Using food as a general preventive is an intriguing subject. True, much anecdotal evidence (“I did this, and that happened”) suggests that eating some foods and avoiding others can raise or lower your risk of some serious diseases. But anecdotes aren’t science.

The more important indicator is the evidence from scientific studies tracking groups of people on different diets to see how things such as eating or avoiding fat, fiber, meat, dairy foods, salt, and other foods affect their risk of specific diseases.

Sometimes, the studies show a strange effect (meat fat increases the risk of colon cancer, high-fat dairy foods lower the risk). Sometimes studies show no effect at all. And sometimes — I like this category best — they turn up results nobody expected.

For example, in 1996, a study was designed to see whether a diet high in selenium would reduce the risk of skin cancer. After four years, the answer was “Not so you’d notice.” But then researchers noticed — by accident — that people who ate lots of high-selenium foods had a lower risk of lung, breast, and prostate cancers.

Naturally, researchers immediately set up another study, which happily confirmed the unexpected results of the first. Foods that serve up a health benefit in addition to basic good nutrition have been christened “functional foods.”

Fruits and veggies rich in vitamin A are naturally functional foods that prevent night blindness (the inability to see clearly in low light) along with their low-calorie, low-fat goodness. A second kind of functional food is one created to produce a specific medical result, such as a food that can actually deliver a vaccine

Battling deficiency diseases

The simplest example of food’s ability to act as preventive medicine is its ability to ward off a deficiency disease, a condition that occurs when you don’t get sufficient amounts of a specific nutrient. For example, people deprived of vitamin C develop scurvy, the vitamin C–deficiency disease.

The identifying characteristic of a deficiency disease is that simply adding the missing nutrient to your diet can cure it; scurvy disappears when people eat foods such as citrus fruits that are high in vitamin C.

Fighting off cancer with food

Is there really an anticancer diet? Right now, the answer seems to be a definite maybe. The problem is that cancer isn’t one disease; it’s many. Some foods seem to protect against some specific cancers, but none seem to protect against all. For example:

• Fruits and vegetables: Plants contain some potential anticancer substances, such as antioxidants (chemicals that prevent molecular fragments called free radicals from hooking up to form cancer-causing compounds); hormone-like compounds that displace natural and synthetic estrogens; and sulfur compounds that interfere with biochemical reactions leading to the birth and growth of cancer cells.

• Foods high in dietary fiber: Human beings can’t digest dietary fiber, but friendly bacteria living in your gut can. Chomping away on the fiber, the bacteria excrete fatty acids that appear to keep cells from turning cancerous. In addition, fiber helps speed food through your body, reducing the formation of carcinogenic compounds.

For more than 30 years, doctors have assumed that eating lots of dietary fiber reduces the risk of colon cancer, but in 1999, data from the long-running Nurses’ Health Study at Boston’s Brigham and Women’s Hospital and Harvard’s School of Public Health threw this into question.

By 2005, several very large studies — one with more than 350,000 people! — confirmed that dietary fiber has no protective effect against colon cancer. But even if dietary fiber doesn’t fight cancer, it does prevent constipation. One out of two ain’t bad.

• Low-fat foods: Dietary fat appears to increase the proliferation of various types of body cells, a situation that may lead to the out-of-control reproduction of cells known as cancer. But all fats may not be equally guilty. In several studies, fat from meat seems linked to an increased risk of colon cancer, but fat from dairy foods comes up clean. In the end, the link between dietary fat and cancer remains up in the nutritional air . . . so to speak.

The American Cancer Society Advisory Committee on Diet, Nutrition, and Cancer Prevention issued a set of nutrition guidelines that shows how to use food to reduce the risk of cancer. These are the American Cancer Society’s recommendations:

• Choose most of the foods you eat from plant sources. Eat five or more servings of fruits and vegetables every day. Eat other foods from plant sources, such as breads, cereals, grain products, rice, pasta, or beans, several times a day.

• Limit your intake of high-fat foods, particularly from animal sources. Choose foods low in fat; limit consumption of meats, especially high-fat meats.

• Be physically active. Achieve and maintain a healthy weight. Be at least moderately active for 30 minutes or more on most days of the week. Stay within your healthy weight range.

• If you drink alcohol, drink in moderation. Moderate consumption means no more than one drink a day for a woman, two for a man.

CA-A Cancer Journal for Clinicians, November/December 1996.

DASH and healthy blood pressure

More than 50 million Americans have high blood pressure (also referred to as hypertension), a major risk factor for heart disease, stroke, and heart or kidney failure. The traditional treatment for hypertension has included drugs (some with unpleasant side effects), reduced sodium intake, weight reduction, alcohol only in moderation, and regular exercise.

Recent data from a National Heart, Lung, and Blood Institute (NHLBI) study, “Dietary Approaches to Stop Hypertension” — DASH, for short — offer strong evidence that the diet that protects your heart and reduces your risk of some forms of cancer may also help control blood pressure.

The DASH diet is rich in fruits and vegetables, plus low-fat dairy products. No surprise there. But the diet is lower in fat than the ordinary low-fat diet. The USDA/U.S. Department of Health and Human Services Dietary Guidelines for Americans 2005 recommend that you get no more than 35 percent of your total calories from fat.

DASH says to aim for no more than 27 percent. The difference seems to make a difference. Your blood pressure is measured in two numbers that look something like this: 130/80.

The first number is your systolic pressure, the force exerted against artery walls when your heart beats and pushes blood out into your blood vessels. The second, lower number is the diastolic pressure, the force exerted between beats.

When male and female volunteers with high blood pressure followed the DASH diet during clinical trials at medical centers in Boston, Massachusetts; Durham, North Carolina; Baltimore, Maryland; and Baton Rouge, Louisiana, their systolic blood pressures dropped an average 11.4 points and their diastolic pressures an average 5.5 points.

And unlike medication, the diet produced no unpleasant side effects — except, of course, for that occasional dream of chocolate ice cream with real whipped cream, pound cake . . . Oh well, nothing’s perfect.

Conquering the common cold

This section is not about chicken soup. That issue has been settled, and Dr. Mom was right. In the 1980s, Dr. Marvin Sackler of Mount Sinai Medical Center in Miami, Florida, published the first serious study showing that cold sufferers who got hot chicken soup felt better faster than those who got plain hot water, and dozens of studies since have said, man, he’s right.

Nobody really knows why it works, but who cares? It works. So let’s move on to other foods that make you feel better when you have the sniffles — for example, sweet foods. Scientists do know why sweeteners — white sugar, brown sugar, honey, molasses — soothe a sore throat.

All sugars are demulcents, substances that coat and soothe the irritated mucous membranes. Lemons aren’t sweet, and they have less vitamin C than orange juice, but their popularity in the form of hot lemonade (tea with lemon and sugar) and sour lemon drops is unmatched. Why?

Because a lemon’s sharp flavor cuts through to your taste buds and makes the sugary stuff more palatable. In addition, the sour taste makes saliva flow, and that also soothes your throat.

Hot stuff — such as peppers, horseradish (freshly grated is definitely the most potent), and onions — contain mustard oils that irritate the membranes lining your nose and mouth and even make your eyes water. As a result, it’s easier to blow your nose or cough up mucus.

Finally, there’s coffee, a real boon to snifflers. When you’re sick, your body piles up cytokines, chemicals that carry messages among immune system cells that fight infection. When cytokines pile up in brain tissue, you get sleepy, which may explain why you’re so drowsy when you have a cold.

True, rest can help to boost your immune system and fight off the cold, but once in a while you have to get up. Like to go to work. The caffeine in even a single cup of regular coffee (or one cup of decaf if, like me, you don’t ordinarily drink regular coffee) can make you more alert.

Caffeine is also a mood elevator and a vasoconstrictor (a chemical that helps shrink swollen, throbbing blood vessels in your head.) That’s why it may help relieve a headache. When I have a cold, one cup of espresso with tons of sugar can make life bearable.

But nothing’s perfect: Drinking coffee may intensify the side effects of OTC (over-the-counter) cold remedies containing decongestants and/or caffeine that make some people feel jittery. Check the label warnings and directions before using coffee with your cold medicine.

Vasoconstrictors reduce the diameter of certain blood vessels and may restrict proper circulation. Couldn’t hurt to check with your doctor, too, if you’re taking meds for a chronic condition such as high blood pressure.