Vitamin D occupies an unusual position in nutritional science because it functions more like a hormone than a vitamin. Unlike true vitamins, the body synthesizes it from sunlight exposure, and it acts on receptors throughout the body to regulate calcium absorption, immune function, mood regulation, and cell growth. When levels are low, the effects are widespread and often misattributed.
Who Is Most at Risk
Women are more likely to be vitamin D deficient than men for several reasons. Darker skin pigmentation requires longer sun exposure to produce equivalent vitamin D. Those who cover most of their skin for cultural or sun-protection reasons absorb less. Women who spend significant time indoors, particularly during winter months in northern latitudes, often have levels that drop well below optimal.
The symptoms most commonly associated with deficiency include persistent fatigue that sleep does not resolve, bone and joint pain, muscle weakness, low mood or seasonal depression, and frequent illness. Because these symptoms overlap with so many other conditions, deficiency often goes undiagnosed through multiple medical appointments.
Testing and Supplementation
A 25-hydroxyvitamin D blood test provides an accurate picture of status. Levels below 20 ng/mL indicate deficiency. Most functional medicine practitioners aim for levels between 40 and 60 ng/mL for optimal function.
Supplementation with vitamin D3, the most bioavailable form, combined with vitamin K2 to direct calcium to bones rather than arteries, is the standard approach. Daily doses of 2000 to 4000 IU are commonly recommended, though optimal dosing should be guided by testing results.
