Diet or Exercise? Which is More Important?

(This is an excerpt from my upcoming book, Fitness without Fear, publishing April 2021)

In chapter four, Let’s Go for a Walk, we briefly talked about Herman Pontzer’s work on the health of modern hunter gatherer societies. One aspect I mentioned and did not elaborate on was his findings on daily calorie expenditure of these societies as compared to modern man. What his research has shown is that, relative to lean body mass and age, the total energy expenditure of these individuals is not very different from Westerners with both active or sedentary lifestyles. This means that the average sedentary American and your average gym going American both burn about the same number of calories as your average hunter gatherer.[1]

That last bit may have perked your ears up a bit and quite honestly, it kind of blew my mind a bit as well. In another comprehensive review article on the energy expenditure on primates, Pontzer demonstrated that this constrained total energy expenditure is species specific with a definitive upper limit regardless of additional physical activity.[2]

I highlighted that last statement because I realize it’s complicated. What it means in layman’s terms is that regardless of how much exercise you get, there is a limit on how many calories you can burn in a day. This means we have to control our calorie intake if we want to control our weight.

To summarize what we learned in chapter four, we evolved to walk upright and have big brains. These two things allowed us to navigate and travel long distances to hunt and gather food. We store fat more easily than other primates to prevent starvation on our long journeys. Another survival advantage were our complex societies that allowed for task and food sharing. Our brains and bodies are intrinsically linked to perform those tasks. Without those tasks, our bodies and our brains do not function optimally.

The crux of that research is that we require physical activity for physical health, brain health, and optimal cognitive function. It is not however the most effective way to decrease excess body fat which is important for our overall health. Although physical activity does help improve health markers in the overweight, it is not to the same degree that occurs in healthy weight individuals. Physical activity and fat loss are optimal for reducing disease and joint damage.[3] Excess weight can cause a number of orthopedic issues that affect the spine, shoulders, knees, feet, wrists, and ankles. These include osteoarthritis, chronic pain, and a greater risk of injury.[4] Joint pain also prevents us from being more physically active which in turn, decreases our resistance to disease.

For the record, I do not advocate that everyone need to walk around with six pack abs or look like a fitness competitor. Those levels of body fat are not particularly healthy either, especially for women. As I stated at the end of chapter three, you can use BMI or waist circumference to determine if you are at a healthy weight.

Why doesn’t increasing our physical activity significantly increase the number of calories we burn?

Our bodies do increase daily energy expenditure (DEE) with exercise and the fitter you are, the higher your DEE is likely to be . . . to a point. Our bodies are masters at adapting to stress and one of the ways we adapt to regular exercise is to become more fuel efficient, ie, over time we will burn fewer calories for the same amount of activity. This happens a couple of ways:

  1. Our bodies get better at burning fat for fuel rather than sugar which makes us more fuel efficient. Fat contains more than twice the calories per gram than sugar and so relying on fat burning pathways actually slows down how much fuel we use. For example, it has been shown in runners that energy expenditure increases to a certain point when they begin training. After several weeks, this plateaus and does not increase despite large increases in daily activity.[5]
  2. Introducing a moderate to vigorous exercise routine to individuals often results in them being less active in other parts of their day. They sit more and move less overall whether it be not getting up as often or simply not fidgeting as much. In other words, they rest more to make up for the energy they just expended.[6]
  3. At higher levels of activity, our bodies may begin to shunt calories during exercise away from non-muscular functions such as hormone production, immune system activities, and other maintenance activities.[7]

This upper limit on energy expenditure in humans regardless of activity level is important in that less than half of the energy we expend is due to physical activity. The majority of the calories we burn are spent on our body’s basic operation and maintenance activities.[8]The less healthy we are, the more maintenance we require. Instead of having energy available for fighting off infections and moving more, energy is mostly spent on things an otherwise healthy body doesn’t need. In order to be optimally healthy, we have to move.

Our bodies are evolved to require daily physical activity, and consequently exercise does not make our bodies work more so much as it makes them work better. . . . Instead exercise regulates the way the body spends energy and coordinates vital tasks.” -Herman Pontzer[9]

Beyond a certain point, no matter how many additional calories you add, how much sleep you get, or how well-trained you are, excess exercise will result in overtraining syndrome, a rather serious condition that results in chronic fatigue, depression, and damage to the nervous system and immune function. This typically happens in high level athletes, however it can happen in anyone with a physically demanding job. What is happening here is that the level of activity the individual is engaging in requires more calories than your body is willing to part with and as a result, begins to shut down.[10]

On the other end of the spectrum, how do we account for a relatively high daily energy expenditure in heavier-set, more sedentary individuals?

This one is a bit easier to answer and you can think of this as the antithesis of the overtrained elite athlete. In individuals with higher body fat percentages, regular activities are much more physically taxing and so they are performed less often and with more rest in between efforts.[11] And it’s not surprising, imagine how difficult it would be to do all of your daily activities with an extra hundred pounds or two strapped to your back. Additionally, larger bodies not only require more calories for basic movement, they also require more physiological maintenance and repair than average individuals and this accounts for a large portion of the calories burned.[12] Like the overtrained athlete, there is a limit on how energy can be spent on repair and maintenance.

To answer the title question, it really depends on where your deficits lie. If you are inactive, you need to exercise for your health. If you have excess bodyfat, you need to control your diet. Ultimately we simply cannot choose diet OR exercise; they both play important, but separate roles in our health and body composition.

[1] H. Pontzer, B. M. Wood, and D. A. Raichlen, “Hunter-Gatherers as Models in Public Health,” Obesity Reviews 19, no. S1 (December 1, 2018): 24–30.

[2] Herman Pontzer, “The Crown Joules: Energetics, Ecology, and Evolution in Humans and Other Primates,” Evolutionary Anthropology: Issues, News, and Reviews 26, no. 1 (January 1, 2017): 16-17.

[3] Pedro L Valenzuela et al., “Joint Association of Physical Activity and Body Mass Index with Cardiovascular Risk: A Nationwide Population-Based Cross-Sectional Study,” European Journal of Preventive Cardiology, no. zwaa151 (January 22, 2021)

[4] Harvard Health Publishing, “Why Weight Matters When It Comes to Joint Pain,” Harvard Health, accessed January 27, 2021.

[5] Klaas R Westerterp, “Exercise, Energy Balance and Body Composition,” European Journal of Clinical Nutrition 72, no. 9 (September 2018): 1247.

[6] K R Westerterp, “Control of Energy Expenditure in Humans,” European Journal of Clinical Nutrition 71, no. 3 (March 1, 2017): 340–3.; Pontzer, “The Crown Joules: Energetics, Ecology, and Evolution in Humans and Other Primates,”: 18.

[7] Herman Pontzer et al., “Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans,” Current Biology : CB 26, no. 3 (February 8, 2016): 414.

[8] Westerterp, “Control of Energy Expenditure in Humans,”: 340-3.

[9] Herman Pontzer, “Evolved to Exercise,” Scientific American, January 2019: 29.

[10] Jeffrey B Kreher and Jennifer B Schwartz, “Overtraining Syndrome: A Practical Guide,” Sports Health 4, no. 2 (March 2012): 128–37.; Pontzer, “The Crown Joules: Energetics, Ecology, and Evolution in Humans and Other Primates,”: 18-9.

[11] Westerterp, “Exercise, Energy Balance and Body Composition,”: 1248.

[12] Westerterp, “Control of Energy Expenditure in Humans,”: 340-3.

Recommended Reading:

  • O’Mara, Shane, In Praise of Walking
  • Pontzer, “Economy and Endurance in Human Evolution.” 
  • Westerterp, “Physical Activity and Physical Activity Induced Energy Expenditure in Humans: Measurement, Determinants, and Effects.”

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