the study 's lead researcher said the results are a surprise ' , but it 's no excuse to pile on the pounds .
for years we have been told that the first step to staying healthy is to keep to a healthy weight - that means making sure your body mass index -lrb- bmi -rrb- is within the normal range of 18 to 24 .
few people are advised to gain weight , unless their bmi is below 18 , but could that change .