When LeBlanc and colleagues tested five pills from the same bottle, the supplements had anywhere from 52 percent to 135 percent of the stated amounts. When five pills were averaged, however, two-thirds were within the stated range.
Although the vitamin supplement industry remains largely unregulated, some manufacturers volunteer to have the quality of their supplements tested by the independent, not-for-profit U.S. Pharmacopeial Convention (USP), which sets standards for supplements.
One of the study samples had USP verification. "Generally, the pills that were from this manufacturer were more accurate," LeBlanc said.
Supplements from compounding pharmacies were the least accurate, LeBlanc said.
Duffy MacKay, vice president of scientific and regulatory affairs at the Council for Responsible Nutrition in Washington D.C., a trade group representing supplement manufacturers and ingredient suppliers, said that there is no excuse for supplements to deviate from their labels. Still, he said, "I am comforted that there are no safety concerns."
Dr. Susan Zweig, an endocrinologist and clinical assistant professor of medicine at NYU Langone Medical Center in New York City, expressed more concern.
"It is hard to get toxic levels vitamin D, but it is certainly possible," Zweig said. "Vitamin D is stored in fat so it can build up, unlike other vitamins that we pass when we urinate." People with liver or kidney conditions may be at higher risk for vitamin D toxicity, she said.
"It is shocking how variable vitamin D is from brand to brand and pill to pill," she added.
Zweig's advice? "If there is any question about whether you need more vitamin D or if you are on the right dose, see your doctor and have your blood levels checked."
MacKay said the study shows there is value in third-party certification. "Bigger and more well-known brands don't use thi
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