The secret to a long life is having a waistline no larger than half your height, scientists claim. A recent study has shown that those with a waist height ratio of 80% or more could reduce life expectancy by up to 20 years. So an average 30 year old with a height of 5ft10in should have a waist of no more than 35in. The reason is the accumulation of fat around waist area is a risk factor for diabetes and heart disease.