In order to understand how the obesity epidemic arose, we will briefly examine how body weight is regulated. The key to understanding body weight regulation is understanding energy balance. The body’s state of energy balance is determined by the amount of energy ingested in food in relation to the amount of energy expended in metabolism and physical activity.39, 40
In order to maintain a stable body weight, energy intake must, over time, exactly equal energy expenditure. Negative energy balance (in which energy expenditure exceeds energy intake) results in weight loss, whereas positive energy balance (in which energy intake exceeds energy expenditure) results in weight gain.
The body appears to have some ability to actively regulate or adjust energy balance since altering one component of energy balance can affect other components. For example, chronic changes in the amount of food consumed lead to changes in metabolism that serve to oppose a change in body weight.41
Similarly, chronic changes in physical activity can affect food intake.42, 43
However, these compensatory physiological changes are not sufficient to completely prevent changes in body weight in the face of strong, persistent positive or negative energy balance.42, 43
Our physiological system seems to protect more against body weight loss than against body weight gain. This makes sense in that for most of mankind’s history, starvation was a much more serious problem than obesity.44
Each component of energy balance can be influenced by genetic, epigenetic and environmental factors. We know, for example, that genes can affect each component of energy balance45
and can explain some of the differences between individuals in body weight and body composition. While we can conclude that our genes are permissive for weight gain, the gradual weight gain of the population does not seem to be primarily due to genetic factors.
The extent to which the body’s physiological regulatory mechanisms serve to maintain a healthy weight depends on the environment. In an environment in which high levels of physical activity are necessary for securing food and shelter and for transportation, and in which food is inconsistently available, the body’s physiological regulatory mechanisms appear to work best and serve to help facilitate sufficient food intake to avoid loss of body mass. However, as the environment has gradually changed to one in which high levels of physical activity are not required in daily life and in which food is abundant, inexpensive and served in large portions, the physiological regulation of body weight appears to be insufficient to oppose positive energy balance, weight gain and obesity. In these situations, becoming obese is an adaptation to the environmental conditions and appears to represent a new “settling point”.
Obesity researchers are increasingly recognizing the importance of the physical and social environment in facilitating weight gain and obesity. Our current food environment is one in which food is inexpensive, abundant and served in very large portions.40
Similarly, we have created a physical activity environment with only a rare need for significant energy expenditure for food, shelter, and transportation.40
These environmental influences make it easy for us to overeat and under exercise. The body’s physiological system for adjusting energy balance is not sufficiently strong in most people to completely oppose the positive energy balance that results.
Similarly, evidence suggests that obesity is being facilitated by our social environment. Christakis and Fowler demonstrated that socials networks influence whether or not we develop obesity.46
With both the physical and social environment facilitating weight gain, it is not surprising that more and more people are gaining weight and becoming obese.
Our environment arose as an unintended consequence of our societal progress. In fact, our environment was likely shaped in large part because of our biological preferences for high energy foods and lack of biological preference to be physically active. The environment we have created is one to which our ancestors aspired, and includes a consistent supply of good-tasting, inexpensive, available food and the ability to not have to work hard to secure food, shelter and transportation.
The realization that the environment is facilitating obesity has increased interest in modifying the environment to help address the obesity epidemic. While research in this area is only beginning, it represents an exciting new approach to obesity. There are, however, some cautions. First, it is unlikely that modifying the environment alone will solve our problem with obesity. The problem is that so many factors that have contributed to obesity are things that enrich our lives in other ways. For example, we have instant access to information throughout the world through televisions, computers, and personal digital assistants. The fact that these tools contribute to reduced physical activity and thus promote weight gain has only recently been realized. Similarly, the increase in families in which both parents work has increased and contributed to the rise of “fast food restaurants” because few people have the time or energy after work to prepare home cooked meals. New York City mandated that restaurants place calorie counts on menu items to help them control their intake. However, one study found no change in the number of calories purchased at fast food restaurants before and after menu labeling.47
It is unlikely that we can ever “go back in time” by giving up these things. It is more likely that we will learn how to modify the environment to support and sustain specific behavioral changes in the population to help people maintain healthy weights.
The need to deal with both the environment and behavior is illustrated in . Our biology developed to work best in a different environment – one where food was inconsistent and high levels of physical activity were required to secure food, shelter and for transportation. In previous environments, physical activity was the “driver” for achieving energy balance, and food intake was “pulled” along.39
We developed multiply physiological systems to facilitate eating with no need for physiological systems for food restriction, and no need to develop a biological preference to be physically active when physical activity was not required. Essentially, our biology tells us to eat whenever food is available and to rest whenever physical activity is not required. In previous environments, this biology was adequate to allow most people to maintain a healthy weight without conscious effort. Body weight regulation was achieved for most with simple physiological control.
Both the environment and behavior must be addressed in assessing energy balance.
The situation is different in today’s environment which requires very little physical activity. Securing food and shelter and moving around our environment do not require the high levels of physical activity required in the past.48
Technology has made it possible to be productive while being largely sedentary. Under such conditions, weight gain can only be prevented with conscious efforts to eat less or to be physically active without the need to be physically active. The minority of Americans who are maintaining a healthy weight are likely exercising cognitive control of eating and physical activity patterns to eat less than they would otherwise and to be physically active without the necessity of doing so. In today’s environment, maintaining a healthy body weight cannot be left to physiological processes but requires cognitive effort. This does not mean that we should not look for ways to modify the environment to make it easy for people to avoid overeating and a sedentary lifestyle. It does mean that we have to focus not exclusively on changing individual behavior or on changing the environment, but on the combination. We must change the environment to facilitate and sustain the behavior changes required to avoid obesity.