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Vitamin D and lower-extremity function in the elderly

 

Clinical bottom line


Bandolier has already reported on the effects of vitamin D supplements on fractures and falls. Reductions in fractures may be due in part to the known beneficial effect of vitamin D on calcium homeostasis, but also in part to a reduction in falls due to a beneficial effect on muscle strength. A new study lends further support for a direct effect of vitamin D on muscle

Reference

HA Bischoff-Ferrari et al. Higher 25-hydroxyvitamin D concentrations are associated with better lower-extremity function in both active and inactive persons aged 60 years or more. Am J Clin Nutr 2004 80: 752-758.


Study

The study was a population-based survey of 4,100 ambulatory, non-institutionalised adults, aged 60 to 90 years, who took part in the third National Health and Nutrition Examination Survey in the United States, and for whom data were available for physical performance relevant to lower-extremity function, and serum concentrations of 25-hydroxyvitaminD [25(OH)D] (the active form of vitamin D).

Lower-extremity function was assessed by timing an 8-foot walk (8-foot walk test; mean of two tests) and five repetitions of rising from a chair and sitting (sit-to-stand test). Serum 25(OH)D concentrations were determined by radioimmunoassay from venous blood samples. Information on medical comorbidities, activity levels, calcium intake, BMI, race/ethnicity and family income was also collected.

Results

The study population was 49% female, 58% non-Hispanic white, 19% non-Hispanic black, and 20% Mexican-American, with a mean age of 71 (8) years, and 25% classified as inactive. Mean 25(OH)D levels were significantly lower in inactive than active subjects, and lower in women than men. They were higher in whites than Mexican-Americans, and lowest in blacks. Performance in lower extremity function tests decreased with increasing age and increasing comorbidity.

Table 1: Lower extremity function for quintiles of 25(OD)D concentrations

Quintile

25(OD)D range

nmol/L

Reduction in time for test (s)*(95 %CI)
% change lowest to highest
P for trend
8-foot walk test
1
8.7 to 43.4
0 (reference)
2
43.7 to 56.7
0.17 (0.02 to 0.36)
3
56.9 to 69.4
0.22 (0.05-0.39)
4
69.6 to 85.9
0.20 (0.05 to 0.36)
5
86.1 to 400.1
0.27 (0.09 to 0.44)
5.6
0.001
Sit -to-stand test
1
8.7 to 43.4
0 (reference)
2
43.7 to 56.7
0.67 (0.24 to 1.11)
3
56.9 to 69.4
0.37 (0.16 to 0.91)
4
69.6 to 85.9
0.57 (0.13 to 1.01)
5
86.1 to 400.1
0.67 (0.23 to 1.11)
3.9
0.017
* adjusted for sex, age, BMI, race/ethnicity, poverty-income ratio, calcium intake, medical comorbidities, arthritis, use of walking device, month of assessment, activity level

 

While active subjects were faster than inactive subjects, improvement in performance with increasing 25(OH)D concentrations was seen for both tests. Regression plots show that improvement occurs across the whole range of 25(OH)D concentrations, but mostly between 22 and 40 nmol/L.

Comment

What this study does not tell us is how a 4 to 6% improvement in lower-extremity function translates into improved outcome for an individual, but we do know that lower-extremity function has strong predictive value for disability, falls and hip fractures in older adults. There is a plausible physiological mechanism by which vitamin D may affect muscle function, and a causal relation between vitamin D and musculoskeletal function has been shown in randomised controlled trials, while positive associations are seen in other cross-sectional studies. In an ageing population anything that will help reduce disability, falls and hip fractures is worth considering. It is estimated that 30-40% of older adults with hip fractures have low levels of vitamin D. Supplements are generally well-tolerated, safe and inexpensive, and even at low-doses could increase 25(OH)D concentrations to at least 40 nmol/L in most elderly people.

How much vitamin D do we need?

As a rough guide, our daily intake of vitamin D should be at least 5 g (200IU), although recommended intakes vary with age and from country to country. Since it is stored in the liver, high intakes (mg quantities/day) sustained for several weeks or months can be toxic.

Sources of vitamin D

It can be difficult to get enough vitamin D from the diet, and some foods such as breakfast cereals and margarine (and milk in the US) are fortified. Good food sources include cod liver oil (a teaspoonful provides over 30 g, so dosing twice a week might suffice), oily fish such as salmon, herring, sardines and mackerel, liver and egg yolks.

The most important source is from the sun via the action of UV light on the skin. Exposing the face, arms, hands or back to the sun for 10 to 15 minutes, two or three times a week, should provide enough vitamin D, but the effect is reduced for example by cloud cover (50%), shade (60%), and sun protection creams (100% for SPF 8 and above). Dark skin also reduces vitamin D synthesis.

Most people eating a healthy balanced diet and getting out into the sunshine should be fine. The elderly, however, are at risk of deficiency because the skin becomes less efficient at synthesising vitamin D and the kidneys less efficient at converting it to the active form with increasing age. Elderly people may also be less likely to get out into the sun or to eat enough foods rich in the vitamin.