Key points from article :
Life expectancy for obese people lower than for those in healthy BMI range.
4.2 years shorter for men, and 3.5 years shorter for women.
BMI was associated with all causes of death categories (except accidents).
Argument that a higher BMI is protective in older people still controversial.
Study involved nearly 2 million people in the UK.
The study was published in The Lancet Diabetes and Endocrinology journal.