Vitamin D increases calcium absorption, and blood levels of vitamin D are directly related to the strength of bones.¹ Mild deficiency of vitamin D is common in the fit, active elderly population and leads to an acceleration of age-related loss of bone mass and an increased risk of fracture.² In double-blind research, vitamin D supplementation has reduced bone loss in women who consume insufficient vitamin D from food and slowed bone loss in people with osteoporosis^3,4 and in postmenopausal women.⁵ However, the effect of vitamin D supplementation on osteoporosis risk remains surprisingly unclear,^6,7 with some trials reporting little if any benefit.⁸ Moreover, trials reporting reduced risk of fracture have usually combined vitamin D with calcium supplementation,⁹ making it difficult to assess how much benefit is caused by supplementation with vitamin D alone.¹⁰

Impaired balance and increased body sway are important causes of falls in elderly people with osteoporosis.¹¹ Vitamin D works with calcium to prevent some musculoskeletal causes of falls.¹² In a double-blind trial, elderly women who were given 800 IU per day of vitamin D and 1,200 mg per day of calcium had a significantly lower rate of falls and subsequent fractures than did women given the same amount of calcium alone.¹³ Vitamin D in the amount of 800 IU per day effectively prevented falls in a double-blind study of elderly nursing home residents, but lower amounts were ineffective.¹⁴

Despite inconsistency in the research, many doctors recommend 400 to 800 IU per day of supplemental vitamin D, depending upon dietary intake and exposure to sunlight.

One trial studying postmenopausal women combined hormone replacement therapy with magnesium (600 mg per day), calcium (500 mg per day), vitamin C, B vitamins, vitamin D, zinc, copper, manganese, boron, and other nutrients for an eight- to nine-month period.¹⁵ In addition, participants were told to avoid processed foods, limit protein intake, emphasize vegetable over animal protein, and limit consumption of salt, sugar, alcohol, coffee, tea, chocolate, and tobacco. Bone density increased a remarkable 11%, compared to only 0.7% in women receiving hormone replacement alone.

Vitamin D is needed to regulate immune activity and research has shown it has an important role in preventing autoimmune diseases, including type 1 diabetes.²¹ Vitamin D receptors have been found in the pancreas where insulin is made and some, but not all, preliminary evidence suggests that supplementation might reduce the risk of developing type 1 diabetes.^22,23 Case reports even suggest a combination of high-dose vitamin D and omega-3 fatty acids may completely reverse the onset of type 1 diabetes.^24,25

Vitamin D deficiency is common in people with type 1 diabetes and associated with poor glucose control. Supplementation with cholecalciferol (vitamin D3), in doses ranging from 2,000 IU per day to about 6,000 IU per day, has been shown in randomized controlled trials to slow the decline of pancreatic function in people with newly diagnosed type 1 diabetes who have not yet suffered an extensive loss of pancreatic function.²⁶ Furthermore, the majority of the research shows vitamin D3, in doses ranging from 2,000–10,000 IU per day, improves short and long term glycemic control in people with type 1 diabetes, particularly in those with low vitamin D status, possibly by preserving beta cell function and increasing insulin production.^{27,28,29,30,31,32}

Vitamin D malabsorption is common in Crohn’s³³ and can lead to a deficiency of the vitamin.³⁴ Successful treatment with vitamin D for osteomalacia (bone brittleness caused by vitamin D deficiency) triggered by Crohn’s disease has been reported.³⁵ Another study found 1,000 IU per day of vitamin D prevented bone loss in people with Crohn’s, while an unsupplemented group experienced significant bone loss.³⁶ In addition, in a double-blind trial, vitamin D supplementation (1,200 IU per day for 12 months) prevented relapses in patients with Crohn's disease in remission. The patients in that study had normal vitamin D status prior to receiving vitamin D supplementation.³⁷ In a preliminary study of patients with mild-to-moderate Crohn's disease, vitamin D supplementation for 24 weeks was associated with an improvement in disease activity. The amount used in that study was 1,000 to 5,000 IU per day, based on blood levels of the vitamin.³⁸ A doctor should be consulted to determine the right level of vitamin D for supplementation.