Vitamin D and muscle function in older people

Vitamin D deficiency is a worldwide problem particularly among older people, who are more susceptible due to diminished vitamin D synthesis in the skin and low dietary intake of vitamin D. Vitamin D has been associated with various (patho)physiological functions including muscle function. It is important to determine the effect of vitamin D on muscle function in older people because of the possible consequences for mobility. In 70 frail vitamin D insufficient older women the most important determinants of mobility were knee extension strength, physical activity and use of a walking aid. Fifty-three percent of the women had experienced a fall in the previous 6 months. Low knee extension strength, worse performance on the mobility tests, use of benzodiazepines and use of a walking aid were associated with fall occurrence. In 802 independently living volunteers between 40 and 80 yrs of age, 36% of the men and 51% of the women had a low vitamin D status (serum hydroxyvitamin D <50 nmol/L). Overall, male gender and season were the most important determinants of serum 25-hydroxyvitamin D concentration. This gender difference was most outspoken in the summer and disappeared in the winter and spring. In men, season and physical activity were the only significant determinants of vitamin D status, which shows that in healthy Dutch male volunteers up to an age of 80 years, most of vitamin D is produced in the skin via sun-exposure due to outdoor activities. In contrast, in the women no seasonal variation in vitamin D status was found and household and sport physical activity were positively associated with vitamin D and blood estradiol concentration negatively. In this population of men and women between 40-80 yrs of age, a significant association between serum 25-hydroxyvitamin D and lean mass was found below a 25-hydroxyvitamin D level of 60 nmol/L, and absent above this threshold. For each nmol/L 25-hydroxyvitamin D higher, lean mass increased by almost 80 grams, after correcting for various confounders. We also found a significant association between 25-hydroxyvitamin D and handgrip strength and physical performance below 60 nmol/L, but the magnitude of the effect was smaller as compared with lean mass. These results indicate that there may be a ceiling effect in the relation between 25-hydroxyvitamin D and muscle function. In a randomized controlled trial in 70 frail older women, (25-hydroxyvitamin D concentration 20 - 50 nmol/L), 6 months of 400 IU/d cholecalciferol + 500 mg/d calcium supplementation significantly improved 25-hydroxyvitamin D concentration as compared with calcium mono-therapy (77.2 vs 41.6 nmol/L respectively). At baseline, a significant association between 25-hydroxyvitamin D and handgrip strength, knee extension strength, leg extension power and mobility was found. Improving serum 25-hydroxyvitamin D concentration did not improve handgrip strength, isometric knee extension strength, leg extension power, nor functional mobility as compared with calcium mono-therapy after 6 months. However, the group was relatively small and standard deviations in the outcome measures were large. Further, presence of comorbidity, affecting muscle strength and mobility as well, might explain the negative results.