In the presence of excess calories, the body will subsequently convert and store them as triglycerides in adipose tissue. Over time, if this process continues, obesity results. Obesity as defined by some as a body mass index (BMI) greater than 30. However, because BMI is not an accurate reflection of adipose mass,1 it may not be the best objective measure of obesity. Obesity results as a consequence of increasing both the size and number of adipocytes. As stated, the overconsumption of macronutrients contributes to obesity.
However, not all macronutrients contribute to obesity in an equal manner. For instance, a highfat meal that results in a positive energy balance will stimulate fat storage without a subsequent match in fat oxidation. Furthermore, the deposition of excess dietary triglycerides into adipose tissue is associated with a very low metabolic cost (0–2%). In contrast, Flatt et al. reported that the conversion of carbohydrate into fat is an energy-requiring process in which 25% of the energy content is lost as heat. Of the three macronutrients, protein requires the most metabolic cost to be converted to and subsequently stored as fat. In studies involving twins, energy intake and obesity has been explored. Authors studying both monozygotic and dizygotic twins reported that there may be macronutrient-specific familial influences, but not conclusively. Also, greater-than-average caloric intake was associated with increased levels of body fat, despite likely genetic influences on both phenotypes. However, the authors concluded by stating that more research is needed to determine the genetic influence on energy intake. Finally, Tholin et al. studied genetic and environmental influences on eating behavior in a large cohort of monozygotic and dizygotic twins. The study concluded that genetic factors are of great importance of in the eating behavior in young adult male twins.
In conclusion, it has been reported that the increase in the prevalence of obesity has concomitantly occurred with an increase in portion sizes of foods that we consume. Furthermore, it has been shown that eating low-energy-dense foods such as fruits and vegetables helps sustain satiety while concurrently reducing energy intake, which appears to be a more effective weight loss strategy than fat reduction and decreased portion sizes. Energy expenditure, also known as physical activity, is one of the two components of the energy balance equation. Both energy intake and energy expenditure are profoundly important when discussing the etiology of obesity. Energy expenditure is composed of three categories: basal metabolic rate, thermic effect of food, and physical activity. Physical activity can also be broken down into two distinct sub-classes. The first is activity thermogenesis, which is volitional exercise. The second category is coined non-activity exercise thermogenesis, which is all of the activity that one performs that is not related to “sporting-like” exercise. Examples of this include fidgeting, housework, etc. It should be noted that non-activity exercise thermogenesis is often difficult to quantify. Activity thermogenesis accounts for approximately 15–50% of total daily expenditure in the sedentary to very active population respectively.
It further has been estimated that spontaneous minor activity performed during the day can account for 20% of the differences of energy expenditure in a 24-hour time frame. Equally important, Castaneda et al. reports that minimal amounts of spontaneous physical activity are a major predictor of accumulating fat mass during overfeeding in humans. An inverse association between physical activity and weight as a result of epidemiological studies has also been shown. Similarly, Meredith et al. reported a negative association between aerobic exercise at 65–80% maximal oxygen (VO2max) uptake and body composition. Additionally, a meta-analysis demonstrated that weight training as well as aerobic exercise is effective in facilitating weight loss, but also can increase or maintain lean mass. Thus, adding physical activity to promote weight loss encourages favorable changes in body composition. Sedentary lifestyle is commonly mentioned as a significant cause of the mounting prevalence of obesity.
In a study by Slentz et al. researchers reaffirmed what many have already suspected. If individuals partake in a modest exercise program similar to those suggested by the Centers for Disease Control and the American College of Sports Medicine, significant increases in visceral fat can be avoided and exercise that is a modest increase beyond their recommendations can facilitate significant decreases in visceral, subcutaneous, and total abdominal fat without changes in daily dietary intake. In a study published in 2005, researchers looked at the comparison of monozygotic twins in relation to lipoprotein and weight variation between vigorously active and sedentary siblings. The authors reported vigorous exercise could possibly reduce genetic influences on body mass index. Therefore, it is of critical importance to investigate energy expenditure in relation to obesity, especially when it has been reported that approximately 22–29% of adults report that they engage in no leisure-time physical activity.
