A few months back I wrote a blog post titled, Exercise Is Pointless, So Why Do It?
I was inspired to write this post after reading Herman Pontzer’s book Burn. Much of the book is based upon research Pontzer and his team did around energy expenditure in a hunter gatherer tribe called the Hadza. The Hadza are one of the few hunter gatherer tribes left on the planet. They are lean and free of many of the chronic diseases most modern civilizations suffer from today and because of this many researchers look to them as a proxy for what the optimal human lifestyle may look like.
One conclusion you might draw from looking at the Hadza is that the reason they are so lean is due to the amount of physical activity they do daily. Due to the lack of modern technology in their lives, daily tasks necessary for survival are physically demanding, much more so than modern societies. With so much physical activity as part of their daily lives you would think they would be burning a ton of calories, much more than modern Western societies, and therefore keeping them much leaner than they Western counterparts.
This is what Pontzer and team believed as well. They sought to confirm their hypothesis by strapping accelerometers onto the Hadza and tracked their energy expenditure using sophisticated technology called doubly labeled water.
When they went back and looked at the data they found that even with their physically demanding lifestyle they did not burn any more calories than the average Western individual with their very sedentary lifestyle!
How is that possible?!?!?!
Enter something Pontzer calls the constrained model of energy expenditure.
Traditionally it is thought that the more physically active you are the more calories you will burn. In other words if you go for a run and burn 400 calories you would expect that your total energy expenditure for the day would increase by 400 calories. This is referred to as the additive model of energy expenditure.
In Pontzer’s constrained model of energy expenditure he theorizes that you may actually burn no additional calories if you have hit the maximum amount of calories you can burn in a given day. Instead your body compensates for those additional calories by downregulating other physiological processes in the body.
With so much of the Western world focusing, rightfully so, on getting a handle on their weight, this news that exercise may not help them burn any additional calories can come as quite a shock, and ultimately lead them to discount exercise as meaningful way to reach their goal.
Now I don’t think Pontzer actually thinks that exercise is not helpful for those seeking to reduce their weight. In fact to his credit in the book he gives several benefits to exercise beyond the effect of burning calories. However the message that grabs headlines and gets clicks is that exercising more is pointless and what we have been told for years around physical activity is wrong.
In my previous blog post I threw out a potential explanation for what Pontzer was observing in the Hadza with regards energy expenditure.
The body is keenly aware of its external environment and often will adjust based on the signals it is receiving. What Pontzer was observing in the Hadza was the body adjusting to compensate for an activity level that was higher than the energy it was receiving.
Think of your sleep/wake cycle. I don’t care what your current bedtime or wake time is now, if I stuck you out to live with the Hadza for a few weeks your body would naturally get tired when the sun set and you would naturally wake up when the sun came up.
Why would this be any different for something like the amount of food we have available to us?
If the body thinks there isn’t a lot of food around based on the calories we are consuming vs the amount of energy we are expending then it will most likely try and do everything it can to balance things out so that we have enough calories around for its most critical physiological processes.
In other words the body will constrain your energy expenditure!
If however, calories seem to be abundant and we have plenty of food around, the body will have no problem wasting some energy and things might look more like the additive model of energy expenditure.
My gripe with Pontzer’s book and research is that the Hazda are actually demonstrating this constrained model of energy expenditure because their food availability is scarce while at the same time their energy expenditure is high. The body is protecting itself by saving scarce amounts of calories for critical processes it needs to keep them alive.
Fast forward a few months later from when I wrote that blog, and a paper was just published that backs up my hypothesis. Guess who is one of the authors of the paper? Pontzer!
In this recent paper  Pontzer and team compared energy balance, energy expenditure, and total activity across 584 free living adults ages 50-74 years old. They determined energy balance by measuring weight gain, loss, and maintenance. People who gained over 3% of their body weight were considered to be in a positive energy balance (they were consuming more calories than they were burning). People who maintained their body weight within 3% were considered to be in a neutral energy balance (the amount of energy they were burning was about equal to the energy they were consuming). People who lost over 3% of their body weight were considered to be in a negative energy balance (the amount of energy they were consuming was less than what they were burning).
What Pontzer and team found in this study was that the additive model of energy expenditure held true as long as people were in a positive or neutral energy balance. This means for those people that maintained or gained weight that the more calories they expended via physical activity the greater their total energy expenditure. The energy constrained model only held true in the individuals that were in a negative energy balance! This is exactly the case I was making for why we see the constrained model of energy expenditure in the Hadza!
Now I fully admit that comparing this current study to the one done on the Hadza directly is not entirely fair. The study population is different, the methods used to measure things like activity are different, the diets of the participants are different and so on. However Pontzer’s original research and subsequent application of his constrained model of energy expenditure is being applied to Western modern societies, so I think it’s fair enough to challenge this constrained model by using a study done on this exact population.
The other thing that bothers me about this topic is that we are applying the constrained model of energy expenditure to a population that is clearly not in a negative energy balance!
How do I know that?
Globally between 1980 and 2013 the prevalence of people who are overweight and obese have increased by 28% in adults and 47% in children . That is globally which includes non-westernized societies.
In addition we have this recent paper stating how obesity is threatening our national security and military by both impairing the physical fitness and health of existing soldiers and also by limiting the number of people who meet the standards to become recruits .
Then we have the compounding problem of the amount of weight people have gained over the past 2+ years due to the downstream effects of the pandemic .
We are not becoming more obese if most of us are in a negative energy balance!
Telling a population that is battling these types of rates of obesity that physical activity is not going to help battle their weight gain is quite harmful in my opinion. The fact of the matter is that it will help them.
I also know that many people believe exercise is not about caloric burn, and that regardless of whether the constrained model of energy expenditure is true or not, you should not be exercising to lose weight.
I can respect this opinion as well. However there is no doubt that exercise contributes to weight loss and weight maintenance in many other ways besides the caloric burn associated with it.
Here is a short list of how:
- It improves sleep
- Exercise contributes to the maintenance and growth of lean mass
- It improves your hormone profile
- It regulates your appetite
- It can be a great facilitator of social connection
- It can improve your mental health
- It can improve your cardiovascular health
- It makes you more capable in everyday life
All these improvements in your health will end up contributing to better weight loss and weight maintenance. Take calories completely out of the picture…exercise and movement is still beneficial!
Finally you might be wondering if you need to potentially worry about the constrained model of energy expenditure if you are in a negative energy balance (aka you are trying to lose weight).
Ultimately if your goal is to lose weight, all that matters is if that is occurring or not. If your weight is going down, or if your body composition is changing in the right direction, then clearly you are not running into issues caused by the constrained model of energy expenditure.
In the most recent study done by Pontzer on Western individuals, once their total energy expenditure reached 2500 calories per day, doing any additional physical activity did not result in any additional caloric burn when in a negative energy balance (keep in mind that this does not mean the benefits from exercise that I listed above don’t apply though!)
So does that mean once you hit 2500 calories expended for the day you should sit on your butt and not move?
As long as what you are consuming from a food perspective is less than your energy expended you are still going to lose weight, and movement and exercise will still contribute to that goal.
If you find yourself in a situation where weight loss has stalled despite moving and exercising a lot and not eating a lot of food could you be running into this constrained model of energy expenditure?
Maybe, although I am going to guess it’s more rare than you might think. It is more likely, in my experience, that something else is causing your weight loss progress to stall.
If you do think that you are running up against the constrained model of energy expenditure though and that is the cause, then in my opinion it is time to take a break from your diet. I would take this as a sign that you have been dieting either too long, or your negative energy balance is too big. I would encourage you to try to at least eat at caloric maintenance or put yourself in a surplus for a few months while simultaneously maintaining your energy expenditure.
That might sound scary but as we can see here when you are at maintenance or in a surplus the additive model of energy expenditure will take over and you will more than likely burn off those additional calories anyways. By doing this though, you send the signal of food abundance to the body telling it that it is safe, there is plenty of food around. It should then be more willing to allow you to lose more weight once you again lower your calories back down…after a few months!
There are a few messages I want you to take away from this blog post…
- Metabolism is not simple, it is not clear cut, no one model is going to explain everything we need to accept the fact that many factors play a role
- We cannot take results from research out of context and try to apply them broadly to all populations
- Movement and exercise play an important role in weight loss regardless of how many calories you burn
- More than likely you are not running into the constrained model of energy expenditure, and even if you are you can reverse out of it and get yourself back on track by eating more food
In my opinion the constrained model of energy expenditure is just another example of trying to explain the weight problem the Western world suffers from in an overly simplistic way. It also is contrary to the prevailing narrative so that means it gets clicks and views and in today’s world that often translates into money. In reality though, there is probably a very small percentage of people who fall into this constrained model, and even if they are falling into this constrained model it does not mean that they still can’t reach their weight loss goal and it does not mean the movement and exercise are pointless for achieving that goal.
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