High vitamin D levels significantly reduce people's risk of diabetes, new research suggests.
Lead author Dr Sue Park, from Seoul National University, said: "We found that participants with blood levels of [vitamin D] that were above 30 ng/ml had one-third of the risk of diabetes and those with levels above 50 ng/ml had one-fifth of the risk of developing diabetes."
The researchers argue their findings suggest adults should have at least 30ng of vitamin D per millimetre of blood, which is 10ng/ml higher than the level recommended by the Institute of Medicine.
Some 77 per cent of adults in the US are deficient in vitamin D, which is double the rate of 1980.
Previous research suggests vitamin D strengths people's immune systems.
Type 1 diabetes is an autoimmune condition that occurs when a person's body attacks the cells that produce insulin. Type 2 diabetes is more related to lifestyle factors, such as carrying too much weight.
The researchers analysed 903 healthy adults with an average age of 74 between 1997 and 1999.
The participants were followed until 2009.
Their vitamin D and glucose levels were measured during regular visits.
The participants' blood vitamin D levels were assessed in a previous study between 1977 and 1979.
All of the participants were Caucasian and living in southern California.
The findings were published in the journal PLOS One.
This comes after research released last January suggested vitamin D is an "inexpensive solution" to drugs.
Scientists discovered the sunshine supplement repairs and prevents damage to the heart caused by diabetes and high blood pressure.
Vitamin D stimulates the production of nitric acid, which is involved in regulating blood flow and preventing the formation of clots, according to the first study of its kind.
It also reduces 'internal stress' in the cardiovascular system, which could avoid heart-related incidents, the research adds.
Study author Dr Tadeusz Malinski, from Ohio University, said: "There are not many, if any, known systems which can be used to restore cardiovascular cells which are already damaged, and vitamin D can do it.
"This is a very inexpensive solution to repair the cardiovascular system.
We don't have to develop a new drug. We already have it."