by Jack Norris, RD
- Ideal Vitamin D Levels
- Dietary Sources of Vitamin D
- Vitamin D Levels in Vegetarians
- Older People
- Tanning Beds
- Vitamin D3 vs. Vitamin D2
- Vitamin D Supplements and Meals
- Vitamin D2 in UV Treated Mushrooms
- Vitamin D in Fortified Foods
Ideal Vitamin D Levels
Traditionally, vitamin D recommendations have been based on how much was required to prevent the most obvious diseases of vitamin D deficiency, rickets, and osteomalacia. Some research suggests that higher vitamin D levels might help prevent fibromyalgia, rheumatoid arthritis, multiple sclerosis, upper respiratory tract infections, premenstrual syndrome, polycystic ovary disease, psoriasis, muscle weakness, lower back pain, diabetes, high blood pressure, and cancer (1), and asthma (2). Vitamin D supplementation might also improve mood (1). Because of these findings, some researchers have suggested that ideal vitamin D levels in the blood are between 80 to 100 nmol/l (32 to 40 ng/ml) (1).
In 2011, the Institute of Medicine (IOM) released a report in which they reviewed the scientific literature. They conclude that a serum vitamin D level of < 30 nmol/l (12 ng/ml) puts adults at risk for deficiency with regard to bone health, but that “Practically all persons are sufficient at serum 25(OH)D levels of at least 50 nmol/L (20 ng/mL),” and that optimal vitamin D levels are 50 to 125 nmol/l (20 to 50 ng/ml). They also increased the RDA for vitamin D from 400 to 600 IU (3).
In a separate paper, members of the IOM committee said (4):
For outcomes beyond bone health, however, including cancer, cardiovascular disease, diabetes, and autoimmune disorders, the evidence was found to be inconsistent and inconclusive as to causality.
A 2010 review in the Journal of the American Dietetic Association came to similar conclusions about vitamin D and cancer prevention (5); i.e., that the data is too inconsistent to draw conclusions.
Dietary Sources of Vitamin D
Most Americans get vitamin D through sunshine, fortified milk, and fortified margarine. The only significant, natural sources of vitamin D in foods are fatty fish (e.g. cod liver oil, mackerel, salmon, sardines), eggs (if chickens have been fed vitamin D), and mushrooms (if treated with UV rays). The vegan diet contains little, if any, vitamin D without fortified foods or supplements.
Vitamin D Levels in Vegetarians
Vitamin D appears to be more important for bones in conjunction with lower calcium intakes (typical in most vegan diets) than in diets that have large amounts of calcium (6).
A report from the EPIC-Oxford cohort found that vegans had the lowest average vitamin D levels of any diet group (see the table below). The study measured vitamin D intake from foods, including fortified foods, and found that vegans had the lowest intake. The report found a correlation between vitamin D intake from food and higher blood levels. For all groups, vitamin D levels were lower, on average, when measured in the winter and spring compared to the summer and autumn. Although the results were adjusted for dietary supplement use, there was no data on whether those supplements contained vitamin D (7).
Although the vegans had lower vitamin D levels than did meat-eaters, their vitamin D levels would be considered in the healthy range of 50 to 125 nmol/l according to the Institute of Medicine (3). The researchers noted that the average vitamin D levels in vegans was “comparable, if not slightly higher, than that reported in other studies among the general British population,” although this could be due to differences in the measurement method. The researchers surmised that the lower calcium intake in vegans may have lowered their vitamin D levels further because more 25(OH)D would be converted into the active vitamin D hormone to increase calcium absorption.
|Average Vitamin D Levels in EPIC-Oxford7|
|25(0H)D (nmol/l)||77 (75-79)||72 (68-76)||66 (63-69)||56 (51-61)|
|Adjusted for season and year of blood sample collection, age, sex, case–control status, Body Mass Index, smoking status, summer outdoor activity, vigorous exercise, current use of hormones, and supplement use.|
A 2007 report from EPIC-Oxford found that vitamin D levels were not associated with the risk of bone fracture (37). After about 5 years of follow-up, they sent a questionnaire to participants. Of the respondents, 730 reported experiencing a bone fracture. These subjects were then matched with 1,445 controls. Blood levels taken at baseline were assessed for vitamin D levels. They found no evidence of an association between plasma 25-hydroxyvitamin D and risk of fracture in either men or women. They noted that the distribution of fractures seen in this population was similar to that previously reported for England and Wales, and that a lack of association could possibly be due to the young average age of the participants (about 52 years old at baseline).
Danish Vegans (2018)
|Average Vitamin D Levels in Danish Vegans36
|| Median 25(0H)D in nmol/l (25th/75th percentile)
|| 67 (54-78)
|| 57 (40-71)
Adventist Health Study-2 (2009)
Despite low intakes of vitamin D by vegetarians, Adventist Health Study-2 (AHS2) showed no difference in vitamin D levels between vegetarians and non-vegetarians among people aged 51 to 70 years living in the United States. See the table below. About 4% of the vegetarians were vegan. Once again, on average, the vegetarians’ vitamin D levels were adequate.
The study showed that dietary vitamin D intake was a minor factor in someone’s vitamin D status at the levels that vitamin D is normally found in the diet. All groups got between 119 and 165 IU of vitamin D per day (the DRI is 600 IU) from their diet.
|Average Vitamin D Levels in Adventist Health Study-28|
|25(0H)D in nmol/l (SE)||51 (2.8)||53 (5.1)||49 (4.0)|
|25(0H)D in nmol/l (SE)||79 (2.7)||77 (4.4)||77 (2.6)|
|SE – standard error|
The variable causing the greatest difference in 25(OH)D concentrations was not diet, but ethnicity. This is likely due to people with dark skin needing much longer amounts of time in the sun to produce adequate vitamin D.
For many years, people thought that extra amounts of vitamin D made by the sun during the summer could be stored in the body and used during the winter. But it is important to remember that these days many of us spend very little time in the sun without sunscreen.
On average, it appears that most people, including vegans, are sustaining vitamin D levels over the winter that the Institute of Medicine considers healthy. In some cases, they are not.
For example, in a 2000 experiment, vegans in Finland were not able to maintain healthy levels of vitamin D during the winter (9). A follow-up study found an increase in lumbar spine density in 4 out of the 5 vegans who took 5 µg (200 IU) per day of vitamin D2 for 11 months (10). A dose of 5 µg (200 IU) per day was also required to keep vitamin D levels above 40 nmol/l (16 ng/ml) in Ireland during the winter (11).
It should be noted that the American Academy of Dermatology urges people not to get vitamin D via sunshine because of the increased risk of cancer (12). That said, not all researchers recommend complete avoidance of the sun.
According to Dr. Jacqueline Chan, increasing the surface of the skin exposed to the sun proportionately decreases the amount of time needed in the sun to produce the same amount of vitamin D. The duration of the sun exposure should be no more than about half the amount of time it takes for the skin to turn pink (12). Dr. Chan also says that in order to make vitamin D, “The sun must shine directly on skin without being blocked by sunscreen, glass and most plastics. Glass and most plastics block UVB, the part of the spectrum that converts pro-vitamin D3 but allow passage of UVA which contributes to skin cancer.” (12)
Dark Skinned People
An article in USA Today, Your Health: Skin color matters in the vitamin D debate (updated 4/19/2009), quotes vitamin D researcher Dr. Michael Holick as saying:
“Though someone in Boston with pale skin can get adequate vitamin D by exposing their arms and legs to the sun for 10 to 15 minutes twice a week in the summer, someone with the darkest skin might need two hours of exposure each time[.]”
This was the most specific statement I could find by a vitamin D researcher on how much sun a dark-skinned person needs to produce adequate vitamin D. As Holick notes in the article, this much sun is impractical and could cause skin cancer. While dark-skinned people have lower rates of skin cancer than whites, they are more likely to get diagnosed past the time that the cancer can be cured.
It is probably best for dark-skinned people to rely on vitamin D supplements rather than exposing themselves to the sun for more than a few minutes at a time. Monitoring vitamin D levels, if possible, would be ideal for knowing if supplements are needed.
Elderly people need 30 minutes a day of direct sunlight in order to produce adequate vitamin D (13).
A 2009 study from Ireland of people aged 64 years or older showed that 5 mcg (200 IU) per day was needed to keep most of the participants’ vitamin D levels above 40 nmol/l (16 ng/ml) over the winter (based on the lower standard deviation; 14).
The abstract of a 1982 study indicates that a daily dose of 11.2 µg (450 IU) of vitamin D2 was able to increase vitamin D levels in elderly subjects (15)
A 2010 study found that breast milk was not a sufficient source of vitamin D (16). A 1985 study recommended exposing babies to 30 minutes of sun a week wearing only a diaper in order to provide sufficient vitamin D (17), however, the American Academy of Pediatrics recommends no sun exposure for infants. The authors of the 2010 study recommend that all infants get the RDA for vitamin D of 400 IU via infant formula or vitamin D drops.
Some tanning beds can produce vitamin D with the type of UV rays they emit, but most do not. Experts generally recommend against using tanning beds to produces vitamin D because of their inefficiency and an increased risk of skin cancer. Click here for more information on tanning beds, vitamin D, and skin cancer.
Vitamin D3 vs. Vitamin D2
There are two types of vitamin D supplements:
- Vitamin D3 – cholecalciferol
- Derived from animals (usually from sheep’s wool or fish oil). It is the form of vitamin D produced in the skin when exposed to UV rays.
- Vitashine and Deva carry vegan vitamin D3 that has been verified by the UK Vegan Society. Some companies claim to produce a vegan D3 but have questionable ingredients (more info).
- Vitamin D2 – ergocalciferol
There has been a long-running debate on whether vitamin D3 supplements are more effective than vitamin D2. The research has been trickling in since 1998, and in 2012 a thorough meta-analysis by Tripkovic et al. was published making some things clear (19):
- In large boluses of 50,000 IU or more, vitamin D3 is much more effective than D2 at raising and maintaining vitamin D levels.
- In daily amounts of 1,000 to 4,000 IU per day, vitamin D3 is somewhat better than D2 at raising vitamin D3 levels.
There are some things to consider regarding this research, but first we need some background on vitamin D.
Vitamin D2 and D3 are not biologically active. Once ingested or created in the skin, in order to become biologically active, they have to be converted to the hormone, calcitriol. The first step in this process is for the liver to convert the vitamin into 25-hydroxyvitamin D, also known as 25(OH)D. When we talk about “vitamin D levels” going up or down, we are talking about the 25-hydroxyvitamin D. Then, when the body senses a need for more calcitriol, the kidney converts 25-hydroxyvitamin D into calcitriol.
Throughout this process, the part of vitamin D that distinguishes D2 from D3, also known as the side chain, stays attached to the molecule. So, calcitriol is either calcitriol-D2 or calcitriol-D3, and to our knowledge there is no difference in biological activity. However, there is a theory that once converted into calcitriol and then degraded, the calcitriol-D3 can be retroconverted back into 25-hydroxyvitamin D3, but the D2 version of calcitriol cannot be retroconverted or cannot be at nearly the rate of D3. This could explain why infrequent, large boluses of vitamin D2 quickly disappear from the system – the vitamin D is converted to calcitriol, used, and then degraded without replenishing the 25-hydroxyvitamin D.
There are some other things to consider about the research comparing D2 to D3, the most important of which is that vitamin D2 is probably adequate for most people. Even though D3 might increase vitamin D levels somewhat better than D2, D2 still increases the levels well into and beyond the range that is considered optimal by the Institute of Medicine.
Additionally, much of the research is done on people who already have adequate vitamin D levels. For example, in one of the more recent studies that was included in the meta-analysis mentioned above, a 2011 study by Binkley et al. (20), the average vitamin D levels started out above 72 nmol/l (29 ng/dl) which is well above adequate levels of 50 nmol/l (20 ng/ml) recommended by the Institute of Medicine. It could very well be that at levels this high, the degrading of the vitamin D2 is of no concern (unless someone goes a long time without being able to replenish stores).
All studies in the meta-analysis used vitamin D doses much higher than the DRI of 600 IU per day. At doses closer to the DRI, in people who have low vitamin D levels, it’s possible that vitamin D2 and D3 might be virtually indistinguishable in their ability to raise vitamin D back to healthy levels.
Finally, there is some anecdotal evidence. Although I have heard from some vegans who have had a hard time raising their vitamin D levels using D2, many others have succeeded. For example, in June 2010, a vegan who had been diagnosed with vitamin D deficiency wrote me saying that his weekly 50,000 IU of vitamin D2 prescribed by his doctor for 12 weeks succeeded in raising his vitamin D levels from 32.5 nmol/l (13 ng/ml) in January to 180 nmol/l (72 ng/ml) in May. For long-term maintenance, his doctor recommended 1200 IU per day. I heard from another person in December 2010 who raised her levels from 30 to 67 nmol/l (8.1 to 27 ng/ml) with 4,000 IU of vitamin D2 per day for 2 months.
For those vegans whose vitamin D levels do not respond well to vitamin D2, there is a vegan vitamin D3 supplement available from Vitashine, mentioned at the top of this section.
For historical purposes and because some people might want more details, I have left in the research I previously discussed regarding D2 vs. D3 here:
A 2013 study from Germany found that with supplementing daily for 8 weeks at about 2,000 IU, D3 was more effective at raising vitamin D levels (21). The average 25(OH)D levels increased 46 nmol/l in the D3 group (for an average of 89 nmol/l) but only 30 nmol/l in the D2 group (for an average of 68 nmol/l). Vitamin D2 supplementation appeared to decrease the amount of circulating vitamin D3. PTH levels were not different between groups. Because D2 raised levels into the recommended range (50 – 125 nmol/l), it seems preliminary to assume that D3 is more healthy based on these results, though evidence is mounting that D3, even at smaller doses, can raise vitamin D levels higher or more quickly than D2.
In a 2011 study from Germany, 28,000 IU of vitamin D2 were fed to subjects either in the form of a supplement or from mushrooms, one time per week for four weeks. Vitamin D levels increased from 34 to 57 nmol/l in the mushroom group, and from 29 to 58 nmol/l in the supplement group. The placebo group’s vitamin D2 levels decreased over the course of the study (22).
A 2011 study by Heaney found that a weekly dose of 55,000 IU of vitamin D3 raised vitamin D levels significantly better than did a weekly dose of 48,000 IU of vitamin D2 (23). The differences in the amounts given were not enough to explain the discrepancy between the increases in vitamin D3. However, some things should be noted. 25(OH)D levels for the D2 group started out at 76.5 nmol/l (30.6 ng/ml), while those in the D3 group started out with levels at 65.0 nmol/l (26.0 ng/ml). In other words, both groups were already replete (according to the Institute of Medicine). The 25(OH)D levels in the D2 group increased to about 130 nmol/l (50 ng/ml) over the course of the study. Finally, two of the authors have financial ties to BTR Group, Inc., a manufacturer of Maximum D3. That is not to say that any data was fudged, just that financial ties can possibly bias one’s perspectives.
Biancuzzo et al. (24) (2010) tested changes in 25(OH)D from a daily dose of 1,000 IU of vitamin D2 or D3 from either orange juice or supplement capsules for 11 weeks at the end of winter. The placebo group received nothing and their 25(OH)D levels decreased slightly. The average 25(OH)D levels of the other four groups (D2 from orange juice, D2 from capsules, D3 from orange juice, D3 from capsules) went up about 25 nmol/l (10 ng/ml) with no significant differences between groups.
In a 2009 review, Dr. Jacqueline Chan sums up the studies on vitamin D2 vs. D3, “Treatment for most of the studies finding D2 to be less effective than D3 were extremely large boluses given only once, whereas in studies finding them equally effective, the treatment was daily amounts between 400 and 2,000 IU (25).”
Glendenning et al. (26) (2009) compared 1,000 IU of D2 vs. D3 in people with vitamin D insufficiency who had hip fractures. After three months, those who supplemented with D3 had a 31% or 52% (depending on how they were measured) greater increase in 25(OH)D levels than those supplementing with D2. However, parathyroid hormone levels (which can cause bone loss) did not differ between groups, leading the researchers to question whether the difference in 25(OH)D levels were of biological importance.
Thatcher et al. (27) (2009) gave children with rickets one oral dose of 50,000 IU of vitamin D2 or D3. After three days, 25(OH)D levels rose from approximately 50 to 72 nmol/l (20 to 29 ng/ml) for both groups. Calcitriol (the actual vitamin D hormone) levels also increased similarly in both groups (by about 70%), however, calcium absorption did not increase, leading the researchers to conclude the rickets were not caused by vitamin D deficiency. This should not be a surprise since the baseline average level of 50 nmol/l (20 ng/ml) of 25(OH)D should be adequate to prevent rickets.
Gordon et al. (28) (2008), treated 40 infants and toddlers with vitamin D deficiency. Each were assigned to one of three 6-week regimens: 2,000 IU oral vitamin D2 daily, 50,000 IU vitamin D2 weekly, or 2,000 IU vitamin D3 daily. At the end of the trial, participants’ 25(OH)D levels went from an average of 42.5 to 90 nmol/l (17 to 36 ng/ml), and there were no significant differences between treatment groups.
Holick et al. (18) (2007) found that a daily dose of 1,000 IU of vitamin D2 over 11 weeks increased 25(OH)D levels from 42 to 67 nmol/l (16.9 to 26.8 ng/ml). Vitamin D3 increased levels similarly, from 49 to 72 nmol/l (19.6 to 28.9 ng/ml). It took 6 weeks for 25(OH)D levels to plateau on that regimen. The study was conducted in Boston and started in February.
In a 2004 study by Armas et al. (29), subjects were given one dose of 50,000 IU of vitamin D2 or vitamin D3. Vitamin D2 was absorbed just as well as vitamin D3. However, after three days, blood levels of 25(OH)D started dropping rapidly in the subjects who were given vitamin D2, whereas those who received vitamin D3 sustained high levels for two weeks before dropping gradually.
Trang et al. (30) (1998) found that a daily dose of 4,000 IU of vitamin D3 for two weeks was 1.7 times more effective in raising 25(OH)D levels (which increased 22.5 ± 5 nmol/l (9.0 ± 2 ng/ml)) than 4,000 IU of vitamin D2 (which increased levels 10.5 ± 5 nmol/l (4.2 ± 2 ng/ml)).
Vitamin D Supplements and Meals
Because vitamin D is a fat soluble vitamin, taking vitamin D supplements with foods that contain fat might increase absorption.
A 2015 randomized controlled trial found that taking vitamin D3 supplements in a 50,000 IU dose with a 30% fat (as calories) meal increased absorption 32% over taking with a meal containing no fat (31).
A 2010 study found that patients whose vitamin D levels didn’t respond to prescribed supplements (some D2 and some D3) did respond after 2 to 3 months of being instructed to take the supplements with meals (32). This study had some methodological issues, such as no control group, and should be viewed with caution.
Vitamin D2 in UV Treated Mushrooms
Food manufacturers are now creating large amounts of vitamin D2 in mushrooms by exposing them to commercial ultraviolet light or direct sunlight (33, 34). The vitamin D is well-retained in the mushrooms over the course of the typical storage life of fresh mushrooms, up to two weeks (33, 35). This vitamin D is effective in improving vitamin D status and no different from a vitamin D2 supplement (22).
Vitamin D in Fortified Foods
- The Daily Value for vitamin D is 10 mcg (400 IU). Therefore, if a food label says it has 25% of
the daily value, it means it has 2.5 mcg (100 IU) per serving.
- Vitamin D fortified soy, almond, or rice milk normally has 2-3 mcg (80-120 IU) per cup.
2. Urashima M, Segawa T, Okazaki M, Kurihara M, Wada Y, Ida H. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010 May;91(5):1255-60. Epub 2010 Mar 10.
3. Ross AC, Taylor CL, Yaktine AL, Del Valle HB, Editors. Dietary Reference Intakes for Calcium and Vitamin D. Committee to Review Dietary Reference Intakes for Vitamin D and Calcium. Food and Nutrition Board. Institute Of Medicine of The National Academies. The National Academies Press. Washington, D.C. 2011.
7. Crowe FL, Steur M, Allen NE, Appleby PN, Travis RC, Key TJ. Plasma concentrations of 25-hydroxyvitamin D in meat eaters, fish eaters, vegetarians and vegans: results from the EPIC-Oxford study. Public Health Nutr. 2011 Feb;14(2):340-6.
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9. Outila TA, Karkkainen MU, Seppanen RH, Lamberg-Allardt CJ. Dietary intake of vitamin D in premenopausal, healthy vegans was insufficient to maintain concentrations of serum 25-hydroxyvitamin D and intact parathyroid hormone within normal ranges during the winter in Finland. J Am Diet Assoc. 2000 Apr;100(4):434-41.
11. Cashman KD, Hill TR, Lucey AJ, Taylor N, Seamans KM, Muldowney S, Fitzgerald AP, Flynn A, Barnes MS, Horigan G, Bonham MP, Duffy EM, Strain JJ, Wallace JM, Kiely M. Estimation of the dietary requirement for vitamin D in healthy adults.Am J Clin Nutr. 2008 Dec;88(6):1535-42.
14. Cashman KD, Wallace JM, Horigan G, Hill TR, Barnes MS, Lucey AJ, Bonham MP, Taylor N, Duffy EM, Seamans K, Muldowney S, Fitzgerald AP, Flynn A, Strain JJ, Kiely M. Estimation of the dietary requirement for vitamin D in free-living adults >=64 y of age. Am J Clin Nutr. 2009 May;89(5):1366-74.
15. Toss G, Andersson R, Diffey BL, Fall PA, Larko O, Larsson L. Oral vitamin D and ultraviolet radiation for the prevention of vitamin D deficiency in the elderly. Acta Med Scand. 1982;212(3):157-61. (Abstract only.)
16. Liang L, Chantry C, Styne DM, Stephensen CB. Prevalence and risk factors for vitamin D deficiency among healthy infants and young children in Sacramento, California. Eur J Pediatr. 2010 Nov;169(11):1337-44.
17. Specker BL, Valanis B, Hertzberg V, Edwards N, Tsang RC. Sunshine exposure and serum 25-hydroxyvitamin D concentrations in exclusively breast-fed infants. J Pediatr. 1985 Sep;107(3):372-6. (Abstract only.)
18. Holick MF, Biancuzzo RM, Chen TC, Klein EK, Young A, Bibuld D, Reitz R, Salameh W, Ameri A, Tannenbaum AD. Vitamin D2 is as effective as vitamin D3 in maintaining circulating concentrations of 25-hydroxyvitamin D. 2008 Mar;93(3):677-81. Epub 2007 Dec 18.
19. Tripkovic L, Lambert H, Hart K, Smith CP, Bucca G, Penson S, Chope G, Hyppönen E, Berry J, Vieth R, Lanham-New S. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012 Jun;95(6):1357-64.
20. Binkley N, Gemar D, Engelke J, Gangnon R, Ramamurthy R, Krueger D, Drezner MK. Evaluation of ergocalciferol or cholecalciferol dosing, 1,600 IU daily or 50,000 IU monthly in older adults. J Clin Endocrinol Metab. 2011 Apr;96(4):981-8.
21. Lehmann U, Hirche F, Stangl GI, Hinz K, Westphal S, Dierkes J. Bioavailability of vitamin D(2) and D(3) in healthy volunteers, a randomized placebo-controlled trial. J Clin Endocrinol Metab. 2013 Nov;98(11):4339-45. doi: 10.1210/jc.2012-4287. Epub 2013 Sep 3.
22. Urbain P, Singler F, Ihorst G, Biesalski HK, Bertz H. Bioavailability of vitamin D₂ from UV-B-irradiated button mushrooms in healthy adults deficient in serum 25-hydroxyvitamin D: a randomized controlled trial. Eur J Clin Nutr. 2011 Aug;65(8):965-71. doi: 10.1038/ejcn.2011.53. Epub 2011 May 4.
24. Biancuzzo RM, Young A, Bibuld D, Cai MH, Winter MR, Klein EK, Ameri A, Reitz R, Salameh W, Chen TC, Holick MF. Fortification of orange juice with vitamin D(2) or vitamin D(3) is as effective as an oral supplement in maintaining vitamin D status in adults. Am J Clin Nutr. 2010 Jun;91(6):1621-6. Epub 2010 Apr 28.
26. Glendenning P, Chew GT, Seymour HM, Gillett MJ, Goldswain PR, Inderjeeth CA, Vasikaran SD, Taranto M, Musk AA, Fraser WD. Serum 25-hydroxyvitamin D levels in vitamin D-insufficient hip fracture patients after supplementation with ergocalciferol and cholecalciferol. Bone. 2009 Nov;45(5):870-5. Epub 2009 Jul 23. (Abstract)
27. Thacher TD, Obadofin MO, O’Brien KO, Abrams SA. The effect of vitamin D2 and vitamin D3 on intestinal calcium absorption in Nigerian children with rickets. J Clin Endocrinol Metab. 2009 Sep;94(9):3314-21. Epub 2009 Jun 30.
28. Gordon CM, Williams AL, Feldman HA, May J, Sinclair L, Vasquez A, Cox JE. Treatment of hypovitaminosis D in infants and toddlers. J Clin Endocrinol Metab. 2008 Jul;93(7):2716-21. Epub 2008 Apr 15.
30. Trang HM, Cole DE, Rubin LA, Pierratos A, Siu S, Vieth R. Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2. Am J Clin Nutr. 1998 Oct;68(4):854-8. (Abstract)
33. Kalaras MD, Beelman RB, Elias RJ. Effects of postharvest pulsed UV light treatment of white button mushrooms (Agaricus bisporus) on vitamin D2 content and quality attributes. J Agric Food Chem. 2012 Jan 11;60(1):220-5.
34. Simon RR, Phillips KM, Horst RL, Munro IC. Vitamin D mushrooms: comparison of the composition of button mushrooms (Agaricus bisporus) treated postharvest with UVB light or sunlight. J Agric Food Chem. 2011 Aug 24;59(16):8724-32.
35. Roberts JS, Teichert A, McHugh TH. Vitamin D2 formation from post-harvest UV-B treatment of mushrooms (Agaricus bisporus) and retention during storage. J Agric Food Chem. 2008 Jun 25;56(12):4541-4.
36. Hansen TH, Madsen MTB, Jørgensen NR, Cohen AS, Hansen T, Vestergaard H, Pedersen O, Allin KH. Bone turnover, calcium homeostasis, and vitamin D status in Danish vegans. Eur J Clin Nutr. 2018 Jan 23.
37. Roddam AW, Neale R, Appleby P, Allen NE, Tipper S, Key TJ. Association between plasma 25-hydroxyvitamin D levels and fracture risk: the EPIC-Oxford study. Am J Epidemiol. 2007 Dec 1;166(11):1327-36.