Increased intakes of vitamin D from foods but not supplements are associated with reduced prevalence of depression for postmenopausal women, says a new study.
The prevalence of depressive symptoms was 21% lower for women with daily vitamin D intakes exceeding 800 International Units (IU) per day, compared with intakes of less than 100 IU per day, according to findings published in the American Journal of Clinical Nutrition.
The study's findings are based on data from 81,189 participants of the Women's Health Initiative (WHI) Observational Study.
Led by Harvard's JoAnn Manson, the researchers add that the apparent benefits of vitamin D were primarily associated with dietary intakes of the sunshine vitamin, with supplemental vitamin D intakes producing "less consistent" results.
The researchers called for future studies to establish if boosting vitamin levels could prevent or treat depression, or both.
Data from the US National Health and Nutrition Examination Survey revealed that people with vitamin D deficiency were at a 85 percent increased risk of having current depressive episodes, compared with people with sufficient levels, according to findings published in the International Archives of Medicine.
Vitamin D is somewhat of an unusual "vitamin," because it can be made in the body from sunlight and most foods do not contain vitamin D. Synthesis of vitamin D in the body requires exposure to ultraviolet light and can be influenced by genetics, skin color, and sun exposure.
Consequently, the sunlight is the best and only natural source of vitamin D.
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive.
Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
While our bodies do manufacture vitamin D on exposure to sunshine (UV-B radiation with a wavelength between 290 and 315 nm), the levels in some northern countries are so weak during the winter months that our body makes no vitamin D at all, meaning that dietary supplements and fortified foods are seen by many as the best way to boost intakes of vitamin D.
Dr Manson and her co-workers performed a cross-sectional and prospective investigation of vitamin D intakes and depression. Vitamin D intakes were measured at the start of the study using food-frequency and supplement-use questionnaires, while symptoms of depression were tested using a well-established scale and assessing the use of antidepressant medication.
Results showed that, in addition to the general 21% reduction in prevalence of depressive symptoms for intakes over 800 IU, when the researchers limited they calculations to women who were depression-free at the start of the study, a 20% lower prevalence of depression was observed for intakes of vitamin D from foods over 400 IU.
"Overall, our findings support a potential inverse association of vitamin D, primarily from food sources, and depressive symptoms in postmenopausal women," they concluded.
Source: American Journal of Clinical Nutrition