Inadequate levels of vitamin D may help to explain not only morbidities such as osteoporosis but also less-appreciated effects of vitamin D insufficiency that worsen bodily functions and are commonly thought to be related to aging alone, Dr. Neil Binkley said at a conference sponsored by the American Society for Bone and Mineral Research.
“I would like to suggest to you that vitamin D inadequacy might be contributing to what we are currently accepting as old age-related morbidity,” said Dr. Binkley, co-director of the University of Wisconsin Osteoporosis Clinical Center and Research Program, Madison.
The prevalence of densitometric osteopenia markedly increases with advancing age, and at any given bone density, age has a “profound impact” on the risk of fracture, he said (J. Clin. Invest. 1988;81:1804–9).
But many conditions other than osteoporosis that are affected by vitamin D status have been labeled as “age-related” morbidities, including sarcopenia, falling, overactive bladder, swallowing dysfunction, worsening lung function, macular degeneration, and cognitive decline.
“I'd ask you to always consider that perhaps some of these other age-related morbidities are what are causing this dramatic effect of age on fracture,” he said.
▸ Sarcopenia. The expression of vitamin D receptors declines in muscle with aging. In muscle, vitamin D also may be involved with calcium transport and actin-myosin interaction.
A study of 1,008 older adults has suggested that vitamin D inadequacy is associated with sarcopenia. After a 3-year follow-up, men and women with baseline 25(OH)D levels less than 25 nmol/L were more than twice as likely to develop sarcopenia (based on either grip strength or muscle mass) than were those with a higher level of 25(OH)D (J. Clin. Endocrinol. Metab. 2003;88:5766–72).
▸ Falling. It is not known whether vitamin D status and muscle strength are causally related, but “it is, however, clear that vitamin D status is related to the risk of falling in both older men and older women,” Dr. Binkley said.
The risk of falling is increased by orthopedic disabilities, visual impairment, central or peripheral neurologic dysfunction, and muscle weakness, which may be the main risk factor, he said. A meta-analysis of double-blind, randomized, trials showed that vitamin D reduced the risk of falling by 22% (JAMA 2004;291:1999–2006).
▸ Overactive bladder. Bladder dysfunction also may be associated with muscle weakness, leading to poorer coordination of the muscles used to control urination. Overactive bladder affects 30%–40% of adults older than 75 years of age and two-thirds of nursing home residents; it is defined as urinary urgency with or without incontinence, usually with frequency and nocturia.
In a study of nearly 6,000 community-dwelling women aged 40 years or older, those in the highest quintiles of vitamin D intake had the lowest risk of developing overactive bladder (Neurourol. Urodyn. 2004;23:204–10).
▸ Difficulty swallowing. Up to 40% of individuals older than 60 years have problems swallowing, which can lead to undernutrition, sarcopenia, and aspiration pneumonia. Dysphagia associated with aging classically has been felt to reflect neurologic disease such as Parkinson's or stroke, but more recent work has shown that even normal healthy adults swallow more slowly and generate lower tongue pressures than do younger adults.
“I think it's at least plausible that this decreased muscle function might be causally related to the increased risk of dysphagia observed with advancing age,” Dr. Binkley suggested.
But no research has been conducted on vitamin D status and the risk of dysphagia of aging, he said.
▸ Pulmonary function. Both the forced expiratory volume in the first second after taking a deep breath and forced vital capacity are known to decline with aging; poor results on such tests are associated with substantial morbidity and mortality. In a study of people in the National Health and Nutrition Examination Survey III (NHANES III) who were aged 60 years or older, both of those measures of lung function were significantly higher among people in the highest quintile of serum 25(OH)D concentration than in individuals in the lowest quintile of the vitamin (Chest 2005;128:3792–8).
Biologically plausible ways in which vitamin D might protect against a decline in pulmonary function include the possibility of a decline in respiratory muscle function with inadequate levels of vitamin D, lung tissue remodeling, or a reduction in airway inflammation.
▸ Age-related macular degeneration. In a yet-to-be published study involving 7,752 people who participated in NHANES III, the risk of developing age-related macular degeneration declined steadily from the lowest to the highest quintiles of serum 25(OH)D concentration.
▸ Dementia/cognitive decline. A poster that was presented at the conference showed that higher scores on the Mini-Mental State Examination in 32 patients in a memory clinic were significantly and positively correlated with higher vitamin D concentrations.
In the observational study, 25(OH)D levels below 30 ng/mL—the generally recommended cutoff for vitamin D sufficiency—were detected in 25 patients, Dr. Binkley said.
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