You might have heard recently that exercise doesn’t help you lose weight.
This might be quite surprising because for years we have been sold the idea that being active is quite important when it comes to achieving your optimal body composition. Exercise, combined with a good diet, is supposed to lead you to the promised land.
Everywhere you look on the internet it seems like people are saying that exercise is actually not doing a damn thing for your weight loss goals. Then you hear accounts of how people are killing themselves in the gym and never get the physique they desire.
Most of the negative press exercise is getting has come as a side effect of the new book Burn by Herman Pontzer. In the book Pontzer uses data he has collected from a hunter gatherer tribe called the Hadza about how many calories they burn (get it?) per day to show that human energy expenditure is essentially capped.
We traditionally think exercise works in an additive fashion, like the graph on the left.
As physical activity (exercise volume) increases, so does the amount of energy your body expends. More energy expenditure, more calories burned, the more calories you burn, the easier it may be to be in a caloric deficit and lose weight.
However what Pontzer is proposing, and the whole point of the book Burn is that energy expenditure is not additive at all and instead is constrained. This graph on the right basically tells us energy expenditure is capped, so more physical activity will burn more calories, but only till you reach a certain limit and then total calorie expenditure essentially levels off. The consequence of this theory means that if the exercise you are doing is putting you over this “caloric burn limit”, it is actually not helping you burn more calories, therefore not helping you be in a caloric deficit, so exercise is pointless for fat loss!
At this point you might be feeling a lot like this…
I can relate. Could the message we have been given that exercise is an important component of weight loss be wrong?
Let’s dive into some of the data and take a look.
Hazda men walk an average of 12.4 km/day, or 7.7 miles/day, while Hazda woman walk an average of 6.2 km/day, or 3.8 miles/day.
We know that in the US, men take around 7431 steps per day, while women take around 5756 steps per day. If we translate that into kilometers and miles, men walk 5.6 km/day, or 3.4 miles/per day, and women walk 4.3 km/day or 2.6 miles/day (https://journals.lww.com/acsm-msse/pages/articleviewer.aspx?year=2009&issue=07000&article=00005&type=Fulltext).
For men there is about a 7km or 4 mile difference between the Hazda and US, while for women there is almost no difference.
Pontzer took the daily expenditure data of both people living in market economies (western civilization) and people living in places where farming was the primary driver of the economy. Why select a farming based society for a comparison? We can assume that farmers are also going to be much more active due to the demands of their jobs than the average person living in a market economy. In the study Ponzer graphed them against their body mass which in turn produced a trend line of what we would expect energy expenditure to be for a given body mass in market and farming population.
In the traditional model of energy expenditure you would think that the Hazda, since they are moving much more (especially the men) than western society, would be way above the trend line, but they weren’t, they fell right along the trend line 🙉.
What’s even more fascinating was that the farming populations had a greater energy expenditure than what would be predicted by the trend line. This is important as we will come back to this later on.
If we ignore the farming populations for the moment and just focus on the Hazda and market populations, how could it be that the Hazda and market populations have the same daily energy expenditure despite the additional movement the Hazda seem to do?
As I mentioned above, Pontzer’s theory is that the human body has a set number of calories it can expend, once it reaches that number it cannot burn more, no matter how much more you move.
Our total daily energy expenditure can be broken down into several different pieces.
Let me explain the abbreviations in the diagram above to learn more about what makes up our energy expenditure.
BMR – Stands for basal metabolic rate. This is how many calories your body burns just to keep you alive, it is not influenced by your movement.
NEAT – Stands for non-exercise activity thermogenesis. This is all the calories you burn moving throughout the day outside of exercise. Examples would be, walking to the bathroom, doing the dishes, blinking, tapping, waving at someone, etc.
TEF – Stands for thermic effect of food. This is how many calories your body burns to digest the food you eat.
EAT – Stands for exercise activity thermogenesis. This is how many calories you burn through formal exercise.
NREE – Stands for non-resting energy expenditure. This is all the calories you burn through movement.
REE – Stands for resting energy expenditure. This is all the calories you burn if you did not move for 24 hours, it’s essentially equivalent to your BMR.
According to Pontzer’s theory, as NREE increases your BMR will have to decrease in order to keep you from exceeding your caloric burn limit. So what does the body do in order to decrease your BMR?
Luckily Pontzer has created a nice infographic explaining what happens.
As an individual moves from sedentary to active, many of the compensations are a good thing. We see less energy spent on inflammation, a reduction in stress, and a reduction in excess sex hormones. According to Pontzer we are trading calorie utilization of potentially harmful processes in our body (contributing to our BMR) to utilize those calories to fuel our movement. I would say that is a positive trade off for sure.
At some point though the need to conserve calories becomes negative and our bodies will start to slow down or stop certain non-essential processes in the body. Hormone levels drop below adequate levels, our immune system becomes compromised, men will have low testosterone, females will stop ovulating, and a host of other problems may arise.
All this happens to essentially make sure we don’t let our daily energy expenditure become a runaway train that endlessly increases.
I won’t deny that the body will enact these protective mechanisms if it thinks there is not enough energy around to fuel the activity we are demanding of our body. However I think that the situation the Hazda are living in is quite different than the one those in the Western world are living in.
Let me explain…
What if we think about the constrained energy model a little differently, what if the reason the Hazda don’t burn more energy is that they don’t have access to enough calories to do so?
For the Hazda, and the very few other hunter-gathering societies left in the world, food is not a certainty (just like it wasn’t for all of our ancestors). Somedays the hunt is successful, somedays the men return to camp empty handed. On those days the only food available to eat is what was left over from a previous hunt, or what the females are able to gather, which is mostly fibrous tubers (envision a potato with a lot of fiber). In other words the Hazda can expend a huge amount of energy, but get nothing in return for it.
It makes perfect sense then that the Hazda’s bodies would enact the protective mechanisms Pontzer talks about to help balance the amount of energy spent hunting and gathering with what is (or is not) coming in via food.
This is VERY different from everyone living in Western industrialized societies. Here, most exercisers go for an hour-long workout and eat before, during and after. In other words they can easily replace the energy expended during physical activity and then some because food is plentiful. We are not sending any signals to our body that food is scarce, so it has no need to try to balance out the energy expended vs. the energy coming in.
In fact, Pontzer himself acknowledges that when the body has plenty of energy (it has plenty of fat on board) that daily energy expenditure does seem to rise with activity and is not subject to the compensations we see in the Hazda.
“The apparent effect of body fat on the timing or degree of metabolic compensation could reflect a kind of buffering: greater energy reserves (e.g., more fat) might blunt physiological signals of energy stress and thus diminish the speed or degree of metabolic response.”
I think this energy availability theory can also explain the faming population outliers from Pontzer’s study on energy expenditure.
Those farming societies have much higher energy burn than what would be predicted by Pontzers model.
Not surprisingly, Pontzer never addresses this oddity in his paper, and completely leaves this data out in Burn. Unfortunately the papers with the daily energy expenditures for these farming populations , do not provide energy intake data for the farming populations so we don’t know how much they ate.
However we can speculate…
If they are farming they are certainly working hard, as farming is a very physically demanding occupation. However, unlike hunting and gathering there is a better guarantee of food in return for all that work (as long as they are good farmers). We can therefore assume that their caloric intake can better match their caloric burn. The result, less compensation, and a higher BMR. Again this is just speculation on my part, so I could be completely wrong.
As a final note in all this constrained model of energy expenditure topic, a recent paper that was just published attempted to quantify how much the average person constrains when expending extra energy (Pontzer was also one of the authors on this paper!)  The ultimate conclusion was that the average person compensates about 28% of the energy expended during physical activity. This means that 72% of the calories expended turn into extra calories burned. That’s not bad, and it’s certainly a long way away from 0%!
The author’s also noted that the amount and degree of compensation does appear to correlate with body composition. You may recall that Pontzer also theorized that the amount of compensation that occurs might be influenced by body composition. Pontzer theorized that individuals with lower body fat might compensate more than those with higher body fat (aka more energy available) . However this recent paper presented data that said the opposite, people with higher body fat might compensate more than those with lower body fat.
So which one is right?
My guess is that it probably happens at both extremes. If you have way too low of body fat you might compensate more because you are lacking energy and the body is trying to do what it can to survive by minimizing the amount of energy expended. On the opposite end of the spectrum where you have a lot of body fat, your body might compensate in order to keep that body fat around.
Why do that?
Body fat is a great insurance mechanism for survival, your body doesn’t want to give away its insurance if it doesn’t have to, so it compensates to prevent that from happening as best it can. We certainly need more investigation in this area to understand more about what is happening.
At this point you might be wondering if we have any evidence that exercise does promote weight loss?
Yup, we do!
In the MIDWEST-2 study participants burned 400-600 calories 5 days per week doing exercise and lost and average of 13.5 lbs of body fat in 10 months . What makes this even more impressive was that there was NO DIETARY INTERVENTION in this study! That’s right, 13.5 lbs of body fat lost with no changes in diet! Granted that’s a lot of physical activity, still, it certainly seems like exercise is not pointless for weight loss.
The other thing to keep in mind here is that exercise does more than cause you to lose weight, it is the only way you are going to add muscle mass, especially if you are strength training. The effects of muscle mass on health and health outcomes are numerous. In addition exercise will improve sleep, decreases cardiovascular risk, cancer risk, alzheimer’s risk, provides an excellent opportunity for social connection, gets you outside in nature, and gives you more “wiggle room” in your diet so you can enjoy a piece of cake on your birthday with little to no ill effects. Finally exercise is the best thing you can do for weight maintenance as well!
Yes, I agree that maybe the numbers don’t always add up, maybe we should expend more calories than we actually do, but that is no reason to not exercise. In my opinion we cannot analyze energy expenditure of hunter gatherers and then apply it to a different society like western populations with access to an endless number of calories. Those populations are sending very different signals to their bodies, and as with everything in physiology, context is important!
Usually I side with the ancestral model in most of the content I put out. However in this case I am not. If we were to put our bodies in a situation like the Hazda, which greatly resembles how our ancestors lived, yes we would probably experience a fair degree of compensation, and maybe in that situation I would say sure, lay off the activity a bit. However, that is not the situation we live in. We live in a situation where food is easy to come across, and it’s quite calorically dense. Calories are not a problem and neither is too much movement. So let’s not demonize exercise. Most of us need more movement, not less, and it appears that even if we do compensate a bit for that movement it will most likely aid our weight loss goals and will certainly improve our health in other ways!
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