A new research by Medical College of Georgia has shown that black tea contains higher concentrations of fluoride than previously thought.

'The additional fluoride from drinking two to four cups of tea a day won't harm anyone; it's the very heavy tea drinkers who could get in trouble,' said Dr. Gary Whitford, Regents Professor of oral biology in the School of Dentistry.

Most published reports show 1 to 5 milligrams of fluoride per litre of black tea, but a new study shows that number could be as high as 9 milligrams.

Fluoride is known to help prevent dental cavities, but long-term ingestion of excessive amounts could cause bone problems.

Whitford discovered that the fluoride concentration in black tea had long been underestimated when he began analysing data from four patients with advanced skeletal fluorosis, a disease caused by excessive fluoride consumption and characterized by joint and bone pain and damage.

'When we tested the patients' tea brands using a traditional method, we found the fluoride concentrations to be very low, so we wondered if that method was detecting all of the fluoride,' Whitford said, noting that the tea plant, Camellia sinensis, creates a quandary when measuring fluoride.

Unique among other plants, it accumulates huge concentrations of fluoride and aluminium in its leaves. When the leaves are brewed for tea, some of the minerals leach into the beverage.