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. Author manuscript; available in PMC: 2013 Jun 11.
Published in final edited form as: Am J Mens Health. 2012 Mar 7;6(5):420–426. doi: 10.1177/1557988312437240

Predictors of Serum Vitamin D Levels in African American and European American Men in Chicago

Adam B Murphy 1, Brian Kelley 1, Yaw A Nyame 1, Iman K Martin 2, Demetria J Smith 2, Lauren Castaneda 2, Gregory J Zagaja 3, Courtney M P Hollowell 4, Rick A Kittles 2
PMCID: PMC3678722  NIHMSID: NIHMS462392  PMID: 22398989

Abstract

Vitamin D deficiency is epidemiologically linked to prostate, breast, and colon cancer. When compared with European American (EA) men, African American (AA) men have increased risk of prostate cancer, but few studies evaluate vitamin D status in AA men. The authors evaluate the biological and environmental predictors of vitamin D deficiency in AA and EA men in Chicago, Illinois, a low ultraviolet radiation environment. Blood samples were collected from 492 men, aged between 40 and 79 years, from urology clinics at three hospitals in Chicago, along with demographic and medical information, body mass index, and skin melanin content using a portable narrow-band reflectometer. Vitamin D intake and ultraviolet radiation exposure were assessed using validated questionnaires. The results demonstrated that Black race, cold season of blood draw, elevated body mass index, and lack of vitamin D supplementation increase the risk of vitamin D deficiency. Supplementation is a high-impact, modifiable risk factor. Race and sunlight exposure should be taken into account for recommended daily allowances for vitamin D intake.

Keywords: health inequality/disparity, health care issues, health promotion and disease prevention, nutrition, preventive medicine, public health

Introduction

Vitamin D regulates parathyroid hormone levels and is known to have a role in bone formation, resorption, and mineralization. Vitamin D deficiency results in decreased bone density, and it is the primary cause of rickets in children and osteomalacia and osteoporosis in adults (Heaney, 2004). In recent years, scientists have been investigating the role of vitamin D in disease prevention. Vitamin D deficiency has been implicated in diabetes, hypertension, end-stage renal disease, tuberculosis, and peripheral artery disease (Holick, 2006; Holick & Chen, 2008; Melamed et al., 2009; Reis, Michos, von Muhlen, & Miller, 2008). Studies have also found increased incidence of breast, colon, and prostate cancer among people living at higher latitudes in the United States (Holick, 2006; Schwartz & Hulka, 1990).Miller et al. (1992) found that prostate cells have the vitamin D receptor (VDR), and the VDR has been found in a number of other nonrenal tissues. Other studies suggest that 1,25(OH)2-D, the most active metabolite of vitamin D, appears to promote cell differentiation and inhibit proliferation (Holick, 2006). Multiple studies have assessed determinants of low 25-hydroxyvitamin D (25-OH D) levels (Benjamin et al., 2009; Bischoff-Ferrari, Dietrich, Orav, & Dawson-Hughes, 2004; Chapuy et al., 1997; Dawson-Hughes, 2004; Dawson-Hughes et al., 2005; Hannan et al., 2008; Harris, Soteriades, Coolidge, Mudgal, & Dawson-Hughes, 2000; Holick, 2006; Holick et al., 2005; Kumari, Judd, & Tangpricha, 2008; Malabanan, Veronikis, & Holick, 1998; Nesby-O’Dell et al., 2002; Saadi et al., 2006; Vieth, Ladak, & Walfish, 2003; Zadshir, Tareen, Pan, Norris, & Martins, 2005). These studies have been carried out mainly in Caucasian women and have demonstrated that age, sex, ethnicity, latitude, season, body mass index (BMI), and dietary and supplemental vitamin D intake were important factors influencing vitamin D status. There are a few studies evaluating the prevalence of vitamin D insufficiency in men (Benjamin et al., 2009; Holick & Chen, 2008; Miller et al., 1992; National Institutes of Health, 2010; Reis et al., 2008; Schwartz & Hulka, 1990) and far fewer in African American (AA) men (Benjamin et al., 2009).

There are a number of factors that affect circulating levels of serum vitamin D. In addition to genetic variation in the vitamin D pathway, three primary modifiers of serum vitamin D levels are diet, exposure to UV radiation (UVR), and skin color. The National Institutes of Health (NIH) lists fish liver oils and fatty fish, such as salmon, tuna, and mackerel, as the best sources of dietary vitamin D. Vitamin D can also be found in eggs, irradiated mushrooms, yeast, and to lesser amounts in fortified cow’s milk (NIH, 2010). On November 30, 2010, the Institute of Medicine (2011) released their updated Dietary Reference Intake, which recommends that individuals younger than 70 years of age obtain 600 IU of vitamin D from their diet. This represents a 200 IU increase in previous recommended daily allowance for vitamin D3. This is only expected to increase serum 25-hydroxyvitamin D (25-OH D) levels by 1.4 ng/mL on average. Also, because of a relative paucity of Level 1 evidence for defining vitamin D deficiency, the Institute of Medicine has lowered the deficiency cutoff for serum 25-OH D levels from > 30 ng/mL to >20 ng/mL. Obesity is associated with vitamin D deficiency. Only 28% of obese adolescents reached sufficient vitamin D serum levels when given daily supplementation of vitamin D (800 IU; Harel, Flanagan, Forcier, & Harel, 2011). In a large Swedish prostate cancer study, it was shown that men living at high latitude have high prevalence of vitamin D deficiency. Taken together, with data on Swedish prostate cancer and with studies done on women from the United Arab Emirates, the data suggest that these recommendations might need to be altered based on BMI, sun exposure, and skin color (Saadi et al., 2006).

Exposure to UVR accounts for approximately 90% of circulating levels of 25-OH D (Holick, 2003). This exposure is affected by the time that one spends outside, the amount of clothing one wears, and the use of sunscreen. Individuals who live farther from the equator receive less UV exposure on average than those who reside closer to it. Altitude, local weather trends, and latitude also affect UVR exposure. Living north of about 37° latitude limits UV-B radiation exposure from around November through February because the sun’s zenith angle is so low that the atmosphere absorbs most UV-B rays before it reaches the Earth’s surface. Darker skin pigmentation resulting from increased melanin production in the skin melanocytes can reduce the efficacy of UV-B radiation–induced vitamin D3 synthesis. Skin with high melanin content can reduce vitamin D3 synthesis by up to 99%, much in the way that SPF-15 (sun protection factor-15) sunscreen does (Holick, 2006).

Although vitamin D deficiency affects a significant portion of various populations worldwide, AAs have been identified as a group with a particularly high risk of vitamin D deficiency (Holick, 2006). A study done in young women in Boston found the serum 25-OH D levels of Black women to be less than half that of their White counterparts regardless of season (Harris et al., 2000). Additionally, many of the diseases thought to be associated with vitamin D deficiency are more prevalent among AAs (Melamed et al., 2009). These observations highlight the importance of investigating the epidemiology of vitamin D deficiency in AAs; however, there are surprisingly few studies involving this population, especially AA men. Previous studies have not had a sufficient number of AAs to allow ancestry-specific statistical analysis, or they have not thoroughly investigated the effect of nongenetic factors (Harris et al., 2000; John, Schwartz, Koo, Van Den Berg, & Ingles, 2005). In this study, we explore the three potential environmental modifiers of serum vitamin D levels in AA men to help answer this question.

Study Population

This is a cross-sectional study evaluating modifiers of serum vitamin D within a larger cross-sectional study evaluating the biological and environmental mediators of serum vitamin D and prostate cancer risk. The study population consists of 40- to 79-year-old, ambulatory, unrelated men in Chicago, Illinois. Recruitment took place at Northwestern Memorial Hospital, Cook County Health and Hospitals System, and University of Chicago Hospital, in Chicago, Illinois, through the respective outpatient urology clinics. A subset of the control subjects was recruited through prostate cancer screening events. All patients recruited were males that self-identified as AA, European American (EA), or Hispanic and consented to the venipuncture and the institutional review board (IRB)–approved study. We excluded the 66 Hispanic men from this analysis to await further accrual. Exclusion criteria were patients with hyperparathyroidism, liver failure, chronic kidney disease, history of rickets, cancers except nonmelanoma skin cancer, and history of inborn error of calcium or vitamin D metabolism. Five hundred and fifty-eight patients (282 AA, 210 EAs, and 66 Hispanic men) were recruited in total. The study was approved by each of the institution’s IRB, and all participants provided written informed consent.

Methods

Research coordinators conducted in-person interviews at the time of recruitment and administered structured questionnaires that ascertained calcium and vitamin D supplementation intake, ancestry, family history of cancer, medical history, occupation, income, education, alcohol consumption and tobacco use, marital status, and lifetime history of sun exposure. Because of previous data implicating increased body fat content as a contributing factor to vitamin D deficiency (Holick, 2006), standing height and weight were measured for BMI calculations. UVR exposure was assessed using a validated questionnaire that recorded reported cumulative sun exposure over various age ranges. Skin color was also measured, as it has been shown that increased skin pigmentation reduces the cutaneous synthesis of vitamin D (Clemens, Adams, Henderson, & Holick, 1982). Skin pigmentation was measured using a portable narrow-band reflectometer called the Dermaspectrometer (Cyberderm, Broomall, PA). The Dermaspectrometer measures skin color through skin reflectance, where output is expressed in terms of erythema (E) and melanin (M) indices from 0% to 100%, where higher values denote higher pigment content (Takiwaki, 1998). Three measurements of skin pigmentation were taken at the inner upper arm and three at the center of the forehead to establish constitutive and facultative pigmentation, respectively. The difference between facultative and constitutive M indices was used to calculate an M index, which represents the additional melanin produced in sun-exposed skin, and it would be positively correlated with the cumulative sun exposure a person has experienced over his or her lifetime. This melanin content–derived index is an objective, quantitative index to measure cumulative lifetime UVR exposure. The reported and the melanin content–derived UVR exposure indices were used in all analyses.

A Block calcium and vitamin D screener adapted from National Health and Nutrition Examination Survey 1999–2001 dietary recall data and validated for use in the AA population assessed usual 25-OH D intake during the reference year, defined as the year prior to recruitment into the study (Block et al., 1986; Block, Hartman, & Naughton, 1990; Coates et al., 1991). The screener consisted of 19 food items, 3 supplements questions, and questions to adjust for food fortification practices.

A peripheral blood sample was collected at the time of recruitment for serum 25-OH D measurement. Serum samples were stored in small test tubes at −20°C until 25-OH D measurement. Total 25-OH D was assessed by chemiluminescent immunoassay by the Associated Regional and University Pathologists laboratory in conjunction with the University of Utah. The season of blood draw was evaluated in two seasons as cold (1 November through 30 April) and warm (1 May through 31 October) based on UVR data from Chicago.

Results

In our cohort of EA and AA men with vitamin D data, 81.4% of all men meet the laboratory definition of vitamin D deficiency with levels <30 ng/mL (see Table 1). Ninety-three percent of AA men and 66% of EA would be considered deficient. Using the Institute of Medicine definition of deficiency being <20 ng/mL, 18% of the EA men were deficient versus 63% of AA men. The mean and median serum 25-OH D level was 21 ng/mL. The median level for AA men was 17.2 ng/mL, whereas for EA men it was 24.2 ng/mL (p < .001). There was a seasonal pattern witnessed in serum 25-OH D levels (see Figure 1). The mean and median for dietary vitamin D intake was 248 IU/day and 174 IU/day, respectively. The mean total vitamin D intake (dietary and supplements) was 410 IU/day, with a median of 166 IU/day. For AA men, the mean total vitamin D intake was 240 IU/day, with a median of 74 IU/day. EA men had a mean total vitamin D intake of 572 IU/day (median of 225 IU/day). The differences between EA and AA men in supplemental intakes were statistically significant (Table 1).

Table 1.

Characteristic Comparison Between African American and European American Men

Characteristic African American
Men (n = 282)		 European American
Men (n = 210)		 p Value
Age (years) 59.0 ± 11.1 60.8 ± 8.8 .958
BMI (kg/m2) 28.6 ± 5.6 27.8 ±5.4 .041*
Serum 25-OH vitamin D level (ng/mL) 17.2 ± 8.1 24.2 ± 10.1 <.001*
Dietary vitamin D intake (IU/day) 93.3 ± 89.8 100.4 ± 101.8 .115
Supplemental vitamin D intake 168.5 ± 582.5 475.2 ± 2965.8 <.001*
Multivitamin use, n (%) (contains 400 IU vitamin D3) 92 (43) 110 (53) .036*
Vitamin D supplement use, n (%) 98 (35) 115 (55) .019*
Calcium supplement use, n (%) 105 (49) 122 (59) .036*
Sunscreen use, n (%) 37 (13) 142 (68) <.001*
High sun exposure, n (%) (reported UV-exposure
index ≥ 10)		 56 (20) 21 (10) .023*
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Note. BMI = body mass index; IU = international unit; UV = ultraviolet radiation; 25-OH vitamin D = 25-hydroxyvitamin D.

*

p <. 05.

Figure 1.

Figure 1

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European American and African American vitamin D levels by month

Pearson correlations for the total group reveal that higher vitamin D levels were significantly negatively correlated with BMI and positively associated with income, the use of vitamin D supplementation, and total lifetime sun exposure as measured by melanin content differences in sun-exposed and nonexposed skin (all p < .01). The distributions of vitamin D level, dietary and supplemental vitamin D intake, and income were relatively distinct between EA and AA men. Because of this, we stratified the pairwise correlations by race with separate analyses for AA and EA men (see Table 2). In AA men, none of the covariates reached statistical significance for their correlation with 25-OH D levels. Among EA men, only income was positively correlated with vitamin D levels (p = .04).

Table 2.

Pearson Correlations With Serum 25-OH Vitamin D and Other Covariates in African American Men (Unshaded Area) and European American Men (Shaded Area)

Age 25-OH D Dietary
Vitamin
D Intake		 Supplemental
Vitamin D
Intake		 BMI M Index Reported
UV
Exposure		 Comorbidity
Index		 Pack Years of
Smoking		 Income
Age 1.00 .13 −.03 .11 −.05 .09 .05 .37* .27* −.02
25-OH D .01 1.00 .07 .14 −.08 .11 .06 −.01 −.11 .28*
Dietary vitamin D intake −.17 .13 1.00 .03 −.08 .05 −.06 .08 −.06 .02
Supplemental vitamin D intake −.05 .23 .04 1.00 .07 .003 −.001 −.08 −.13 .01
BMI −.13 .01 −.06 .28* 1.00 .05 −.01 .19* .02 −.05
M index .16* −.03 −.09 .05 .07 1.00 −.03 .13 (p< .06) −.10 .09
Reported UV exposure .02 .03 .06 .05 .10 −.06 1.00 −.05 .11 −.05
Comorbidity index .39* −.04 .01 .08 .20* .07 −.03 1.00 .21* −.08
Pack-years of smoking −.07 −.05 −.07 −.02 −.06 .10 −.04 −.05 1.00 −.19*
Income −.08 .16 −.05 −.05 −.19* −.23* .01 −.13* −.03 1.00
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Note. BMI = body mass index; 25-OH D = serum 25 hydroxyvitamin D level; M index = melanin content in sun-exposed skin - melanin content in nonexposed skin (correlated with total lifetime sun exposure)

*

p < .05.

On univariate linear regression (Table 3), 25-OH D levels were most strongly predicted by AA race (β = −6.95, p < .0005). The linear model was constructed with gradual addition of the covariates. The relationship between AA race (β = −4.85, p = .02) and 25-OH D level was weakened by addition of season of blood draw and sunscreen use. With the addition of total vitamin D supplement intake to the model, AA race retains borderline significance (β = −3.68, p = .066). With the addition of baseline skin melanin content and reported lifetime sun exposure (both p < .05), AA race loses its significance (p = .74).

Table 3.

Linear Regression Model With Serum 25-OH D Concentration as Outcome (R2 = 0.367)

β Estimate Standard Error p Value
Total vitamin D intake (IU/day), dietary and supplemental 0.002 0.003 .001*
Age (years) 0.050 0.057 .379
Vitamin D supplement use (0 = no, 1 = yes) 2.89 1.89 .129
AA race (0 = EA, 1 = AA) −4.46 1.27 <.001*
BMI (kg/m2) −0.201 0.092 .029*
Reported UV exposure 0.281 0.184 .128
Use of sunscreen 0.867 1.19 .47
High UVR months 4.33 1.02 <.001*
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Note.BMI = body mass index; IU = International Unit; AA = African American; EA = European American; UVR = ultraviolet radiation. In AA men only total vitamin D intake and high UVR months were significant in the model (both p ≤ .02). In EA men only income and reported UV exposure were significant (both p < .01).

*

p < .05.

When stratified by race, the linear regression (R2 = 0.30) for EA reveals that 25-OH D levels are most strongly predicted by season of blood draw (β = 6.74, p < .0005). This was followed in strength by the M index–derived lifetime sun exposure (β = 5.8, p = .02), income (β = 1.13, p = .002), and BMI (β = −0.47, p = .009). The linear regression for AA men reveals that 25-OH D levels (R2 = 0.43) were associated with previous residence in the tropics (β = 5.60, p = .04), dietary vitamin D intake (β = 0.02, p = .03), and supplemental vitamin D intake (β = 0.05, p = .01).

On binomial logistic regression for vitamin D deficiency (25-OH D < 20 ng/mL), we constructed the regression model with season as the predictor and also stratified the analysis by race. In EA men, vitamin D deficiency was predicted by cold season of blood draw (odds ratio [OR] = 3.34, p = .02) and by the lack of vitamin D supplement use (OR = 2.38, p = .035). In AA men, vitamin D deficiency was predicted by the lack of vitamin D supplement use only (OR = 5501.0 p = .05).

Discussion

Forty-four percent of our population was vitamin D deficient using the recently defined Institute of Medicine deficiency cutoff (25-OH D < 20 ng/mL). Using the laboratory standard definition (<30 ng/mL), more than 90% of the AA men have deficiency. The median total vitamin D intake was 166 IU/day in our population. The median 25-OH D level was 21 ng/mL, which is well below the usual 30 ng/mL vitamin D sufficiency level. If we assume that the men consumed an additional 434 IU to get everyone in our population to the 600 IU recommended daily allowance, we would expect an increase of 3.04 ng/mL in serum 25-OH D levels (0.7 ng/mL per 100 IU of calciferol consumed). This would leave 37.1% of men deficient using <20 ng/mL as the deficiency cutoff. This is not an appropriate recommendation for this population of men.

Vitamin D level is predicted by season, AA race, income, BMI, and vitamin D supplemental intake. Because the distributions of vitamin D levels were different between EA and AA men, we ran regressions stratifying by race. When stratifying by season and controlling for age, we find that EA and AA men have a different constellation of variables associated with their vitamin D status. For EA men, lifetime sun exposure (M index) and income reach statistical significance with small beta coefficients (β< 1, p < .02, for both) with the largest effect for season of blood draw (β = 3.40, p = .047). When stratified by season, vitamin D supplement use (β = 6.34, p = .02) reaches statistical significance during the cold months in EA men. In AA men, total vitamin D intake and sunscreen use are statistically significant but with small beta coefficients (β < 0.03, p < .03, for both) with the largest effect for season of blood draw (β = 3.40, p < .047). This is likely due to the low amount of dietary and supplement use among AA men.

In our analysis of vitamin D deficiency (25-OH D level < 30 ng/mL), we found that season of blood draw and lack of vitamin D supplement use was significant for EA men; lack of vitamin D supplement use predicted deficiency among AAs. The season of blood draw did not reach statistical significance in AA men (p = .18) but has an OR of 32.8 for deficiency during the cold months relative to the warm months. Since season is not an easily modifiable risk factor, supplementation may be the easiest way to overcome this issue. In addition to its well-established negative effects on the musculoskeletal system, vitamin D deficiency is associated with an increased risk of colorectal and breast cancer, autoimmune diseases, and cardiovascular disease (Lowe et al., 2005; Zittermann, 2003). AA people are at increased risk for many of these diseases. It is essential, therefore, to maintain normal vitamin D status.

Measures to prevent vitamin D deficiency include increased skin exposure to sunlight, increased fortification of food items with vitamin D, and vitamin D supplementation. In the absence of adequate exposure to sunlight, there is mounting evidence that at least 1,000 IU of dietary or supplemental vitamin D intake is required daily to prevent vitamin D deficiency (Glerup et al., 2000; Holick, 2002; Hollis, 2005; Vieth, 1999). Given the serum levels of 25-OH D in our population and an estimated 0.7 ng/mL increase in serum 25-OH D level per 100 IU of vitamin D3 , we estimate 2,000 IU per day to reach minimum sufficiency (≥20 ng/mL) standards in nearly 95% of the Chicago population. Vitamin D supplementation currently remains the most appropriate mode for preventing vitamin D deficiency in high-risk groups such as AAs and individuals living in UV-poor environments. The optimal dosing regimen, including single or intermittent high-dose supplementation of vitamin D, needs to be defined in these groups. Race and residence in UV-poor environments should be taken into account when suggesting daily allowances of vitamin D.

Limitations

A limitation of our study may be the fact that dietary intake accuracy is threatened by recall bias. We use the validated semiquantitative Block Food Frequency Questionnaire for assessing vitamin D and calcium intake. Another limitation is that our determination of serum 25-OH D relied on a single measurement, which may not adequately reflect long-term exposure.

Conclusion

Vitamin D deficiency is present in 44% of Chicago-area men across all ethnicities using the strictest definition (serum 25-OH D < 20 ng/mL). Among AA men, 63% are vitamin D deficient using the 20 ng/mL cutoff and 93% of AA men are deficient using normal deficiency standards (<30 ng/mL), which is a significant cause for concern. Sunlight exposure through season or reported UV exposure in EA and vitamin D supplementation in both racial groups are statistically significantly associated with lower risk of vitamin D deficiency and higher overall 25-OH D levels and represent modifiable risk factors. Vitamin D supplementation counteracts the risk of vitamin D deficiency among AA men. Race and sunlight exposure should be taken into account for recommended daily allowances for vitamin D intake.

Acknowledgments

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article:

This study was funded by the U.S. Department of Defense (Grant W81XWH-10-1-0532) and Northwestern University SPORE NIH grant (P50CA090386).

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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