I've heard that it's entertaining to be in the same room with a dietitian and a dermatologist when the topic of vitamin D comes up.

Traditionally, our primary source of vitamin D comes from exposing our bare, unprotected skin to the sun. Our Paleolithic ancestors didn't have sunscreen, after all. Come to think of it, neither did our grandparents.

Does that mean you have to court sun damage to get enough vitamin D? Of course not. However, this is one of those rare cases where food may not be the answer, because few foods naturally contain vitamin D.

Fatty fish such as salmon and tuna are decent sources, and many breakfast cereals and dairy products are D-fortified. Beyond that, you have supplements — and the sun.

When the sun's ultraviolet rays strike your skin's surface, a type of cholesterol is converted to inactive vitamin D3 (cholecalciferol), which moves into your bloodstream. To make it usable by your body, your liver converts it to calcidiol, then your kidneys turn it into the active form, calcitriol. The vitamin D you swallow goes through the same process. Finally, it travels to your intestines to aid absorption of calcium. That's vital, because almost all of our body processes depend on calcium.

Vitamin D's most established benefit is healthy bones, mostly because it helps keep our blood stocked with the right levels of the bone-building minerals calcium and phosphate. That's important at all ages.

Beyond that, almost every single cell in our body can use vitamin D, and that's one reason it's being heralded as a possible "miracle pill" for everything from the flu to diabetes.

This is a hot area of research, and the most promising prospects are colon- and breast-cancer prevention, cardiovascular health, prevention of autoimmune diseases (such as multiple sclerosis) and muscle strength and balance.

Does this mean that more D is better? Not necessarily, but it does mean we should be sure to get enough. And if you're reading this from Seattle, you might not be.

The northern latitudes just don't get enough sun. We get even less if we have the healthy habit of protecting our skin when we do see the sun. Your deficiency risk also jumps if you have dark skin, are an older adult or are overweight or obese.

From ages 1 to 70, the daily recommended intake of vitamin D is 600 IU a day. That increases to 800 IU at age 71. It's possible we need more.

Many experts say that going out into the midday sun for 10 minutes in shorts and a tank top a few times a week will keep you supplied with vitamin D — but that you should never let your skin sunburn. Other experts say supplements are safer. You have a choice between D2 (ergocalciferol) and D3 (cholecaliferol), but our bodies may use D3 more effectively.

If you have concerns about your vitamin D status, talk to your doctor about having your blood levels of 25-hydroxyvitamin D tested. It will give you a better baseline for planning your D strategy.

Next time: What is preventive health?

Carrie Dennett: nutritionbycarrie@gmail.com; Dennett is a graduate student in the Nutritional Sciences Program at UW; her blog is nutritionbycarrie.com.