Pretty much every single person on this forum should read through this entire article. It's lengthy, but if everyone read it, there'd be a lot less of the repetitive questions you see around here. I understand that some of the article might be tough to understand to those who aren't used to reading this sort of thing, so if you have questions, speak up!
I feel like I'm spamming the board today, but I'm coming across a lot of good articles that have been sitting in my inbox that I want to share. This is another from the consistent Lyle McDonald.
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Feature Article:
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
TEE = RMR + TEF + TEA + SPA
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.
