LOS ANGELES: New research is strengthening evidence that following mom's admonition to eat your vegetables may be some of the best health advice around.

A large study of 500,000 American retirees has found that just one extra serving of fruit or vegetables a day may reduce the risk of developing head and neck cancer.

Numerous studies have demonstrated that diet plays a role in cancer. Cancer experts now believe that up to two-thirds of all cancers come from lifestyle factors such as smoking, diet and lack of exercise.

"It may not sound like news that vegetables protect from cancer, but there is actually some controversy in the literature. It is important that we do these large studies," said Alan Kristal, associate head of the cancer prevention program at Fred Hutchinson Cancer Research Center in Seattle.

Researchers at the National Cancer Institute queried men and women aged 50 and older about their diets, then followed participants for five years to record all diagnoses of head and neck cancer, which is the sixth-leading cause of cancer-related death worldwide.

Tobacco and alcohol use increase the risk of head and neck cancers, which affect the mouth, nose, sinuses and throat.

The study found eating six servings of fruit and vegetables per day per 1,000 calories cut the risk of head and neck cancer by 29% compared to eating one and a half servings.

The typical adult consumes around 2,000 calories a day. "Increasing consumption by just one serving of fruit or vegetables per 1,000 calories per day was associated with a 6% reduction in head and neck cancer risk, said Neal Freedman, cancer prevention fellow at the NCI.

A second study of food consumption in more than 183,000 residents of California and Hawaii found that a diet high in flavonols might help reduce pancreatic cancer risk, especially in smokers. Flavonols are common in plant-based foods but are found in highest concentrations in onions, apples, berries, kale and broccoli.