Recent studies show that vitamin D helps prevent flu. This information is so new that it will be some years before California and federal public-health agencies such as the Centers for Disease Control are able to alert the public that D is crucial for immunity.

Emory University scientists recently examined the research and concluded that D not only prevents but, in fact, treats flu. And the November 2009 Harvard Heart Letter says, "Having enough D in circulation can help the body fight off the flu, tuberculosis and infections of the upper respiratory tract."

Unfortunately, only three out of 10 American children have enough D to use this immunity. Worse, only two out of 10 Latino children and only one out of 10 African-American children studied have adequate D, because melanin in the skin blocks the sunlight that makes the stuff.

Vitamin D is needed by every cell in the body, and is actually not a vitamin but a hormone. Here is a list of health problems which research finds related to D deficiency: acne, asthma, arthritis, cancer, diabetes, fibromyalgia, heart disease, high blood pressure, MS, muscle weakness, obesity, osteoporosis, TB, ADD, autism, depression, schizophrenia, Alzheimer's and Parkinson's.

It's shocking: in just 10 years, the average blood-serum level of D in Americans went from low to lower; that is, from 30 to 24 nanograms per milliliter (ng/mL). For comparison, Kaiser Permanente aims for blood levels from 40 to 70 ng/mL. Farmers and lifeguards are the gold standard, because they make D from sunlight and typically have 100 ng/mL.

The American College of Nutrition advises 5,000 International Units (IU) of D a day for adults, and in the Medical Journal of Clinical Nutrition, Dr. C. L. Wagner advises 6,4000 IU for nursing mothers. In order to get 2,000 IU per day simply from food you would need to eat six servings of salmon, or 10 servings of tuna or drink 20 glasses of fortified milk.

All of these are all much higher than the out-of-date FDA standard of only 200 IU a day for children and adults that makes no allowance for age, weight, fish consumption, sun exposure or melanin level in the skin.

School food only provides 100 to 200 IU a day because it is based on the one-size-fits-all FDA standards. Therefore, it's up to parents to ensure that their children get enough D. Keep in mind that vitamin D3 is more effective than vitamin D2. Vitamin D3 is made from fish liver oil, or lanolin. Lanolin has no fishy taste; it's usually a powder inside a capsule. Parents can pull apart the capsule and mix the powder into food.

Note that D supplementation calls for sufficient dietary calcium, magnesium and other minerals. Anyone with liver or kidney disease, or sarcoidosis, needs medical supervision.

If you want to know your or your child's current blood level of D, the 25-OH-D test will tell you. This reliable $40 home "blood spot" D test hurts no more than pulling a splinter. Go to GrassrootsHealth.net, click on "D-Action," and scroll down to the "Join Now" button. This is a five-year study, but you can buy just one test. Results are sent by email.

Why are so many people deficient? Modern life. Americans eat less fish than ever, work and play inside more than ever, use sunscreen, weigh more (excess body fat sequesters D), and no longer use supplements like cod liver oil. Plus, we cannot make D during the winter because the sun's ultraviolet B rays travel at an oblique angle, a longer path that means the UVB rays are absorbed before reaching us.

In spring, summer and fall, when the sun is directly overhead, a pale person wearing a bathing suit can make as much as 20,000 IU of D in 20 minutes. People with more melanin make less, and need more time in the sun.

We can store D in our bodies, but by January or February most people have used up what they stored, which is among the reasons why that is called the flu "season." New evidence suggests that this need not be a necessary evil if you commit to giving yourself and your family enough D.

Retired from teaching, Sonoma Valley resident Lauren Ayers now has more time to work for better school cafeteria food, see GoodSchoolFood.org.

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