How much does muscle matter for energy expenditure and fat loss

photo of man with muscular body

Since muscle mass is energy-intensive, building more mass should help you lose fat right?

Muscle fibers are divided between low and fast-twitch muscle fibers. Slow-twitch muscle fibers have a lot more mitochondria than fast-twitch, meaning they burn more fat at rest.

If you list weight, you build both slow and fast-twitch muscle fibers, which should speed up your metabolic rate.

As a side note, there are different kinds of fast-twitch fibers as well, some of which are also relatively good at burning fat, but not to the extent of slow-twitch.

So how much does muscle mass matter for fat loss?

You can watch or listen to this article here or read the whole thing below.

You can listen to it here.

Elia in 1992, investigated the resting metabolic rate of individual organs and tissues and found that specific organs, such as the liver, brain, heart and kidney had the highest metabolic rate, whereas the skeletal muscle actually had a relatively low metabolic rate.

Ki (resting metabolic rate) values for major organs and tissues in the Reference Man (Snyder et al., 1975), including liver (200), brain (240), heart (440), kidneys (440), skeletal muscle (13), adipose tissue (4.5) and residual mass (12, all units are in kcal/kg per day)” (R)

Here’s the layout of the energy expenditure of each organ per day.

  • Liver (average weight: 1.5kg) = 300 cal per day
  • Brain (average weight: 1.5kg) = 360 cal per day
  • Heart (average weight: 0.3kg) = 132 cal per day
  • Kidneys (average weight: 0.255kg) = 112 cal per day
  • Skeletal muscle
    • Average: 33kg = 429 cal
    • Built: 43kg = 559 cal
    • Difference = 130 cal per day
So, clearly, building loads of muscle will not speed up your metabolic rate to the extent that you'll get sliced. What actually matters is your physical activity energy expenditure > thyroid, androgens, stress hormones > muscle mass.

Physical energy expenditure

This includes all the activities you do in the wakeful state. This is coined, NEAT; non-exercise activity thermogenesis.

Accordingly, NEAT corresponds to all the energy expended with occupation, leisure time activity, sitting, standing, ambulation, toe-tapping, shoveling snow, playing the guitar, dancing, singing, washing, and more (15, 16). Some authors categorize NEAT into three main subcomponents comprising body posture, ambulation, and all other spontaneous movements, including “fidgeting” (38).” (R)

The reason why NEAT is so important is because this is the only variable that can be significantly altered.

The importance of NEAT becomes apparent when considering the following: the variability in BMR between individuals of similar age, BMI and of equal gender ranges around 7-9% (39), while the contribution of TEF (thermic effect of food) is maximally 15%. Thus, BMR and TEF are relatively fixed in amount and account for approximately three quarters of daily TEE variance. As EAT is believed to be negligible on a population level, NEAT consequently represents the most variable component of TEE within and across subjects. It is responsible for 6-10% of TEE in individuals with a mainly sedentary lifestyle and for 50% or more in highly active subjects.” (R)

In other words, NEAT can vary by about 2000 calories between different individuals. You can literally boost your energy expenditure by a lot just by moving more. I would add here that spontaneous movement is what’s important here, not forced movement because you think it’s good for you.

For an average worker spending most of her/his time in a seated position, occupational NEAT is relatively low and estimated energy costs range up to a maximum of 700 kcal day-1 (Figure 3). A comparable person working mainly in a standing position can increase her/his occupational NEAT to up to 1400 kcal day-1, while an agricultural occupation would theoretically result in NEAT levels ranging around 2000 kcal day-1 and more (Figure 3) (36). Thus, occupations relying on intense physical activity can expend 1500 kcal day-1 more than a sedentary job.” (R)

  • Seated work – NEAT up to 700 kcal per day – TDEE roughly 2700 calories

  • Standing work – NEAT up to 1400 kcal per day – TDEE roughly 3400 calories

  • Hard manual labor – NEAT up to (and perhaps higher than) 2000 kcal per day – TDEE roughly 4000 calories

TDEE – total daily expenditure, given that someone has a BMR of 2000 calories.

Changes in NEAT by over- or underfeeding

Eating more can increase NEAT and undereating, such as when you’re in a caloric deficit, NEAT usually goes down.

Interestingly, in the majority of studies, alterations in adaptive thermogenesis in response to overfeeding are in large part explained by raised non-resting energy expenditure (i.e., NEAT), although the ability to adapt NEAT to manipulations of energy balance is highly variable between subjects.” (R)

A substantial percentage of this variability is explained by differences in spontaneous physical activity including “fidgeting,” as the latter accounted for 100-800 kcal day-1 in subjects.” (R)

As you can see, not everyone gets a boost in NEAT from overeating. A lot of people like to claim that you should overeat to “speed up the metabolic rate”, whatever that means. What they’re referring to (even if they’re unaware of it) is NEAT. Overfeeding can increase energy expenditure by increasing NEAT. But that is not the case for everyone.

Check out the following findings:

Using sophisticated methods and measuring NEAT over a representative time span, the authors overfed 16 volunteers (12 males, 4 females; age ranging from 25-36 years) by 1000 kcal day-1 in excess of their weight maintenance requirements (20% of the calories came from protein, 40% from fat, 40% from carbohydrates). The energy surplus was paralleled by a mean TEE increment of 554 kcal day-1. In this study 14% of this TEE increase was attributable to a rise in REE, approximating by 79 kcal day-1. A further 25% of the TEE increment (136 kcal day-1) corresponded to an increase in TEF, probably due to the relatively high percentage of protein intake. The most prominent effect was, however, attributable to enhanced physical activity thermogenesis, corresponding to around 336 kcal day-1. the change in NEAT varied remarkably between subjects, ranging from -98 to +692 kcal day-1.” (R)

The above protocol was followed for 8 weeks, which should give your metabolism plenty of time to speed up. Yet, resting energy expenditure increased by only 79 calories per day. The biggest change came from the increase in NEAT, which accounted for an average of 336 calories.

Some of the individuals actually experienced a drop in their total energy expenditure, whereas others experienced almost a 700 calorie increase. Point being, not everyone is the same.

If you can’t turn the calories into energy effective, then you won’t have the energy for more spontaneous activities.

Calorie deficits and long term adaptions to NEAT

The following study found that NEAT remained depressed even after 6 months after a 2-year diet. Dieting/undereating for too long will sap your energy and your NEAT will stay low. The body goes into conservative mode and won’t upregulate processes that will expend too many calories.

For instance, Leibel et al. in a second arm of their “weight clamping” experiment evaluated the effect of a 10% body mass reduction, followed by weight maintenance at this level (56). While overfeeding was accompanied by a 16% increase of adjusted TEE, weight reduction from underfeeding induced a 15% decrement (57). As compared to overfeeding, it is well accepted that both REE and non-resting energy expenditure are affected by energy deprivation, which is supported by recent data from animal models (55, 58, 63). These findings are supported by results from the so-called Biosphere 2 experiment, where the authors demonstrated a major reduction of TEE and spontaneous physical activity with body mass reduction over a two-year time period, which persisted after six months of body weight regain.” (R)

Thyroid function, metabolic rate and NEAT

There is only a relatively small difference between the metabolic rate of someone that is hypothyroid vs euthyroid.

Someone that is severely hypothyroid, with a TSH of around 10, has a metabolic rate about 15% slower than someone that is euthyroid (R). That’s about a 300 calorie difference. It will definitely make a difference in the long run, but it’s not like your BMR goes from 1500 to 3500 or something like that.

In a hyperthyroid state (e.g. someone with Graves disease), resting energy expenditure is 140% over baseline, so that would mean a BMR of about 2800 calories vs 2000. That’s a much bigger difference.

Kim et al. [40] studied six Korean women with Graves disease during the course of a year after diagnosis and measured EE by indirect calorimetry at baseline, then at 4, 8, 12, 26, and 52 weeks after treatment with methimazole. At baseline, REE was 140% of predicted REE, which reduced to 113% at week 52. Free T3, total T3, and free T4 were significantly correlated with REE as well as peripheral deiodinase activity [40]. In another study, metabolic changes in REE were measured (by indirect calorimetry) during the treatment course of hyperthyroidism in 21 Chinese women. EE was found to be significantly reduced (28.7 ± 4.0 kcal/kg to 21.5 ± 4.1 kcal/kg; P < 0.001), and respiratory quotient was increased (0.76 to 0.81; P = 0.037), indicating a shift in substrate utilization from fat to carbohydrate oxidation.” (R)

One of the main reasons why hyperthyroid individuals are leaner is because they move more. They have more energy. Thyroid hormone T3 enhances the oxidation of glucose and fat, creating more ATP. More energy allows people to be more spontaneously active.

In conclusion, hyperthyroidism is associated with increased spontaneous physical activity and NEAT.” (R)

Other factors that influence BMR and NEAT

Additional factors that influence the metabolic rate and NEAT include dopamine, histamine, prolactin, serotonin, cortisol, testosterone and DHT.

Giving men testosterone and DHT promotes fat loss (R, R, R).

Someone with high dopamine and histamine is more likely to be active than someone with low dopamine and or histamine. Testosterone and more specifically DHT, promotes dopamine release, which promotes physical activity.

Prolactin and serotonin (gut serotonin or the use of SSRIs) are known to cause weight gain in humans (R, R)

Mike Fave and I discuss these factors in our podcast, Calories in vs calories out? Nope – what really matters for fat loss.

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