*Corresponding Author:
Carol L. Wagner, MD
Medical University of South Carolina, 165 Ashley Avenue,
MSC 917, Charleston, SC 29425, USA.
Tel: (001) 843-792-2112; Fax: (001) 843-792-8801
Email: wagnercl@musc.edu

In the past three decades, the number of peer reviewed articles concerning vitamin D has gone from under a hundred per year to thousands. With the increase in published articles has come an increase in controversy. Some suggest that vitamin D as the preprohormone of the active form—calcitriol or 1,25-dihydroxy-vitamin D (1,25(OH)₂D)—is an enabler of various processes in the body (1), that include both innate and adaptive immune functions and explain vitamin D’s link with long latency diseases such as multiple sclerosis (2, 3), diabetes (4-7), rheumatoid arthritis (8, 9), certain cancers (prostate (10) and colon (11, 12) cancers), autism (13-16), and infections such as tuberculosis (17), and derangement in placental function that manifests as preeclampsia and premature birth, and later childhood asthma (18-22). There are as many articles showing benefit as there are negative studies. It is confusing and problematic for health care providers and policy makers. While no one would dispute vitamin D’s endocrine function in helping maintain calcium and phosphate metabolism (23), the confusion surrounds whether vitamin D has the capacity to change an individual’s health trajectory.

We have become a bit more sophisticated in our understanding of vitamin D and realize that there are genetic variations in the way vitamin D is processed, whether made in the epidermis through UVB exposure or through diet or dietary supplementation, where vitamin D is absorbed from the intestines. The vitamin D binding protein is the controller of how vitamin D and its metabolites—25-hyroxy-vitamin D or 25(OH)D and 1,25(OH)₂D—are carried around the body—how tightly bound and how easily or not vitamin D and its metabolites are delivered to cells throughout the body. It is the vitamin D receptor that is found in the nucleus of cells that binds to 1,25(OH)₂D. Genetic variations impact the carrying of vitamin D and its delivery. Once bound to calcitriol, the VDR partners with another protein called retinoid X receptor (RXR), forming a complex that then binds to the vitamin D response elements on DNA, and thereby regulates the activity of vitamin D-responsive genes. The vitamin D responsive genes are enabled by calcitriol and the complex, and in turning these downstream genes on or off, the complex help control calcium and phosphate absorption and other processes. It is the “other processes” that has been an area of contention, with variable effects in demonstrating health effects of vitamin D.

Part of the problem has been that there is little agreement on how much vitamin D is needed by the body to maintain the various non-calcium related functions. How does one assess the effect of vitamin D on long-latency diseases that may take months to years to manifest? Focusing on observational or epidemiological studies has shown association between vitamin D deficiency and as described earlier, diseases such as multiple sclerosis, cancers, autism, and pregnancy complications. Randomized controlled trials using various dosing regimens has provided mixed results—those study populations with the greatest vitamin D deficiency appear to have the greatest improvement (24), and in contrast, those study populations without frank deficiency and who are prescribed—what some consider minimal dosing—have shown minimal effects. When vitamin D supplementation is initiated also appears to impact results; for example, if dosing of higher dose vitamin D occurs early during pregnancy affecting placentation appears to impact maternal and fetal health differently than if supplementation occurs later in pregnancy (25-28). There have been numerous meta-analyses performed on the various topics—vitamin D and cardiovascular health, all-cause mortality risk, pregnancy complications and other long latency diseases such as cancers, multiple sclerosis, and autism with variable results (15, 29-33). Most meta-analyses do not consider VDBP or VDR genotype differences, and so the conclusions are based on a combined reference group of people without such knowledge. In such meta-analyses, there is a wide range of dosing—from no vitamin D given—true placebo to minimal doses given—400 international units (IU), the amount that is prescribed for a neonate in the US that is also prescribed for pregnant and lactating women and is the typical amount found in prenatal vitamins (34). Other studies have included a randomization schedule with higher doses given up to 10,000 IU/day in healthy males during winter months (35, 36); but there are also differential dosing schedules—weekly, biweekly, monthly, quarterly, and even high dose yearly dosing—all with variable results (34). The tests themselves to measure the metabolite 25(OH)D, which is considered the gold standard for assessing vitamin D status given its 2-3-week half-life, can be measured a number of ways and not all methods are equivalent or accurate (37, 38).

Another area of controversy in the vitamin D realm is the vitamin D requirements of the lactating woman and her breastfeeding infant. The status quo is that all breastfeeding infants should receive 400 IU vitamin D/day. This seems like an easy prescription yet adherence or compliance of giving infant daily drops of vitamin D is quite low in certain areas of the world, including the US (39, 40). It begs the question of why does one really have to supplement a breastfeeding infant when all other nutrients and bioactive substances are well provided; mothers who are exclusively breastfeeding mention this inconsistency frequently. The data from the 1980’s was that breast milk provided minimally sufficient vitamin D (41), but what was the vitamin D status of those mothers? Data from our laboratory suggests that breast milk is marginally sufficient in vitamin D when mother herself has marginal sufficiency (42-45). When we provide a vitamin D supplement to mother in the amount that increases the vitamin D content in her milk (and remember, it is the parent compound—vitamin D itself—cholecalciferol or ergocalciferol—that gets into the breast milk—with minimal amounts of 25(OH)D), then mother is safely sufficient, her milk is enriched, and her infant receives at least 400 IU/L breast milk and has vitamin D status comparable to an infant who receives daily 400 IU/day vitamin D supplementation and whose mother is taking 400 IU/day herself (43-45).

Despite the findings of two earlier pilot studies and a larger, National Institutes of Health (NIH)-sponsored study (43-45), the recommendations put forth by various agencies have not been changed. Why should we care? Does it matter if mother herself is vitamin D sufficient or not, if her baby is receiving daily vitamin D supplementation? Some would argue no, as long as the baby is sufficient. Yet, if we look at breast milk not only as a nutritive medium but as a bioactive substance that continues what began in utero as an ex-utero extension of processes impacting life-long immune function (46), we would answer yes: that through the actions of vitamin D on the mother herself and the refinement of her immune function and the immune signalling that then occurs through her milk become vital processes to her recipient infant. We know that maternal vitamin D status during pregnancy impacts neonatal immune function (47); why not maternal vitamin D status and its resulting effect on maternal immune status being a specific delivery/signalling system to the recipient infant? When we make mother vitamin D replete, we no longer have to supplement her breastfeeding infant, and quite possibly we are insuring an immune signalling process that will have life-long impact. It is certainly something worth studying.

Disclosures

Drs. Wagner and Hollis serve as scientific consultants for Church & Dwight, Inc, Princeton, NJ, and Dr. Wagner was an invited speaker at the Mead Johnson Pediatric Nutrition Institute seminar on September 27, 2018.

PEER REVIEW

Not commissioned. Externally peer reviewed.

References

1. Heaney RP, Is Vitamin D Inadequacy in Early Life an Instance of the "Barker Hypothesis"?" Nutr Today, 2016. 51(1): 14-17.
2. Munger K, Zhang S, O'Reilly E, Hernan M, Olek M, Willett W, et al., Vitamin D intake and incidence of multiple sclerosis. Neurology, 2004. 62: p.60-65.
3. Ebers GC, Sadovnick AD, Veith R, Vitamin D intake and incidence of multiple sclerosis. Neurology, 2004. 63(5): p. 939.
4. Hypponen E, Vitamin D and increasing incidence of type 1 diabetes-evidence for an association? Diabetes Obes Metab, 2010. 12(9): p. 737-743.
5. Omar DF, Kamal MM, El-Hefnawy MH, El-Mesallamy HO, Serum Vitamin D and Its Upregulated Protein, Thioredoxin Interacting Protein, Are Associated With Beta-Cell Dysfunction in Adult Patients With Type 1 and Type 2 Diabetes. Can J Diabetes, 2018. 42(6): p. 588-594.
6. Lee WC, Mokhtar SS, Munisamy S, Yahaya S, Rasool AHG, Vitamin D status and oxidative stress in diabetes mellitus. Cell Mol Biol (Noisy-le-grand), 2018. 64(7): p. 60-69.
7. Yu Y, Tian L, Xiao Y, Huang G, Zhang M, Effect of Vitamin D Supplementation on Some Inflammatory Biomarkers in Type 2 Diabetes Mellitus Subjects: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Ann Nutr Metab, 2018. 73(1): p. 62-73.
8. Cutolo M, Otsa K, Uprus M, Paolino S, Seriolo B, Vitamin D in rheumatoid arthritis. Autoimmun Rev, 2007. 7: p. 59 - 64.
9. Ramagopalan SV, Goldacre R, Disanto G, Giovannoni G, Goldacre MJ, Hospital admissions for vitamin D related conditions and subsequent immune-mediated disease: record-linkage studies. BMC Med, 2013.11: p. 171.
10. Trump DL, Chadha MK, Sunga AY, Fakih MG, Ashraf U, et al., Vitamin D deficiency and insufficiency among patients with prostate cancer. BJU International, 2009. 104(7): p. 909-914.
11. Garland C, Comstock G, Garland F, Helsing K, Shaw E, Gorham E, Serum 25(OH)D and colon cancer: Eight-year prospective study. Lancet, 1989. 2(8673): p. 1176-1178.
12. Ma Y, Zhang P, Wang F, Yang J, Liu Z, Qin H, Association between vitamin D and risk of colorectal cancer: a systematic review of prospective studies. J clin oncol, 2011. 29(28): p. 3775-82.
13. Cannell JJ, Hollis BW, Use of vitamin D in clinical practice. Altern Med Rev, 2008. 13(1): p. 6-20.
14. Saad K, Abdel-Rahman AA, Elserogy YM, Al-Atram AA, Cannell JJ, Vitamin D status in autism spectrum disorders and the efficacy of vitamin D supplementation in autistic children. Nutr Neurosci, 2016.19(8): p. 346-351.
15. Cannell JJ, Vitamin D and autism, what's new? Rev Endocr Metab Disord, 2017. 18(2): p. 183-193.
16. El-Ansary A, Cannell JJ, Bjorklund G, Bhat RS, Al Dbass AM, et al., In the search for reliable biomarkers for the early diagnosis of autism spectrum disorder: the role of vitamin D. Metab Brain Dis, 2018. 33(3): p. 917-931.
17. Liu P, Stenger S, Tang D, Modlin R, Cutting edge: vitamin D-mediated human antimicrobial activity against Mycobacterium tuberculosis is dependent on the induction of cathelicidin. J Immunol, 2007. 179: p. 2060 - 2063.
18. Mirzakhani H, O'Connor G, Bacharier LB, Zeiger RS, Schatz MX, et al., Asthma control status in pregnancy, body mass index, and maternal vitamin D levels. J Allergy Clin Immunol, 2017. 140(5): p. 1453-6.
19. Weiss ST, Litonjua AA, Vitamin D in Host Defense: Implications for Future Research. Am J Respir Cell Mol Biol, 2017. 56(6): p. 692-693.
20. Wolsk HM, Harshfield BJ, Laranjo N, Carey VJ, O'Connor G, et al., Vitamin D supplementation in pregnancy, prenatal 25(OH)D levels, race, and subsequent asthma or recurrent wheeze in offspring: Secondary analyses from the Vitamin D Antenatal Asthma Reduction Trial. J Allergy Clin Immunol, 2017. 140(5): p. 1423-1429.
21. Litonjua AA, Weiss ST, Vitamin D status through the first 10 years of life: A vital piece of the puzzle in asthma inception. J Allergy Clin Immunol, 2017. 139(2): p. 459-61.
22. Wolsk HM, Chawes BL, Litonjua AA, Hollis BW, Waage J, et al., Prenatal vitamin D supplementation reduces risk of asthma/recurrent wheeze in early childhood: A combined analysis of two randomized controlled trials. PLoS One, 2017. 12(10): e0186657.
23. Food and Nutrition Board. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary Reference Intakes for Vitamin D and Calcium. Washington, D.C.: National Academy Press; 2010.
24. Rostami M, Tehrani FR, Simbar M, Bidhendi Yarandi R, et al., Effectiveness of Prenatal Vitamin D Deficiency Screening and Treatment Program: A Stratified Randomized Field Trial. J Clin Endocrinol Metab, 2018. 103(8): p. 2936-48.
25. Roth DE, Morris SK, Zlotkin S, Gernand AD, Ahmed T, et al., Vitamin D Supplementation in Pregnancy and Lactation and Infant Growth. N Engl J Med, 2018. 379(6): p. 535-46.
26. Ganguly A, Tamblyn JA, Finn-Sell S, Chan SY, Westwood M, et al., Vitamin D, the placenta and early pregnancy: effects on trophoblast function. J Endocrinol, 2018. 236(2): R93-R103.
27. Hewison M, The earlier the better: preconception vitamin D and protection against pregnancy loss. Lancet Diabetes Endocrinol, 2018. 6(9): p. 680-681.
28. Hewison M, Wagner CL, Hollis BW, Vitamin D Supplementation in Pregnancy and Lactation and Infant Growth. N Engl J Med, 2018. 379(19): p. 1880-1881.
29. Angellotti E, D'Alessio D, Dawson-Hughes B, Chu Y, Nelson J, et al., Effect of vitamin D supplementation on cardiovascular risk in type 2 diabetes. Clin Nutr, 2018.
30. Garland CF, Kim JJ, Mohr SB, Gorham ED, Grant WB, et al., Meta-analysis of all-cause mortality according to serum 25-hydroxyvitamin D. Am J Public Health, 2014. 104(8): e43-50.
31. Amegah AK, Klevor MK, Wagner CL, Maternal vitamin D insufficiency and risk of adverse pregnancy and birth outcomes: A systematic review and meta-analysis of longitudinal studies. PLoS One, 2017. 12(3): e0173605.
32. Rafiq S, Jeppesen PB, Body Mass Index, Vitamin D, and Type 2 Diabetes: A Systematic Review and Meta-Analysis. Nutrients. 2018. 10(9).
33. Handel AE, Ramagopalan SV, Vitamin D and multiple sclerosis: an interaction between genes and environment. Mult Scler. 2012. 18(1): p. 2-4.
34. Hollis BW, Wagner CL, Clinical review: The role of the parent compound vitamin D with respect to metabolism and function: Why clinical dose intervals can affect clinical outcomes. J Clin Endocrinol Metab, 2013. 98(12): p. 4619-4628.
35. Vieth R, Vitamin D supplementation, 25-hydroxy-vitamin D concentrations, and safety. Am J Clin Nutr, 1999. 69: p. 842-856.
36. Heaney R, Davies K, Chen T, Holick M, Barger-Lux M, Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr, 2003. 77: p. 204 -210.
37. Tai SS, Bedner M, Phinney KW, Development of a candidate reference measurement procedure for the determination of 25-hydroxyvitamin D3 and 25-hydroxyvitamin D2 in human serum using isotope-dilution liquid chromatography-tandem mass spectrometry. Anal Chem, 2010. 82(5): p. 1942-1948.
38. Carter GD, Accuracy of 25-hydroxyvitamin D assays: confronting the issues. Curr Drug Targets, 2011. 12(1): p. 19-28.
39. Perrine CG, Sharma AJ, Jefferds ME, Serdula MK, Scanlon KS, Adherence to vitamin D recommendations among US infants. Pediatrics, 2010. 125(4): p. 627-32.
40. Appelgren KE, Nietert PJ, Hulsey TC, Hollis BW, Wagner CL, Analyzing adherence to prenatal supplement: does pill count measure up? Int J Endocrinol, 2010. 2010: 631971.
41. Greer FR, Marshall S, Bone mineral content, serum vitamin D metabolite concentrations and ultraviolet B light exposure in infants fed human milk with and without vitamin D2 supplements. J Pediatrics, 1989. 114: p. 204-212.
42. Hollis B, Wagner C, Assessment of dietary vitamin D requirements during pregnancy and Lactation. Am J Clin Nutr, 2004. 79: p. 717 - 726.
43. Hollis BW, Wagner CL, Vitamin D requirements during lactation: high-dose maternal supplementation as therapy to prevent hypovitaminosis D for both the mother and the nursing infant. Am J Clin Nutr, 2004. 80(6 Suppl): p. 1752S-1758S.
44. Wagner CL, Hulsey TC, Fanning D, Ebeling M, Hollis BW, High-dose vitamin D3 supplementation in a cohort of breastfeeding mothers and their infants: a 6-month follow-up pilot study. Breastfeed Med, 2006. 1(2): p. 59-70.
45. Hollis BW, Wagner CL, Howard CR, Ebeling M, Shary JR, et al; Maternal Versus Infant Vitamin D supplementation during lactation: A Randomized Controlled Trial. Pediatrics, 2015. 136(4): p. 625-34.
46. Wagner CL, Amniotic fluid and human milk: a continuum of effect? J Pediatr Gastroenterol Nutr, 2002. 34(5): p. 513-514.
47. Hornsby E, Pfeffer PE, Laranjo N, Cruikshank W, Tuzova M, et al., Vitamin D supplementation during pregnancy: Effect on the neonatal immune system in a randomized controlled trial. J Allergy Clin Immunol, 2018. 141(1): p. 269-278.

PEER REVIEW

Not commissioned. Externally peer reviewed.

References

1. Munger K, Zhang S, O'Reilly E, Hernan M, Olek M, Willett W, Ascherio A, Vitamin D intake and incidence of multiple sclerosis. Neurology, 2004. 62: p.60-65.

2. Ebers GC, Sadovnick AD, Veith R, Vitamin D intake and incidence of multiple sclerosis. Neurology, 2004. 63(5): p. 939

3. Hypponen E, Vitamin D and increasing incidence of type 1 diabetes-evidence for an association? Diabetes Obes Metab, 2010. 12(9): p. 737-743.

4. Omar DF, Kamal MM, El-Hefnawy MH, El-Mesallamy HO, Serum Vitamin D and Its Upregulated Protein, Thioredoxin Interacting Protein, Are Associated With Beta-Cell Dysfunction in Adult Patients With Type 1 and Type 2 Diabetes. Can J Diabetes, 2018. 42(6): p. 588-594.

5. Lee WC, Mokhtar SS, Munisamy S, Yahaya S, Rasool AHG, Vitamin D status and oxidative stress in diabetes mellitus. Cell Mol Biol (Noisy-le-grand), 2018. 64(7): p. 60-69.

6. Yu Y, Tian L, Xiao Y, Huang G, Zhang M, Effect of Vitamin D Supplementation on Some Inflammatory Biomarkers in Type 2 Diabetes Mellitus Subjects: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Ann Nutr Metab, 2018. 73(1): p. 62-73.

7. Cutolo M, Otsa K, Uprus M, Paolino S, Seriolo B, Vitamin D in rheumatoid arthritis. Autoimmun Rev, 2007. 7: p. 59 - 64.

8. Ramagopalan SV, Goldacre R, Disanto G, Giovannoni G, Goldacre MJ, Hospital admissions for vitamin D related conditions and subsequent immune-mediated disease: record-linkage studies. BMC Med, 2013.11: p. 171.

9. Trump DL, Chadha MK, Sunga AY, Fakih MG, Ashraf U, et al; Vitamin D deficiency and insufficiency among patients with prostate cancer. BJU International, 2009. 104(7): p. 909-914.

10. Garland C, Comstock G, Garland F, Helsing K, Shaw E, Gorham E, Serum 25(OH)D and colon cancer: Eight-year prospective study. Lancet, 1989. 2(8673): p. 1176-1178.

11. Ma Y, Zhang P, Wang F, Yang J, Liu Z, Qin H, Association between vitamin D and risk of colorectal cancer: a systematic review of prospective studies. J clin oncol, 2011. 29(28): p. 3775-82.

12. Cannell JJ, Hollis BW, Use of vitamin D in clinical practice. Altern Med Rev, 2008. 13(1): p. 6-20.

13. Saad K, Abdel-Rahman AA, Elserogy YM, Al-Atram AA, Cannell JJ, Vitamin D status in autism spectrum disorders and the efficacy of vitamin D supplementation in autistic children. Nutr Neurosci, 2016.19(8): p. 346-351.

14. Cannell JJ, Vitamin D and autism, what's new? Rev Endocr Metab Disord, 2017. 18(2): p. 183-193.

15. El-Ansary A, Cannell JJ, Bjorklund G, Bhat RS, Al Dbass AM, et al; In the search for reliable biomarkers for the early diagnosis of autism spectrum disorder: the role of vitamin D. Metab Brain Dis, 2018. 33(3): p. 917-931.

16. Liu P, Stenger S, Tang D, Modlin R. Cutting edge: vitamin D-mediated human antimicrobial activity against Mycobacterium tuberculosis is dependent on the induction of cathelicidin. J Immunol, 2007. 179: p. 2060 - 2063.

17. Mirzakhani H, O'Connor G, Bacharier LB, Zeiger RS, Schatz MX, et al; Asthma control status in pregnancy, body mass index, and maternal vitamin D levels. J Allergy Clin Immunol, 2017. 140(5): p. 1453-6.

18. Weiss ST, Litonjua AA. Vitamin D in Host Defense: Implications for Future Research. Am J Respir Cell Mol Biol, 2017. 56(6): p. 692-693.

19. Wolsk HM, Harshfield BJ, Laranjo N, Carey VJ, O'Connor G, et al; Vitamin D supplementation in pregnancy, prenatal 25(OH)D levels, race, and subsequent asthma or recurrent wheeze in offspring: Secondary analyses from the Vitamin D Antenatal Asthma Reduction Trial. J Allergy Clin Immunol, 2017. 140(5): p. 1423-1429.

20. Litonjua AA, Weiss ST, Vitamin D status through the first 10 years of life: A vital piece of the puzzle in asthma inception. J Allergy Clin Immunol, 2017. 139(2): p. 459-61.

21. Wolsk HM, Chawes BL, Litonjua AA, Hollis BW, Waage J, et al; Prenatal vitamin D supplementation reduces risk of asthma/recurrent wheeze in early childhood: A combined analysis of two randomized controlled trials. PLoS One, 2017. 12(10): e0186657.

22. Food and Nutrition Board. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary Reference Intakes for Vitamin D and Calcium. Washington, D.C.: National Academy Press; 2010.

23. Rostami M, Tehrani FR, Simbar M, Bidhendi Yarandi R, et al; Effectiveness of Prenatal Vitamin D Deficiency Screening and Treatment Program: A Stratified Randomized Field Trial. J Clin Endocrinol Metab, 2018. 103(8): p. 2936-48.

24. Roth DE, Morris SK, Zlotkin S, Gernand AD, Ahmed T, et al; Vitamin D Supplementation in Pregnancy and Lactation and Infant Growth. N Engl J Med, 2018. 379(6): p. 535-46.

25. Ganguly A, Tamblyn JA, Finn-Sell S, Chan SY, Westwood M, et al; Vitamin D, the placenta and early pregnancy: effects on trophoblast function. J Endocrinol, 2018. 236(2): R93-R103.

26. Hewison M. The earlier the better: preconception vitamin D and protection against pregnancy loss. Lancet Diabetes Endocrinol, 2018. 6(9): p. 680-681.

27. Hewison M, Wagner CL, Hollis BW, Vitamin D Supplementation in Pregnancy and Lactation and Infant Growth. N Engl J Med, 2018. 379(19): p. 1880-1881.

28. Angellotti E, D'Alessio D, Dawson-Hughes B, Chu Y, Nelson J, et al; Effect of vitamin D supplementation on cardiovascular risk in type 2 diabetes. Clin Nutr, 2018.

29. Garland CF, Kim JJ, Mohr SB, Gorham ED, Grant WB, et al; Meta-analysis of all-cause mortality according to serum 25-hydroxyvitamin D. Am J Public Health, 2014. 104(8): e43-50.

30. Amegah AK, Klevor MK, Wagner CL, Maternal vitamin D insufficiency and risk of adverse pregnancy and birth outcomes: A systematic review and meta-analysis of longitudinal studies. PLoS One, 2017. 12(3): e0173605.

31. Rafiq S, Jeppesen PB, Body Mass Index, Vitamin D, and Type 2 Diabetes: A Systematic Review and Meta-Analysis. Nutrients. 2018. 10(9).

32. Handel AE, Ramagopalan SV, Vitamin D and multiple sclerosis: an interaction between genes and environment. Mult Scler. 2012. 18(1): p. 2-4.

33. Hollis BW, Wagner CL, Clinical review: The role of the parent compound vitamin D with respect to metabolism and function: Why clinical dose intervals can affect clinical outcomes. J Clin Endocrinol Metab, 2013. 98(12): p. 4619-4628.

34. Vieth R, Vitamin D supplementation, 25-hydroxy-vitamin D concentrations, and safety. Am J Clin Nutr, 1999. 69: p. 842-856.

35. Heaney R, Davies K, Chen T, Holick M, Barger-Lux M, Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr, 2003. 77: p. 204 -210.

36. Tai SS, Bedner M, Phinney KW, Development of a candidate reference measurement procedure for the determination of 25-hydroxyvitamin D3 and 25-hydroxyvitamin D2 in human serum using isotope-dilution liquid chromatography-tandem mass spectrometry. Anal Chem, 2010. 82(5): p. 1942-1948.

37. Carter GD, Accuracy of 25-hydroxyvitamin D assays: confronting the issues. Curr Drug Targets, 2011. 12(1): p. 19-28.

38. Perrine CG, Sharma AJ, Jefferds ME, Serdula MK, Scanlon KS, Adherence to vitamin D recommendations among US infants. Pediatrics, 2010. 125(4): p. 627-32.

39. Appelgren KE, Nietert PJ, Hulsey TC, Hollis BW, Wagner CL, Analyzing adherence to prenatal supplement: does pill count measure up? Int J Endocrinol, 2010. 2010: 631971.

40. Greer FR, Marshall S, Bone mineral content, serum vitamin D metabolite concentrations and ultraviolet B light exposure in infants fed human milk with and without vitamin D2 supplements. J Pediatrics, 1989. 114: p. 204-212.

41. Hollis B, Wagner C, Assessment of dietary vitamin D requirements during pregnancy and Lactation. Am J Clin Nutr, 2004. 79: p. 717 - 726.

42. Hollis BW, Wagner CL, Vitamin D requirements during lactation: high-dose maternal supplementation as therapy to prevent hypovitaminosis D for both the mother and the nursing infant. Am J Clin Nutr, 2004. 80(6 Suppl): p. 1752S-1758S.

43. Wagner CL, Hulsey TC, Fanning D, Ebeling M, Hollis BW, High-dose vitamin D3 supplementation in a cohort of breastfeeding mothers and their infants: a 6-month follow-up pilot study. Breastfeed Med, 2006. 1(2): p. 59-70.

44. Hollis BW, Wagner CL, Howard CR, Ebeling M, Shary JR, et al; Maternal Versus Infant Vitamin D supplementation during lactation: A Randomized Controlled Trial. Pediatrics, 2015. 136(4): p. 625-34.

45. Wagner CL, Amniotic fluid and human milk: a continuum of effect? J Pediatr Gastroenterol Nutr, 2002. 34(5): p. 513-514.

46. Hornsby E, Pfeffer PE, Laranjo N, Cruikshank W, Tuzova M, et al; Vitamin D supplementation during pregnancy: Effect on the neonatal immune system in a randomized controlled trial. J Allergy Clin Immunol, 2018. 141(1): p. 269-278