Why is vitamin D status important in older adults?
Vitamin D supplementation has been found to reduce the incidence of osteoporotic fractures in the elderly. This may occur via increased bone mineralization and/or improved muscle function and reduction in falls. According to the Institute of Medicine, vitamin D deficiency is defined as a serum 25-hydroxyvitamin D (25[OH]D) level < 20 ng/mL (50 nmol/L). National and international groups have suggested that a 25(OH)D level ≥ 30 ng/mL (75 nmol/L) is needed to minimize fracture and fall risk. There are insufficient data to recommend a safe upper limit of 25(OH)D.
It has been estimated that > 40% of community-dwelling older women and men in the United States are vitamin D deficient, and the prevalence is even higher in nursing home residents. There are multiple causes of vitamin D deficiency in older adults, including decreased sun exposure; decreased skin synthesis; decreased intake; impaired absorption, transport, or liver hydroxylation of oral vitamin D; medications altering vitamin D metabolism (e.g., drugs that induce p450 enzymatic activity leading to decreased circulating levels of 25[OH]D); chronic illnesses associated with malabsorption; and liver and kidney diseases.
Bone mineral density is adversely affected when serum 25(OH)D levels are < 30 ng/mL. Although vitamin D 3 supplementation has been found to raise serum 25(OH)D levels above 30 ng/mL, recent meta-analyses did not find that vitamin D 3 (alone or in combination with calcium) supplementation reduced fracture risk among community-dwelling older adults without known vitamin D 3 deficiency, osteoporosis or prior fracture. There is currently no evidence for antifracture efficacy of vitamin D 2 supplementation.
Vitamin D deficiency also causes muscle weakness. Proximal muscle strength is linearly related to serum 25(OH)D when levels are < 30 ng/mL. Vitamin D supplementation has been associated with a 22% reduction in falls. Nursing home residents randomized to receive 800 IU/day of vitamin D 2 plus calcium had a 72% reduction in falls.
In addition to its important role in muscle and bone metabolism, vitamin D deficiency is postulated to influence immune function, cancer risk, parathyroid hormone and renin production, and insulin secretion. Epidemiological studies demonstrate higher mortality in patients with insufficient or deficient levels of 25(OH)D.