Physiological and Genetic Influences

Why is it so difficult for some people to lose weight and for others to gain weight? One theory is that every person has a “set point” of energy balance. This set point can also be called a fat-stat or lipostat, meaning the brain senses body fatness and triggers changes in energy intake or expenditure to maintain body fat within a target range. Some believe that this theory explains why after dieting, most people return to their original weight not long after stopping the diet. Another theory is referred to as the “settling” point system, which takes into account (more so than the “set-point” theory) the contribution of the obesogenic environment to weight gain. In this model, the reservoir of body fatness responds to energy intake or energy expenditure, such that if a person is exposed to a greater amount of food, body fatness increases, or if a person watches more television, body fatness increases. A major problem with these theories is that they overgeneralize and do not consider the fact that not all individuals respond the same way to changes in food intake or energy expenditure. This brings up the importance of the interactions between genes and the environment.

Not all individuals who take a weight-loss drug lose weight and not all people who smoke are thin. An explanation for these discrepancies is that each individual’s genes respond differently to a specific environment. Alternatively, environmental factors can influence a person’s gene profile, which is exemplified by the effects of the prenatal environment on body weight, percentage of body fat, and disease incidence later in life.^[1]

One example is a study that found that the offspring of women who were overweight during pregnancy had a greater propensity for being overweight and for developing Type 2 diabetes. Thus, undernutrition and overnutrition during pregnancy influence body weight and disease risk for offspring later in life. They do so by adapting their energy metabolism to the early nutrient and hormonal environment in the womb.

Psychological/Behavioral Influence

Sedentary behavior is defined as participating in pursuits in which energy expenditure is no more than one-and-one-half times the amount of energy expended while at rest and includes sitting, reclining, or lying down while awake. Of course, the sedentary lifestyle of many North Americans contributes to their average energy expenditure in daily life. Simply put, the more you sit, the less energy you expend. A study published in a 2008 issue of the American Journal of Epidemiology reports that 55 percent of Americans spend 7.7 hours in sedentary behavior daily. The statistics in Canada are comparable. ^[2]

Canadians partake in an excessive amount of screen time, which is a sedentary behavior that not only reduces energy expenditure but also contributes to weight gain because of the exposure to aggressive advertising campaigns for unhealthy foods.

Societal Influence

Many societal factors influence the number of calories burned in a day. Escalators, moving walkways, and elevators (not to mention cars!) are common modes of transportation that reduce average daily energy expenditure. Office work, high-stress jobs, and occupations requiring extended working hours are all societal pressures that reduce the time allotted for exercise in large populations. Even the remote controls that many have for various electronic devices in their homes contribute to society being less active. Other obesogenic factors were discussed in the weight management section of this chapter.

As mentioned previously in the chapter, socioeconomic status is inversely proportional to weight gain. One reason for this relationship is that inhabitants of low-income neighborhoods have reduced access to safe streets and parks for walking. Another is that fitness clubs are expensive and few are found in lower-income neighborhoods. The recent and long-lasting economic crisis in this country is predicted to have profound negative effects on average body weight.

Fitness vs Fatness

When thinking about the relationship between health and disease, it’s also important to consider if low cardiorespiratory fitness levels lead to the increased risk of CVD and other negative health outcomes.

In a meta-analysis, researchers investigated the relationship between cardiorespiratory fitness (CRF) and weight status on all-cause mortality. Upon completing the meta-analysis, researchers established that CRF is a better indicator of all-cause mortality risk than body mass index (BMI); the risk of all-cause mortality is heightened by low CRF levels, not by elevated weight.^[3] These results demonstrate that weight loss is not necessary to see improvements in one’s health status. Individuals who struggle with losing weight can still achieve numerous health benefits as long as they remain physically active and maintain moderate CRF levels. The idea that reduced fat does not equate to fitness is further supported by studies showing that weight loss via liposuction without the implementation of lifestyle changes (i.e. improved physical activity levels) does not improve one’s cardiometabolic health profile or risk of developing CVD.^[4] The results of this study suggest that body weight is not a core issue, but instead, it is the increased levels of sedentarism and low physical activity levels that lead to the increased morbidity and mortality rates. This is really important to consider because of the societal stigma surrounding body weight. Society currently places too much importance on weight status rather than focusing on CRF and health status. Hence, we need to shift our focus from weight and work towards simply promoting increased physical activity participation for all populations on a global scale.