The short answer: being a little bit fat isn't so bad, especially if you're already a senior citizen, but the fatter you are, the shorter your life expectancy. Let's dive into the details.
The new study, published in The Lancet, is a combined evaluation (a meta-analysis) of 239 studies that included over 10 million people from four continents: Asia, Australia, Europe and North America. All the studies followed their subjects for a long time, averaging nearly 14 years of observation. The authors (a large consortium called "The Global BMI Mortality Collaboration") wanted to exclude people who might have already been sick, so their study only looked at people who (a) had never smoked, and (b) who lived at least five years after the study began.
This left them with nearly 4 million people, of whom 385,879 died at some time during the course of the study. From this large data set, the researchers computed the risk of death as a function of body mass index (BMI).
[Aside: BMI is a simple function of your height and weight. For example, someone who stands 5'11" and weights 170 has a BMI of 23.7. A height of 5'6" and weight of 150 gets you a BMI of 24.2. You can calculate your own BMI using this calculator.]
The study divided people into six groups:
- underweight, BMI 15–18.5
- normal, BMI 18.5–24.9
- overweight, BMI 25–29.9
- obesity grade 1, BMI 30–34.9
- obesity grade 2, BMI 35–39.9
- obesity grade 3, BMI 40 or above
The main outcome that they studied was mortality (death) from any cause. Of course, one can argue that this is too simplistic, since if someone dies from, say, an auto accident, it probably wasn't due to their weight. But the results were consistent across all four continents, which argues that the study design was probably good. Here are the main findings for each group:
- BMI 15–18.5: 47% increased risk of death
- BMI 18.5–24.9: no increase (normal)
- BMI 25–29.9: 11% increased risk of death
- BMI 30–34.9: 44% increased risk of death
- BMI 35–39.9: 92% increased risk of death
- BMI 40 or above: 171% increased risk of death