The fourth vitamin to be discovered, vitamin D, is technically not a vitamin as one’s body can produce it – it just requires sunlight. No one is suggesting that sunlight is a vitamin per se. And only a relatively short span of radiation from the sun is involved, the so-called UVB of the ultraviolet spectrum. The intensity of UVB available depends on the weather, season, location on earth, and time of day; in the United States it maximizes between 10 am and 4 pm between April and October.
With moderate direct exposure to the summer sun (say 5 - 30 minutes twice a week), the body will make 10,000 to 20,000 IU. Sunscreen can effectively block UVB absorption; for an individual with frequent sun exposure (greater than twice per week), it might be prudent to place sunscreen after the first 10 – 15 minutes of sun exposure to avoid skin cancer but allow vitamin D production. As our bodies can store vitamin D, it is thought that sufficient exposure during spring, summer, and early fall should be sufficient to provide needed vitamin D during the winter months.
So this is another recent modification in our evolution – to stay indoors a lot more than our ancestors, decreasing our vitamin D production. The National Academies Institute of Medicine has no guidelines for vitamin D through sun exposure; they do have RDA but it is based on food intake.
The reaction of cholesterol (in the form of 7-dehydrocholesterol) in the skin with sunlight actually produces several fat-soluble related compounds, the most important being cholecalciferol, vitamin D3, and ergocalciferol, vitamin D2. The term “vitamin D” includes both of these compounds.
Very few foods in nature contain vitamin D, although some food products have vitamin D as an additive. To manufacture vitamin D industrially, 7-dehydrocholesterol, a substance typically obtained from fish liver,or lanolin extracted from shorn sheep wool, is exposed to UVB light, producing vitamin D3. Vitamin D cannot be manufactured directly; it requires the photochemical process.
To become biologically active, vitamin D has to undergo two transformative reactions, one in the liver, then another in the kidney.
Vitamin D deficiency to the extent of causing rickets or osteomalacia is rare in the developed world but what we might call vitamin-D insufficiency, a lower than ideal biologically active form of vitamin D, appears to be quite common, particularly in the elderly.
Vitamin D toxicity is also rare. There is a feedback loop associated with vitamin D production in the skin that lowers its production as adequate amounts are reached. This natural regulatorymechanism doesn’t apply to supplementation but for daily supplemental intake of 2,000 IU (about 50 micrograms) per day, there is very little risk of toxicity.
As vitamin D is fat soluble, it requires the presence of fat for absorption; some supplements encapsulate cholecalciferol, vitamin D3, with fat; otherwise often it is recommended to take with a meal containing some degree of fat.
We have learned relatively recently that vitamin D has a lot larger effect on the body than just calcium absorption; for example, it has to do with modulation of cell growth, neuromuscular and immune function, and reduction of inflammation. And mood states.
Vitamin D and psychiatric disorders
Vitamin D acts on receptors in a variety of regions in the brain such as the prefrontal cortex, hippocampus, cingulate gyrus, thalamus, hypothalamus, and substantia nigra and as such can influence neurochemistry cognition, emotion, and behavior. Vitamin D deficiency in early life affects neuronal differentiation, and brain structure and function and appears to have some influence on disorders with a developmental basis, such as autistic spectrum disorder and schizophrenia ontogeny and brain structure and function.
The initial suggestion that vitamin D may be linked to clinical depression was based on the relation between low vitamin D and high prevalence of seasonal affective disorder (now considered to be a depressive disorder with seasonal pattern) in winter at high latitudes. One treatment modality for clinical depression with seasonal pattern is light therapy, although no ultra-violet light is used. Vitamin D insufficiency is not considered to be directly causative for this disorder.
However, vitamin D concentrations have been shown to be low in many patients suffering from mood disorders and have been associated with poor cognitive function . For example, data from the third National Health and Nutrition Examination Survey were used to assess association between serum vitamin D and depression in 7,970 residents of the United States. In that study, the likelihood of having depression in persons with vitamin D deficiency was found to be significantly higher compared to those with vitamin D sufficiency.
One thorough systematic review and meta-analysis of observational studies and randomized controlled trials was conducted and found that vitamin D insufficiency was strongly associated with clinical depression. Another systematic review and meta-analysis showed a statistically significant improvement in depression with Vitamin D supplements.
Use of vitamin D as adjunctive therapy, i.e. together with an antidepressant medication in patients with vitamin D insufficiency has shown to be superior to an antidepressant alone.
What to do
This is another situation where recent changes in human lifestyle – here being indoors more than outdoors, can lead to a nutrient deficiency. Because it is so common to have a vitamin D insufficiency and the health consequences, specifically mood states, I recommend more time in the outdoors, including some limited time (say 10 minutes a day) with face and arms without sunscreen.
If you do not spend regular time in the sun, I do recommend a vitamin D3 supplement to be taken before, during, or directly after a meal. I think it wise to take these supplements during the winter months in any case.
Should you question whether or not you may be clinically depressed, professional assessment certainly is recommended as always; initial workup may include serum vitamin D levels (usually 25(OH)D is measured but various labs use different techniques resulting in varying “normal” level ranges).
A strict ethical vegan, however, faces a dilemma as the sources of vitamin D3 supplementation (and all “fortified products such as almond milk and tofu) are animal-based. Some literature supports vitamin D2 intake as sufficient, but good studies are too scarce to suggest this as the sole source for supplementation; vitamin D2 can be obtained from certain mushrooms set out in the sun for 10 minutes or so prior to consumption and there are supplements available from this source. It would appear that lifestyle emphasis on “fun in the sun” is indicated for vegans.
 Takeuchi A, Okano T, Sayamoto M, Sawamura S, Kobayashi T, Motosugi M, Yamakawa T; Okano; Sayamoto; Sawamura; Kobayashi; Motosugi; Yamakawa (1986). "Tissue distribution of 7-dehydrocholesterol, vitamin D3 and 25-hydroxyvitamin D3 in several species of fishes". Journal of nutritional science and vitaminology. 32 (1): 13–22.
 Ross, A. C., Manson, J. E., Abrams, S. A., Aloia, J. F., Brannon, P. M., Clinton, S. K., . . . Shapses, S. A. (2011). The 2011 Dietary Reference Intakes for Calcium and Vitamin D: What Dietetics Practitioners Need to Know⁎⁎This article is a summary of the Institute of Medicine report entitled Dietary Reference Intakes for Calcium and Vitamin D (available at http://www.iom.edu/Reports/2010/Dietary-Reference-Intakes-for-Calcium-and-Vitamin-D.aspx) for dietetics practitioners; a similar summary for clinicians has also been published (Ross AC, Manson JE, Abrams SA, Aloia JF, Brannon PM, Clinton SK, Durazo-Arvizu RA, Gallagher JC, Gallo RL, Jones G, Kovacs CS, Mayne ST, Rosen CJ, Shapses SA. The 2011 report on Dietary Reference Intakes for calcium and vitamin D from the Institute of Medicine: What clinicians need to know. J Clin Endocrinol Metab. 2011;96:53-58).Journal of the American Dietetic Association, 111(4), 524-527. doi:10.1016/j.jada.2011.01.004
 DRI – Dietary Reference Intakes – Calcium and Vitamin D20122 DRI – Dietary Reference Intakes – Calcium and Vitamin D . Institute of Medicine of the National Academies, , ISBN: 13‐978‐0‐309‐16394‐1. (2012). Nutrition & Food Science, 42(2), 131-131. doi:10.1108/nfs.2012.42.2.131.2
 Yue, W., Xiang, L., Zhang, Y., Ji, Y., & Li, X. (2014). Association of Serum 25-Hydroxyvitamin D with Symptoms of Depression After 6 Months in Stroke Patients. Neurochem Res Neurochemical Research, 39(11), 2218-2224. doi:10.1007/s11064-014-1423-y
 Eyles, D. W., Burne, T. H., & Mcgrath, J. J. (2013). Vitamin D, effects on brain development, adult brain function and the links between low levels of vitamin D and neuropsychiatric disease. Frontiers in Neuroendocrinology, 34(1), 47-64. doi:10.1016/j.yfrne.2012.07.001
 Gabbard, Glen O. Treatment of Psychiatric Disorders. 2 (3rd ed.). Washington, DC: American Psychiatric Publishing. p. 1296.
 Stumpf WE, Privette TH: Light, vitamin D and psychiatry. Role of 1,25 dihydroxyvitamin D3 (soltriol) in etiology and therapy of seasonal affective disorder and other mental processes. Psychopharmacology (Berl) 1989, 97:285–294.
 Wilkins, C. H., Sheline, Y. I., Roe, C. M., Birge, S. J., & Morris, J. C. (2006). Vitamin D Deficiency Is Associated With Low Mood and Worse Cognitive Performance in Older Adults. The American Journal of Geriatric Psychiatry, 14(12), 1032-1040. doi:10.1097/01.jgp.0000240986.74642.7c
 Przybelski, R. J., & Binkley, N. C. (2007). Is vitamin D important for preserving cognition? A positive correlation of serum 25-hydroxyvitamin D concentration with cognitive function. Archives of Biochemistry and Biophysics, 460(2), 202-205. doi:10.1016/j.abb.2006.12.018
 Ganji, V., Milone, C., Cody, M. M., Mccarty, F., & Wang, Y. T. (2010). Serum vitamin D concentrations are related to depression in young adult US population: The Third National Health and Nutrition Examination Survey. Int Arch Med International Archives of Medicine, 3(1), 29. doi:10.1186/1755-7682-3-29
 Anglin, R. E., Samaan, Z., Walter, S. D., & Mcdonald, S. D. (2013). Vitamin D deficiency and depression in adults: Systematic review and meta-analysis. The British Journal of Psychiatry, 202(2), 100-107. doi:10.1192/bjp.bp.111.106666
 Spedding, S. (2014). Vitamin D and Depression: A Systematic Review and Meta-Analysis Comparing Studies with and without Biological Flaws. Nutrients, 6(4), 1501-1518. doi:10.3390/nu6041501
 Khoraminya, N., Tehrani-Doost, M., Jazayeri, S., Hosseini, A., & Djazayery, A. (2012). Therapeutic effects of vitamin D as adjunctive therapy to fluoxetine in patients with major depressive disorder. Australian & New Zealand Journal of Psychiatry, 47(3), 271-275. doi:10.1177/0004867412465022