He added: "As we get older our skin is less efficient at forming vitamin D and our diet may also become less varied, with a lower natural vitamin D content. Most importantly, however, the dermal production of vitamin D following a standard exposure to UVB light decreases with age because of atrophic skin changes. When we are older we may need to spend more time outdoors to stimulate the same levels of vitamin D we had when we were younger."

Vitamin D deficiency exists when the concentration of 25-hydroxy-vitamin D (25-OH-D) in the blood serum occurs at 12ng/ml (nanograms/millilitre) or less. The normal concentration of 25-hydroxy-vitamin D in the blood serum is 25-50ng/ml.

This study was carried out in collaboration with colleagues from the Shanghai Institute of Biological Sciences in China. The team recruited 3,262 community residents aged 50-70 from Beijing and Shanghai in China as part of the Nutrition and Health of Aging Population in China (NHAPC) project.

Dr Franco added: "Vitamin D deficiency is now recognised as a worldwide concern and metabolic syndrome has become a global epidemic. More research is needed to find out why older people are more likely to have lower levels of vitamin D and how this is linked to the development of metabolic syndrome and related metabolic diseases."

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