Abstract Background Nutritional deficiencies are common after bariatric surgery, but few studies have examined them preoperatively. The objective of this study was to evaluate several vitamins, nutrients, and nutritional markers and their determinants in patients with obesity considering bariatric surgery. Methods Preoperative values of fasting plasma glucose, insulin, lipid profile, 25-hydroxyvitamin D (25(OH)D), parathyroid hormone, thyroid-stimulating hormone, calcium, phosphate, albumin, magnesium, total proteins, liver function tests, iron, ferritin, folate, vitamin A, vitamin B12, selenium, and zinc were evaluated in 267 Caucasian outpatients (74.2% women, aged 40.5±12.6 years) who were considering bariatric surgery. The determinants of nutrient variability were analyzed by linear regression for nutrients with a prevalence of deficiency>10%, i.e., serum 25(OH)D, iron, phosphate, magnesium, and vitamin A. Results Prevalence of inadequate concentrations was high for 25(OH)D (67.9% with values≤20 ng/mL), magnesium (35.4%), phosphate (21.6%), iron (18.8%), and vitamin A (16.9%). Multiple deficiencies were common; 28.5%, 12.1%, and 6.3% of patients had 2, 3, and 4 deficiencies, respectively. In multivariate analyses, metabolic characteristics had an important impact on deficiencies, with lower values of 25(OH)D and vitamin A with increasing body mass index, lower values of 25(OH)D and magnesium with increasing fasting plasma glucose, and a positive correlation between vitamin A and triglycerides. Elevated TSH was associated with low iron concentrations. Conclusion At all ages, micronutrient deficiencies were common, with high prevalence of concentration inadequacies for 25(OH)D, magnesium, phosphate, iron, and vitamin A. High body mass index and high fasting plasma glucose increased the risk of deficiencies, particularly for 25(OH)D. Preoperative screening and correction of deficiencies should be advised.