People can get the nutrient from their food, vitamin pills or being out in the sunshine. Researchers found that the lung cancer patients with high intake who had surgery during the summer were more than twice as likely to be alive five years later than those with low levels who had operations in winter.
It is one of several recent studies to show the benefits of the "sunshine vitamin" against cancer.
"There are a lot of data emerging from various areas suggesting it is important," said Dr. Edward Giovannucci, professor of nutrition and epidemiology at the Harvard School of Public Health.
He helped conduct what is believed to be the first human study to look at vitamin D and lung cancer survival. The results were reported Tuesday at an American Association for Cancer Research conference in Anaheim.
"This is a very interesting study. It's a new trend — looking at dietary factors as they relate to survival," not just the risk of getting a certain cancer, said Dr. Michael Thun, chief epidemiologist at the American Cancer Society, who had no role in the research.
Vitamin D is made by the skin from sunlight. Getting enough from diet alone is tough — fish and fortified milk are the main sources. Supplements are controversial because too much D can cause medical problems, but many scientists think the recommended daily level of 400 international units is too low.
The nutrient has many features that could explain its possible benefit against cancer, such as stifling cell growth. Doctors had seen evidence suggesting it prevents some cancers, and wanted to know if it also affected survival.
Led by Harvard University's Dr. David Christiani, they studied 456 consecutive patients with early-stage lung cancer at Massachusetts General Hospital, Brigham and Women's Hospital and Dana-Farber Cancer Institute. Patients were interviewed about diet, supplements and timing of their cancer surgery, which was thought to be another indicator of their vitamin D levels.
Those who had high vitamin D levels and summer operations fared the best: five-year survival was 72 percent versus 29 percent for those who had the lowest levels of the nutrient and winter surgery.
This does not mean that people should delay or try to time operations, but taking vitamin D supplements around the time of surgery might be a good idea, said Wei Zhou, a Harvard researcher who presented the study results.
If verified by larger experiments, "this would be considered an important gain," said Thun of the cancer society. "A benefit of this size is important for this highly lethal disease."
Lung cancer is the world's top cancer killer. About 172,500 new cases and 163,510 deaths are expected this year in the United States, and more than 1.3 million cases and nearly 1.2 million deaths worldwide.