Can vitamin D supplements improve bone health?

Maybe, but most likely only in people who have low vitamin D levels.

Vitamin D is essential for calcium absorption in the small intestine. When vitamin D levels are too low, blood calcium levels fall, and more calcium is released from bones to maintain circulating calcium levels in the body. Over time, this process results in bone demineralization^[1] and increases the risk of fractures. Prolonged and severe vitamin D deficiency can result in a condition known as rickets (in children) and osteomalacia (in adults), which is characterized by bone pain, soft bones, and skeletal deformities.

Although severe vitamin D deficiency is somewhat rare in middle- and high-income countries, suboptimal vitamin D levels are not uncommon, likely because of low sun exposure (which decreases the skin’s synthesis of vitamin D) and inadequate dietary intake (very few foods naturally contain vitamin D). Thus, it is plausible that vitamin D supplementation may improve bone health in certain people, but the results to date are mixed.

In some studies, supplementation with up to 800 IU/day of vitamin D reduced the risk of bone fractures in select individuals, such as those with low baseline levels of vitamin D and older adults.^[2][3] There is also evidence that combining vitamin D supplementation with calcium may improve BMD.^[4][5] Conversely, two large trials found that vitamin D supplementation had no effect on bone structure, BMD, or fracture incidence in healthy older adults.^[6][7]