Abstract
Background/Objectives: Dietary patterns play a crucial role in musculoskeletal health; however, the effects of different diets on bone mineral density (BMD), fracture risk, and bone metabolism remain inconsistent across studies. This systematic review and meta-analysis aimed to evaluate the impact of Mediterranean, calorie restriction, high-protein, low-carbohydrate, and ketogenic diets on skeletal outcomes in adults. Methods: A comprehensive search of PubMed/MEDLINE, CENTRAL, and Web of Science was conducted for studies published between January 2000 and June 2025. Eligible randomised controlled trials (RCTs) and cohort studies involving adults (≥18 years) and reporting outcomes related to BMD, fractures, bone turnover markers, and vitamin D or calcium status were included. Risk of bias was assessed using the Cochrane's Risk of Bias tool for RCTs and the Joanna Briggs Institute checklist for observational studies. Random-effects meta-analyses were performed for outcomes reported by ≥3 comparable studies, presenting standardised mean differences (SMDs) for BMD and hazard ratios (HRs) for fractures. Results: Thirty studies met inclusion criteria, comprising 14 RCTs and 16 observational studies with over 500,000 participants. Pooled analyses showed no significant differences in BMD at the femoral neck (SMD = 0.12, 95% CI -0.80 to 1.04), lumbar spine (SMD = 0.04, 95% CI: -1.12 to 1.03), total hip (SMD = -0.07, 95% CI -0.36 to 0.21), or whole body (SMD = 0.03, 95% CI -0.07 to 0.14) across diet categories. However, adherence to a Mediterranean diet was associated with a significantly reduced hazard of hip and overall fractures (pooled HR = 0.95, 95% CI 0.93-0.96). Calorie restriction consistently increased bone resorption markers, whereas Mediterranean and high-protein diets showed neutral or modestly favourable effects. Vitamin D and calcium status were minimally affected across interventions. Conclusions: While dietary patterns exert diverse effects on skeletal health, consistent evidence supports Mediterranean-style diets as protective against fractures. Calorie restriction may elevate bone turnover, whereas ketogenic and high-protein diets show mixed effects on bone. However, across all analyses, high heterogeneity was observed. Further high-quality RCTs are warranted to clarify these relationships and inform dietary guidance for bone health.