Is Vitamin D Fortification of Dairy Products Effective for Improving Vitamin D Status? A Systematic Review and Meta-Analysis of Randomised Controlled Trials

<b>Background/objectives</b>: Given the suboptimal vitamin D intake and status among the United Kingdom population, the Scientific Advisory Committee on Nutrition is seeking suitable food vehicles for vitamin D fortification. Thus, this study aimed to examine the efficacy of vitamin D-fortified dairy products in improving serum 25-hydroxyvitamin D [25(OH)D] concentration using data from randomised controlled trials (RCTs). <b>Methods</b>: PubMed, Embase, and Web of Science were searched from inception until October 2024. Studies were included if they were RCTs with intervention groups administered vitamin D-fortified dairy products and control groups administered unfortified dairy products, as well as examining the effects on serum 25(OH)D concentration. <b>Results</b>: There were 35 RCTs eligible for inclusion, involving 4965 participants (intervention: 2526; control: 2439). The results showed that serum 25(OH)D concentrations were significantly increased by vitamin D₃-fortified milk/milk powder (<i>n</i> = 15, mean difference (MD): 18.31 nmol/L, 95% confidence interval (CI): 13.30-23.33 nmol/L, <i>I</i>² = 95%), vitamin D₃-fortified yoghurt/yoghurt drinks (<i>n</i> = 11, MD: 26.22 nmol/L, 95% CI: 18.67-33.77 nmol/L, <i>I</i>² = 97%), vitamin D₂-fortified milk/milk powder (<i>n</i> = 3, MD: 11.61 nmol/L, 95% CI: 9.31-13.91 nmol/L, <i>I</i>² = 0%), vitamin D-fortified (type not specified) milk/milk powder (<i>n</i> = 8, MD: 13.59 nmol/L, 95% CI: 8.54-18.64, <i>I</i>² = 98%), and vitamin D-fortified (type not specified) yoghurt/yoghurt drinks (<i>n</i> = 4, MD: 27.74 nmol/L, 95% CI: 16.83-38.64 nmol/L, <i>I</i>² = 91%), but insignificantly increased by vitamin D₃-fortified cheese (<i>n</i> = 5, MD: 16.78 nmol/L, 95% CI: -3.61-37.16, <i>I</i>² = 99%). However, the results of vitamin D₃-fortified cheese became significant when leave-one-out analysis was performed by omitting one RCT (MD: 24.13 nmol/L, 95% CI: 4.69-43.58, <i>I</i>² = 90%). <b>Conclusions</b>: These findings provide evidence that vitamin D-fortified dairy products have the potential to improve serum 25(OH)D concentrations in populations.