"Too much vitamin D can make the intestines absorb too much calcium. This may cause high levels of calcium in the blood. High blood calcium can lead to calcium deposits in soft tissues such as the heart and lungs, confusion and disorientation, damage to the kidneys, kidney stones, nausea, vomiting, constipation, poor appetite, weakness, and weight loss."

"There is considerable discussion of the serum concentrations of 25(OH)D associated with deficiency (e.g., rickets), adequacy for bone health, and optimal overall health, and cut points have not been developed by a scientific consensus process. Based on its review of data of vitamin D needs, a committee of the Institute of Medicine concluded that persons are at risk of vitamin D deficiency at serum 25(OH)D concentrations <30 nmol/L (<12 ng/mL). Some are potentially at risk for inadequacy at levels ranging from 30 50 nmol/L (12 20 ng/mL). Practically all people are sufficient at levels "e50 nmol/L ("e20 ng/mL); the committee stated that 50 nmol/L is the serum 25(OH)D level that covers the needs of 97.5% of the population. Serum concentrations >125 nmol/L (>50 ng/mL) are associated with potential adverse effect"

"Vitamin D is found in many foods, including fish, eggs, fortified milk, and cod liver oil. The sun also adds to the body's daily production of vitamin D, and as little as 10 minutes of exposure is thought to prevent deficiency."

"People with dark-colored skin synthesize markedly less vitamin D on exposure to sunlight than those with light-colored skin. Additionally, the elderly have diminished capacity to synthesize vitamin D from sunlight exposure and frequently use sunscreen or protective clothing in order to prevent skin cancer and sun damage. The application of sunscreen with an SPF factor of 8 reduces production of vitamin D by 95%. In latitudes around 40 degrees north or 40 degrees south (Boston is 42 degrees north), there is insufficient UVB radiation available for vitamin D synthesis from November to early March. Ten degrees farther north or south (Edmonton, Canada) the vitamin D winter  extends from mid-October to mid-March."