Question: I am a little confused when it comes to metabolic slowdown. The reason for my confusion is that as far as I can figure, if my LBM remains approximately the same throughout the diet, then my energy expenditure should also remain basically the same. Granted, maintaining LBM is difficult but for arguments sake let’s assume that LBM is maintained within a +/- 5% range. So for an individual with 150lbs of LBM that amounts to 7.5lbs. My assertion(correct or not) is that metabolic slowdown cannot occur beyond what that 7.5lbs of LBM used in the first place?
Is this a faulty assumption? I’ve read on many a website that the body goes into “starvation mode”, however that argument doesn’t sit well with me. Either the body requires X amount of energy to function, or it doesn’t. I think “starvation mode” might simply be reduced activity in general, so for a relatively insane individual (read:athlete) who is willing to push hard on a restrictive diet, metabolic slowdown shouldn’t be an issue?
Answer: I suspect that some of this comes down to an issue of semantics (you sort of get to part of what I’m going to talk about in your second paragraph) but some of it doesn’t. The short answer to your question is that your assumption isn’t entirely correct; even with 100% maintenance of lean body mass (LBM) there can still be some metabolic slowdown. Now here’s the longer answer.
First and foremost, we need to define some terms and what’s meant by metabolic rate since I suspect that’s part of where some of the confusion is coming from. On a daily basis, an individual’s total daily energy expenditure is given by three components, which I’ve discussed in detail in Metabolic Rate Overview. They are
- Resting/Basal Metabolic Rate (RMR/BMR; what I suspect you’re referring to above)
- Thermic Effect of Food (TEF)
- Thermic Effect of Activity (TEA)
Where TEA has now been divided into two distinct components: the thermic effect of exercise and non-exercise activity thermogenesis (NEAT). The distinction being that the first is calories burned during formal exercise and the second, NEAT, is the calories burned during activities such as daily moving around, fidgeting, moving from sitting to standing, etc. I discussed the potentially major impact of NEAT in a recent research review on Role of Nonexercise Activity Thermogenesis in Resistance to Fat Gain in Humans.
Now, each of the above is determined by various factors including body composition, diet, etc. And all of them are affected by dieting and the loss of body mass. Studies have repeatedly shown that individuals who have been dieted down to a given weight will have a lower than predicted metabolic rate compared to someone who didn’t diet to that weight. That is, someone who ‘naturally’ weighs 200 pounds will have a higher total energy expenditure than someone who dieted down to 200 pounds.
So what’s causing this reduction in total energy expenditure. A majority of the ‘metabolic slowdown’ that occurs is due simply to the loss of body mass. Because larger bodies burn more calories (both at rest and during activities) and smaller bodies burn less.
But that’s not the only cause of metabolic slowdown here. There is also an adaptive component of metabolic rate slowdown that is mediated by changes in hormones: leptin, insulin, thyroid, catecholamines. As these change (decrease) on a diet, you find that tissues burn fewer calories per unit mass. I’d mention that not all studies find this, about half do and half don’t. That is, your assumption that a given body composition always burns the identical number of calories on a day to day basis isn’t entirely correct.
Of course, an important question is how much of a change this amounts to. During active weight loss, the impact is relatively greater (because hormones tend to be more greatly affected); at weight maintenance (once a person has stabilized), the impact isn’t huge. In some studies of the post-obese (folks who have been dieted down and maintained at that weight) show a relatively modest 5% or so reduction in RMR. The effect exists but is not massive; it’s also highly variable, with people showing relatively more or less of an effect.
There is also evidence that individuals move around less when they lose/are losing weight. As James Krieger recently wrote on his Weightology.net website, it looks like changes in activity (especially NEAT) are the far larger contribution to the reduction in overall energy expenditure on a day to day basis; the number of calories burned in that activity also appear to be reduced due to improved muscular efficiency.
In that study, decreases in RMR were about 150 calories per day but reductions in activity expenditure were up in the 300 calorie plus range with the total effect being over 400 calories. This is likely why daily activity has such a profound impact on weight maintenance as I discussed in Exercise and Weight/Fat Loss Part 2: since the body is ‘automatically’ decreasing activity energy expenditure, you have to make up for it.
So basically you’re both correct and incorrect. The greatest impact on total daily energy expenditure certainly appears to be due to decreased spontaneous activity during the day. However, there is also an added component of a reduction in resting energy expenditure due to changes in RMR, even with complete maintenance of lean body mass. Some of this is due to simply being smaller, some of it is an adaptive reduction in metabolic rate due to shifting hormone levels (which, again, not all studies find).
And semi-tangentially, a long while back I had written an article as a background primer to something I had intended to write about alcohol. Well, now I don’t have to since Martin Berkhan over at Leangains.com has written it. In his article The Truth about Alcohol, Fat Loss and Muscle Gain he pretty much covers everything you could ever want to know about the topic.