Vitamin D Deficiency and Associated Illnesses

Severe vitamin D deficiency definitively causes bone mineralization diseases, including rickets in children and osteomalacia in adults, as a result of prolonged, severe deficiency 2
The American College of Physicians suggests that osteoporosis and increased fracture risk occur when moderate deficiency leads to elevated parathyroid hormone, triggering bone resorption, although the strength of evidence is moderate 1, 2
The only well-established causal relationships with vitamin D deficiency are skeletal consequences, including bone mineralization diseases 1, 2

Associated Non-Skeletal Conditions

The American Heart Association notes that lower vitamin D levels show associations with multiple conditions, including cardiovascular mortality, stroke, heart failure, and sudden cardiac death, though causality is not proven 3
The relationship between low 25(OH)D levels and cardiovascular disease appears inconsistent across populations, with white populations showing stronger associations than African Americans, based on moderate-strength evidence 1, 4
The American Cancer Society reports that colorectal cancer shows the most consistent inverse relationship with vitamin D levels, with a moderate level of evidence 2
Breast cancer risk may be reduced with levels above 20 ng/mL, though prospective studies show heterogeneous results, and the strength of evidence is low 5, 3
One randomized trial found 60% lower cancer incidence with 1,100 IU/day supplementation over four years, but this was a secondary outcome, and the evidence is considered low 5

The American Diabetes Association notes that type 1 diabetes risk reduction of 78% was observed with 2,000 IU/day supplementation during infancy in one Finnish cohort, based on moderate-strength evidence 5, 3
Multiple sclerosis and rheumatoid arthritis show associations with low vitamin D levels, although the strength of evidence is low 5, 3
Animal models suggest vitamin D modulates immune differentiation and proliferation, with a low level of evidence 5

The American Geriatrics Society reports that falls and functional limitations, particularly in institutionalized elderly populations, are associated with low vitamin D levels, based on moderate-strength evidence 1, 2
Depression, diabetes, and cognitive impairment show associations in observational studies, although the strength of evidence is low 2

The U.S. Preventive Services Task Force concluded that current evidence is inadequate to determine whether screening for and treating asymptomatic low vitamin D levels improves clinical outcomes in community-dwelling adults, with a low level of evidence 1, 7
No consensus exists on the threshold defining deficiency, and associations vary by cut-point used, population, and setting, based on low-strength evidence 1, 7
Most evidence comes from observational studies, not randomized controlled trials with clinical outcomes as primary endpoints, resulting in a low level of evidence 5

Depending on the threshold used, vitamin D deficiency affects 19-77% of the U.S. population, with a moderate level of evidence 1, 2, 6
33% have levels ≤20 ng/mL, and 77% have levels <30 ng/mL, based on moderate-strength evidence 2, 6, 1, 2