Treatment and Prevention of Vitamin D Insufficiency in Cystic Fibrosis Patients: Comparative Efficacy of Ergocalciferol, Cholecalciferol, and UV Light

Abstract

Background: The optimal treatment for correcting or preventing vitamin D insufficiency in cystic fibrosis (CF) patients has not been established.

Objective: The aim of the study was to assess the relative efficacy of three modes of vitamin D therapy: cholecalciferol (D₃), ergocalciferol (D₂), and UV light in raising or maintaining 25(OH)D levels above 30 ng/ml.

Design: Thirty adult CF subjects with vitamin D insufficiency were randomized into one of three treatment arms: D₃, D₂, or UV light. Subjects randomized to D₃ or D₂ ingested 50,000 IU of vitamin D weekly, and those randomized to UV exposed their skin to UV light from a lamp five times a week. Serum was collected for 25(OH)D and PTH at baseline and at 12 wk.

Results: Treatment with D₃ and D₂ raised 25(OH)D levels significantly, from a mean of 21.2 ± 10.18 to 47.1 ± 20.5 ng/ml (P < 0.001) and 24.4 ± 10.3 to 32.7± 9.7 ng/ml (P = 0.01), with 100% and 60% reaching 25(OH)D levels above 30 ng/ml, respectively. Treatment with UV did not raise 25(OH)D levels significantly; however, only 55% of subjects were adherent with UV therapy.

Conclusion: This study demonstrates that CF subjects are able to achieve or maintain optimal vitamin D status (>30 ng/ml) with two oral regimens of either D₃ or D₂ treatment, the former being more efficacious. A confounding variable for this observation is the fact that the D₃ and D₂ capsules contained different carriers, powder-based vs. oil-based, respectively. UV therapy did not alter vitamin D status, possibly due to poor adherence to UV therapy.

Adult cystic fibrosis patients can be given an oral regimen of vitamin D to improve vitamin D status during the winter months.

The prevalence of vitamin D insufficiency in patients with cystic fibrosis (CF) has been reported to be as high as 90% (1,2). This is significantly higher than healthy controls from a similar geographic area (2) or when compared with 25-hydroxyvitamin D [25(OH)D] levels from the Third National Health and Nutrition Examination Survey (NHANES III) (3). This high prevalence occurs despite the routine use of vitamin D supplementation in CF patients (1,4). Poor absorption of fat-soluble vitamins due to pancreatic insufficiency and diminished exposure to sunlight are thought to be among the causes for low vitamin D in this population (5).

Vitamin D insufficiency is associated with decreased bone mineral density (6,7) in patients with CF as well as in other disease states. Low bone mineral density, in turn, is associated with diminished lung function [as measured by forced expiratory volume in the first second (FEV₁)] in these patients (6). A direct association between vitamin D insufficiency and poor lung function has also been reported in non-CF adults (8,9). Furthermore, vitamin D is postulated to enhance the innate immune system (10) and improve insulin sensitivity (11), all issues of clinical relevance in the CF population who suffer from CF-related diabetes, frequent pulmonary infections, and progressive respiratory failure (12).

The best way to overcome vitamin D insufficiency in CF patients has not been established. Current guidelines by the Cystic Fibrosis Foundation Consensus Conference for vitamin D supplementation with high-dose ergocalciferol (vitamin D₂) appear to be inadequate (1). Previous studies in healthy adults (13,14) have suggested that high-dose cholecalciferol (vitamin D₃) is more effective in raising 25(OH)D levels compared with ergocalciferol. In contrast, recent studies have shown that at lower daily replacement doses, ergocalciferol and cholecalciferol are equally efficacious in raising 25(OH)D levels in non-CF children and adults (15,16). An alternative to oral vitamin D supplementation in CF patients is UV light therapy to produce vitamin D₃ in the skin. UV light therapy circumvents the problem of malabsorption in these patients and has been demonstrated to be effective in small pilot studies to improve vitamin D status in subjects with CF (17,18). A portable UV table lamp can provide the UVB radiation that sunlight cannot provide efficiently in northern latitudes during the winter months (19).

The relative efficacy of ergocalciferol, cholecalciferol, and limited UV light therapy in vitamin D replacement of CF patients has not yet been studied. The primary aim of this study was to compare the relative efficacy of these three modes of vitamin D replacement in clinically stable adults with CF.

Subjects and Methods

Subjects with CF were recruited from the Emory Cystic Fibrosis Center from November 2006 to March 2008. Subjects were recruited in the late fall and winter months during these 2 yr to minimize the contribution of endogenous vitamin D₃ production from the sun. The study was approved by the Emory Institutional Review Board, and all subjects gave written informed consent. The study was registered at clinicaltrials.gov under study no. NCT00450073.

Subjects

Patients were included in the study if they were between the ages of 16 and 70 yr, had a screening serum 25(OH)D level between 10 and 40 ng/ml, and had a confirmed diagnosis of CF by genetic analysis or sweat chloride testing. Patients were considered to have pancreatic insufficiency if they were on pancreatic enzyme replacements. Exclusion criteria were history of skin disease or cancer, presence of renal or hepatic disease, recent treatment with high-dose vitamin D, treatment with prednisone, or a history of more than six hospitalizations within the past year. Patients were also excluded if they were planning any trips during the study to sunny climates lasting longer than 2–3 d. Our study included patients with serum 25(OH)D levels up to 40 ng/ml at screening, expecting that CF subjects would experience a decrease in 25(OH)D in the winter (20).

Design

Thirty CF subjects were randomized in six-member blocks to one of three treatment arms: vitamin D₃, vitamin D₂, or UV light therapy. The vitamin D₂ 50,000 IU and vitamin D₃ 50,000 IU capsules were obtained from Pliva, Inc. (East Hanover, NJ) and Tishcon Corp. (Westbury, NY), respectively. Vitamin D₂ was contained in a hard gelatin capsule and embedded in refined soybean oil, glycerin, purified water, D&C yellow no. 10, and FD&C blue no. 1. Vitamin D₃ came in the form of a powder comprised of lactose, microcrystalline cellulose, and magnesium stearate that was contained in a capsule made of hard gelatin. A sample of each vitamin D capsule was assayed by an independent laboratory using HPLC with a reversed phase column. The vitamin D₃ capsules were found to contain 49,300 (± 900) IU of vitamin D₃, and the vitamin D₂ gel capsules were found to contain 47,100 (± 1100) IU of vitamin D₂.

For all subjects, a nonfasting serum was collected at baseline. Subjects randomized to D₂ or D₃ were instructed to take one pill each week for 12 wk. CF subjects randomized to UV light therapy exposed their lower backs in a seated position to a portable UV indoor tanning lamp (Sperti sunlamp “Del Sol”; Sperti, Crescent Springs, KY) at a distance of 14 in. for 3–10 min depending on their skin type five times a week according to a UV light protocol developed to enhance vitamin D status in CF patients (Table 1) (18). The Sperti sunlamp mimics natural sunlight in that it emits 2–5% UVB between the wavelengths of 290 and 320 nm (18). Subjects were contacted on a weekly basis to ensure compliance with their respective treatment regimens. Subject recall was used to assess compliance. Subjects returned for their final nonfasting serum collection within 4 wk of study completion. A focused food frequency questionnaire was used to determine mean daily vitamin D consumption by subjects. For the majority of subjects, this questionnaire was administered at the end of the study. The questionnaire we used was adapted from a previously published version (21). On their final visit, those subjects randomized to UV light therapy had their backs examined for the presence of any burns.

Table 1.

Skin sensitivity types to UVB radiation and the recommended tanning times per session

Skin type scale	Target minutes/session
Type I: always burns, never tans
Type II: always burns easily, tans minimally	3
Type III: burns moderately, tans gradually	6
Type VI: burns minimally, always tans well	9
Type V: rarely burns, tans profusely	12
Type VI: never burns, deeply pigmented	15

Adapted with modification from the U.S. Food and Drug Administration skin type scale (18). 

Analytical methods

Vitamin D insufficiency was defined as a 25(OH)D level below 30 ng/ml. Efficacy for each treatment modality was defined as its ability to raise or maintain 25(OH)D levels above 30 ng/ml during the winter months. Serum 25(OH)D₂, 25(OH)D₃, and total 25(OH)D were determined using liquid chromatography-mass spectrometry by Quest Diagnostics Incorporated (San Juan Capistrano, CA). PTH was done using ELISA (Immutopics International, LLC, San Clemente, CA). Mean daily intake of vitamin D by subjects was estimated using an 11-item food frequency questionnaire. A serving size for each food item was described to the subjects. Subjects were then asked to recall the number of serving sizes they consumed for each food item within a given day, or week, or month. For each food item, the amount of vitamin D contained in each serving size was multiplied by the frequency of intake for each subject. Total vitamin D intake was then calculated and averaged to represent mean intake per day.

Statistical methods

Data were first examined to ensure normality. All recorded variables were normally distributed. We used SAS 9.2 (SAS Institute, Cary, NC) to perform all analyses. The study was originally powered to detect a difference of 7.5 ng/ml in 25(OH)D between groups. Simple ANOVA was used to compare differences in mean covariates by treatment groups and analysis of covariance was used to compare mean differences when controlling for covariates (PROC GLM and PROC MIXED). Least squares means were calculated for all means reported using a Bonferroni correction whenever repeated measures were present. We used Fisher’s Exact Test (PROC FREQ) to calculate P value, comparing count data between treatment arms to examine heterogeneity of treatment groups. To determine whether initial 25(OH)D status effected change in 25(OH)D, we used Pearson’s correlation (PROC CORR). When considering our final models, we used initial 25(OH)D status, age, type of CF mutation, serum calcium, vitamin D intake, and use of vitamin supplement as potential covariates. We tested for interaction with CF mutation, and use of vitamins; however, significant interaction was not present. We conducted analyses using intent to treat. Mixed linear models using random intercept and random slope were used to model data.

Results

Subject demographics

We screened 33 individuals. Of those, 30 entered the study, and one patient each was lost to follow-up in the UV and D₃ groups. The mean age, BMI, FEV₁ percent predicted, and use of pancreatic enzyme replacement (a marker of pancreatic insufficiency) were not significantly different among treatment groups (Table 2). The mean age and distribution of mutations of our subjects were typical of adult patients with CF (5,22).

Table 2.

Baseline characteristics of adult CF patients

	UV	D3	D2	P value
Total (n)	9	9	10
Age (yr)	33.3 ± 15.7	35.7 ± 12.5	27.9 ± 11.9	0.43a
Sex (n)
Male	4	4	5
Female	5	5	5	0.52
BMI (kg/m2)	23.7 ± 3.9	22.5 ± 4.2	22.7 ± 2.9	0.76a
Pretreatment predicted %FEV1	72.4 ± 28.4	65.5 ± 24.5	76.5 ± 25.8	0.66a
Reported MVI/ADEK/Ca+D use, n (%)	8 (88.9)	8 (88.9)	8 (80)	0.68
Average daily vitamin D intake (IU)	370 ± 218	456 ± 290	587 ± 321	0.26a
Mutations (n)
Homo 508	1	2	4
Hetero 508	6	3	3
Neither	2	3	3	0.56
Reported pancreatic enzyme use, n (%)	9 (100)	8 (88.9)	10 (100)	0.24

Values are reported as means ± sd, number, or number (percent). MVI, Multi-vitamins. 

^aP − ANOVA. 

Baseline vitamin D status

Sixty-eight percent of our study population were vitamin D insufficient [25(OH)D <30 ng/ml] at baseline. All groups had similar dietary intakes of vitamin D, between 369 and 587 IU (Table 2). Greater than 80% of the subjects reported daily use of a multivitamin, which did not differ among the three groups. The mean initial 25(OH)D was 22.6 ± 10.8, 21.2 ± 10.1, and 24.4 ± 10.3 ng/ml for UV, D₃, and D₂, respectively, and was not different among the three treatment groups (P = 0.8). Twenty-two percent of the subjects in the D₃ and UV groups were vitamin D sufficient [25(OH)D >30 ng/ml], whereas 40% of subjects in the D₂ group were sufficient (Table 3).

Table 3.

Vitamin D, calcium, and PTH levels before and after intervention

	UV	D3	D2	Pa
Initial 25(OH)D (ng/dl)	22.6 ± 10.8	21.2 ± 10.18	24.4 ± 10.3	0.8
Final 25(OH)D (ng/dl)	28.3 ± 9.2	47.1 ± 20.5	32.7 ± 9.7	0.03
% Sufficient (>30 ng/ml)
Initial (%)	22	22	40	0.09
Final (%)	22	100	60	0.003
Initial Ca (mg/dl)	9.01 ± 0.45	9.06 ± 0.28	9.44 ± 0.42	0.04
Final Ca (mg/dl)	8.97 ± 0.38	8.87 ± 0.50	9.40 ± 0.43	0.24
Initial PTH (pg/ml)	111.1 ± 45.0	49.8 ± 46.7	129.9 ± 47.3	0.01
Final PTH (pg/ml)	110.0 ± 37.6	40.0 ± 47.6	86.1 ± 40.8	0.02

Values are reported as means ± sd, unless otherwise indicated. 

^aP − ANOVA. 

Factors associated with increases in 25(OH)D

The initial 25(OH)D level was associated with the rate of change in 25(OH)D (Table 4). The negative correlation coefficient indicates that those who start out with the highest serum 25(OH)D levels exhibited the smallest increase in 25(OH)D throughout the study period in response to oral vitamin D. We therefore controlled for initial 25(OH)D level when examining the mean rates of change. When controlling for initial serum 25(OH)D status, those receiving D₃ had the largest increase in serum 25(OH)D when compared with the other two treatments (P < 0.01) (Table 4). Those receiving D₃ also had the largest increase in serum 25(OH)D₃ (P < 0.001) (Table 4). Serum 25(OH)D₂ concentrations also increased in those receiving D₂ (P = 0.02) and decreased, but not significantly, in those receiving D₃ or UV. We examined age, BMI, sex, CF mutation, skin type, and vitamin D intake as potential covariates, although none was significant at the 5% level.

Table 4.

Changes in 25(OH)D levels

	Least squares means for total 25(OH)D, 25(OH)D3, and 25(OH)D2 using mixed linear model to account for initial levelsa			Pearson’s correlation between initial 25(OH)D levels and changes in 25(OH)D levels
	UV	D3	D2	Δ 25(OH)D2	Δ 25(OH)D3	Δ 25(OH)D
Initial 25(OH)D	23.1	21.2	24.4	−0.38c	0.24	0.02
Final 25(OH)D	28.2	47.1	32.7
Pb	0.25	<0.001	0.04
Initial 25(OH)D2	1.9	4.3	2.2	0.34	−0.42c	−0.29
Final 25(OH)D2	0.6	1.1	20.4
Pb	0.39	0.38	<0.001
Initial 25(OH)D3	20.2	16.9	22.2	20.2	16.9	22.2
Final 25(OH)D3	27.1	46.0	12.3
Pb	0.11	<0.001	0.03

^aValues are reported as means and were calculated using PROC GLM with a repeated statement. 

^bP compares initial and final levels using least squares means regression. 

^cIndicates significance with Pearson’s correlation. 

Total serum 25(OH)D concentrations after D₂, D₃, or UV light

Subjects randomized to oral forms of vitamin D (D₂ or D₃) exhibited significant increases in total serum 25(OH)D concentrations (Table 4). The most robust increase in total serum 25(OH)D occurred in the D₃-treated group, where the final total serum 25(OH)D was 47.1 ± 3.1 ng/ml, resulting in a significant total net increase of 25(OH)D of 25.3 ± 2.8 ng/ml (P < 0.001). In the D₂-treated group, the total increase in 25(OH)D concentration was less impressive at 8.8 ± 2.8 ng/ml (P = 0.04) (Fig. 1A). In the D₃-treated group, 100% of the subjects reached vitamin D sufficiency [25(OH)D > 30 ng/ml] compared with only 60% of those treated with D₂ and 22% of those treated with UV (P < 0.001). In those randomized to receive UV treatment, the final total serum 25(OH)D was 28.2 ± 3.2 ng/ml for a nonsignificant change of 5.2 ± 3.4 ng/ml (P = 0.25) (Fig. 1A).

Figure 1.

Change in total 25(OH)D, 25(OH)D₃, and 25(OH)D₂ by treatment group. Bold line indicates predicted value for the group using PROC MIXED for mixed linear models and controls for initial levels. Dashed lines represent individual change. The total 25(OH)D concentrations were significantly higher in subjects receiving D₃ and D₂, but not in subjects receiving UV (A). The 25(OH)D₃ concentrations were significantly higher in subjects receiving D₃ and significantly lower in subjects receiving D₂. There were no significant changes in 25(OH)D₃ concentrations in those receiving UV (B). The 25(OH)D₂ concentrations were significantly higher in subjects receiving D₂, and not in subjects receiving D₃ or UV (C).

One subject in the vitamin D₃-treated group had a much greater increase in 25(OH)D compared with others in this group (Fig. 1A). This subject was also the only one in the group that had pancreatic sufficiency. We performed sensitivity analysis after excluding this subject, and our outcomes remained the same.

25(OH)D₂ and 25(OH)D₃ concentrations after D₂, D₃, and UV light

As expected, subjects who received D₃ had significant increases in serum 25(OH) D₃ concentrations from 16.9 to 46.0 ng/ml (P < 0.001). In the D₂-treated group, the serum 25(OH)D₂ level increased by 17.7 ± 3.3 ng/ml, but the 25(OH)D₃ level decreased by 8.9 ± 3.3 ng/ml. This decrease in serum 25(OH)D₃ concentrations was significant compared with those in the D₃-treated group (P < 0.001) (Fig. 1B). Those subjects who received UV light did not have a significant increase in 25(OH)D₃ (P = 0.09).

The 25(OH)D₂ concentrations in subjects receiving D₃ and UV started low initially (over half of the sample in both groups had levels that were not detectable at the beginning of the intervention) at 4.3 and 1.9 ng/ml and did not change significantly with therapy (P = 0.12 and 0.17, respectively).

PTH concentrations after D₂, D₃, and UV light

Initial PTH levels in D₃-treated subjects were nearly half that of D₂- and UV-treated subjects (P = 0.01) (Table 3). When controlling for initial PTH values, there was a significant change in PTH from baseline to final measurements in the D3 and D2 treatment groups compared with the UV treatment group (Fig. 2). There was no significant difference in the change in PTH between the two treatment groups D₂ and D₃ (P = 0.86).

Figure 2.

Change in serum PTH concentrations in response to treatment with ergocalciferol 50,0000 IU once a week for 12 wk (D₂), cholecalciferol 50,000 IU once a week for 12 wk (D₃), or UV light with a Sperti lamp five times a week for 12 wk (UV). Change in PTH were controlled for initial PTH level. Mean change was calculated using least squared means. Subjects receiving both D₂ and D₃ had significantly reduced PTH levels (P = 0.01 and P = 0.05, respectively). There was no significant difference in the change in PTH between the two oral vitamin D treatment groups, D₂ and D₃ (P = 0.86).

Safety

There were no reported adverse events, nor were there any cases of hypercalcemia. All of the individuals randomized to a pill (D₃ or D₂) were 80% compliant or better. Only 55% of those on UV therapy were compliant with therapy more than 80% of the time. When we only considered five of nine subjects who were adherent with UV therapy, then the rise in 25(OH)D was similar to those treated with D₂ (P = 0.93).

Discussion

In this prospective, randomized study of three different regimens to correct vitamin D insufficiency, we found that both oral vitamin D₂ and vitamin D₃ were effective in raising 25(OH)D concentrations, with vitamin D₃ demonstrating the greater response. UV therapy was limited by poor adherence to the study protocol but was equally as effective as oral vitamin D₂ after exclusion of nonadherent subjects. As expected, PTH levels followed the same trend, with the greatest decrease in PTH level occurring in the vitamin D₃-treated group. Our results are in agreement with previous studies in healthy subjects, demonstrating superiority of vitamin D₃ over vitamin D₂ in raising 25(OH)D levels (13,14). Trang et al. (14) showed that when subjects were given 4000 IU vitamin D₃ for 14 d, they were able to raise their 25(OH)D levels 1.7 times more than subjects given the same dose and duration of vitamin D₂ (increase of 9.3 vs. 5.5 ng/ml). Armas et al. (13) compared vitamin D₂ to vitamin D₃ when given as a single dose of 50,000 IU. Despite a similar rise in 25(OH)D levels over the first 3 d for both vitamin D₂ and vitamin D₃, suggesting equal absorption, 25(OH)D levels in the vitamin D₂ group rapidly fell back to baseline by the second week. The area under the curve by d 28 was 204.7 ng · d/ml for vitamin D₃ compared with 60.2 ng · d/ml for vitamin D₂.

The Cystic Fibrosis Foundation consensus statement from 2005 (23) recommends daily oral supplementation with ergocalciferol at a daily dose of 400 to 800 IU. Several studies in CF patients have demonstrated such doses to be inadequate at optimizing 25(OH)D levels. A prospective study of 20 CF patients demonstrated that after 1 yr of treatment with 800 IU vitamin D₃ daily, none of the patients were able to raise their 25(OH)D levels above 30 ng/ml (70 nmol/liter) (24). Rovner et al. (2) showed that 90% of CF patients remained vitamin D insufficient despite routine supplementation with 800 IU/d. Furthermore, in a recent large retrospective study in CF patients (4), it was demonstrated that despite daily cholecalciferol supplementation (mean dose, 647 ± 339 IU), which increased 25(OH)D levels significantly (mean, 14.2 to 25 ng/ml), only 18% of CF patients were able to achieve 25(OH)D levels above 30 ng/ml.

For vitamin D-insufficient CF patients [25(OH)D levels <30 ng/ml], the Cystic Fibrosis Foundation consensus statement recommends treatment with ergocalciferol 50,000 IU either once or twice weekly. Lark et al. (25) demonstrated that CF subjects are only able to absorb 50% of ergocalciferol given as a single dose of 100,000 IU compared with healthy controls. They also demonstrated that CF subjects were less efficient at converting ergocalciferol to 25(OH)D compared with controls (25). Boyle et al. (1) demonstrated that only 8% of vitamin D-insufficient CF patients were able to reach 25(OH)D levels above 30 ng/ml after 8 wk of treatment with weekly 50,000 IU doses of ergocalciferol. Those that did not achieve vitamin D sufficiency were treated for another 8 wk, yet none was able to achieve vitamin D sufficiency. Green et al. (26) demonstrated that ergocalciferol administered once, twice, or three times a week for 8 wk was able to raise 25(OH)D levels above 30 ng/ml in only 33, 26, and 43%, respectively. Furthermore, the proportion of patients with follow-up 25(OH)D levels above 30 ng/ml were the same irrespective of whether or not they had been treated with ergocalciferol. Based on these data, the recommendations for maintenance of vitamin D status and correction of vitamin D insufficiency should be revised to higher levels, preferably with vitamin D₃.

In our study, CF subjects taking vitamin D₂ are able to raise their total 25(OH)D concentration levels; however, this is attenuated by the corresponding decrease in 25(OH)D₃. Circulating vitamin D exists mostly as 25(OH)D₃ representing vitamin D from cutaneous and most dietary sources of vitamin D. Less circulating vitamin D exists as 25(OH)D₂, which represents vitamin D from dietary sources fortified with ergocalciferol only and is limited to very few foods. This finding of lowered 25(OH)D₃ after vitamin D₂ therapy is of concern because ergocalciferol is most commonly available by prescription to correct vitamin D insufficiency in the United States and is recommended by the Cystic Fibrosis Foundation for correction of vitamin D insufficiency. Based on the studies of Trang et al. (14) and Armas et al. (13), some have suggested that ergocalciferol not be used as a supplement to correct vitamin D insufficiency (27).

A recent study by Holick et al. (15) demonstrated equal efficacy of vitamin D₂ and vitamin D₃ in raising 25(OH)D levels. However, vitamin D₂ and D₃ were given daily at much lower doses (1000 IU/d) in that trial and to subjects without chronic malabsorption, as in the CF patients we studied. This suggests that there may be differences in vitamin D₂ and D₃ when given in pharmacological (>10,000 IU) doses rather than in more physiological doses (<2,000 IU), although such studies in CF have not been performed. A recent study (28) demonstrated that at vitamin D₃ intake doses of less than 2000 IU/d there is rapid conversion of vitamin D₃ to 25(OH)D₃, but at doses of more than 2000 IU/d, vitamin D₃ concentrations can be detected in the serum along with a rise in 25(OH)D₃. It has been demonstrated that vitamin D₃ binds vitamin D binding protein (DBP) more avidly than vitamin D₂ (29), thus allowing vitamin D₃ to have a longer circulating half-life. One could infer from these two studies that at lower intake levels, vitamin D’s metabolism becomes independent of how avidly it binds DBP. At higher intake doses, the differential avidity for DBP of vitamin D₃ and vitamin D₂ becomes more evident. Finally, vitamin D₂ may activate the pregnane X receptor, which in turn up-regulates the CYP24, the enzyme that catabolizes vitamin D. The pregnane X receptor is a recently described nuclear receptor that is important for the detoxification of foreign substances in the body (30). Whether vitamin D₂ up-regulates the catabolism of vitamin D metabolites greater than vitamin D₃ is not known.

We found that UV light therapy did not significantly raise 25(OH)D levels. One explanation for the lack of a significant rise in 25(OH)D levels with UV light therapy could be attributed to the high nonadherence rate of subjects assigned to this treatment modality. Over half of the eligible subjects randomized to receive UV light therapy were not adherent with therapy per self-report. When we excluded those individuals, the mean 25(OH)D increase in the UV-treated group was 11 ng/ml (P = 0.04); however, only one of the individuals reached vitamin D sufficiency. In addition, our subjects exposed only their backs to the sunlamp, a much smaller surface area, when compared with a previous study in CF patients showing a change in 25(OH)D from 21 to 44 ng/ml after 8 wk of whole body tanning (17). Therefore, we cannot make definitive conclusions about whether UV light therapy is inferior to oral vitamin D replacement due to nonadherence of the study participants. Increased adherence as well as increased frequency and duration of UV light therapy with increased surface area exposure during therapy sessions would likely result in higher levels of 25(OH)D.

Our study has limitations. We studied adult CF patients with clinically stable disease, and UV light therapy was not supervised. Thus, adherence to UV light therapy was by subject report only. Another limitation of our study is the fact that two different formulations were used for the ergocalciferol and cholecalciferol pills. We cannot therefore rule out a difference in bioavailability of the two pills due to their different formulations. Another limitation is that the dose of vitamin D in the ergocalciferol pill was slightly lower than the cholecalciferol pill (approximately 4% less). However, this small difference in vitamin D cannot completely account for the large difference in 25(OH)D when comparing the cholecalciferol- and ergocalciferol-supplemented groups [cholecalciferol treatment resulted in 88% higher 25(OH)D than the ergocalciferol treatment]. Also, our study was a short-term study, and it does not evaluate long-term effects of vitamin D treatment on bone mineral density and efficiency of intestinal calcium absorption. It is not known what level of 25(OH)D is required to prevent bone loss and maximize calcium absorption in CF. Optimal levels of 25(OH)D have been extrapolated from non-CF populations. A recent study in pediatric CF patients suggests that these patients have normal baseline intestinal calcium absorption that did not improve further with daily 2000 IU D₃ treatment (31).

We conclude that oral vitamin D₃ given at a dose of 50,000 IU once a week for 12 wk is the most efficacious method to increase 25(OH)D and lower PTH values in CF subjects who are vitamin D insufficient. Ergocalciferol and UV light were not as effective in our study and should be reserved as alternative therapies. The optimal means to correct low vitamin D in patients with CF is not yet determined. However, this study along with others suggests that vitamin D₃ should be considered over vitamin D₂. Because vitamin D status is often low in CF patients, a more preventative approach to treatment of vitamin D status should be adopted, especially during the winter.