Vitamin D and Calcium
Cholecalciferol (vitamin D-3) is not actually a vitamin, but a hormone, since the body manufactures it in the skin from 5-dihydrotachysterol. This requires sunlight, but even with a great deal of sunlight dietary intake of calciferols is required to meet the needs of a growing child. Despite the fact that the child receives adequate amount of most vitamins through breast milk, and although the mother is taking vitamin supplements, human milk does not contain high vitamin D levels. Thus, if sufficient quantities are not obtained from the diet, deficiencies develop in which case the bones do not mineralize, or they demineralize. If severe, this can result in osteomalacia in adults and rickets in children, which manifests with bowing in the legs due to the force resulting from weight-bearing, widened epiphyses, and decreased bone density, leading to fractures.
Vitamin D toxicity is extremely rare and when it occurs is the result of an overdose of vitamin supplements. The symptoms that people who do ingest too much vitamin D are related mostly from high calcium levels in the blood (hypercalcemia) that develop. When the toxicity is acute, these symptoms include headache, weakness, nausea, vomiting, irritability and bone pain. When chronic they also include constipation, abdominal cramps, backache, polyuria (high frequency of urination and volume of urine), polydipsia (high consumption of liquids), hyperlipidemia (high LDL cholesterol and tryglycerides), and of course hypercalcemia. Hypertension (high blood pressure) and heart arrhythmias also have been documented.
Calcium is present in adequate quantities in breast milk and although fluoride levels in breast milk are much lower than what is present in the water that the mother consumes, only a tiny amount is needed. Since studies have revealed that such levels are fairly constant in breast milk, regardless of how much the mother consumes, supplementation during the breastfeeding period is not necessary.
Calcium toxicity is extremely difficult to develop, since the calcium absorption from the gastrointestinal tract is regulated based on the body's needs. Thus, the only effects from ingesting excess calcium occurs in the gastrointestinal tract itself as people may develop constipation. Hypercalcemia, on the other hand, may develop as a result of vitamin D overdose, but also as a result of a functional parathyroid tumor, a tumor that secretes parathyroid hormone. This hormone stimulates resorption of bone mineral to raise calcium levels when the levels are too low. Additionally, hypercalcemia can develop as a result of a malignancy producing proteins similar to parathyroid hormone, from disorders of bone in which bone is broken down, and from excessive unloading of from muscles and bones, due to inactivity which also causes bone to demineralize.
One very special type of unloading occurs in astronauts exposed to weightlessness in space flight. This results in a loss of 1-2 percent of bone mineral per day in weightlessness and this is offset only partially by exercise. In addition to an elevated risk of fractures of the process occurs for a month or more, the hypercalcemia from bone demineralization increases the risk of developing kidney stones. Since this can happen fairly quickly, it is of particular concern during spaceflight since it can happen during the actual mission, when medical treatment is limited.