Background: Obesity is associated with vitamin D insufficiency and secondary hyperparathyroidism. Objective: This study assessed whether obesity alters the cutaneous production of vitamin D₃ (cholecalciferol) or the intestinal absorption of vitamin D₂ (ergocalciferol). Design: Healthy, white, obese [body mass index (BMI; in kg/m²) ≥ 30] and matched lean control subjects (BMI ≤ 25) received either whole-body ultraviolet radiation or a pharmacologic dose of vitamin D₂ orally. Results: Obese subjects had significantly lower basal 25-hydroxyvitamin D concentrations and higher parathyroid hormone concentrations than did age-matched control subjects. Evaluation of blood vitamin D₃ concentrations 24 h after whole-body irradiation showed that the incremental increase in vitamin D₃ was 57% lower in obese than in nonobese subjects. The content of the vitamin D₃ precursor 7-dehydrocholesterol in the skin of obese and nonobese subjects did not differ significantly between groups nor did its conversion to previtamin D₃ after irradiation in vitro. The obese and nonobese subjects received an oral dose of 50000 IU (1.25 mg) vitamin D₂. BMI was inversely correlated with serum vitamin D₃ concentrations after irradiation (r = −0.55, P = 0.003) and with peak serum vitamin D₂ concentrations after vitamin D₂ intake (r = −0.56, P = 0.007). Conclusions: Obesity-associated vitamin D insufficiency is likely due to the decreased bioavailability of vitamin D₃ from cutaneous and dietary sources because of its deposition in body fat compartments.

Vitamin D deficiency is now recognized as a pandemic. The major cause of vitamin D deficiency is the lack of appreciation that sun exposure in moderation is the major source of vitamin D for most humans. Very few foods naturally contain vitamin D, and foods that are fortified with vitamin D are often inadequate to satisfy either a child's or an adult's vitamin D requirement. Vitamin D deficiency causes rickets in children and will precipitate and exacerbate osteopenia, osteoporosis, and fractures in adults. Vitamin D deficiency has been associated with increased risk of common cancers, autoimmune diseases, hypertension, and infectious diseases. A circulating level of 25-hydroxyvitamin D of >75 nmol/L, or 30 ng/mL, is required to maximize vitamin D's beneficial effects for health. In the absence of adequate sun exposure, at least 800–1000 IU vitamin D₃/d may be needed to achieve this in children and adults. Vitamin D₂ may be equally effective for maintaining circulating concentrations of 25-hydroxyvitamin D when given in physiologic concentrations.

Background: The cholecalciferol inputs required to achieve or maintain any given serum 25-hydroxycholecalciferol concentration are not known, particularly within ranges comparable to the probable physiologic supply of the vitamin. Objectives: The objectives were to establish the quantitative relation between steady state cholecalciferol input and the resulting serum 25-hydroxycholecalciferol concentration and to estimate the proportion of the daily requirement during winter that is met by cholecalciferol reserves in body tissue stores. Design: Cholecalciferol was administered daily in controlled oral doses labeled at 0, 25, 125, and 250 μg cholecalciferol for ≈20 wk during the winter to 67 men living in Omaha (41.2° N latitude). The time course of serum 25-hydroxycholecalciferol concentration was measured at intervals over the course of treatment. Results: From a mean baseline value of 70.3 nmol/L, equilibrium concentrations of serum 25-hydroxycholecalciferol changed during the winter months in direct proportion to the dose, with a slope of ≈0.70 nmol/L for each additional 1 μg cholecalciferol input. The calculated oral input required to sustain the serum 25-hydroxycholecalciferol concentration present before the study (ie, in the autumn) was 12.5 μg (500 IU)/d, whereas the total amount from all sources (supplement, food, tissue stores) needed to sustain the starting 25-hydroxycholecalciferol concentration was estimated at ≈96 μg (≈3800 IU)/d. By difference, the tissue stores provided ≈78–82 μg/d. Conclusions: Healthy men seem to use 3000–5000 IU cholecalciferol/d, apparently meeting > 80% of their winter cholecalciferol need with cutaneously synthesized accumulations from solar sources during the preceding summer months. Current recommended vitamin D inputs are inadequate to maintain serum 25-hydroxycholecalciferol concentration in the absence of substantial cutaneous production of vitamin D.

Context: Two reports suggested that vitamin D₂ is less effective than vitamin D₃ in maintaining vitamin D status. Objective: Our objective was to determine whether vitamin D₂ was less effective than vitamin D₃ in maintaining serum 25-hydroxyvitamin D levels or increased the catabolism of 25-hydroxyvitamin D₃. Subjects and Design: This was a randomized, placebo-controlled, double-blinded study of healthy adults ages 18–84 yr who received placebo, 1000 IU vitamin D₃, 1000 IU vitamin D₂, or 500 IU vitamin D₂ plus 500 IU vitamin D₃ daily for 11 wk at the end of the winter. Results: Sixty percent of the healthy adults were vitamin D deficient at the start of the study. The circulating levels of 25-hydroxyvitamin D (mean ± sd) increased to the same extent in the groups that received 1000 IU daily as vitamin D₂ (baseline 16.9 ± 10.5 ng/ml; 11 wk 26.8 ± 9.6 ng/ml), vitamin D₃ (baseline 19.6 ± 11.1 ng/ml; 11 wk 28.9 ± 11.0 ng/ml), or a combination of 500 IU vitamin D₂ and 500 IU vitamin D₃ (baseline 20.2 ± 10.4 ng/ml; 11 wk 28.4 ± 7.7 ng/ml). The 25-hydroxyvitamin D₃ levels did not change in the group that received 1000 IU vitamin D₂ daily. The 1000 IU dose of vitamin D₂ or vitamin D₃ did not raise 25-hydroxyvitamin D levels in vitamin D-deficient subjects above 30 ng/ml. Conclusion: A 1000 IU dose of vitamin D₂ daily was as effective as 1000 IU vitamin D₃ in maintaining serum 25-hydroxyvitamin D levels and did not negatively influence serum 25-hydroxyvitamin D₃ levels. Therefore, vitamin D₂ is equally as effective as vitamin D₃ in maintaining 25-hydroxyvitamin D status.

Throughout evolution, exposure to sunlight and the photosynthesis of vitamin D₃ in the skin has been critically important for the evolution of land vertebrates. During exposure to sunlight, the solar UVB photons with energies 290–315 nm are absorbed by 7‐dehydrocholesterol in the skin and converted to previtamin D₃. Previtamin D₃ undergoes a rapid transformation within the plasma membrane to vitamin D₃. Excessive exposure to sunlight will not result in vitamin D intoxication because both previtamin D₃ and vitamin D₃ are photolyzed to several noncalcemic photoproducts. During the winter at latitudes above ∼35°, there is minimal, if any, previtamin D₃ production in the skin. Altitude also has a significant effect on vitamin D₃ production. At 27° N in November, very little (∼0.5%) previtamin D₃ synthesis was detected in Agra (169 m) and Katmandu (1400 m). There was an ∼2‐ and 4‐fold increase in previtamin D₃ production at ∼3400 m and at Everest base camp (5300 m), respectively. Increased skin pigmentation, application of a sunscreen, aging, and clothing have a dramatic effect on previtamin D₃ production in the skin. It is estimated that exposure in a bathing suit to 1 minimal erythemal dose (MED) is equivalent to ingesting between 10,000 and 25,000 IU of vitamin D₂. The importance of sunlight for providing most humans with their vitamin D requirement is well documented by the seasonal variation in circulating levels of 25‐hydroxyvitamin D [25(OH)D]. Vitamin D deficiency [i.e., 25(OH)D < 20 ng/ml] is common in both children and adults worldwide. Exposure to lamps that produce UVB radiation is an excellent source for producing vitamin D₃ in the skin and is especially efficacious in patients with fat malabsorption syndromes. The major cause of vitamin D deficiency globally is an underappreciation of sunlight's role in providing humans with their vitamin D₃ requirement. Very few foods naturally contain vitamin D, and those that do have a very variable vitamin D content. Recently it was observed that wild caught salmon had between 75% and 90% more vitamin D₃ compared with farmed salmon. The associations regarding increased risk of common deadly cancers, autoimmune diseases, infectious diseases, and cardiovascular disease with living at higher latitudes and being prone to vitamin D deficiency should alert all health care professionals about the importance of vitamin D for overall health and well being.