What is obesity? † International Journal of Obesity

What is the definition of obesity, what is obesity and what causes or contributes to obesity are three similar sounding questions with very different answers. Obesity is defined using body mass index (BMI) cutoffs to reflect the associated health risk. Different cut-off values ​​for BMI or the amount or distribution of body fat serve as an indicator of overweight and obesity in descriptive statistics. The BMI cut-off for obesity in adults was approximately 27 kg/m2which was later changed to the current limit value of 30 kg/m2 in the western world. Establishing a single border for the Asian population remains a challenge [1]† The definition of obesity therefore seems unstable. So what is obesity?

The human body has evolved to store excess energy as fat for use during periods of energy deficiency. There seem to be physiological controls to limit this fat accumulation within a certain range [2]† Yet certain individuals exceed that range, while others do not. Obesity is an excessive accumulation of body fat, which results from a deterioration of the energy balance mechanisms [3, 4]† An example of the condition edema can illustrate this point. Edema is not the result of drinking excess water. In terms of health, water management is excellently controlled, but is disrupted in some diseases, leading to excessive water accumulation. Similarly, several centrally and peripherally acting mechanisms influence long- and short-term energy absorption and consumption, including hunger and satiety hormones, various adipokines, cytokines, hormones, and thermogenic adipose tissue. As elaborated before [3, 4], the disruption of one or more of these energy balance regulatory mechanisms can lead to excessive energy storage and ultimately lead to obesity. Obesity is therefore a result of a disturbed energy balance.

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This leads to the third question about the causes and causes of obesity. It is argued that most “causes” of obesity are intrinsic while “contributors” are extrinsic to the human body. Causes can be considered a deterioration in the regulation of energy balance due to defects in physiology or behavior that predispose a person to excessive energy storage. Since genes can influence brain function and behavior [5, 6], genes may play a previously recognized role in the development of obesity. While “contributors” are factors such as energy-rich or tasty foods, or low levels of physical activity, which make it easier to achieve a positive energy balance in the presence of “causes”. For example, active thermogenic adipose tissue is more common in people who are slim and less so in obese [7]† While this has not been established as a causal relationship, a scenario can be suspected. Overnutrition increases total daily energy expenditure (TDEE), and the increase in TDEE shows significant inter-individual variation [8]† When energy intake is excessive, an adequately functioning thermogenic adipose tissue can dissipate excess energy and help maintain a stable body weight, while an inadequate thermogenic response can lead to weight gain. While the underlying reasons are unclear, very different responses in weight gain to identical positive energy balance are well documented [9]† It seems that both causes and contributions are required to express obesity, or at least one of them must be overwhelmingly present. For example, there are mechanisms in a person to resist excessive weight gain or weight loss [2, 10]† Yet those mechanisms can be overwhelmed by an excessive positive or negative energy balance to alter body weight.

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These concepts have practical significance in realistic obesity management and prevention. In obesity research, for example, it should no longer be enough to conclude that obesity is due to overeating. It is important to understand what upstream defect in physiology leads to overeating. A true prevention of obesity would involve preventing the intrinsic causes of obesity, such as leptin deficiency or a delayed onset of satiety hormones. Until medical science learns to regulate these causative factors, we are left with controlling the contributing factors such as energy intake and activity. These will not “prevent” obesity – the defect in the energy balance mechanism, but can help reduce its expression.

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Dhurandhar, N.V. What is obesity?.
Int J Obes (2022). https://doi.org/10.1038/s41366-022-01088-1

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