PITTSBURGH (KDKA) 

Sunshine is the most effective and the only natural source of vitamin D.

Many people are boosting their vitamin D intake because they believe it may actually help protect against both seasonal and H1N1 flu.

At Goldenseal in Squirrel Hill, sales of vitamin D-3 are dramatically up.

Dan Cappello has been a clinical nutritionist for two decades.

"As a nutritionist, I would recommend 5,000 units a day and take the D3 form, not the D2," he said.

Vitamin D stimulates the immune system and disease fighting cells. Those cells make proteins that have anti-viral properties.

The primary food sources of vitamin D are fresh salmon and fortified whole milk, but daily supplements are the easiest way to get the recommended dosages.

"There's a lot of interest right now in taking vitamins to protect yourself against swine flu," Dr. Marc Itskowitz said.

He is getting as lot of questions about vitamin D dietary supplements from patients. But he thinks the jury is still out on vitamin D therapy.

"I think it's something that we should study - people can do it but we can't say at this time that there's evidence that's there's benefit," Itskowitz said.

But if you take supplements, Dr. Itskowitz says care must be taken not to overdo it.

"Patients can overdose on vitamin D. You can develop elevated levels of calcium in the blood from too much vitamin D," he said. "Some patients even get kidney stones from too much vitamin D."

A simple blood test can tell if you have a vitamin D deficiency and as always talk to your family physician or pediatrician.