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An Introduction to Energy Expenditure and Metabolic Rate

An Introduction to Energy Expenditure and Metabolic Rate
Believe it or not, this actually started out as the introduction to this week's research review. By the time I realized how long it was, it made more sense to make it into a separate article so that the research review would actually focus on the paper itself. And, as I mentioned above, it got so long that the research review will have to wait

The topic of metabolic rate and energy expenditure comes up a lot when people discuss body recomposition. Whether it's setting calories to lose fat or gain muscle, knowing how many calories you're burning is a key aspect of knowing where to set calories for optimal results.

Over the course of a day, the number of calories you burn is generally referred to as total energy expenditure (TEE). In previous years, TEE was divided up into three components: RMR, TEF and TEA. In more recent times, a fourth component has been added which is called NEAT or SPA.

Resting metabolic rate (RMR)

RMR refers to the number of calories your body burns at rest, this typically makes up 60-70% of the day's total calorie burn. Essentially, if you laid in bed all day, you'd burn your RMR and not much more. RMR is affected by total weight, fat free mass (not only muscle mass) and hormones such as leptin, insulin, thyroid and the catecholamines. I want to make the point that fat free mass is not synonymous with muscle mass here. Although skeletal muscle might make up 35-40% of your total fat free mass, on a pound per pound basis, it actually burns few calories (the most recent estimate is around 6 cal/lb of muscle). In contrast, organs like liver, kidneys and your brain burn far more calories per pound, although they contribute far less weight than your muscle mass. There is also some inherent variability in RMR between individuals. At the same body weight and composition, RMR can vary by something like 15% each direction, or a few hundred calories total.

Thermic effect of food (TEF)

TEF refers to the calorie burn involved in digestion. While the different nutrients show different levels of TEF (fat is ~3%, carbs 6% and protein can range from 15-25%), the average across a typical diet is usually 10% of total calories eaten. So if you eat 2000 calories per day, you might burn 200 via TEF. There's not a massive amount of variability in TEF (and it takes pretty major shifts in nutrient intake to affect it drastically) although I'd note that insulin resistance is known to reduce TEF by about half. Admittedly this won't amount to a huge difference (200 cal TEF vs. 100 cal TEF on 2000 cal/day) but it doesn't help.

Thermic effect of activity (TEA)

TEA essentially refers to exercise although it rightly includes other daily activities. This generally shows the biggest variance. A sedentary individual may only burn 10% over resting via TEA (a few hundred calories). An elite athlete in heavy training may burn 100% over resting levels with activity or more (1000 or more extra calories per day depending on the training load). If you wanted to be super detailed, you could consider TEA in terms of calories burned during activity and calories burned after activity. But since, as per previous research reviews, the post-exercise calorie burn generally doesn't amount to much, I'm not sure this is terribly useful

Non-exercise activity thermogenesis (NEAT) and spontaneous physical activity (SPA)

As mentioned above, in recent years a fourth component of energy expenditure has been a topic of much interest in research. While the original studies referred to NEAT (non-exercise activity thermogenesis), the literature is now using the term SPA (spontaneous physical activity). You can think of NEAT/SPA as such things as fidgeting, changing from sitting to standing, general moving around. Basically it's activity that is not formal exercise (which is what distinguishes it from TEA).

SPA can vary massively, for example, one study found that SPA varied from 135-685 calories per day between individuals. NEAT has been studied specifically with regards to overfeeding, in response to a 1000 cal/day surplus, some people ramp up NEAT to high levels (700 or more calories per day wasted via activity), avoiding fat gain while others show no such effect. Unfortunately, SPA/NEAT appears to be genetic and biologically determined. Research shows that weight gain or loss doesn't really impact on SPA in a given individual; either you move around a lot or you don't.

In sum, your total daily energy expenditure can be expressed as


The sum of those four values will determine how many calories you burn in a given 24 hour period.

Changes in food intake = changes in TEE

Now, for years research has grappled with the impact of changes in food intake and body weight on each of these components. While I'm going to focus on dieting and weight loss below, just keep in mind that the same basic systems work during weight gain/overfeeding, just in the opposite direction. So while most systems decrease when you restrict calories, they tend to increase when you increase calories.

There is also some evidence that these systems work better in terms of limiting weight loss than they do at limiting weight gain. Except for a small proportion of people who do things like ramp up NEAT/SPA to massive levels and keep from getting fat.

In any case, I want to look at the impact of dieting/weight loss on each of the above components and how this can potentially impact on TEE.


First up is RMR which typically decreases when dieting/when weight is being lost. However, the drop in RMR can occur via two distinct mechanisms.

The first is simply the loss of body mass. A lighter body burns less calories per day both at rest and during exercise. The magnitude of this drop will depend, of course, on the magnitude of the weight loss. One of the biggest reductions in RMR measured was about 25% from baseline, it was found in males who were kept on 50% maintenance calories for 6 months and lost a large proportion of their body weight (they also ended up at 5% bodyfat).

Frankly, there's not a hell of a lot you can do about this, other than not lose weight. I did propose one idea in the Rapid Fat Loss Handbook to the effect of wearing a weighted vest to offset the change in body weight. While this wouldn't impact on true RMR (which is determined by the cellular metabolism of the body's tissues), it would impact on calorie burn during exercise and possibly SPA.

The second and more debatable component is the adaptive component; this refers to a reduction in RMR in excess of what you'd expect based on changes in bodyweight. So say that someone loses 10 pounds and you would predict metabolic rate to drop by 150 cal/day. If their RMR actually drops by 200 cal/day, that extra 50 cal/day is the adaptive component.

The problem is that studies are mixed as to whether or not the adaptive component actually occurs. It generally turns out to depend on such things as how fat the person is, whether you measure RMR while folks are actively dieting or are weight stable, and of course the big genetic effect.

In general, fatter individuals show less of an adaptive component than leaner individuals and this may explain some of the discrepancies in earlier research, studies which found no adaptive component were measuring fatter individuals while those that did were using leaner effects.

As well, early studies often measured RMR while subjects were still actively dieting and many, if not most studies, find that most of the adaptive component disappears when calories are brought back to maintenance. Given that a big part of the adaptive component is related to changes in hormones (e.g. leptin, insulin thyroid) while dieting, and many of those "correct" when calories are brought back to maintenance, it makes sense that most of the effect would go away.

There is also a huge genetic factor here that I've mentioned before; some people's metabolic rates crater harder and faster than others. As I've noted previously, studies also show that there is a relationship between how much metabolic rate goes up during overfeeding and how fast and hard it falls during dieting.

Individuals who show the largest increase in metabolic rate when they overfeed show the smallest decrease during dieting and vice versa; folks who show the smallest increase during overfeeding show the biggest decrease during overfeeding. Researchers have proposed that there are thrifty (gain easily/lose with difficulty) and spendthrift (gain with difficulty/lose easily) phenotypes when it comes to weight loss.

Unfortunately, nobody has really figured out what separates one from the other or how to fix it. It's probably due to levels of and sensitivity to the same hormones I keep going on about, leptin, insulin, thyroid, etc. At this point, it appears that you are what you are. If you are lucky, you gain weight/fat with difficulty and lose it easily. If you're like most people, it's exactly the opposite.

In any case, in general studies show that the adaptive component is not enormous, and mostly goes away when calories are brought back to maintenance (a small decrease is still usually present in the post-obese individual). To put the adaptive component in perspective, during the longest study of starvation known, the adaptive component only amounted to about a 15% drop (in contrast, the reduction due to weight loss was 25%). In the post-obese, this may only be 5% or less below normal. It's not as if the adaptive component is ever enough to offset a large enough deficit.

Again, I'd note the subjects I mentioned above, lean men subject to 50% semi-starvation for 6 months. That study (the seminal Minnesota Semi-starvation study) measured the largest drop in BMR to date, a total of 40% from starting (25% from the loss of body mass with an additional 15% from the adaptive component). Yet, with a 50% daily deficit, fat loss didn't really stop until the men were as lean as they were going to get (at 5% bodyfat)


Not much to say here. When you eat less, TEF will go down, since it's related to how much you're eating. Not a whole lot you can do about this although raising activity while keeping food intake higher would, presumably keep TEF higher. Just keep in mind that the effect isn't massive. With TEF at 10% of total intake, keeping calories 500 cal/day higher only amounts to 50 calories via TEF. And means nearly an hour of hard exercise to compensate and keep the calorie burn up.


Whether or not TEA changes with dieting depends a lot on whether or not the person keeps up their activity levels, of course this can be difficult when energy level is down due to calorie reduction. As above, a lighter body does burn fewer calories during activity and there is some indication that muscular efficiency may change during dieting to reduce caloric expenditure.

Volume or intensity might have to increase to offset this. It's no accident that bodybuilders discovered years ago that their training volume had to go up as they got leaner (they generally increased the duration or frequency of their cardio, sessions); only by increasing activity could the drops in energy expenditure from other components be offset.

In recent years, folks are trying to keep increasing intensity at this time. Unfortunately, in the face of reduced calories, overtraining becomes that much more likely. Trying to do intervals multiple times per week along with heavy resistance training is a good way to blow yourself up completely.


Finally is the NEAT/SPA issue which, being fairly new, is far less well studied. Some studies have found a decrease in SPA (consider how lethargic you often get when you're dieting for weeks on end), others have not. The paper I was going to look at but ran out of room actually addressed this specifically, how calorie restriction affected SPA.

In any case, it's clear that calorie restriction/weight loss affects all aspects of the energy balance equation. This has ramifications for both the rate and predicted amount of weight or fat loss as well as for maintenance of that loss when the diet is over. I want to look at both.

Weight loss

Many people set a caloric intake based on either an estimated or measured total energy expenditure while they are at calorie balance. Then they work the math on what the deficit should product in terms of fat loss and go to town.

But between drops in RMR (from weight loss and the adaptive component), the drop in TEA, changes in efficiency during activity and whatever may happen with SPA, the deficit that is expected and what is actually created may be totally different. I'd note that there are other potential problems here as well, usually related to mis-measurement or mis-estimation (or simply poor adherence) of the day's food intake. I'll have to get Leigh "FatLoss Troubleshoot" Peele in here to talk about that some more

As an example, consider someone with a maintenance intake of 3000 cal/day when they are weight stable and eating at maintenance. Now they decide to cut calories to 2500/day, which should yield a nice 1 lb/week fat loss.

First off, TEF drops by 50 calories/day. If they are genetically unlucky, RMR could drop quite a bit in the first couple of weeks. Their exercise efficiency might go up (meaning less calories burned during activity) and they might find themselves moving around less during the day due to a decrease in SPA. Suddenly a nice 500 cal/day deficit might be cut in half, the expected fat or weight loss will not occur. And that's even before things like water retention are factored in

I'd note that some goofballs use this response to throw out the energy balance equation completely, they argue that the calorie balance numbers don't work out because the real weight loss isn't what's predicted; hence thermodynamics fails.

But this isn't the case; rather, the expenditure part of the equation is changing (usually decreasing) when calories are reduced and weight is being lost. The equation isn't wrong, it simply isn't static. The value that was your maintenance level when you weren't losing weight isn't necessarily the same as when you actually reduce calories and start actively dieting.

Tangentially, I'd note that many people have found that maintenance calorie levels are far more of a range than a value, and this is assuredly related to some of the topics I've discussed above. If someone has a particularly large or NEAT/SPA response in terms of overfeeding, they might find that they are weight stable at a fairly large range of caloric intakes; as their intake goes up, so does TEE due to changes in NEAT/SPA (ok, enough, too many TLAs)

Of real world interest, a number of people on the forum have been playing around with the Bodybugg (a device worn on the arm that estimates total energy expenditure based on movement, temperature, galvanic response, and I suspect a bit of voodoo) and some odd observations are coming out of it.

Many, for whom the Bodybugg shows fairly high maintenance intake values simply aren't seeing the expected fat loss on what should be a nice moderate deficit. In many of those cases, larger deficits work much more effectively. In general, these are women who, for the most part, have screwy physiologies (and a tenacious bodyweight defense system) in the first place.

For example, one member had a predicted maintenance intake of ~2700 calories per day as measured by the Bodybugg. On 1600 calories per day, over a 2 month span, the net weight/fat loss was approximately two pounds. Yet on the very low calorie intake of the Rapid Fat Loss Handbook program, she lost nearly 4 pounds of fat in about 10 days. Clearly something strange is going on.

But even more clearly, regardless of the adaptations to RMR, TEA, TEF and SPA during dieting, sufficient large deficits can overcome them. With moderate deficits, the changes in those values may cut the deficit to almost nil. With a larger deficit (and a much shorter diet, mind you; I'm not recommending massive deficits for prolonged periods), this seems to be able to overcome those adaptations.

Now, I suspect that part of this has to do with some strange goings on hormonally (which I'm currently delving into as one of the never ending projects that consume my days), but some of it simply reflects the above issues. When calories are dropped, a number of different systems adapt and what was previous maintenance is now lower, what should have originally been a decent deficit no longer is such. Unless the deficit is increased beyond a certain point where the adaptations are simply overwhelmed by the size of it.

Weight maintenance

Finally, the above clearly has some implications for maintaining weight loss after the diet is over (yes, diets do eventually end). Coming out of a diet, metabolism is going to be depressed, how much will depend on the same factors I mentioned above.

Practically speaking, you can use morning waking temperature to track this roughly. A waking temperature of 97.8-98.2 is indicative of a healthy 'normal' metabolism (i.e. 100% of the predicted value). Every degree drop from that is a roughly 10% reduction from normal.

Some of the drop is unavoidable as mentioned; the loss of body mass will lower RMR and short of the vest idea, there's not much you can about it. The adaptive component of metabolic rate drop will go away as calories are normalized and hormones also normalize but there may still be some small reduction in overall RMR even at weight maintenance. TEF should increase as food intake comes up, of course more and/or higher intensity activity can be done when calories are increased. I'm not aware of anything looking systematically at SPA after dieting.

In any case, to avoid excessive rebound fat gain following a diet, raising calories somewhat gradually and using a slightly reduced estimate for maintenance is not a bad idea. This can always be adjusted based on body composition changes. So if you'd normally use 15 cal/lb for maintenance, you might choose to use 14 cal/lb initially at the new bodyweight to take into account the reductions in metabolic rate.

This does mean, mind you, that maintaining the new lower body weight or body fat mandates that caloric intake be controlled (or activity increased to compensate for the reductions in metabolic rate). Quite in fact, studies have routinely found that exercise is more effective at preventing weight regain after a diet more than impacting weight loss on the diet.

By Lyle McDonald

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