Effect of vitamin D nutritional status on muscle function and strength in healthy women aged over sixty-five years

Abstract

Background

Vitamin D insufficiency is common in elderly adults, and leads to secondary hyperparathyroidism, bone loss, muscle weakness, and osteoporotic fractures.

Objective

To evaluate the relation between vitamin D nutritional status and muscle function and muscle strength in women aged over 65 years.

Methods

Fifty-four postmenopausal women from Buenos Aires (latitude 34° S), average age (X±DS) 71±4, were included in the study. Determinations of serum calcium, phosphate, 25 hydroxyvitamin D (25OHD), intact parathormone (iPTH) and calciuria / creatininuria ratio in 24-hour urine samples were performed. Muscle function was assessed by means of walking-speed test, standing balance, and sit-to-stand tests. lower extremity muscle strength was determined using a manual dynamometer.

Results

25OHD levels ≥20ng/ml were found to be associated with better lower extremity muscle function and strength. Forty- six % of participants had 25oHD levels ≥20ng/ml. Women with 25OHD levels ≥20ng/ml scored higher on the muscle function tests (11.2±0.9 vs.10.0±2.1; p<0.003) and had stronger knee extensor (13.4±2.7 vs.11.6±2.5 Kg.; p<0.03) and hip abductor (8.3±2.7 vs. 7.3±3.1 Kg; p<0.04) muscles; strength of their hip flexors tended to be higher but did not reach significantly different values (17.0±3.3 vs. 15.4±2.8 Kg.; 0.1>p>0.05). Negative correlation was observed between iPtH and muscle function (r= −0.436; p<0.02).

Conclusion

25OHD levels ≥20ng/ml are needed for a better muscle function and strength. assessing vitamin D nutritional status in adults aged ≥ 65 years would allow correcting hypovitaminosis D and improve muscle function and strength.

Introduction

Vitamin D insufficiency is a frequent clinical condition in elderly adults, and is partly due to reduced sun exposure, low dietary vitamin D intake, and a decrease in the capacity of the skin to synthesize vitamin D in persons over the age of 65 years.

Vitamin D insufficiency is associated with increased fracture risk since it leads to an increase in parathormone secretion and in bone remodeling which in turn results in bone mass loss, thus causing bone fragility (1, 2, 3).

According to the Cochrane Review published in 2003, 50% of fractures were non-vertebral fractures mainly caused by falls, and 30% of falls reported annually were suffered by adults aged over 65 years (4, 5).

There are studies reporting that a 700 to 800 IU/day dose of vitamin D reduced the relative risk of hip fractures by 26% and of non-vertebral fractures by 23% compared with administration of calcium or placebo. Treatment with 400IU/day of vitamin D₂ had no significant effects on the risk of hip and non-vertebral fractures (6, 7). The protective effect of vitamin D against fractures is attributed to its positive effect on calcium homeostasis, which suppresses PTH secretion, inhibiting bone remodeling and increasing bone mineral density, in addition to its effect on factors affecting muscle tissue (8, 9, 10, 11, 12).

Prevalence of vitamin D insufficiency is determined by factors including population characteristics, dietary fortification, latitude, and season of the year. Prevalence of vitamin D insufficiency is high in elderly adults in Argentina. Oliveri et al assessed vitamin D nutritional status in 339 healthy subjects aged > 65 years, living in seven cities located in different regions in Argentina (latitude 26°-55° south). The authors observed that 52 to 87% of community-dwelling elderly subjects had 25 OHD levels <20ng/ml (13). Plantalech et al found that 61% of elderly subjects in Buenos Aires city (latitude 34° S) had 25OHD levels <20ng/ml and only 6% exhibited adequate vitamin D levels (>30ng/ml) (14).

Studies conducted over the last years have defined optimal vitamin D nutritional status as the circulating 25(OH)D level needed to suppress PTH levels and thus attain calcium homeostasis (15). Although the optimal 25(OH)D level required for adequate muscle function and strength has not been established to date, there are reports indicating that 25(OH)D levels >20ng/ml are associated with improved physical performance (16, 17).

The aim of the present study was to analyze the relation between vitamin D nutritional status and muscle function and strength in a group of healthy women aged over 65 years.

Materials and methods

Population

Ninety women aged over 65 years presenting spontaneously at the outpatients offices of the our Section of the Buenos aires University Hospital for bone health assessment and attending Senior Citizen Centers of the Buenos Aires city government (latitude 34° S) were asked to participate in the present study which was conducted from August of 2006 to July of 2007 .

Seventy women were willing to participate; 15 were excluded based on the following exclusion criteria: 1-Administration of vitamin D or any medication known to affect mineral metabolism within the 12 months prior to the onset of the study; 2- Presence of a medical condition affecting vitamin D metabolism (hepatic diseases, renal insufficiency) or a neurological or motor disorder. Women needing mobility walking aids were not included in the study. The study protocol was approved by the Ethics Committee of the Clinical Hospital, and all participants gave their written informed consent. After signing the consent form, 55 women underwent densitometric assessment of femoral neck, total skeleton, and body composition, and biochemical determinations of mineral metabolism. Hypercalciuria and hypercalcemia was detected in only one subject, who was diagnosed with primary hyperparathyroidism and was excluded from the study. The final study population comprised 54 women. The sequence of subject recruitment, assignment to a group, and follow-up is shown in figure 1.

Figure 1.

Subject recruitment and assignment to groups

Methods

Presence of Chronic Diseases

The following medical conditions were evaluated during the patient interviews: presence of cardiovascular diseases (myocardial infarction, congestive cardiac failure, arrhythmias, hypertension), lung diseases (chronic obstructive pulmonary disease, asthma, and emphysema), diabetes, osteoarthritis, and hip fracture. All these diseases are known to be associated with impaired mobility (18).

Sun exposure assessment

Sun exposure was evaluated indirectly in terms of the number of weekly hours each participant spent outdoors. In addition, the subjects were asked about the kind of clothing they wore, which was typed as follows: type 1: pants, skirts, shorts, or dresses; type 2: long sleeves, short sleeves, tank tops; Type 3: face. A modified version of “the rule of nine” was used to estimate the body surface area (BSA) exposed to sunlight (19). The sun exposure index was calculated as follows: hours of sun exposure body X surface area (BSA) exposed to sunlight. All subjects were asked about use of sunscreens (20, 21).

Nutritional Aspects

Protein (mg/day), calcium (mg/day), vitamin D (μg/day) and calorie (kcal/day) intake was determined by a physician nutrition specialist using a food frequency questionnaire.

Muscle function and strength assessment

Lower- extremity muscle function was assessed by using an 8-foot (2.4m) walking- speed test (8-foot walk test; mean of 2 trials), and a sit-to-stand test (time test of 5 repetitions of rising from a chair and sitting down). To test standing balance, the subjects were asked to attempt maintaining their feet in the side-by-side, semi-tandem (heel of one foot beside the big toe of the other foot) and tandem (heel of one foot directly in front of the other foot) positions for 10 seconds each. Subjects were instructed by the physiatrist investigator and familiarized to the testing procedure with three tries before performing the evaluation in order to obtain acceptable reproducible trials.

The above tests were described by JM Guralnik et al and are considered indicators of coordination, balance, and strength of the proximal lower limb muscles (22). The time taken to perform each of the tests was scored from 1 to 4. The value obtained by adding the points scored on each of the tests was the muscle function score; maximum score was 12 and minimum score was 0 indicating that the subject did not have the capacity or ability to perform the test.

Muscle strength of hip flexors and abductors and knee extensor muscles was assessed using a manual dynamometer (Nicholas Muscle Tester (MMT) model 01160). Muscle strength of hip flexors and abductors and knee extensor muscles was determined in triplicate using a handheld dynamometer, taking into account the maximum value recorded in each determination. The results are expressed in kilograms (kg). All the muscle function and strength determinations were performed by the same operator who was blinded to the study subjects' clinical and biochemical data.

Biochemical determinations

Twelve hour fasting blood and 24-hour urine samples were collected at the onset of the study to perform mineral metabolism determinations. Serum calcium and phosphorus and urinary calcium and urinary creatinine ratio (uCa/uCr) were measured using standardized techniques (23, 24, 25). Serum levels of 250HD were determined using radioimmunoassay (DIASORIN) (inter and intra-assay coefficients of variation were 19 and 7.6 % respectively) and intact parathormone (iPTH) was measured using an electrochemical-luminescence assay (ECLIA-R0CHE) (inter and intra-assay coefficients of variation were 3.3 and 2.2% respectively).

Lower limb lean mass assessment

Lower limb lean mass (total, right, and left) was determined by dual energy X-ray absorptiometry (Lunar-DPX, Madison, USA). The presence of sarcopenia (SAR) was determined using the relative skeletal muscle index (RSMI), which was calculated by dividing the appendicular bone free lean body mass in kg (FBLBM) (bone-free lean mass in the arm plus leg) by height in m². The criterion for SAR was defined as having an RSMI value below 5.45 kg/m² (26).

Statistical Analysis

Results were analyzed using statistical SPSS 11.5 software for Windows (SPSS, Inc, Chicago, IL, USA). Results are expressed as mean and standard deviation (X ± SD). Given that the studied variables did not follow a normal distribution pattern, comparison between both groups was performed using Mann-Whitney's non-parametric test. A test of proportions was used to detect differences between percentages, and Pearson and Spearman's correlation coefficients were used to establish association among variables. The third quartile estimation was used to determine the cut-off point. A value of p below 0.05 was considered statistically significant.

Results

The statistical study was performed on data corresponding to 54 postmenopausal women. Results are shown in Table 1. The mean age (X±SD) of the total population was 71±4 years and 25OHD 21±9ng/ml (range:7-46ng/ml). Analysis of 25OHD levels associated with a significant decrease in muscle function and strength showed a cut off point of 20ng/ml. So that the subjects were divided in two groups (G) accordingly: G1 (n=25) =20ng/ml and G2 (n= 29) <20ng/ml (Table 2).

Table 1.

General characteristics of the 54 postmenopausal women (mean ±standard deviation)

Age (years)	71±4
Weight (Kg.)	65±9
Height (cm)	154±6
BMI (Kg/m2)	27±4
250HD(ng/ml)	21±9
iPTH (pg/ml)	44±17
Body Composition
Total Lower limb lean mass (g)	11321±2031
Right lower limb lean mass (g)	5763±809
Left lower limb lean mass (g)	5777±689
RSMI	6.5±0.7
Chronic Diseases (%/n)
Cardiovascular diseases	66(36/54)
Diabetes	7(4/54)
Arthrosis	66(36/54)
Pulmonary diseases	5(3/54)
Nutritional status
Calcium intake (μg/day)	830±700
Calorie intake (Kcal/day)	1778±375
Protein intake (g/day)	96±130
Vitamin D intake (μg/day) ∗	4±2

BMI: body mass index; RSMI: relative skeletal muscle index;

^∗Recommended intake for subjects aged 50-70 years:10μg/day and = 71years:15μg/day(43)

Table 2.

Baseline characteristics of women with 250HD levels = and <20ng/ml (mean ±standard deviation)

	25OHD =20ng/ml (n=25)	25OHD <20ng/ml (n=29)	Value p
Age (years)	70.4±4	71.6±5	0.54
Weight (Kg.)	63.9±9	66.0±9	0.59
Height (cm)	154.4±7	154.7±4	0.72
BMI (Kg/m2)	26.7±3	27.4±4	0.59
250HD (ng/ml)	29.6±6	13.3±4	<0.001
iPTH (pg/ml)	39.0±10	48.9±20	0.07
Body Composition
Total Lower limb lean mass (g)	11421±1027	12236±1592	0.40
Right lower limb lean mass (g)	5690±1027	5825±574	0.41
Left lower limb lean mass (g)	5733±881	5815±482	0.25
RSMI	6.4±0.7	6.5±0.8	0.21
Chronic Diseases (%/n)
Cardiovascular diseases	56 (14/25)	75(22/29)	0.14
Diabetes	4 (1/25)	10(3/29)	0.39
Arthrosis	64 (16/25)	68(20/29)	0.75
Pulmonary diseases	0	10(3/29)
Nutritional status
Calcium intake (mg/day)	757±377	721±348	0.59
Calorie intake (Kcal/day)	1683±348	1865±385	0.10
Protein intake (g/day)	83±17	74±22	0.11
Vitamin D intake (μg/day)∗	5±2	4±2	0.34

BMI: body mass index; RSMI: relative skeletal muscle index;

^∗Recommended intake for subjects aged 50-70 years: 10μg/day and = 71years: 15μg/day (43)

Chronic diseases

Fifty-six percent (14/25) of participants with 250HD levels =20ng/ml reported a history of cardiovascular disease, 4% (1/25) reported non-insulin dependent type II diabetes, and 64% (16/25) reported osteoarthritis. As regards those with 250HD levels <20ng/ml, 75% (22/29) reported cardiovascular disease, 10% (3/29) had non-insulin dependent type II diabetes, 68% (20/29) had osteoarthritis, and 10% (3/29) reported chronic pulmonary disease. No significant differences were observed between groups (Table 2).

Sun exposure

Women with 250HD levels =20ng/ml reported an average exposure to sunlight of 9 weekly hours, and only 24% (6/25) reported using sunscreen. Evaluation of indirect exposure to sunlight in this group showed that the fraction of BSA was 0.26 (0.14-0.43), and average sun exposure score was 2.2. Those with 250HD levels <20ng/ml reported an average exposure to sunlight of 12 weekly hours, and only 10% used sunscreens. BSA of this group was 0.30 (0.14-0.43) and average sun exposure score was 3.3. No statistically significant differences were observed between groups.

Nutritional Aspect

No significant differences in calcium, protein, vitamin D or calorie intake were observed between subjects with 250HD levels =20ng/ml and <20ng/ml. Results were as follows: calcium intake: 757+377 vs. 721±348mg/day, protein intake: 83±17 vs. 74+22 g/day, vitamin D 5+2 vs. 4+2μg/day and calorie intake: 1683+348 vs. 1865+385 Kcal/day respectively (Table 2).

Biochemical determinations

Table 3 shows the results of the biochemical determinations. Significant differences in serum 250HD levels were observed between subjects with 250HD levels =20ng/ml and <20ng/ml (29±6ng/ml vs.13±3ng/ml;p<0.001). Serum iPTH values tended to be higher (+18%) in subjects with 250HD<20ng/ml than in women with 250HD =20ng/ml (48±20 vs. 39±10; ns). No significant differences in serum calcium, serum phosphorus and uCa/uCr were observed between the groups.

Table 3.

Biochemical determination corresponding to the women with levels of 250HD =20ng/ml and 250HD<20ng/ml (mean ±standard deviation)

	iPTH (6-65pg/ml)	25OHD∗	Serum Calcium (8.9-10.4mg/dl)	Serum Phosphorus (2.6-4.4mg/dl)	uCa/uCr (0.25 mg/mg)
250HD =20ng/ml (n= 25)	39±10	29±6	9.8±0.4	3.4±0.4	0.16±0.7
250HD < 20ng/ml (n= 29)	48±20	13±3	9.8±0.4	3.6±0.4	0.13±0.8
p	ns	0.001	ns	ns	ns

^∗250HD: Deficiency: <10ng/ml; Insufficiency D: 10-29ng/ml; Mild Hypovitaminosis D: 20-29ng/ml; Desirable:>40ng/ml (44)

Body Composition determinations

No significant differences in total (11421±1027 vs. 12236±1592 g; p<0.40), right (5690±1027 vs. 5825±574 g; p<0.41), or left (5733±881 vs. 5815±482 g; p<0.25) lower limb lean mass were observed between women with 250HD levels =20ng/ml and <20ng/ml. Likewise, no significant differences in the presence of sarcopenia determined by RSMI were observed between these two groups of women (6.4±0.7 vs. 6.5±0.8 kg/m²;ns) (Table 2).

Muscle Function and Strength

Subjects with 250HD levels =20ng/ml were found to have a better performance on the muscle function tests, and had significantly higher scores compared to the other group: (250HD =20ng/ml: 11.2+0.9 vs. 250HD <20ng/ml: 10.0+2.1; p<0.03) (Table 4). Subjects with 250HD levels <20ng/ml took 0.4 seconds longer on average on the walking-speed test than those with 250HD levels = 20ng/ml.

Figure 2 shows results of lower extremity muscle strength of both groups. Women with 250HD levels =20ng/ml exhibited significantly greater muscle strength in their knee extensor muscles (250HD =20ng/ml: 13.4+2.7 vs. 250HD <20ng/ml: 11.6+2.5; p<0.03) and hip abductors (250HD =20ng/ml: 8.3+2.7 vs. 250HD <20ng/ml: 7.3+3.1; p< 0.04); although their hip flexors tended to be stronger, the difference in strength compared with the other groups did not reach statistical significance (250HD >20ng/ml:17.0+3.3 vs. 250HD <20ng/ml:15.4+2.8; 0.1>p>0.05). No correlation was observed between 250HD levels and muscle function and strength. iPTH correlated negatively with muscle function (r= - 0.436; p <0.02) but not with muscle strength.

Figure 2.

Discussion

The results obtained in this study, which was conducted on a cohort of community-living women aged =65 years, showed that those with 250HD levels above 20ng/ml had better lower extremity strength and function than those with levels below 20ng/ml. These results are in keeping with reports by other authors, confirming that 250HD levels = 20ng/ml are necessary to maintain adequate muscle function and strength (16, 17). The results obtained in the present study showed no significant differences in any of the objective measurements that could affect 250HD levels between the two groups. This could partly be due to the fact that data on exposure to sunlight and vitamin D intake were obtained using questionnaires. This method is subjective and when involving elderly patients, results may be further biased by the influence of memory and the methods used to code and interpret responses. Other authors have also reported a wide range of 250HD levels in spite of similar levels of exposure to sunlight, and have even suggested genetic differences in liver cytochrome P450 enzyme (21, 27).

Analysis of each of the muscle function tests separately showed that women with 250HD levels <20ng/ml performed worse on walking- speed and sit-to-stand tests compared with those whose 250HD levels were =20ng/ml, though no significant differences in performance on the standing balance test were observed between groups. Cummings et al reported that inability to perform the chair sit-to-stand test doubles the risk of hip fracture (RR= 2.1; 95% CI: 1.3-3.2) and Dargent-Molina et al found that walking-speed is a predictor of hip fracture (RR= 1.4; 95% CI: 1.1-1.6) (28, 29).

Walking is a recommended physical activity for a wide range of the population, including the elderly. The benefits of walking include its positive effect on the skeleton and reduced risk of cardiovascular diseases. Krall et al. reported that walking >7.5 miles has positive effects on bone mass and mineral density, associated with reduced risk of hip fracture (30). It has also been demonstrated that poor performance in long distance walking is associated with higher mortality and incident cardiovascular disease in older adults (31). The difference in performance on the walking speed test observed between the groups studied here, and measured in seconds, would be equivalent to a difference of 40 minutes per week for a distance of 7.5 miles. It could thus be hypothesized that subjects with 25OHD <20ng/ml tend to walk less resulting in a lesser protective effect regarding both maintaining bone mass and reducing the risk of hip fracture and cardiovascular diseases.

No differences in performance on the balance test were observed between groups, probably because performance on this test would seemingly be associated with age rather than with vitamin D levels. All the sensorial systems that contribute to maintaining balance and posture undergo age-related degenerative changes (32). However, in vitro and in vivo studies have shown that vitamin D induced synthesis of growth factors improves defects in neuromuscular coordination (33). Pfeifer et. al. found that supplementation with 800IU of vitamin D and 1200mg of calcium per day in a group of elderly women, average age 74±1 years, with deficient 25OHD levels (~ 10ng/ml) resulted in a 72% increase in serum 25OHD levels and 18% decrease in PTH levels, improving balance and decreasing body sway by 9% (34).

The present study shows that women with 25O HD levels <20ng/ml had higher PTH levels than those with 25OHD levels =20ng/ml, though the difference failed to reach statistical significance. Analysis of the relation between muscle function and strength and serum 25OHD and iPTH levels showed that whereas PTH correlated negatively with muscle function (r = -0.436; p <0.02), 25OHD did not. Decreased muscle tone, muscle weakness, and pain, are typical symptoms of clinical conditions that are characterized by increased PTH secretion. For example, patients with primary hyperparathyroidism present muscle symptoms that are similar to those observed in patients with vitamin D deficiency-related myopathy. It remains to be elucidated whether muscle fibers are affected by vitamin D deficiency, high PTH levels, or a combination of both (35, 36, 37). It is well documented that hyperparathyroidism secondary to vitamin D deficiency causes proteolysis of muscle proteins. At the genomic level, vitamin D stimulates protein synthesis. At the nongenomic level, vitamin D activates protein kinase C in the muscle and results in calcium release, increasing the intracellular calcium pool, which is essential to muscle contraction (38). These effects may be altered in vitamin D insufficiency, decreasing muscle function.

Women with vitamin D levels below 20ng/ml showed lower muscle strength in their knee extensor muscles and hip abductors, and tended to have greater strength in their hip flexors. Prospective studies have reported an association between elevated PTH levels and a decrease in hip flexor and knee extensor muscle strength (39, 40). We observed no correlation between PHT levels and muscle strength in the group of leg muscles studied herein nor did we observe differences in leg muscle mass between both groups, in spite of the aforementioned difference in muscle function and strength.

The weaknesses of the present work must be pointed out: 1-the size of the study sample is small; 2- levels of 1,25(OH)2 D and androgenic metabolites associated with muscle function and strength in the elderly adult were not determined (41); 3-Exposure to sunlight was assessed indirectly using a questionnaire, and no statistically significant differences were observed between groups. This self-assessment method is biased by the subjectivity of the respondent. The appropriate method to avoid this bias would be to use devices that are specially designed to accurately measure each subject's exposure to ultraviolet radiation (42).

To conclude, community-dwelling women aged > 65 years with 25OHD levels =20ng/ml living in Buenos Aires had better muscle function and strength. Further studies should be conducted to fully explain the role of vitamin D and PTH in muscle strength and function. Vitamin D insufficiency may affect regularity and effectiveness of walking, a form of physical activity associated with prevention of cardiovascular disease and decrease in falls and hip fracture. It is therefore essential to evaluate the vitamin D nutritional status of elderly people aged over 65 years, since it would allow correcting hypovitaminosis D by adequate vitamin D supplementation, improving lower limb muscle function and strength.

Acknowledgements: We would like to thank Lic. Oscar Alzua, Head of the FLENI Rehabilitation institute, for providing the dynamometer, Ms María del Carmen Degrandi for performing the densitometric determinations, and Lic. Graciela Brito for her technical assistance. This paper was partially funded by a grant of the National Council for Scientific and Technology Research (CoNICET) (PIP 6482) and The National Agency for Scientific and Technological Promotion of Argentina (PICT 523).

Financial disclosure:

None of the authors had any financial interest or support for this paper.