According to this study, overweight people actually live longer than people with normal/below normal weight. Is it really true? What about Okinawa people for example? What about underweight people because of their high metabolism? They are slim and they live very long. I'm confused a little. So what is better for a longer life? Is there any causation between BMI and life expectancy?
It rather depends on what you mean by normal weight. The standard definition is to use Body Mass Index (weight in kg divided by square of height in metres) and describe normal as something like 20-25, overweight as 25-30 and obese as 30+. There are issues about this simple rule of thumb, since it ignores body type, and there seems to be no reason to believe that the power of two in the denominator is the correct number (it certainly is not for children and may not be for tall people). But ignoring those questions and just looking at epidemiological evidence, this study from the Journal of the American Medical Association says
while this meta-study from the Lancet says
suggesting that being moderately overweight is not in itself unhealthy, but being underweight may be.
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Moderately overweight people have significantly lower mortality than those with "normal" BMI's; the really thin and the really fat do worse
A recent meta-analysis in the Journal of the American Medical Association (JAMA) concluded that the moderately fat by current standards (BMI 25-30) had significantly lower mortality than people in other standard weight categories.
The meta-analysis was very careful to exclude the normal confounding factors (e.g. ill people may well have low BMI's because of their illness).
The key conclusions were:
The key numbers from the study are the definitions of the weight categories and the resulting mortality risk. These are given below:
So, moderately overweight seems to (slightly) lower your risk of death compared to the thin, but if you get too fat your risk of death goes up a lot.