It is no news that much of the Western world is gaining a lot of body fat. We have seen the news and graphs showing the ever increasing rates of people who are overweight or obese.
The rate of fat gain among children is even more alarming….
The question everyone is asking (and debating) is why?
There are a lot of hypotheses, the types and amount of food and physical activity are two of the most common.
But what if it is something that is out of our control completely, what if our bodies are just not burning as many calories as it used to?
In other words, what if our metabolisms are slowing down?
A recent study sought to provide some insights to this question (Total Daily Energy Expenditure Has Declined Over the Past Three Decades Due to Declining Basal Expenditure, Not Reduced Activity Expenditure, 2023).
In this study researchers used a very cool database that contains energy metabolism data for 5000 people using one of the gold standards for measuring energy expenditure, something called doubly labeled water. The science behind how this technique works is not really important, just know it is a very accurate way of determining energy expenditure, so the data can be trusted. They then looked at basal energy expenditure data (how many calories you burn just to be alive) from 10,000 people from various research studies over the past 100 years.
Researchers took this data and adjusted it for age, sex, and body weight to put each data point on a level playing ground as these three factors are known to play a key role in basal energy expenditure. After correcting for these three variables researchers found in fact there was a decline in energy expenditure over the past few years.
The answer is….81 calories per day.
Yup a whopping 81 calories a day, less than a tablespoon of peanut butter…11 almonds….one slice of bread…about 1 egg…1/4 of a McDonald’s cheeseburger…1/3 of a grande caramel macchiato from Starbucks.
OK, so let’s put a pin in those 81 calories per day.
The other interesting part about this paper is that researchers proposed a reason as to why our metabolisms are slowing down.
The researchers noted that the other thing that has changed over the past 100 years is the amount of saturated fat intake.
A low fat diet was recommended beginning in the 1970s and this is where we start to see obesity increase
To complement the obesity trends, here are the trends in animal based fat consumption (which is generally mostly saturated fats) and vegetable based fats (which are higher in polyunsaturated fats).
As you can see when obesity started to increase in the 1970s the amount of polyunsaturated fats started to increase and saturated fats really hit rock bottom.
In parallel to this study the same researchers ran another study on mice. They fed them 12 different types of diets and found that the amount of saturated fat intake correlated to a decrease in basal metabolic rate…in only the adult male mice.
So their explanation for why basal metabolic rates have decreased over the past 100 years is based on a result of a mouse study where lower saturated fat intake correlated with a lower basal metabolic rate in a very specific type of mouse, but only the adult males.
I don’t know about you but I am not ready to bet any amount of money on that hypothesis.
Furthermore, there is a bunch of HUMAN research that shows metabolic rates INCREASE when swapping saturated fats for polyunsaturated fats (Polyunsaturated Fatty Acids as Modulators of Fat Mass and Lean Mass in Human Body Composition Regulation and Cardiometabolic Health, n.d.).
Now let me be clear here…I am not suggesting that swapping saturated fats for polyunsaturated fats is beneficial for your health. I know there is a lot of debate about the consequences seed oils, omega-6 fats, and polyunsaturated fats have on our health. Where I stand on that debate is as follows: in general seed oils are highly processed, and I prefer to use the least processed foods I can, so I try to limit highly processed oils, just like I try to limit other highly processed foods.
Speaking of processed foods, that could be a viable explanation for why our metabolisms may have slowed during the past 100 years.
Here is a fact we know is true, the more unprocessed a food is, the more energy it takes for us to digest it. Yes we burn energy when we digest food. Some foods take a lot of energy to digest, while others do not.
For example let’s look at protein for example.
Consider the following three protein sources, a chicken breast, ground beef, and a whey protein powder. All of them are excellent sources of protein, but each requires a different amount of energy to digest. In general, the more processed the food is the less energy it will take. So the whey protein powder will require the least amount of energy to digest, followed by the ground beef, and lastly the steak.
Besides being easier to digest, ultra-processed foods also contain less fiber. The more fiber in a food the more energy it takes to digest (The Thermic Effect of Food: A Review, n.d.).
Let’s consider the increase in ultra-processed food consumption over the past 100 years. From 1800 to 2019 processed and ultra-processed food consumption has gone from less than 5% to more than 60% (United States Dietary Trends Since 1800: Lack of Association Between Saturated Fatty Acid Consumption and Non-Communicable Diseases, 2022). Considering this information I think a more plausible explanation for the decrease in energy expenditure could be the types of food most people are eating.
Let’s also consider that the researchers are using data that is over 100 years old. Do we think that the technology used to measure metabolic rates 100 years ago is as accurate as what we have today?
I am willing to bet not.
What if this decrease in metabolic rate is just due to changes in the accuracy of the technology used to measure it?
Lets disregard all the potential flaws in this study, hell maybe it’s a fact that our metabolisms are slower than 100 years ago.
I am going to make the argument that this DOESN’T MATTER!!!
We are talking about 81 calories per day!!!!
Yes 81 calories per day can add up over years and years. But you could compensate for those 81 calories with a barely noticeable change in your diet. I am not asking you to eat 500 calories less a day, it’s less than a tablespoon of peanut butter, 11 almonds, one slice of bread, about 1 egg, 1/4 of a McDonald’s cheeseburger, 1/3 of a grande caramel macchiato from Starbucks.
In my opinion these 81 calories a day, if true, pales in comparison to the change in our lifestyle over the past 100 years. In my opinion, if you focus on your sleep, nutrition, movement, and stress management and align them with our ancestral norms it will more than correct for any potential metabolic slow down that MAY have occurred.
I prefer to make people feel empowered to take control of their health. Instead of focusing on what’s out of your control, ie metabolic slow down, focus on the things you can control, ie your lifestyle. When you feel empowered, you feel motivated. When you feel motivated it’s easier to make change.
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