Large numbers of senior citizens need to spend more time outdoors to boost their levels of vitamin D and lower their risk of chronic disease, researchers from Warwick University have warned.

"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," said lead researcher Oscar Franco. "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."

In a study published in the journal Diabetes Care, the researchers measured vitamin D levels in 3,000 residents of China between the ages of 50 and 70. They found that a full 94 percent had insufficient blood levels of the vitamin.

"Vitamin D deficiency is becoming a condition that is causing a large burden of disease across the globe with particular deleterious impact among the elderly," Franco said.

Forty-two percent of participants tested also had metabolic syndrome, a cluster of symptoms associated with an elevated risk of cardiovascular disease and diabetes.

"We found that low vitamin D levels were associated with an increased risk of having metabolic syndrome, and was also significantly associated with increased insulin resistance," Franco said.

Vitamin D has long been known to play a vital role in bone health, but recent research suggests that it also helps maintain the body's immune system, lowering the risk of cardiovascular disease, diabetes, cancer and autoimmune disorders.

The body can produce all the vitamin D it needs with enough time in the sun, but fear over skin cancer leads many people to avoid sunlight, or to cover up with clothing or sunblock.

"The amount of sunlight it takes to make enough vitamin D is always less than the amounts that cause reddening or burning, so it should be possible to get the benefits of this vitamin without increasing the risk of skin cancer," said Ed Yong, of Cancer Research U.K.