There are a few reported benefits to being overweight that you may read online, so I want to discuss these. This is sometimes called the ‘obesity paradox’ where some propose there are benefits in regards to death and illness from a range of disorders. This is, in particular, true if you are mildly overweight, from a BMI ~25-30 and those who are over 65 years.
A 2013 study of 2.88 million individuals and more than 270,000 deaths indicated that relative to normal weight, having a BMI of over 35, called grades 2 and 3 obesity were associated with significantly higher all-cause mortality, meaning death from all causes. However, grade 1 obesity of a BMI of 30-35 overall was not associated with higher mortality, and an overweight BMI between 25-30 was associated with significantly lower all-cause mortality. So being overweight actually lowered the risk of dying from any cause by about 6 % vs a ~30% increased risk of all-cause death if you have a BMI >35. So the reduction is there but it is small.
However many other studies disagree with this, as I said before, losing 3-14 years of life depending on how overweight you are and how long for and these counter arguments make a lot of sense. There are often two reasons we might see studies like the one above show a downside to being a lower weight. For example, often people who smoke are thinner, but not healthier and they will die younger. And people with cancer are thinner, but not healthier. So having a low BMI, or being normal weight or underweight does not mean you are healthy at all, you could, in fact, be very sick. The problem is often these trials included those people. When we look at trials that remove those confusing factors then we see that those over a BMI of 25, or overweight – are more at risk in general of death from any cause. And this was supported by a huge 2016 trial in The Lancet of over 10 million people showing that every bit you are overweight increases your risk of death.
So even if there was a 6% potential benefit for those just a bit overweight, which there probably is not when we look at bigger trials, we still have to remember that a much greater amount of their lives, up to 20 years may be marked by more chronic illness like strokes, cancers, arthritis and a subsequent dependence on prescription drugs to keep them alive.
So personally, when I look at the quality and amount of evidence – I don’t think there is any benefit to overall lifespan or death rates.