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Few people view their vitamin D supplement as hormone replacement therapy, but that's exactly what it is, experts told 口袋女优.

The weight of the literature suggests that vitamin D is indeed a hormone, not a nutrient, said , of Boston University.

"By definition, vitamin D is a hormone," Holick told 口袋女优. "The body synthesizes it after sun exposure, and it's activated by the liver and kidneys. That activated form again acts like a hormone to regulate calcium metabolism."

No other vitamin goes through the process of activation that D does before it can be used by the body, Holick said. First, the skin must synthesize vitamin D3, or cholecalciferol, after exposure to UVB radiation. D3 is then metabolized by the liver into 25-hydroxyvitamin D, or 25(OH)D, and then moves on to the kidney where it is converted to the biologically active form 1,25-dihydroxyvitamin D, or 1,25(OH)₂D.

"D3 is the prohormone, 25(OH)D is the major circulating form, and 1,25(OH)₂D is the hormonally active form," Holick said, adding that vitamins A and C do get metabolized, but they don't need to be activated the way D does.

But vitamin D's status as a hormone rather than a nutrient raises questions about the way companies use it to fortify foods, said , a food policy expert from New York University.

Nestle recently submitted comments to the FDA on its panel on foods, arguing that companies shouldn't be permitted to tout vitamin D fortification without more context or details.

"Vitamin D fortification must be understood as a form of hormone replacement therapy," . "As such, it raises questions about efficacy, dose, and side effects that should be asked about all such therapies."

She notes that D is found naturally in very few foods -- fish is one exception -- and even then it only exists in small quantities.

"It is present in most foods as a result of fortification," she said.

Holick disagreed that its status as a hormone should give people pause over vitamin D supplementation, because D deficiency affects such a large proportion of the population. Thus, a national hormone replacement therapy program would only provide benefit, he said.

"It's reasonable to have on the label," Holick said, "because everyone should be taking steps to increase their vitamin D intake." Though Holick's prescription is simpler -- 15 minutes a day in the sun should do the trick for sufficiency.

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