Whenever I see certain headlines around health, I always feel a responsibility to dig a bit deeper to see if there is any truth to them. There are just too many eye-catching headlines that are backed by very flaky “evidence”.
So when I came across this YouTube video with the caption…
“The carbohydrate quantity in your diet may influence your metabolic rate, a new study finds.“
I knew I had to dig deeper.
As I did, I saw this Tweet referencing the same study…
Carbs slow your metabolism…🤔
Now I knew I had to dig further.
First let’s take a step back and lay some groundwork around the research these two claims were made on.
If I told you there was a hormone which controlled whether our body chose to burn or store fat it would seem logical that if we can control that hormone that we might be able to more easily manipulate the amount of fat people are carrying around these days.
Well there is such a hormone, and it is called insulin (to be fair insulin does much more than control fat usage) and for a while now there has been a group of people who have been theorizing that if we can keep insulin low and therefore always be burning fat that people could escape the consistent fat gain that is plaguing them.
In fact, I have written about this exact topic before, and for a lot more background and deep insight into this theory you can read this post.
The TL:DR of that post is insulin is not the only factor in determining whether you lose body fat or gain body fat. Human metabolism is quite complex, and ultimately if you end up burning more calories than you consume you will lose body fat regardless of how high or low you insulin is.
I will also be the first to admit that the amount of calories you burn and the amount of calories you eat, is also not the only determining factor to body fat loss or gain, as I said metabolism is complex, and there is no one variable we can manipulate to cause fat loss or gain.
Both sides of this debate, those who support the insulin model of obesity, and those that support the calories model of obesity, are still trying to prove each other wrong and the study referenced in the video and tweet is the latest example of this.
In the study (Do Lower-Carbohydrate Diets Increase Total Energy Expenditure? An Updated and Reanalyzed Meta-Analysis of 29 Controlled-Feeding Studies, 2021) scientists recruited 27 people and put them in a 60% caloric deficit in order to induce a 10-14% loss in body weight. After the participants lost the required weight they were then put on one of 3 diets intended to keep them weight stable, a high-carbohydrate diet with 60% of calories coming from carbohydrates, a moderate carbohydrate diet with 40% of calories coming from carbohydrates, or a low carbohydrate diet with 20% of calories coming from carbohydrates. All 3 diets had 20% of calories coming from protein and the meaning calories came from fat.
Before beginning the high, moderate, or low carbohydrate diet, researchers performed fat biopsies and measured the mitochondrial respiration from that sample. The participants then began their specific diets and at the 10-15 week mark researchers took another fat biopsy to again assess their mitochondrial respiration.
What researchers found was that all the participants had an increase in mitochondrial respiration regardless of the amount of carbohydrates they were consuming, however those on the high carbohydrate diet had the smallest increase in mitochondrial respiration from the fat biopsy.
What is mitochondrial respiration?
You might recall what mitochondria are from high school biology, but if not, they are what is commonly referred to as the “powerhouses of the cell”, they produce energy for our cells to keep us alive.
Respiration is a fancy term used to describe the fuel used by the mitochondria to produce that energy.
In other words the higher the level of mitochondrial respiration, the more calories you are burning or the higher your metabolism.
The researchers came to the conclusion…
“Our study suggests that a high-carbohydrate diet, possibly through an increase in insulin secretion, lowers mitochondrial respiratory function—a metabolic state that would favor deposition rather than oxidation of fat and predispose to weight gain.”
Basically they are saying that the reason the participants on the high carbohydrate diet have the smallest increase in mitochondrial respiration is because of the higher amounts of insulin needed to deal with the higher carbohydrates.
This is odd to me because…THEY DID NOT EVEN MEASURE THE INSULIN OF THE PARTICIPANTS IN THE STUDY!
How can you propose that insulin is the driving factor if you can’t even correlate the mitochondrial respiration rates to levels of insulin for each diet? They are just assuming that because they ate more carbs they had higher insulin levels. While that is generally true, it is not universally true in all scenarios…and science is based on assumptions.
Fortunately they do somewhat redeem themselves in the second to last sentence of the paper saying…
“Furthermore, we cannot definitively attribute the observed effects to insulin reduction per se, as other aspects of diet (e.g., fatty acid profile) or indirect influences (e.g., the microbiome) may be mechanistically involved.”
In addition to not measuring insulin researchers also only looked at mitochondrial respiration in fat biopsies OUTSIDE of the body. This does not come anywhere close to assessing energy use throughout the entire body. For one, mitochondria are part of nearly every cell in our body and our body consists of a lot more than body fat.
What about the mitochondria in other tissues besides body fat?
In addition our body has ways of burning energy that occurs outside of the mitochondria.
Unfortunately this study only looks at metabolism in a very specific type of tissue and is doing so outside the context of that tissue being in the body.
In my opinion this study provides some interesting results, but it does not come close to showing what headlines claim…a low carb diet will cause you to burn more calories and therefore cause you to maintain or lose weight easier. There are just too many gaps in this study for me to come even close to that conclusion.
This is not to say that I am against low carb diets or that we should not pay attention to insulin (or any other hormone). What I don’t think is right is demonizing insulin and trying to blame a complex problem, the rise in fat gain, and decline in health, on one single hormone.
The fact of the matter is that insulin is an incredibly important hormone and in healthy individuals it should rise and fall based on the foods we eat in healthy individuals.
When healthy individuals eat something like a banana and their blood sugar goes up, the body responds by elevating insulin, stopping the burning of body fat and triggering your body’s cells to take in the sugar from the blood to lower their blood sugar.
This is healthy.
Yes you temporarily stopped burning fat within your body but that’s OK, in a healthy person insulin will lower back down restoring the use of fat.
However, in the situation where insulin is chronically elevated and never comes down then this can become an issue because the body will never be able to access its fat stores. This can result in all kinds of health issues, and yes, also make it very hard to lose body fat and much easier to store body fat.
In these cases a low-carb diet can help lower their chronically elevated insulin, and that can be a GAME CHANGER for their health. In addition though, low carb diets make it easier for people to regulate the amount of calories they are consuming, particularly if it’s a whole foods based low carb diet. With a whole foods low-carb diet you’re left with eating non-starchy vegetables, protein, and healthy fats. This makes it nearly impossible to overeat calories.
Could you make the case that in a low-carb diet that because insulin is low and you consume less calories than you are burning that the two factors work together to help regulate body fat?
And I think that is the real magic behind a low-carb diet.
Insulin is not the sole cause of weight gain, just like the amount of calories you eat and burn is not the sole cause either. In one person it could be insulin, and in another person it could be calories. In many people it could be both! For others it could be some other hormone. For another person it could be neither and it could be their sleep, stress, and lack of movement.
Weight loss and gain can be very hard to decipher, there are lots of variables at play, but I believe if we start with the basics, eat whole foods, move our bodies throughout the day, exercise a few times a week, get good quality sleep for 7-8 hours, keep stress in balance, and find joy in the company of others we don’t need to worry about the specifics as much. Our bodies can find a natural homeostasis, a happy place, where we can both thrive and be healthy.
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Do Lower-Carbohydrate Diets Increase Total Energy Expenditure? An Updated and Reanalyzed Meta-Analysis of 29 Controlled-Feeding Studies. (2021, March 11). PubMed. Retrieved June 22, 2023, from https://pubmed.ncbi.nlm.nih.gov/33274750/