The Hidden Connection Between Vitamin B12 and Vitamin D Deficiency Why These Two Nutrients Matter Vitamin B12 and Vitamin D are two of the most common deficiencies seen today — and both play vital roles in energy, mood regulation, and immune function. When either one runs low, symptoms like fatigue, brain fog, and mood changes can appear. But when both  are low, the effects can compound and create deeper metabolic and hormonal imbalances. Vitamin D: The Hormone-Like Vitamin D