Do overweight people live longer?

Question

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?

Answer 1

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

Underweight and obesity, particularly higher levels of obesity, were associated with increased mortality relative to the normal weight category. The impact of obesity on mortality may have decreased over time, perhaps because of improvements in public health and medical care

while this meta-study from the Lancet says

Patients with a low body-mass index had an increased relative risk for total mortality and cardiovascular mortality, overweight had the lowest risk for total mortality and cardiovascular mortality compared with those for people with a normal BMI. Obese patients had no increased risk for total mortality or cardiovascular mortality. Patients with severe obesity did not have increased total mortality but they had the highest risk for cardiovascular mortality.

suggesting that being moderately overweight is not in itself unhealthy, but being underweight may be.

Answer 2

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:

Relative to normal weight, both obesity (all grades) and grades 2 and 3 obesity were associated with significantly higher all-cause mortality. Grade 1 obesity overall was not associated with higher mortality, and overweight was associated with significantly lower all-cause mortality.

The key numbers from the study are the definitions of the weight categories and the resulting mortality risk. These are given below:

1. Normal, BMI 18.5-25, mortality risk 1
2. overweight, BMI 25-30, mortality risk 0.94
3. obese grade 1, BMI 30-35, mortality 0.95
4. obese grade 2 and 3, BMI >35, mortality 1.29

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.