RMR
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).
TEF
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.
TEA
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.
SPA
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.
