Weight gain is generally associated with detrimental health outcomes. In particular, weight gain in the form of body fat increases the risk of a number of lifestyle diseases particularly cardiovascular disease and type 2 diabetes, and can also increase the risk of mortality. However, the relationship between body weight, disease and mortality is not linear, but fits a somewhat ‘U-shaped’ curve. While very high body weights do increase the risk of lifestyle diseases and mortality, very low body weight also pose similar risks to mortality. However, the diseases that an individual is at risk of developing vary from one end of the curve to the other. The obesity paradox describes the situation that arises from those with a larger body weight being protected from diseases and mortality compared to those of a lower body weight. For example, those who are classed as overweight (up to class I obese) have a lower risk of mortality than those of normal weight, when body weight is adjusted for height (the body mass index, BMI).
The reason for lower risk of the overweight category when compared to the normal weight category is threefold. Firstly BMI measurements have been gerrymandered downwards. This has effectively taken a large number of individuals who would previously have been considered of normal weight, and pushed them into the overweight category. This manipulation of the BMI categories has its roots in the World Health Organisation and has effectively skewed the ‘U-shape’ relationship between body weight and mortality. The second reason that overweight individuals are protected from mortality is due to the reverse causation caused by weight loss during serious illness. Rapid weight loss, particularly muscle wastage that occurs during serious illness and old age increases the risk of mortality significantly. Lastly, selective survival can bias the mortality figures. The selective survival advantage involves the bias that is inherent in survival statistics if all variables are not considered.
Therefore the desirable body mass index set by the World Health Organisation at 18.5 to 25 kg/m2 creates the obesity paradox, because this is not normal weight for most individuals, but actually underweight. On top of this, the statistical bias inherent in including mortality of the terminally ill and elderly causes their to be a higher mortality risk in the underweight. On top of this, survival bias creates further distortion, because although body weight is one factor that influences mortality, there are others. For example sex, diet, exposure to pollution and race all determine risk of mortality. More importantly, subcutaneous fat is not a risk factor for disease and mortality, whereas abdominal obesity is. African Americans have a higher rate of mortality before middle age, compared to white Americans. However, after middle age the opposite is true. African Americans tend to have higher subcutaneous to abdominal fat ratios than white Americans, and so body weight is not a good predictor of mortality in this group.
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