Long before I ever dreamed of becoming a registered dietitian, I saw my first case of nutritional deficiency. My pet guinea pig died of scurvy — a fatal disease caused by a lack of vitamin C. I never saw his receding gums, joint pain, neuropathy (nerve pain) or swollen, discolored "black legs." I only saw (too late) his lack of energy and vigor before he died.

Over the years, I have known of just one human case of scurvy — a man who ate only canned and packaged foods which lack vitamin C.

Everyone gets enough vitamin C these days, right? Wrong, according to research by registered dietitian and vitamin C researcher Carol Johnson of Arizona State University. She found more than a third of supposedly healthy college students she tested had low levels of vitamin C in their blood.

Big deal? Literally. Research by Johnson suggests that even slight deficits of vitamin C may be one reason for our continued battle with obesity.

Vitamin C affects how our body uses fat, explains Johnson. It is used in the body to make carnitine — a substance that helps our cells burn fat for energy. When vitamin C is lacking, carnitine cannot do its job.

Johnson instructed one group of college student "guinea pigs" not to eat foods high in vitamin C, such as citrus fruit, strawberries, tomatoes, broccoli or sweet bell peppers. Another group was given a daily supplement of 500 milligrams vitamin C.

In just two weeks, the students on low intakes of vitamin C had less measured energy levels and burned less fat during exercise than the students with adequate vitamin C. Could it be, suggests Johnson, that our typical diet devoid of fresh fruits and vegetables is making us too tired to exercise and less able to burn fat when we do?

We need more research in this area, admits Johnson. But it's interesting that overweight people tend to have lower levels of vitamin C in their blood.

How much do we need? Children need 15 to 65 milligrams of vitamin C a day, depending on age. Adult women and men need 75 to 90 milligrams, respectively. Smokers need 125 milligrams a day. Higher intakes up to 2,000 milligrams a day are probably not harmful, although body tissues are well-saturated with just 200 milligrams a day.

Where do we get it? Good sources of vitamin C (in milligrams) include: one red or yellow bell pepper (340), one cup cooked broccoli (120), one cup brussels sprouts (100), one cup strawberries (80), one kiwi (75), one orange (70).

And that morning glass of O.J.? Be careful, says Johnson. Any process that introduces light, air or heat destroys vitamin C. She found the most vitamin C retained in reconstituted frozen orange juice; the least in cartons and jugs of fresh orange juice. Whatever juice you drink, finish it within a week.

So what's the best way to get the most active form of vitamin C from our food? "Peel an orange and eat it," says Johnson.

If only I had known that for my guinea pig.

Barbara Quinn is a registered dietitian at the Community Hospital of the Monterey Peninsula, Calif.