A MATTER OF FITNESS, NOT FATNESS:

Obesity, at least when operationally defined as exceeding a specific amount of body fat and/or body mass index, is associated with certain health benefits. Examples include the now rarely needed but obvious protection against starvation in times of food scarcity, protection against osteoporosis, fractures, frailty, and premature mortality in the elderly, as well as reduced mortality rates in the face of certain severe illnesses or injuries [6]. The assumption that adiposity per se increases mortality risk is also not well supported by the scientific evidence. Many epidemiologic studies have shown that people who are overweight or moderately obese live at least as long as normal weight individuals, and often longer [13-15]. Additionally, life expectancy increased dramatically during the same time period in which body weight rose, and the World Health Organization projects life expectancy will continue to rise in coming decades [16].

From a physiological standpoint, fat gain is seen as a solution for maintaining homeostasis and re-establishing energy balance in the current obesogenic environment. The ability of fat cells to produce molecules involved in genuine regulatory processes has been known for decades. During weight gain, many adaptations over time can promote the re-equilibration of energy balance. These include increases in fat oxidation, sympathetic nervous system activity, insulinemia at euglycemia, leptinemia, and overall energy expenditure [6]. The problem related to fat gain as a physiological compensation to chronic unhealthy lifestyle habits is that it cannot occur with the same metabolic efficiency as exercise. Specifically, fat gain relies more on increased concentration of substrates (e.g. free fatty acids) and hormones (e.g. insulin and leptin) to re-establish energy balance by increasing total energy expenditure, which likely underlies the occurrence of the metabolic syndrome which often accompanies obesity. Thus, while increasing fat reserves may help to restore energy balance, it can also lead to increased risk of chronic disease. These observations emphasize the importance of adhering to a healthy lifestyle in order to maintain body weight stability rather than relying on the overuse of regulatory systems.

2. What are the Adverse Effects of Weight Loss?

Although counter-intuitive, many prospective observational studies suggest that weight loss increases rather than decreases the risk of premature death [17-19]. Paradoxically, most short-term weight loss intervention studies do find improvements in many health indicators. However, given that intentional weight loss is generally accompanied by a change in dietary and physical activity behaviors, it is not known whether or to what extent the improvements can be attributed to the weight loss per se. The case of liposuction can certainly provide relevant information about the effects of subcutaneous fat loss in the absence of behavior change. In their study, Klein et al. [20] evaluated the effects of large-volume abdominal liposuction on metabolic risk factors in obese women before and 10 to 12 weeks after liposuction. Although the participants lost 10.5 kg of fat, liposuction did not improve obesityassociated metabolic abnormalities, suggesting that decreasing adipose tissue mass alone (and especially reducing subcutaneous fat stores) without behavior change will not achieve the metabolic benefits of weight loss. In contrast, most health indicators can be improved through changing health behaviors, regardless of whether weight is lost or not. For instance, it is well-known that physical activity participation without weight loss has the capacity to reduce visceral adiposity and substantially improve the cardio-metabolic risk profile [21]. Similarly bariatric surgery, which dramatically changes the way that food is consumed and digested, has been shown to result in rapid improvements in glycemic control just days after surgery, long before any appreciable weight loss has taken place [22]. These observations agree with the "fat but fit" [23] and "metabolically healthy but obese" [24] concepts and stress the importance of regular physical activity and a healthy diet as key components in any health promotion and disease prevention strategy, regardless of body weight.

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