The Flu Fighters in Your Food

New Research Points to Ways to Boost Immunity by Making Sure Your Diet Has the Right Nutrients


While many people are still waiting for swine-flu vaccine to become available in their area, there is a lot they can do in their own kitchens to help fight off disease and build a strong immune system.

Scientists in the growing field of nutritional immunology are unveiling new evidence of the complex role that nutrition plays in fighting off infectious diseases like influenza. A diet rich in nutrients such as vitamin A (beta carotene), found in colorful fruits and vegetables, and zinc, found in seafood, nuts and whole grains, can provide the critical fuel the body needs to fight off disease, heal injuries, and survive illness when it does strike, experts say.

Key Ingredients

Scientists are still studying all the complex ways in which nutrients interact with the immune system. There is still much that they don't know about minerals such as zinc, for instance, including how they are absorbed and all the roles they play in the body. But scientists do know that certain vitamins and minerals can improve the body's ability to fight off infection: Studies in healthy elderly adults, for example, have shown an improved immune response to vaccination and fewer infections after receiving extra doses of vitamin E.

To create immune cells to fight off a specific infection, the body has to rapidly draw nutrients from the bloodstream, says Anuraj Shankar, a researcher at the Harvard School of Public Health. "If you don't have an adequate intake of vitamins and minerals, you won't be able to produce the number of immune cells you need, and the immune cells you do produce may be compromised," Dr. Shankar says. That makes it impossible to mount an effective response to infection, he says.

The benefits of good nutrition may have been recognized first by Hippocrates, the ancient Greek physician who declared "let food be thy medicine, and medicine be thy food." An 18th century naval surgeon's discovery that citrus fruits could cure scurvy in sailors was later recognized as a vitamin C deficiency, and after the 1930s, when dairies began to fortify milk with vitamin D, the disease known as rickets was virtually eliminated in the U.S.

Researchers warn that malnourished people may be a breeding ground for more dangerous infectious diseases. Animal studies at the University of North Carolina show that in a host with poor nutrition, viruses mutate in the face of a weak immune response to become more powerful. And once those mutations occur, even well-nourished hosts are susceptible to the newly virulent virus. "A lot of people may think malnutrition on the other side of the world isn't their problem," says Melinda A. Beck, a researcher at the University of North Carolina, Chapel Hill. But malnutrition "is a driving force in emerging infectious diseases that are spreading around the world," she says.

The human body doesn't have to be starving to suffer from malnutrition. Studies show that obesity, in addition to its other health risks, may also make people more susceptible to infections like the flu. A diet heavy on processed and fast foods may be low in the vitamins and minerals important for health. And diets that are high in saturated fat appear to actually depress the body's immune response, increasing the risk of infections.

Dr. Beck says studies of mice show that only 4% of lean animals infected with the flu virus die. That compares with a death rate of between 40% and 60% in obese mice infected with the virus. And after a small study showed that obese people vaccinated for the flu didn't mount a strong immune response, the University of North Carolina is expanding its trials to compare vaccination response rates in lean and obese people.

When obese people fall ill, "their immune function may not be strong enough to mount an effective response," says Donald Hensrud, a Mayo Clinic specialist in preventive and internal medicine and editor-in-chief of "The Mayo Clinic Diet," a new book promoting weight loss through a healthy diet that allows unlimited quantities of fruits and vegetables.

Warning on Supplements

Dr. Hensrud and other experts caution against loading up on supplements to add vitamins and minerals to the diet. While a multivitamin is a good addition to any balanced diet, individual supplements and vitamin pills may not be as well absorbed by the body as nutrients in foods. Some supplements also can have toxic effects in too-high quantities. An excess of zinc, for example, can interfere with absorption of other nutrients, including iron and copper. And too much of the mineral selenium can cause nerve damage and has been linked recently to an increased risk of diabetes.

There is no single test to measure if your body has enough vitamins and minerals, and assays for individual nutrients are generally expensive and unreliable. Blood tests used to screen for blood-cell abnormalities can pick up changes that are linked to possible vitamin or mineral deficiencies, but they can't necessarily identify the cause.

Scientists have long known that some vitamins, minerals and other nutrients can play a key role in the immune system by acting as antioxidants. These protect and repair cells from oxidative stress, the damage caused by molecules known as free radicals.

But nutrients work in ways beyond acting as antioxidants, says Dr. Beck. For example, vitamin A can enhance the immune system "by stimulating specific proteins necessary for immune function by activating specific genes," she says. So, if vitamin A is deficient, then the immune cells that require vitamin A to function properly won't work as efficiently. Animal studies show that a deficiency of vitamin B-6, which helps maintain the health of organs that make white blood cells, can decrease antibody production and suppress the immune response. And selenium in small amounts can help stimulate immune cells and may prevent the growth of some tumors.

 "The key to supplements for strengthening the immune system is time. You have to be using them for months before the infection for them to be effective."

Nutritional experts generally agree that the best way to get the right balance of nutrients is a balanced diet that includes plenty of fresh fruits and vegetables, lean proteins and dietary fiber. The federal Centers for Disease Control and Prevention's Web site offers a calculator to determine how many servings are ideal based on calorie needs for age, sex and activity level. Harvard's Nutrition Source Web site includes a healthy eating pyramid based on the most up-to-date knowledge of nutrition requirements. And the National Institutes of Health's Office of Dietary Supplements Web site ( offers detailed information on the risks and benefits of supplements, along with tables that list food sources for each vitamin and mineral.

A survey by the CDC in 2007 showed that the majority of adults consume less than the government's recommended five servings of fruits and vegetables daily. But quantity matters: A 2004 Harvard study of 110,000 men and women showed that people who averaged eight or more servings of fruit and vegetables daily were 30% less likely to have had a heart attack or stroke than those who had only 1.5 servings daily.

Nutrition experts say to boost immunity it is also important to avoid processed foods, and to minimize trans fats and unhealthy saturated fats from animal products and vegetable oils like palm and coconut. Instead, they say, people should eat foods rich in unsaturated fats such as olive oil.

Contradictory Advice

Some advice for a healthy diet can seem contradictory. For example, heart-healthy diets typically include unsaturated fats such as omega-3 fatty acids, which are found in fish such as salmon and trout and in flaxseed and walnuts. For people who don't want those foods, nutritionists may recommend fish-oil supplements, which can be beneficial in suppressing chronic inflammation in the body, a condition that can lead to coronary artery disease and arthritis.

But those same anti-inflammatory properties of fish oil can also suppress the immune responses necessary to combat an acute viral infection. Studies at the University of North Carolina have shown that mice fed with fish oil have an impaired resistance to infections, including the flu. "If I suppress the immune response and get a viral infection, I'm worse off," says Dr. Beck, who is studying the links between fish oil and resistance to influenza.

One nutrient hard to get in food is vitamin D. Even with the fortification of milk, orange juice and other food products, some experts have been sounding the alarm in recent years about wide deficiencies, especially in children. Tests are available for about $100 to determine vitamin D levels, but their accuracy is in question. And just how much vitamin D different people need is the subject of considerable debate. The federal government's current recommendations range from 200 international units daily for children to 600 IUs for adults, with a safe upper limit of 2,000 IUs daily. The American Academy of Pediatrics recommends 400 IUs for children, and vitamin D experts at Oregon State University and elsewhere recommend 2,000 IUs daily for all adults. The Institute of Medicine, a government advisory group, is expected next year to update the recommendations.

Adrian Gombart, a researcher at Oregon State University's Linus Pauling Institute, says vitamin D, in addition to building strong bones and fighting off a variety of diseases, appears to activate proteins that help the body fight off infection. "Vitamin D won't prevent you from getting the flu, but it might allow you to mount an optimal immune response, suffer less of the effects, and resolve the infection more quickly," says Dr. Gombart, who is researching the nutrient's role in stimulating immune cells.

Immune to Harm? provides more information on this important subject.

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