Everyone’s favorite scapegoat, the fast – or slow – metabolism is always to blame. It’s either too slow, making fitness a difficult goal, or someone else’s is fast, making their achievements seem effortless. Either way, it’s a convenient but misapplied excuse.
When talking about a fast metabolism, most people conjure up an image of someone who eats whatever, whenever, exercises, and still maintains an ideal physique.
As always, it’s not so simple. Specifically, the way your metabolism functions isn’t in a straightforward slow or fast mode. Hence, there’s no magical “fast” switch that’s permanently enabled for those that do little to stay fit.
So who or what can we blame!? Not ourselves, that’s clearly out of the question.
Instead, let’s take a quick look at some research and figure out if there is such a thing as a fast metabolism, and what biological factors – if any – set you apart from those to whom ideal fitness comes easy.
Fast or Slow?
Before blaming fitness goal failures on your metabolism, it’s worth looking at how your metabolism works.
Simply put, the metabolism is the chemical processes that occurs within a living organism in order to maintain life. In plain language, it is the accumulation of all energy expended by your body, also known as Total Daily Energy Expenditure (TDEE).
In turn, your total daily energy expenditure consists of three components; your Resting Metabolic Rate (RMR); the Thermic Effect of Food (TEE); and your Active Energy Expenditure (AEE). Hence, your total daily energy – measured in calories – output is the sum of all three.
So whether you consider your metabolism fast or slow, you’re really just saying that these processes burn too many or too few calories.
Resting Metabolic Rate
Your RMR – also known as the Basal Metabolic Rate – accounts for most of your metabolic activity. You can think of it as your metabolism’s foundation. In laymen terms, it can be explained as the calories your body burns to sustain basic biochemical functions when the body is at rest.
So if you spend all day in bed and expend no energy, the calories that you will burn performing internal functions is your RMR. How important is the RMR in respect to your metabolism?
In 2016, the Pennington Biomedical Research Center conducted a seminal study1, which encompassed a comprehensive review of our understanding of TDEE.
The research suggested that your RMR is responsible for almost 70% of your total daily calorie expenditure!2 Further, the research suggested that “fat-free mass is by far the strongest determinant of RMR and accounts for ∼70% of its variance.”3
Hence, your RMR is the biggest factor when determining how your metabolism functions.
Moreover, due to the RMR’s major role in your metabolic function, it’s no surprise that any fluctuations in fat-free mass can play a vital role.
Further, the idea of a slow or fast metabolism may have originated because of the RMR’s variance Specifically, a 2004 University of Vermont study4 found that 96% of people stay within 16% of their RMR, which translates to roughly 200-300 calories.
Hence, most people (68%) stay within just 8% of the average RMR, and the vast majority (96%) stay within 16%.
In other words, 96% of people stay within 200–300 calories of the average RMR. Subsequently, some people have a slightly higher functioning RMR and therefore a faster metabolism. However, that advantage translates to an inconsequential amount.
Go High Go Low
As mentioned above, on average your RMR accounts for about 70% of your total daily energy expenditure. Commanding such a significant portion, the different options at your disposal are worth consideration if you want to speed up your RMR.
Of all the resting energy spent, roughly two-thirds goes toward maintaining a steady body temperature.
In fact, a 2012 Northwestern University study5 found that a core temperature increase of one degree Celsius translates to roughly 13% greater energy expenditure at rest. So if you’re looking to increase your RMR, consider high-intensity exercises.
The second component, the thermic effect, is the energy your body spends on absorbing, digesting, and storing food. According to a Pennington Biomedical Research Center study6, thermic effect accounts for up to 10% of your TDEE.
The three macronutrients – carbohydrates, protein, and fat – have a different thermic effect and therefore vary in how much energy the body spends breaking them down. For example, it takes 5-10% of your thermic rate to break down carbohydrates, 0-3% to break down fats, and 20-30% to break down protein.
Since protein requires the most energy of the three macronutrients, a higher intake – especially on a calorie deficit – is attributed to preventing muscular atrophy and promoting fat loss.
By the way, the popular concept of “negative calorie foods” is by definition not possible.
Purportedly, negative calorie foods are so low in calories that your body spends more calories digesting them than they’re worth.
This isn’t possible for one simple reason. As displayed above, the absolute highest thermic effect taps out at 30%. As a result, there is no food out there that requires more than 30% of its total caloric content to be digested7.
Active Energy Expenditure
Last but not least, the third component is your active energy expenditure. Your AEE consists of two parts; energy expended from physical activity, and energy expended from what we’ll call incidental physical activity.
The latter, otherwise known as non-exercise activity thermogenesis (NEAT) – are those activities that happen in the regular course of life. They may include the tasks required by your occupation, or any other motion your body completes as the day goes on.
As a result, AEE is a highly variable component and also one that we know least about. Energy spent on intentional physical exercise, as well as NEAT, can range widely, and be difficult to ascertain.
In fact, two separate studies have reflected the difficulty.8 9 Specifically, the findings – conducted in respiratory chambers and in free-living environments – suggest that NEAT accounts for 4–17% of the TDEE, or about 100–700 kcal/day.
Such a wide range isn’t surprising when considering all the relevant factors. For example; age, weight, sex, activity type and intensity, are a few to consider when calculating energy expenditure. Consequently, AEE is highly variable and difficult to factor into your fitness regimen.
When considering the research, it’s clear that your metabolism’s speed isn’t to blame for any fitness goal failures. In fact, it’s role should be an encouraging sign to anyone striving toward greater physique and health.
As the research suggests, there are ways to galvanize the process. Eat foods with greater thermic effect, do exercises with greater intensity. Don’t let your metabolism slow you down!