Gwyneth Paltrow secretly suffered from a bone disorder.

The 'Iron Man 2' actress was diagnosed with the onset of osteopenia - a condition where bone mineral density is lower than normal - and was ordered by doctors to spend more time in the sun to boost her vitamin D levels.

She said: "I suffered a pretty severe Tibial plateau fracture a few years ago (requiring surgery) which lead the orthopaedic surgeon to give me a bone scan, at which point it was discovered I had the beginning stages of osteopenia.

"This led my western/eastern doctors in New York to test my Vitamin D levels, which turned out to be the lowest they had ever seen, not a good thing. I went on a prescription strength level of Vitamin D and was told to. spend a bit of time in the sun!"

Gwyneth - who has two children, daughter Apple, six, and four-year-old son Moses, with husband Chris Martin - admits the advice left her "confused" as she had always been told to stay out of the sun.

She added in her GOOP newsletter: "I was curious if this was safe, having been told for years to stay away from its dangerous rays, not to mention a tad bit confused."

However, after following the doctor's advice, the 37-year-old blonde beauty managed to reverse her Vitamin D deficiency.