Introduction to Body Weight and Health

On December 26, 2018, 33-year-old Colin O’Brady of Portland, Oregon, became the first person to cross the landmass of Antarctica solo, unassisted, and without any resupply shipments. Others had crossed the continent with the help of a wind sail to propel them over the ice or with resupply drops along the way—both ways of saving precious energy—but O’Brady completed the 926-mile trek only on skis, pulling a sled packed with food, fuel, and supplies the entire way. Speed was important, because he was racing another man, Louis Rudd, a 49-year-old British Army captain. And of course, he didn’t want to run out of food hundreds of miles from the finish. Rudd finished the Antarctica crossing just two days after O’Brady.


Colin O'Brady, a Caucasian man wearing a red hooded coat and a striped winter cap, looks off into the distance while sunlight shines on his face. In the background, snowy mountains are visible against a bright blue sky.

Figure 7.1. Endurance athlete Colin O’Brady, photographed in March 2016.

To prepare for the expedition, O’Brady and his team had to make careful calculations to estimate his nutrient and caloric needs. He’d be skiing all day, every day in below zero temperatures and against constant wind for about two months. O’Brady estimated that he’d burn about 10,000 calories per day on his journey, and he knew that if he didn’t pack enough food, he wouldn’t have the strength to complete this epic test of endurance in extreme conditions. Previous explorers died in the Antarctic because they didn’t pack enough food.

A photograph of the vast Antarctic landscape shows miles of snow-covered terrain, with wind visibly swirling snow around the surface. An intensely blue sky is visible above the horizon.

Figure 7.2. O’Brady and Rudd raced across a landscape similar to that shown in this photo from Antarctica—a polar desert and the coldest, windiest, driest continent on earth.

But O’Brady also knew that the more food he packed, the heavier his sled would be—ironically making him burn more calories, plus slowing him down and prolonging his trip. So he focused on choosing foods that were calorie- and nutrient-dense but lightweight: oatmeal with added oil and protein powder; freeze-dried dinners reconstituted with melted snow; and 4,500-calorie slabs of a custom-made “Colin bar” made from coconut oil, nuts, seeds, and dried fruit.

At the start of his journey, O’Brady’s sled weighed 375 pounds and contained enough food to provide him with 8,000 calories per day. That was a bit short of the 10,000 calories he estimated he’d burn every day, so to build up some additional energy stores, he gained 15 pounds prior to his trip. In the end, after 54 days of skiing through ice and snow, he lost 25 pounds during his Antarctic crossing. He was successful, and while his fitness level and determination were surely critical to his feat, the trip would have been impossible without an adequate supply of calories.

In our daily lives, we need far fewer calories than an Antarctic adventurer, and we don’t need to schlep a two-month supply of food on our backs wherever we go. Thankfully, we also get to enjoy fresher and more interesting food options. But each of us, every day, whether we’re aware of it or not, is attempting to balance the calories we consume with the calories we burn, just like Colin O’Brady. This is the concept of energy balance—one we’ll be exploring throughout this unit. If adults eat roughly the same number of calories as they burn each day, their body weight will generally stay very constant. If they burn more calories than they eat, like O’Brady on his expedition, they’ll lose weight. If they consume more calories than they burn, they’ll gain weight.

Energy balance may seem like a simple concept, but in practice, how many calories a person eats and expends each day is influenced by so many different factors that it can be frustratingly difficult to apply. The human body has many sophisticated mechanisms that have evolved to help us protect and maintain our body weight, and in the history of our species, these mechanisms have been vital to our survival in times of food shortages. But today, we live in a world where highly palatable food is easily available and convenient, and we’re constantly bombarded with marketing messages enticing us to eat more of it. At the same time, the diet industry and media send messages saying that being thin is the same as being healthy and that restrictive dieting is the virtuous path to wellness, which ironically may cause weight gain and worsen people’s mental and physical health. Meanwhile, disparities in access to healthy food, safe places for physical activity, and healthcare have grown, all of which are associated with body weight and health. Many other factors, including the use of some medications, exposure to certain chemicals in the food supply and environment, inadequate sleep, excess stress, microbiome changes, and experiencing traumatic events, have been linked to body weight.

In this unit, we’ll explore these complex concepts and concerns and seek some answers, but we acknowledge that we raise more questions than we resolve. The truth is that researchers don’t fully understand why body weight has increased over the last several decades, much less how to change this trend. What is clear is that body weight is extraordinarily complex, and people have long been harmed by oversimplified messages to “eat less and move more” to reach a “normal” or “healthy” body weight. This approach implies that obesity is a failure of personal responsibility, but the last several decades have shown that blaming individuals for the size and shape of their bodies has not changed the trajectory of increasing body weight, and it has likely worsened people’s health by harming their relationships with food and perpetuating weight stigma.

We have done our best to approach this topic with care, recognizing that it can be very personal. We know that the language used to describe people’s bodies can be hurtful, and that encouraging people to engage in restrictive dieting practices for the purposes of making their bodies smaller can cause harm. Researchers’ understanding of body weight regulation is incomplete and still evolving, and we have tried to include different perspectives in this unit. We hope you’ll come away with an appreciation of the complexities, nuances, and uncertainties of this topic.


Unit Learning Objectives

After completing this unit, you will be able to:

  • Explain the concept of energy balance and how it relates to body weight, including the many factors that can influence both energy intake and expenditure.
  • Describe the characteristics of a healthy body composition, how it is estimated or measured, and limitations to these measures.
  • Describe the concerns with being underweight and overweight, appreciating that body weight may affect a person’s health, at least in part due to the experience of living in a world with unrealistic expectations around body size and shape.
  • Explain how weight bias causes harm and can contribute to greater stress, increased body weight, and poorer health outcomes.
  • Describe the major types of eating disorders, including their risk factors, consequences, and treatment.
  • Identify the global trends in obesity worldwide, and identify possible causes and solutions.
  • Explain the challenge of and best practices for managing body weight and health, including non-diet and non-weight centric approaches.
  • Discuss the importance of a moderate approach when it comes to nutrition and weight management, recognizing that all foods can fit into a healthful diet.
  • Recognize that nutrition and its effect on our physical body is only one dimension of health, and others are equally important, including exercise, sleep, finding purpose, freedom from excessive stress, and community relationships.


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Nutrition: Science and Everyday Application Copyright © 2020 by Alice Callahan, PhD; Heather Leonard, PhD, RDN; and Tamberly Powell, MS, RDN is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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