BOMSS (British Obesity and Metabolic Surgery Society) have produced a document for guidelines regarding pre-op nutritional status blood testing, post-op testing and possible supplementation regimes for bariatric surgeries including gastric balloon, gastric band, sleeve gastrectomy, gastric bypass and BPD/DS.
It is in recognition of the fat there has been little consensus regarding the supplementation and testing needed for bariatric surgery and the need for pre-op deficiencies to be diagnosed and treated as part of the pre-op work-up.
It should be stressed that these are guidelines and that each bariatric team might have their own requirements as well.
The document (although lengthy) is most informative and can be accessed here.