Dr. Michael Holick, professor of medicine at Boston University and one of the most-cited vitamin D researchers worldwide, argues that the current deficiency threshold of 20 ng/mL is too low — his research suggests optimal immune and bone health requires 40-60 ng/mL, a level that only 18% of Americans achieve. A 2020 meta-analysis in the BMJ (52 RCTs, n=75,454) found that daily vitamin D supplementation (1,000-4,000 IU) reduced acute respiratory infections by 12% overall and by 25% in severely deficient individuals. Vitamin D deficiency, defined as serum 25-hydroxyvitamin D below 20 ng/mL, affects approximately 42% of the US adult population. Among Black Americans, the prevalence exceeds 80%.
The Institute of Medicine set 20 ng/mL as the threshold for sufficiency. The Endocrine Society uses 30 ng/mL. For practical purposes, most clinicians target 30-50 ng/mL.
Supplementation dose depends on starting level, body weight, and individual absorption. Dr. Roger Bouillon, professor emeritus of endocrinology at KU Leuven, published a 2022 Endocrine Reviews analysis showing that a general starting dose of 1,000-2,000 IU daily maintains levels in most adults. For documented deficiency (below 20 ng/mL), loading doses of 5,000-10,000 IU daily for 8-12 weeks are standard before stepping down to maintenance.
The VITAL trial (NEJM 2019, n=25,871), led by Dr. Manson, is the largest vitamin D supplementation trial ever conducted. After 5.3 years of follow-up, it found no significant reduction in cancer incidence or cardiovascular events with 2,000 IU/day. However, secondary analyses showed reduced cancer mortality by 25% — a finding that prompted the ongoing VITAL Follow-Up Study.
Testing frequency is debated. The USPSTF found insufficient evidence to recommend routine screening. Dr. Clifford Rosen, professor of medicine at Tufts University and a member of the IOM vitamin D committee, argues that population-wide testing produces more harm than benefit through overdiagnosis and unnecessary supplementation. At minimum, testing is recommended for high-risk individuals: dark skin, limited sun exposure, obesity, malabsorption, and older adults.
Toxicity is rare but possible with chronic intake above 10,000 IU daily. A BMJ Case Reports review (2018) by Dr. Asha Sharma at the University of Alberta documented that toxicity typically presents with serum levels above 150 ng/mL, causing hypercalcemia. At standard supplementation doses (1,000-4,000 IU daily), toxicity does not occur.