Girdle measuring was a widespread practice in Europe and the British Isles as late as the 16th century. It was a technique of magical healing used by wizards, witches, and wise men and women that involved, oddly enough, measuring a patients girdle or belt.
A patient would approach these people with some medical complaint, which would be traced to the presence of evil spirits or fairies that had invaded their body. Changes in the patients girth would indicate this (and here I am blaming fluctuating weight on eating too much.Now I find it must be a simple infestation of fairy folk!). After the magical 'doctor' had exorcised the being using various charms they would take another girdle/belt measurement to ensure it had worked. The cures used by them could involve the recitation of charms, cutting up the patients girdle, then burying the pieces in the ground (possibly next to the patient, if there was a more earthy cause for their illness...).
The idea of measuring a girdle to determine a sickness may seem to be quite ridiculous, particularly since it was invariably attributed to supernatural entities, but a change in size could indicate something such as an infection or cancerous growth. It is the diagnosis and consequent treatment, but not the method of determining that is askew.
The popularity of girdle measurers declined as people realised that there were 'real' reasons behind their poor health. They should have a resurgence though, as soon as the world realises that these guys have provided you with the ultimate fat day excuse - 'It's not because I had a 3 course dessert last night, it's because I'm hosting a hive of evil spirits'.