LONDON — The ever-increasing popularity of vitamin D may have reached its plateau, suggested a report issued by Euromonitor International in late March, in part because of the vitamin’s ubiquity across the marketplace.

Accordingly, sales of vitamin D supplements may fall, especially sales of single-letter vitamin D, the report concluded.

“Vitamin D supplements see rising competition from fortified foods and other supplements such as fish oils and multivitamins,” the report read. “Moving forward, the recent reports on vitamin D intake will not make single vitamin D supplements disappear from the shelves. People with a medical deficiency and those faithful to the vitamin craze will support future sales."

According to Euromonitor International, retail value sales of vitamin D supplements reached $591 million worldwide in 2010, up 21% from the previous year using fixed 2010 exchange rates. The United States is the largest consumer of vitamin D supplements, comprising 76% of total retail value sales; Canada is the second-largest consumer with a value share of 9%.