat gain. And the people who showed the smallest increase in NEAT showed the most. I’d note in this regard that the four worst responders were the 4 female subjects. Given the physiological issues I discussed in The Women’s Book Vol 1, this is no real surprise.
I’d mention that beyond identifying and quantifying NEAT, the study didn’t attempt to determine mechanistically what caused it. Later studies have attempted to do such without much success. It has been shown that the propensity to increase NEAT seems to have a genetic component. The subconscious component of NEAT either increases or it doesn’t and there’s not much you can do about it.
Addendum: While NEAT was originally conceptualized as representing only unconscious activities such as fidgeting, it would later come to incorporate all daily activities that weren’t formal exercise. As well, research long after this paper would establish environmental and physiological factors underlying NEAT.
But ultimately, this explained what was going on: individual variability in the increase in NEAT was helping to offset any calorie balance. Simply, the energy out side of the energy balance equation went up in response to overfeeding.
The people using this paper to deny the existence of energy balance are making the mistaken assumption that energy out can’t change and that’s wrong. Importantly, there was a huge individual variance in the increase which explains why two people on the same surplus can gain vastly different amounts of body fat.
Energy balance always holds and every calorie can always be accounted for. It’s simply that some people store excess calories as fat and others burn them off through NEAT. But it always holds. It has to.
I don’t have much to add to what I wrote above and there’s not much practical that can be gleaned from this. Mainly it’s a point of interest that goes a long way towards explaining individuals differences in weight/fat gain in response to a calorie surplus.
It goes even further in explaining why people’s incorrect assumptions about how energy balance are incorrect. The mistake they chronically make, among others, is assuming that energy out is unchanging. This is wrong. As importantly is the fact that individual changes in the energy out side of the equation appear to be enormously variable. With most of the variability being in the component now called NEAT.
If two people are overfed the identical surplus and their changes in NEAT are 10 fold different, with one burning off the excess, clearly the resultant weight and fat gain will differ. Energy balance will always hold and the only thing that is incorrect is people’s understanding of the system.
And basically those people who show a big increase in NEAT have won the lottery, at least in terms of avoiding fat gain in the modern environment. Mind you, if they actively want to gain weight, this increase can be a hindrance because the body will ramp up their unconscious activity levels and burn the calories off as heat.
To that I’d add that, given the likely mechanisms driving NEAT, there is probably a link between increases in NEAT and the decrease in appetite that is so often seen in these folks. Even when they try to overeat, invariably their appetite just shuts off. So they get a double benefit in terms of avoiding fat gain.
In a related vein, studies have examined if the increase in NEAT to overfeeding is linked to the known decrease in energy expenditure that occurs during dieting. And there is one. Specifically the same individuals who show the greatest increase in energy expenditure with overfeeding show the SMALLEST decrease in energy expenditure with dieting or fasting. And it works the other way: the people who show the smallest increase with overeating show the largest decrease during dieting.
This led to the concept of spendthrift and thrifty metabolisms I mentioned above. The spendthrift metabolism resists weight and fat agin with large increases in energy expenditure to overfeeding but loses fat and weight easily due to a smaller decrease in metabolic rate with dieting. The thrifty metabolism is the opposite.
Clearly that first individual will find it much easier to stay lean/thin in the modern environment compared to the second. Even if the first does gain weight, they will have no problem losing it if they wish. The thrifty metabolism individual has gotten the short end of the stick in both directions. They will gain weight and fat with more ease and find it more difficult to lose. Research has continued to try to determine what physiology makes up the “Spendthrift metabolism” with the idea of finding a way to induce it for thrifty individuals. Little to nothing has come of it so far.
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. But as I also stated, NEAT now encompasses conscious activities that people can do. Even small things like taking stairs instead of the escalator or elevator, parking further away or light activities like gardening can burn sufficient calories throughout the day and add up over time.
And even if you didn’t get anything practical out of this article, perhaps you learned something NEAT.