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Once vitamin D levels were above 40 ng/ml, they were monitored annually and the dose adjusted if they fell below target. To avoid potential side effects of a vitamin D overdose, researchers monitored both the participants’ vitamin D and calcium levels throughout the study. The dose was reduced or supplementation was discontinued if vitamin D levels rose above 80 ng/mL. Because an excess of vitamin D can lead to hypercalcemia (increased calcium levels in the blood), kidney failure and cardiac arrhythmias.
