The vitamin D council recommend vitamin D serum level between 50 and 80 ng/mL (125 and 200 nmol/L) year round. The body needs vitamin D for many reasons but two important reasons are to absorb calcium and to activate killer T cells to kill off bacteria and viruses. Vitamin D deficiency is linked to numerous health conditions from cold and flu to cancer so you want to make sure your serum levels are optimal.
To avoid symptoms of vitamin D supplementation either increase magnesium in the diet or take magnesium supplements. If you have kidney disease or severe heart disease take magnesium only under a doctor's supervision. A few excerpts below with more information....
Excerpt from the Vitamin D council - Magnesium...
"Magnesium deficiency is epidemic in the United States, with the majority of Americans unknowingly deficient. This can be problematic when optimizing vitamin D levels. In fact, it is common for rising vitamin D levels to bring up symptoms of an underlying magnesium deficiency.
Vitamin D has no known side effects. If one experiences any of the following symptoms due to supplementing with vitamin D - or with sun exposure - a magnesium deficiency is most likely the reason why:
irritability or anxiety
Excerpt from Baby Centre - Magnesium in your pregnancy diet...
"When you're pregnant, magnesium helps build and repair your body's tissues. A severe deficiency during pregnancy may lead to preeclampsia, poor fetal growth, and even infant mortality.
Magnesium and calcium work in combination: Magnesium relaxes muscles, while calcium stimulates muscles to contract. Research suggests that proper levels of magnesium during pregnancy can help keep the uterus from contracting prematurely."
Excerpt from The Journal Of Nutrition - High Prevalence of Vitamin D Insufficiency in Black and White Pregnant Women Residing in the Northern United States and Their Neonates...
"In utero or early life vitamin D deficiency has been linked to an increased risk of type 1 diabetes (11), asthma (12), and schizophrenia (13,14). Fascinating new data also show that vitamin D regulates placental development and function (15), which suggests that maternal vitamin D status may be associated with adverse outcomes of pregnancy, such as miscarriage, preeclampsia, and preterm birth."