This review from 2020 included 11 eligible studies evaluating the effectiveness of boron supplementation (alone or with other micronutrients) on growth and maintenance of bone in humans through control of calcium, vitamin D and sex steroid hormone metabolism. Authors concluded supplementation of 3 mg / day is considered useful to support bone health (in order to prevent and maintain adequate bone mineral density).
Rondanelli, M., Faliva, M. A., Peroni, G., Infantino, V., Gasparri, C., Iannello, G., Perna, S., Riva, A., Petrangolini, G., & Tartara, A. (2020). Pivotal role of boron supplementation on bone health: A narrative review. Journal of trace elements in medicine and biology : organ of the Society for Minerals and Trace Elements (GMS), 62, 126577. https://doi.org/10.1016/j.jtemb.2020.126577