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Follow this eat-right plan to fortify your immune system

  • Story Highlights
  • A balanced diet can help maintain a healthy immune system in flu season
  • If you don't eat enough protein, you'll produce fewer protective white blood cells
  • High-fat diets appear to decrease immune system function
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By Stacey Colino
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Cooking Light

When cold and flu season comes around, many people head to their medicine cabinets in search of relief. But a trip to the kitchen may be the smarter move.

The nutrients that lend fruits and vegetables their colors serve as antioxidants that promote immune function.

"Nutrition plays an important part in maintaining immune function," explains George L. Blackburn, M.D., Ph.D., associate director of the division of nutrition at Harvard Medical School in Boston, Massachusetts. "Insufficiency in one or more essential nutrients may prevent the immune system from functioning at its peak."

The immune system consists of a finely orchestrated, complex collection of tissues and cells that protect your body from allergens, bacteria, viruses, and other potentially harmful organisms, collectively known as antigens. Skin and the membranes that line entrances to the body -- nasal passages, eyes, and respiratory and digestive tracts -- are the first line of defense, providing a physical barrier against invaders. Internally, specialized white blood cells fight antigens that make it past the skin: T-lymphocytes continuously patrol the body in search of antigens; B-lymphocytes manufacture antibodies, special blood proteins that neutralize or destroy germs; and neutrophils and macrophages scavenge antigens from the blood for delivery to the lymphatic system, which disposes of them. To work smoothly, these cells depend on you keeping your body in top shape.

"There's no question the immune system fundamentally is influenced by overall health -- and a balanced diet is key," says David Katz, M.D., M.P.H., director of the Yale Prevention Research Center in New Haven, Connecticut. "Not only are essential nutrients critical for the production and maintenance of key germ-fighting cells in the immune system, but a balanced diet also has a strong effect on vascular function, and the immune system is dependent on blood flow," Katz says. The bloodstream is the route along which infection-fighting cells travel throughout the body to where they're needed.

Given the complexity of the immune system, there isn't one specific food that will magically make you repel cold germs and flu viruses. Instead, eating a healthful, balanced diet is your best investment in immunity.CookingLight.com: How nutritional remedies can help manage cold symptoms

Fill your plate with fruits and vegetables

The vitamins (especially A and C) and the phytochemicals that lend fruits and vegetables their colors serve as antioxidants that promote immune function, says Charles Stephensen, Ph.D., a research scientist with the usda's Western Human Nutrition Research Center at the University of California, Davis. "These nutrients help ensure that lymphocytes can divide and reproduce properly in response to a virus and that the neutrophils and macrophages that engulf and kill invading bacteria can do their job," Stephensen says.

What to do: Eat five to nine servings of fruits and vegetables per day. To maximize the variety of vitamins, minerals, and antioxidants, aim to consume two different colors of vegetables and fruits with each meal, Blackburn suggests. "Cover two-thirds of your plate with vegetables, fruit, whole grains, and beans, and one-third with lean protein," he says.

Choose lean protein sources

The amino acids that are found in protein form the building blocks of all the body's cells -- including the cells that power your immune system. If you don't consume enough protein, you'll manufacture fewer white blood cells to combat antigens. "One of the ways immune cells fight against pathogens is by increasing their numbers," says Simin Nikbin Meydani, Ph.D., associate director of the Jean Mayer Research Center on Aging at Tufts University in Boston, Massachusetts. "To increase immune cell proliferation, you need protein and amino acids."

What to do: Consume 0.8 to 1 gram of protein per kilogram (kg) of your body weight. That means if you weigh 130 pounds, which equals about 59kg, consume at least 47 grams of protein per day. But remember that quality counts: To avoid saturated fat, choose three- to four-ounce portions of lean protein such as fish, seafood, poultry (without the skin), eggs, lentils, beans, and soy products.

Focus on healthful fats

High-fat diets appear to impair the immune system by decreasing the function of T-lymphocytes.

Reducing fat, on the other hand, can boost immune function by enhancing T-lymphocyte function. However, the type of fat you consume is equally important as the amount. Trans fats (found in margarines and many commercial baked goods) can contribute to chronic low-grade inflammation in the body. "The immune system can become tied up dealing with inflammation -- and the damage to cells and tissues that results -- rather than defending the body," Katz says.

What to do: Limit your total fat intake to 30 percent of daily calories, with five to 10 percent from saturated fats. For the remaining 20 to 25 percent, look for sources of unsaturated fats, such as canola oil, olive oil, nuts, avocados, and seeds. And increase your intake of omega-3 fatty acids (from fatty fish like salmon, halibut, and sardines), which help fight inflammation and free your immune system to defend against antigens.

Make time for tea

Green tea is a rich source of a type of antioxidant called a catechin, and preliminary research has found that a specific catechin -- epigallocatechin gallate (egcg) -- may give the beverage antigen-fighting abilities. When researchers at the University of Sherbrooke in Canada added green tea to lab samples of the adenovirus (one of many viruses that causes colds), they discovered that egcg inhibited the virus' ability to replicate. Similarly, researchers in South Korea found that egcg can also stop the influenza virus from replicating. While these studies were conducted in petri dishes instead of human subjects, some researchers believe you can reasonably bank on green tea's benefits -- particularly when it's consumed in place of colas or other sugary beverages with little nutritional benefit. "Green tea catechins can improve lymphocyte responses and seem to have anti-inflammatory effects," Stephensen says.

What to do: Trade at least one cup of coffee each day for green tea. To derive the optimal amounts of catechins from your tea, let the bag steep for at least three minutes in hot water.

Maintain a proper energy balance

Eat enough -- yet not too many or too few -- calories per day. "Considerable evidence shows crash dieting, anorexia, or nutrient deficiencies increase a person's susceptibility to infections, but overconsumption of calories can also have harmful effects on cell production in the immune system," Meydani says. "This could be because overconsumption of calories leads to increased production of compounds called prostaglandins, which have a suppressive effect on T-lymphocyte cell production." Fewer T-cells patrolling the body increases the chances of an antigen taking hold.

What to do: To find the proper number of calories for you, go to www.mypyramid.gov, which makes recommendations based on age, gender, and activity level.

Take the one supplement that works

Although the researchers we spoke with panned most supplements, they all agreed that a multivitamin is well worth taking. If you have even a marginal deficiency of certain nutrients --particularly the B vitamins, A, C, E, selenium, iron, and zinc -- your immune system's function could be impaired. "This can happen without seeing any obvious signs of deficiency -- until you become sick with a virus or bacterial infection," Meydani says.

What to do: Choose a multivitamin specifically formulated for your gender or age. For example, multivitamins made for men and post-menopausal women contain less iron than those made for younger women. And always look for the United States Pharmacopeia seal, says Douglas Heimburger, M.D., a professor of nutrition sciences and medicine at the University of Alabama at Birmingham. "It tells you that certain pharmaceutical standards have been met and proven," Heimburger says.

CookingLight.com: How to choose the right multivitamin for you:

Consume friendly bacteria

A yogurt or kefir drink per day might help keep infections at bay. That's because these foods contain probiotics, bacteria that stimulate immunity cells in the gastrointestinal tract. "Normal, healthy bacteria that colonize the GI tract help you resist bad bacteria and detoxify harmful substances," explains Susanna Cunningham-Rundles, Ph.D., a professor of immunology at Weill Medical College of Cornell University in New York City. In addition to their protective effect in the GI tract, probiotics also may help stimulate immune-cell production system-wide. In a recent study of 33 women from the University of Vienna, Austria, those who ate ordinary yogurt daily for two weeks raised their T-lymphocyte cell count by nearly 30 percent.

What to do: Look for yogurt or kefir that contains "live active cultures," indicating helpful bacteria.CookingLight.com: Add live cultures to your diet with these tips and recipes

How Exercise Helps

"The immune system's cells don't function normally when a person is overweight or has high levels of cholesterol and triglycerides," says Gabriel Fernandes, Ph.D., a professor of medicine at the University of Texas Health Science Center at San Antonio. "Immune cells are not able to recognize bacteria or viruses and destroy them," Fernandes says. Along with diet, exercise is key to shedding extra pounds.

What to do: "Increase your physical activity from 30 minutes a day to 60 to burn more calories," Fernandes says. In a yearlong study of 550 men and women, researchers from the University of South Carolina found those who exercised moderately were 25 percent less likely to develop a cold compared with those who rarely exercised. (The subjects simply walked at a brisk pace.) As with diet, moderation is critical; too much exercise or exercising to the point of exhaustion can boost the body's production of adrenaline and cortisol, two hormones that temporarily suppress immune function. E-mail to a friend E-mail to a friend

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Copyright 2009 Cooking Light magazine. All rights reserved.

Stacey Colino is a writer in Chevy Chase, Maryland. Her work has appeared in dozens of national magazines.

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