Obesity rates in developed nations are currently at an all-time high, and this has lead to predictable increases in obesity-related disease such as hypertension, diabetes, and heart disease. Despite what some social media health gurus might tell you, the cause of this world-wide phenomenon is not a mystery. It is not seed oils, food dyes, GMOs, artificial sweeteners, etc. The answer is much simpler, but the solution is incredibly complex. As countries develop economically, we commonly see changes that impact both the “energy in” and “energy out” sides of the weight management equation. On the “energy in” side, there is commonly an increase in the availability of highly-palatable, affordable, calorie-dense foods. On the “energy out” side, industrialized economies tend to see decreases in overall physical activity as daily tasks tend to shift to a more sedentary nature.
But the question remains, which of these factors has been the greatest contributor to the rise of obesity worldwide? Is it the energy in, or energy out?
The Research
To answer this question, we will be looking at this study, which compares intakes and expenditures of various populations across the development spectrum, from hunter-gatherers to industrialized nations. In total, the sample consisted of 4,213 individuals from 34 diverse populations. To measure energy expenditure, the researchers used a doubly-labelled water database, which is the gold standard for measuring energy expenditure. Participants in this study were relatively weight-stable for the 7-14 day measurement periods, and so total energy expenditure and weight change were used to estimate energy intakes.
Energy expenditures were higher in developed nations, but this was mainly driven by larger body sizes. When controlled for body composition and size, energy expenditure decreased slightly in developed nations. This might not sound like a ground-breaking revelation, populations who are constantly on the move like hunter-gatherers expend more energy, duh. But if I were to ask you to guess how much of a difference in energy expenditure there was between these populations, one that regularly averages 25,000 and 30,000 steps per day, and one that has a large proportion of its people categorized as sedentary, what would you guess the difference was? I think most people would guess at least a double-digit percentage difference. However, the real difference in energy expenditure found in this study between populations with the lowest and highest activity levels was only about 6% when controlling for body size.
How could this be? How could somebody expend so many more calories doing physical activity, but yet not have that result in substantially more calories expended overall? Let us compare two approaches to understanding how physical activity contributes to overall energy expenditure.
Within the past decade, our understanding of this phenomenon has evolved. The intuitive approach would be that if our normal energy expenditure was 2000 calories, and we completed a workout that expended 500 calories, our total energy expenditure for that day would be 2500 calories. This is commonly referred to as the additive model (left). What we actually see when we directly measure this is something that resembles the constrained energy model(right). Essentially, the body experiences adaptive reductions in other aspects of energy expenditure such as sleeping, basal metabolic rate, and non-exercise activity thermogenesis. These compensate for the increase in physical activity, and in fact, we see larger compensation as we move towards the higher ends of the physical activity spectrum.
So we see from this analysis that the “calorie out” side of the equation is not as big of a contributor to the obesity crisis as we may have initially thought. Even going from mostly sedentary to extremely active does not make a huge difference in our ability to affect energy balance. To be clear, there IS a difference, just smaller than we would predict.
What about calorie intake then? This is where things get interesting. In the examined populations, ultra-processed food consumption (foods that are typically calorie-dense and easy to overconsume) ranged from 0-5% of total calories to 40-60% in the more developed populations. The percentage of calories coming from ultra processed foods was strongly correlated with body fat percentage when controlling for other factors such as age, sex, energy expenditure, and economic development.
Overall, the analysis suggests that increased energy intake is roughly 10 times more important than declining energy expenditure in driving the modern obesity crisis. The authors note that there are plenty of reasons why this might be the case. There is evidence that the hyperpalatability, energy density, nutrient composition, and appearance of UPF might disrupt satiety signaling and encourage overconsumption. Many processing methods may also increase the percentage of calories consumed that are actually absorbed by the body rather than excreted.
Take-Home Points
When it comes to weight management, focusing on your calorie intake is by far the most important variable. This can most effectively be done by reducing the amount of ultra processed energy-dense foods to the greatest extent possible, as it is incredibly easy to overconsume them. This doesn’t mean that physical activity is useless, as there are dozens of benefits that are independent of weight loss such as lean mass preservation and improved cardiometabolic health. Additionally, while physical activity does not seem to move the needle for weight loss, it seems to be a pretty good tool for the prevention of weight gain. This might seem paradoxical based on what we just discussed, but this is likely caused by the improved appetite regulation that seems to come with increasing physical activity.
