As a biochemist, I have come up with a radical new diet plan: Eat less and exercise more. Yet, recent work by a research team led by Herman Pontzer at Hunter College exposed the flaws in my newfangled diet before I could even try it out. As it so happens, an emerging body of data indicates that exercise contributes very little to weight loss.
This surprising, counterintuitive finding has important implications for medical practitioners trying to combat a worldwide obesity epidemic. It also highlights the elegant design of the human body and supports the growing case for human exceptionalism.
The Obesity Epidemic
Some of the latest statistics indicate that worldwide, 1 in 3 people are overweight and 1 in 10 suffer from obesity. This problem has serious consequences because obesity plays a part in the etiology of type 2 diabetes, cardiovascular disease, and certain forms of cancer.
Of course, the cause of obesity is straightforward: People consume more calories than they need. One common-sense solution is to have people exercise more. Presumably the obesity epidemic is linked to a sedentary lifestyle. Throughout most of human history, our forebearers lived physically demanding lives. In contrast, people today engage in limited physical activity. Presumably, this inactivity lowers daily energy expenditure, leading to excessive weight gain, as caloric intake exceeds caloric outtake. Ready access to energy-dense foods only serves to exacerbate this caloric imbalance.
But as it turns out, exercise appears to have little to no bearing on weight loss, defying conventional wisdom. While exercise has many health benefits, weight loss doesn’t appear to be one of them. Why? Because, based on the latest research, increasing our physical activity doesn’t lead to a greater caloric expenditure. As a corollary, the only way to lose weight is to restrict caloric intake.
Constrained Energy Expenditure
Over the course of the last few years, researchers at Hunter College have sought to understand what, if any, aspect of the Western lifestyle contributes to obesity. In the process, they have learned that the sedentary lifestyle in the West is not the problem. They discovered that when people transition from an inactive lifestyle to one characterized by moderate activity, a small increase in energy expenditure occurs. But, beyond that point, energy expenditure plateaus. Additional activity doesn’t translate into increased energy expenditure; instead total energy outlay appears to be constrained.
For example, in 2012 the research team from Hunter College published the results of a study in which they examined the energy expenditure of the Hadza people, indigenous hunter-gatherers who live in the woodland and savanna of northern Tanzania. Anthropologists think that their way of living closely resembles the lifestyle of the first modern humans. As expected, the investigators determined that the Hadza are much more active than people who live Western lifestyles. Despite that difference, the average daily energy expenditure of the Hadza was no different than people from the Western world (once corrected for age, body size, and body composition).1
In a broader study, the Hunter College scientists found the same trend when examining average daily energy expenditure for a sample of 332 people from Africa and North America. The sample included 25- to 45-year-old men and women representing people with a variety of lifestyles. After correcting for age, body size, and composition, average daily energy expenditure appeared to be constant, regardless of the amount of daily activity.2
The Hunter College researchers speculate that as physical activity increases, our bodies conserve calories by reducing (1) our basal metabolic rate, (2) our repair processes, and (3) our growth rate. Additionally, women also conserve energy by reducing estrogen production and (for women who are nursing) decreasing lactation. The researchers also speculate that men and women may reduce energy expenditure by altering our posturing behaviors.
Constrained Energy Expenditure and the Case for Human Design
In many ways, constrained energy expenditure functions as an ingenious design to ensure human survival. For most of human history, our ancestors lived as hunter-gatherers—a highly active, physically demanding way of life. Yet when hunting and foraging for food, day-to-day success is not guaranteed. Humans could never have endured as a species if our daily energy expenditures didn’t plateau. When caloric intake is low (because of food scarcity), reducing activity level is not an option for hunter-gatherers because reduced activity makes it even less likely that they will find enough food to provide the minimal daily caloric intake. When food is scarce, the only way to endure is to double down foraging efforts. But increased foraging wouldn’t be possible if caloric expenditures increased linearly with activity. Constraining caloric output by slowing down basal metabolic rates and other processes allows hunter-gatherers to maintain high activity levels even when food isn’t plentiful.
As a creationist, I see constrained energy expenditure as an ingenious biological design befitting a Creator who made human beings to be fearfully and wonderfully made.
Constrained Energy Expenditure and the Case for Human Exceptionalism
When it comes to constraining daily energy expenditure, humans aren’t unique. It appears as if all primates limit their daily energy outlay. For example, the daily energy expenditures of primates in the wild is no different than the caloric output of primates living out their lives in a zoo or in a laboratory setting.
But what does make us unique is the magnitude of our daily energy expenditure. Humans require about 600 more calories per day than chimpanzees and nearly 1,000 more calories than orangutans.3 The primary reason for this difference is our large brain size. Maintenance of our large brains requires an energy outlay not demanded of other primates. Compared to other primates, we have accelerated metabolic processes.
But our large brain size (and our advanced cognitive abilities, capacities for symbolism, and theory of mind that go along with it) allow us to thrive in the face of this additional energy demand. The first anatomically modern humans were adept at shaping their diets to consist of calorie-rich foods. Cooking their food also allowed them to extract more calories and other nutrients from the food they collected. They also shared food with one another. These practices reflect our unique nature as human beings and arise from our symbolism and capacity for theory of mind—properties that reflect the image of God.
The unexpected insight into the relationship between physical activity and energy expenditure points to insights about human beings that are initially unexpected for those of us who view humans as the product of God’s handiwork. Constrained energy expenditure doesn’t make much sense if we think about it in the context of a Western lifestyle. But when we consider it in light of the way human beings have lived for much of human history, it makes perfect sense. And the difference in our average energy expenditure compared to other primates highlights our unique and exceptional nature, adding to the weight (pun intended) of evidence for human exceptionalism.
Returning to my diet plan: I guess it doesn’t take a biochemist to know what do to lose weight—just eat less.