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The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 60:1414-1419 (2005)
© 2005 The Gerontological Society of America

25(OH)D Serum Levels Decline With Age Earlier in Women Than in Men and Less Efficiently Prevent Compensatory Hyperparathyroidism in Older Adults

Dario Maggio1,, Antonio Cherubini1, Fulvio Lauretani2, Roberto Cosimo Russo2, Benedetta Bartali2, Marco Pierandrei1, Carmelinda Ruggiero1, Maria Carmela Macchiarulo1, Ruben Giorgino3, Salvatore Minisola4 and Luigi Ferrucci5

1 Gerontology and Geriatrics, University of Perugia, Italy.
2 Laboratory of Clinical Epidemiology, INRCA (Istituto Nazionale Riposo e Cura Anziani) Geriatric Department, Florence, Italy.
3 Procter and Gamble, Rome, Italy.
4 Department of Clinical Sciences, Policlinico Umberto I, University of Rome La Sapienza, Italy.
5 Longitudinal Studies Section, Clinical Research Branch, National Institute on Aging, Baltimore, Maryland.

Address correspondence to Dario Maggio, MD, Gerontology and Geriatrics, University of Perugia, Policlinico Monteluce, Via Brunamonti, 06122 Perugia, Italy. E-mail: dariomaggio1{at}tin.it


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
Background. Although a host of factors are known to influence 25-hydroxyvitamin D [25(OH)D] serum levels, few studies addressed the distinctive sex-specific influence of aging, and the age-specific relationship of parathyroid hormone (PTH) with 25(OH)D. The aims of this research were to evaluate changes of 25(OH)D and PTH levels with age in a large population-based sample of men and women and to test the hypothesis that 25(OH)D serum concentrations needed to offset age-associated hyperparathyroidism are significantly higher in older than in younger persons.

Methods. In 1107 participants of the InCHIANTI (Invecchiare in Chianti, i.e., Aging in the Chianti area) study, we collected information on dietary intake, daylight exposure, and disability, and measured renal function and serum 25(OH)D and PTH.

Results. In women, the age-related decline of 25(OH)D was already evident shortly after age 50, whereas in men it started only after age 70 and was substantially less steep. Age, daylight exposure, winter season, and disability were independent predictors of low 25(OH)D levels. For any given level of 25(OH)D, PTH levels were progressively and consistently higher in older than in younger participants.

Conclusions. These findings suggest that the age-associated fall of serum 25(OH)D starts earlier in women than in men and that higher levels of 25(OH)D are required in older compared to younger persons to avoid the age-associated compensatory hyperparathyroidism.

THERE is much uncertainty on how to define vitamin D deficiency, mainly because it is difficult to identify a meaningful threshold of circulating 25-hydroxyvitamin D [25(OH)D] levels below which the risk of negative health outcomes increases. Different 25(OH)D thresholds have been used in the past. Until recently, the most popular was around 37.5 nmol/L (1–5), but the further we advance in the understanding of the pathophysiology of vitamin D deficiency, the higher the cutoff is set to define it (5). For instance, Vieth and colleagues (6) have recently suggested that maximal parathyroid hormone (PTH) suppression occurs when circulating 25(OH)D reaches 75 nmol/L, whereas Heaney (7) has shown impaired intestinal calcium absorption when circulating 25(OH)D is below 80 nmol/L. Finally, data from Bischoff-Ferrari and colleagues (8) demonstrate that bone density is optimal when serum 25(OH)D levels exceed 80 nmol/L.

Independent of the cutoff defining vitamin D deficiency, low levels of 25(OH)D are extremely common in older persons, particularly in the oldest old and in the female sex, due to specific physiological and lifestyle factors linked to advanced age, such as impaired production of 7-dehydrocholesterol in the skin, insufficient exposure to sunlight (and/or excess clothing), poor dietary intake of vitamin D, as well as to chronic diseases, pharmacological treatments, and disability (1–5,9–12). Although vitamin D deficiency has originally been reported to be more prevalent at higher latitudes (13–15), even free-living older Southern Europeans are at significant risk of developing it (16).

Vitamin D deficiency has both direct and indirect consequences on bone cell function. Direct effects mainly relate to reduced recruitment and differentiation of osteoclastic progenitors into mature osteoclasts, mostly mediated by osteoblast secretion of activating factors, and to diminished synthesis of specific collagenous and noncollagenous proteins, again in the osteoblast (17). The main indirect consequences of vitamin D deficiency on bone cell function are defective mineralization of osteoid seams, due to inadequate intestinal absorption of calcium and phosphate, and an age-related form of compensatory hyperparathyroidism, which drives an accelerated bone loss (6,13). Recently, it has been suggested that the minimal 25(OH)D serum concentrations needed to avoid compensatory hyperparathyroidism is significantly higher at older ages (6). This hypothesis was originally formulated using data from a selected clinical sample, but has never been verified in a population-based sample of older persons.

The aims of this investigation were to address the distinctive sex-specific influence of aging on 25(OH)D serum levels and to test the hypothesis that 25(OH)D concentrations needed to offset the compensatory hyperparathyroidism are higher in older than in younger persons.


   METHODS
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 Abstract
 Methods
Results
 Discussion
 References
 
Study Population
The InCHIANTI (Invecchiare in Chianti) is a longitudinal study performed in two Italian towns near Florence, Greve in Chianti and Bagno a Ripoli (latitude 43°45'). The study population consisted of a random sample of participants aged 65 years or older living in the two catchment areas, and 30 men and women randomly selected in each decade of age between 20 and 60 years and in the age group 61–64. A detailed description of the design and data collection methods of InCHIANTI has been previously reported (18). The study protocol was approved by the INRCA (Istituto Nazionale di Riposo e Cura dell'Anziano, i.e., National Institute of Research and Care on Aging) ethical committee. All participants received an extensive description of the study and all gave their informed consent.

Of 1530 persons originally sampled, 1453 (94%) agreed to participate in the study. Of these, 1343 accepted to donate a blood sample and 1273 had valid data for all variables used in the present analysis. To better discriminate the effect of aging on vitamin D status from that of a series of confounding variables, we excluded from the analysis all participants who were taking calcium, vitamin D, and drugs interfering with calcium metabolism. Also excluded were persons suffering from diseases affecting bone metabolism (such as primary hyperparathyroidism, malabsorption, chronic renal failure, liver insufficiency) and homebound or institutionalized persons. After these exclusions, the study population consisted of 1107 participants, 497 men and 610 women. The 166 persons excluded were older and more disabled than were controls. They also had higher serum creatinine, lower serum calcium, and lower vitamin D intake, and reported lower sunlight exposure.

Assays of Clinical Chemistry
Blood samples were drawn in the morning following a 12-hour overnight fast. Total proteins were measured by automated colorimetric assay (Roche Diagnostics, GmbH, Mannheim, Germany; interassay coefficient of variation [CV] < 1% and lower detection limit of 0.2 g/dl), and the relative serum albumin concentration (expressed as a percentage) was measured from an agarose electrophoretic technique (Hydragel Protein [E] 15/30; Sebia, Issy-les-Moulineaux, France, with a mean interassay CV of 0.8%).

Commercial enzymatic assays (Roche Diagnostics) were used for measuring serum calcium and creatinine concentrations with a Modular P800 Hitachi Analyzer (Hitachi, Ltd, Tokyo, Japan). Both serum calcium and creatinine were measured by colorimetric assay. For calcium levels, the interassay CV and the analytical sensitivity were 1.5% and 0.2 mg/dl, respectively. For creatinine, the interassay CV and the analytical sensitivity were 2.3% and 0.1 mg/dl, respectively.

A 24-hour urine collection was performed, and creatinine clearance was calculated. In a minority of participants (42 participants), clearance was estimated by means of the method of Cockcroft and Gault (19).

Serum samples frozen and stored at –80°C were used for 25(OH)D and PTH measurements. Serum levels of 25(OH)D were measured by radioimmunoassay (DiaSorin Inc., Stillwater, MN). Intra- and interassay CVs were 8.1% and 10.2%, respectively (20). Serum intact PTH levels were measured using a two-site immunoradiometric assay (N-tact PTHSP; DiaSorin Inc.) kit. The assay uses two affinity-purified polyclonal antibodies, one specific for the amino-terminal 1–34 portion of PTH molecule, and another specific for the 39–84 sequence of the hormone. The assay sensitivity was 1.2 ng/L. Intra- and interassay CVs were <3.0 and 5.5%, respectively (20).

Evaluation of Nutritional and Functional Parameters
To evaluate total energy, calcium, and vitamin D intakes, trained dieticians administered a food-frequency questionnaire, derived from the European Prospective Investigation of Cancer (EPIC) study, to the participants or their proxies (21).

In the interview and medical visit, information was collected on factors affecting vitamin D status, such as age, body mass index, and self-reported difficulty in performing eight instrumental activities of daily living (IADLs) (22). Participants were categorized in two groups: those with none to mild difficulty in performing IADLs (score 0–3) and those with moderate to severe difficulty (score >3). Daylight exposure, for both working and recreational purposes, was estimated from self-report of the average number of hours per day spent outdoors over the last year. The study population was divided into tertiles: the lowest tertile included participants who reported spending <1 hour per day outdoors; the intermediate tertile included those who reported spending between 1 and 4 hours per day; and the highest tertile included those who reported spending >4 hours per day. Seasonal period of the year was dichotomized into the December–April and May–November periods.

Statistical Analysis
The SAS 8.2 package was used for analysis (SAS Institute, Inc., Cary, NC). Data are presented as mean ± standard deviation [SD] or percentages. Continuous variables were compared using the t test or the Mann–Whitney U test. Categorical variables were compared by Pearson's chi-square tests. All analyses concerning 25(OH)D were sex-specific. To test whether the cross-sectional relationship between age and 25(OH)D levels was linear, a quadratic term for age was included in the model. Additionally, we fitted generalized additive models where age as a linear predictor was replaced by a smoothing function of age. Relationships that were found to be nonlinear were examined using piecewise regression models, allowing for two different slopes, before and after a certain age threshold. Sex-specific age thresholds for these models were identified using recursive method as those that maximized the fit of the model. Associations between potential risk factors of vitamin D deficiency and 25(OH)D serum levels were evaluated in multivariate linear regression models. Fully adjusted models were reduced to parsimonious models by removing independent variables not associated with the dependent variable by backward selection method.

Correlation between 25(OH)D and PTH levels (both log transformed, because they were not normally distributed) was assessed using linear regression. To test the hypothesis that the relationship between 25(OH)D and PTH was different at different ages, we included an Age x 25(OH)D interaction term in the model predicting PTH, and adjusted for covariates. Since no difference between sexes was observed in the relationship between PTH levels and age, a single multivariate linear regression model was used to evaluate factors independently associated with PTH levels.


   RESULTS
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 Results
Discussion
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Descriptive characteristics of the study population are reported in Table 1. In the overall study population, the mean serum 25(OH)D concentration was 53 ± 34.6 nmol/L. The levels were higher in men (60.1 ± 34.9 nmol/L) than in women (47.1 ± 33.4 nmol/L) (p <.0001). Vitamin D average daily intake was 80 I.U. In both men and women, the serum levels of 25(OH)D declined with age. Quadratic terms for age inserted in sex-specific models predicting 25(OH)D were statistically significant, indicating that the decline was not linear. To further explore the effect of age on circulating 25(OH)D, we fitted piecewise regression models allowing two different slopes in different age ranges. The age thresholds that separated the two slopes and obtained the best fit were 50 years in women and 70 years in men. Below these age thresholds, the serum level of 25(OH)D was not statistically associated with age, whereas above these age thresholds absolute annual decline of serum 25(OH)D was 0.025 nmol/L in men and 0.032 nmol/L in women (Figure 1).


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  		Table 1. Baseline Characteristics of the InCHIANTI Participants (497 Men and 610 Women).

 


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  		Figure 1. Top: Age related decline of 25-hydroxyvitamin D [25(OH)D] in men. Bottom: Age related decline of 25(OH)D in women

 
In the univariate analysis, variables associated with 25(OH)D were age (r = –0.42; p <.0001), daylight exposure (r = 0.12; p <.01), winter season ( r = –0.23; p <.0001), body mass index ( r = –0.10; p <.05), caloric intake (r = 0.10; p <.01), vitamin D intake (r = 0.15; p <.001), and IADL impairment (r = –0.27; p <.0001) in women. In men, the associated variables were age (r = –0.19; p <.0001), daylight exposure (r = 0.20; p <.0001), winter season (r = –0.24; p <.0001), caloric intake (r = 0.20; p <.0001), and impairment in IADL (r = –0.21; p <.0001), whereas there was a trend toward association with vitamin D intake (r = 0.08; p =.06).

In the multivariate logistic regression analysis, age (>50 years in females and >70 in males), sunlight exposure, and winter season were independently associated with 25(OH)D in both sexes, whereas IADL impairment was associated with 25(OH)D in women only. These variables explained 23% of the variance in 25(OH)D levels in men and 32% in women (Table 2).


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  		Table 2. Multivariate Logistic Regression Analysis of Factors Associated With Vitamin D Levels in Each Sex.

 
In the whole population, serum PTH was inversely correlated with 25(OH)D (Figure 2), and their relationship was linear (r = –0.18, p <.0001 in men; r = –0.36, p <.0001 in women). However, the magnitude of the difference in PTH serum levels according to 25(OH)D quartiles was substantially different in different age groups. In particular, for any given level of 25(OH)D, PTH levels were progressively and consistently higher in older than in younger participants, suggesting that higher levels of 25(OH)D are required to maintain normal levels of PTH in older compared to younger persons (Figure 3). To formally test this hypothesis, we introduced an Age x 25(OH)D interaction term in a multiple linear regression equation predicting PTH. Adjusting for age; levels of total calcium, albumin, creatinine, and 25(OH)D; calcium intake; vitamin D intake; winter season; and IADL the Age x 25(OH)D interaction was a statistical, independent predictor of the outcome (ß = –0.005; p <.0005). Moreover, age (ß = 0.0025; p <.0001) and serum creatinine levels (ß = 0.32; p <.0005) were independently associated with PTH levels.



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  		Figure 2. Correlation between 25-hydroxyvitamin D [25(OH)D] and parathyroid hormone (PTH) in the two sexes

 


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  		Figure 3. Serum parathyroid hormone (PTH) levels according to 25-hydroxyvitamin D [25(OH)D] quartiles in different age strata

 

   DISCUSSION
Top
 Abstract
 Methods
 Results
 Discussion
References
 
Using data from a population-based sample of persons living in the Chianti area, known for its temperate climate and sunny countryside, we found a high prevalence of low 25(OH)D levels. Serum levels diminish with age in both sexes, but substantially earlier in women, who seem to suffer from this steeper decline starting in the perimenopausal period. In men, the decline becomes apparent 20 years later than in women, starting from 70 years of age.

The distinctive pattern of age-related decline in 25(OH)D in men and women is unlikely to be explained by differences in the hormonal milieu between the two sexes. Although estrogens may modulate renal 1-alfa-hydoxylase activity (17,23), 17-beta-estradiol is not recognized as a modulator of vitamin D or 25(OH)D production. Skin synthesis of 7-dehydrocholesterol is not influenced by estrogens, although skin thinning, an age-related factor capable of lowering serum 25(OH)D, does occur as a consequence of menopause (24–27).

Probably, the impact of menopause is more cultural than biological. In many Western cultures, early and mid-postmenopausal years may be characterized by changes in women's habits and attitudes, especially toward walking outdoors, wearing covering clothes, and using cosmetics and sunscreens, all factors that affect exposure of skin to sunlight. These lifestyle changes, probably motivated by the fear of wrinkles and skin cancer, might put women at significant risk of low ultraviolet B irradiation early after menopause (28–30). Indeed, there is some evidence that sun protective behavior is more frequent in women and increases with age, probably because women are more concerned with the detrimental effects of sunlight on skin (31,32). This behavior may lead to vitamin D insufficiency/deficiency (33,34).

The results of our analysis corroborate previous observations that main predictors of low 25(OH)D levels are age, disability, poor daylight exposure, and winter season (9,13,16). In particular, the fact that disability and poor daylight exposure were independent risk factors in the oldest participants suggests that disability exerts its negative effects through mechanisms different from a simple decrease in sunlight exposure. Indeed, disability is associated with unfavorable outcomes, such as malnutrition, hospital admission, institutionalization, and frailty (35,36). Therefore, it is no surprise that low performance in IADLs, which inevitably reflects poor physical health status, may predict vitamin D deficiency. Lack of correlation between 25(OH)D levels and vitamin D intake is likely explained by the very low daily intake of cholecalcipherol, which was on average 80 I.U. in both women and men.

The crucial finding of our study concerns the modulating effect of age on the PTH–25(OH)D relationship. Although we could not identify a clear threshold in 25(OH)D levels below which calcium homeostasis is stressed and PTH increases, we found that the oldest segment of the population needs higher 25(OH)D levels to offset age-associated hyperparathyroidism, which inevitably determines bone loss and increases the risk of osteoporosis. Although the precise mechanisms explaining this phenomenon remain unclear, age-related changes, such as reduced renal function (with the consequent decrease in production of 1,25(OH)2D) and resistance to suppression of PTH secretion mediated by 25(OH)D and 1,25(OH)2D, are possible causative factors (37–40). Independent of the causes, our data provide justification for the increase in vitamin D dietary requirements with older age.

Some limitations of our study should be acknowledged. First, data reported were collected in the context of a cross-sectional survey and, therefore, are subjected to the common biases inherent to this kind of investigation. Second, because of the exclusion criteria set for this investigation, the frailest segment of the population was excluded. Therefore, our results may not be generalized to the entire elderly population. Third, we did not measure some specific biochemical (i.e., ionized calcium, 1,25(OH)2D) parameters that might have fostered interpretation of our results, in particular those regarding changes of the PTH–25(OH)D axis with age.

Conclusion
Our findings demonstrate that low 25(OH)D levels are highly prevalent in the elderly population and tend to occur much earlier in life in women than in men. Age, poor sunlight exposure, and winter season are predictors of low 25(OH)D levels in both sexes. Disability predicted low 25(OH)D levels only in women. More importantly, we demonstrated that, for the same low levels of 25(OH)D, compensatory elevation of PTH is of larger magnitude in older persons. These findings underline how critical it is that older persons take an adequate amount of vitamin D. Implementation of food fortification policies, almost completely neglected in Italy and in many other European Community countries, is clearly warranted.


   Acknowledgments
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 Methods
 Results
 Discussion
 References
 
This research was partly supported by an unrestricted grant from Procter and Gamble.

Dario Maggio and Antonio Cherubini were equal contributors to the research.


   Footnotes
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Decision Editor: John E. Morley, MB, BCh

Received September 29, 2004

Accepted November 30, 2004


   References
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 Discussion
 References
 

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