IS THERE REALLY SUCH A THING AS A “FAST” OR “SLOW” METABOLISM?
In today’s world you have, without a doubt, heard people say that they are a “hardgainer” if they can eat anything and everything without putting on a pound. Inversely, they will lose weight with the tiniest reduction in caloric intake. These people are often referred to as having “fast” metabolisms.
You've also likely heard people with a “slow” metabolism say that they’ll gain weight if they simply look at a carb, and that they only have success with weight loss if they drasticallyreduce their caloric intake.
The question: Is this all in our heads OR is there actually something to these claims?
Short Answer: “yeah, there does, in fact, seem to be a noticeable difference between certain types of individuals”
More Applicable Answer: What ACTUALLY stands out and separates people who have been known to have a “fast” or “slow” metabolism is the relative CHANGE in energy expenditure due to changes in energy intake. It’s not as simple as “some people just burn more calories than others all the time no matter what”
LET’s dive deeper…
To dive deeper, I'm going to reference a recent study/secondary analysis by Hollstein et al called “Reduced adaptive thermogenesis during acute protein-imbalanced overfeeding is a metabolic hallmark of the human thrifty phenotype”.
The title introduces us to the diet culturally defined “slow metabolism” phenotype that is aptly named: “thrifty”.
Logically, it compares fairly directly to the definition of a financially thrifty individual: (a person or their behavior) using money and other resources carefully and not wastefully.
As it relates to energy expenditure, this study identified half of its participants as those who had “robust adaptive mechanisms” in response to energy restriction. Essentially, when fasting (in a metabolic ward) for 24 hours these individuals had a significant drop in energy expenditure. Meaning their body’s innate response was to conserve energyin an energy restricted environment.
Another observation was that these individuals did NOT show (as) much “adaptive thermogenesis” when overfed, either with high or low protein. This suggests these individuals would be likely to “resistant” to weight or fat loss in a deficit and privy to weight or fat gain in a surplus.
The other phenotype is called “spendthrift” and these individuals would be those that diet culture considers having a “fast metabolism” due to being “wasteful” or “bad at conserving energy”.
In this study the energy expenditure of the spendthrift group did not significantly drop during the 24 hour fast AND they showed a greater increase in energy expenditure when both overfed. This suggests these individuals would be more likely to experience weight or fat loss in a deficit and be less likely to gain weight or fat in a surplus.
In summary (of the referenced study) we have…
Thrifty Phenotype:
~235 calorie per day reduction in energy expenditure during 24 hour fast
~40 calorie per day increase in energy expenditure during low protein overfeeding
~235 calorie per day increase in energy expenditure during low protein overfeeding
“Energy out” decreases more significantly when “energy in” decreases
“Energy out” only increases slightly when “energy in” increases
These individuals would likely have a more difficult time losing weight and a more difficult time preventing weight gain; I.E “slow metabolism”
Spendthrift Phenotype:
~90 calorie per day reduction in energy expenditure during 24 hour fast
~100 calorie per day increase in energy expenditure during low protein overfeeding
~300 calorie per day increase in energy expenditure during low protein overfeeding
“Energy out” only decreases slightly when “energy in” decreases
“Energy out” increases more significantly when “energy in” increases
These individuals would likely have an easier time losing weight and a more difficult time gaining it; I.E “fast metabolism”
So…What does this mean?
So, it’s pretty clear that some people have a more adaptive response to changes in energy intake than others.
However, the study also showed a few things that makes the “thrifty” cohort seem “less unfavorable”; and let’s be honest, its not lookin to hot for those who fall under that umbrella:
1) Both groups had very similar weights and body compositions at baseline.
2) When in energy balance, or eating roughly maintenance calories, the total daily energy intake of the thrifty phenotypes was larger, on average, than the spendthrifts.
3) Total energy expenditure during overfeeding was higher in the thrifty group
Based on this it can’t really be said that some, plain and simply, have faster or slower metabolisms than others. But it can be said that the RESPONSE to different diet interventions can lead to more resistance / struggle depending on your phenotype and your goal.
All in all, this adds to the existing body of research that suggests energy metabolism is extremely complex and thus so is weight management.
Final thoughts
Hopefully this helps to show some who think they are “broken” or that they’re doing something wrong that this very well may be a normal phenomenon.
That weight loss or gain is best handled on an individualized level.
That some people may have to drop or increase their calories much more substantially to achieve weight loss or weight gain.
That some can or should diet more conservatively for a longer period of time while others should be much more aggressive within a smaller window.
That some may need multiple or many fat loss – diet break / maintenance cycles.
That, as with most things in the health and fitness world – the answer to our question is by no means definitive. If anything, it leads to more questions. But THAT’S SCIENCE!
UNTIL NEXT TIME - COACH JOHN