Intermountain Health’s TARGET-D randomized trial found measured, personalized vitamin D3 supplementation cut the risk of a repeat heart attack by roughly 50% in patients who previously experienced an MI. The large-scale study from Salt Lake City adjusted dosing to achieve target serum levels and reported significant reductions in recurrent myocardial infarction, suggesting a practical, low-cost intervention could alter secondary prevention strategies if findings replicate. Researchers called for guideline reassessment and further multicenter validation.