Adequate vitamin D levels are defined as having blood levels of vitamin D (25(OH)D) in the range of 50-125 nmol/L (12-20 ng/mL).[1] The only way to determine vitamin D status is through a blood test.
Where you live may influence your risk of Vitamin D deficiency. Vitamin D synthesis rates tend to vary with geographical location, which influences the amount of UVB exposure from the sun. People further away from the equator have lower rates of vitamin D synthesis,[2] and weather patterns associated with more cloud cover and darkness also can also reduce how much vitamin D is made through sun exposure.[3]
Breastfeeding[4] - see “Should vitamin D supplements be given to infants and children?” below.
Medical conditions that limit the absorption of dietary fats.[5] As a fat-soluble vitamin, the absorption of vitamin D is dependent on the ability of the gut to absorb fats from the diet.[6]
Dark skin pigmentation. Darker skin color reduces absorption of UVB, limiting production of vitamin D by the skin during sunlight exposure.[1]
Obesity. People with obesity (defined as a BMI of 30 or more) tend to have lower vitamin D levels compared to people without obesity, which is thought to be caused by an increased sequestration of vitamin D in subcutaneous fat stores.[1]
Gastric bypass surgery. During gastric bypass surgery, parts of the gastrointestinal tract that absorb vitamin D are bypassed, limiting absorption and potentially causing a deficiency.[7][8]
Aging. The ability to make vitamin D in the skin during sun exposure decreases with age, by an amount of 13% per decade, according to one study comparing vitamin D synthesis after a single incidence of sun exposure in younger and older adults.[9]