Madonna is reported to have started injecting herself with vitamins to boost her energy levels.

The singer, who turns 49 on Thursday, is said to have surprised passengers on a recent flight from New York to London by injecting herself with a vitamin shot in her arm.

Nutritionists said that such a drastic practice could have potentially harmful long-term effects on her health.

The singer is said to have refused food on the seven-hour flight and only drank bottled water.

Scroll down for more...

Mad Madge: The health-obsessed singer is said to have injected herself with a cocktail of vitamins during a plane journey

A source told Closer magazine: "Seven hours was a long time to go without eating. Madonna was quiet, talking to the air staff only to ask for water.

"Just before landing, she brought out the phials of vitamins and injected herself."

It is understood that Madonna was injecting herself with B6 and B12 vitamins to boost her energy.

Such a drastic health measure is apparently all the rage among celebrities.

Geri Halliwell has in the past admitted to giving herself 'regular injections of a cocktail of vitamins', and Lindsay Lohan and Britney spears are also said to be fans of the health fad - in Britney's case to boost her energy before she goes on stage.

But Claire Williamson, a nutrition scientist at the British Nutrition Foundation called the practice in healthy individuals completely unnecessary and also said it could be dangerous.

She said: "There is a risk of direct toxicity from excessive intakes of some nutrients. Furthermore, high intakes of some nutrients can inhibit the absorption or metabolism of others.

"The long-term effects of such a regime are also unknown. It is known, however, that high doses of some nutrients may have adverse long-term effects e.g. large amounts of vitamin A over a long time may have adverse effects on bone health, making bones more likely to fracture.

"There is no reason why multivitamin and mineral injections should be any more beneficial than supplements and may indeed be dangerous."

An individual's inability to absorb vitamins is linked to their body fat. Where a person has a limited amount of fat in their diet, the fat-soluble vitamins cannot be absorbed properly by the body.

This is why some people who have a low fat intake can suffer from vitamin deficiency. Madonna is known to have a strict macrobiotic diet and adhere to a draconian fitness regime which gives her little fat and sinewy muscles.

Nutrition scientist Claire MacEvilly has said of injecting vitamins: "Vitamin injections are an extreme and dangerous fad which has come from the US - it is not very popular over here yet and hopefully will not take off.

"Overdosing on the non-soluble vitamins by injecting them is easy to do and is a real concern. It can cause cramps, nosebleeds, nausea, blurred vision, dry skin, liver disease, weight loss, kidney stones and permanent kidney damage, irritability and jaundice.

"In the most extreme cases it can lead to death.'

Madonna's publicists were not available to comment.

Madonna and her husband Guy Ritchie will celebrate her birthday today with a party for 30 of her close friends including Sting and Trudie Styler at Ashcombe House, her country pile in Wiltshire.

Guests will be entertained by a show from the Devil's Horsemen, a troupe of expert film industry horses and riders.