Background <p>Vitamin D deficiency is common in people with obesity and can worsen after bariatric surgery. As the reproductive years and post-menopausal status can place additional demands on vitamin D requirements, we studied vitamin D status after bariatric surgery in women under 45 years of age compared with women over 45.</p> Methods <p>We conducted an observational cohort study of 305 women undergoing primary bariatric surgery at a university teaching hospital in North West England. Participants were stratified by age into women under 45 years (Wu45, <i>n</i> = 123) and over 45 years (Wo45, <i>n</i> = 182). Patients were routinely prescribed daily calcium and vitamin D supplementation after bariatric surgery. Serum 25-hydroxyvitamin D, adjusted calcium, parathyroid hormone (PTH) and metabolic parameters were measured preoperatively and at intervals over 24 months postoperatively.</p> Results <p>After bariatric surgery, vitamin D levels rose significantly within 4 months but were lower in Wu45 at 12 and 24 months (<i>p</i> &lt; 0.05). Adjusted calcium levels declined over time, with Wu45 showing significantly lower levels at 12 and 24 months. PTH levels, initially lower in Wu45, increased and equalized with Wo45’s levels by 12 months.</p> Conclusions <p>Women under 45 are at increased risk of vitamin D and calcium deficiencies after bariatric surgery. This may reflect higher physiological demands and variable adherence to supplementation. Patient education and tailored supplementation strategies may be required to prevent long-term micronutrient complications.</p>