Many people, when they are feeling miserable from a cold or the flu, get the urge to gorge on food. But picking the right foods can benefit and even speed healing.
"This is more or less a new area," said Kerry Neville, a Seattle dietitian and spokeswoman for the American Dietetic Association. "There has been some good research, and we'll be seeing more. But it remains to be seen how much of this can actually be helpful."
Teasing out how and where food can benefit is difficult because our immune systems -- a coordinated system of signals sent and received, feedback loops and multiple redundancies to ensure that foreign molecules are identified and destroyed if they are harmful -- are so complex. A breakdown in any part of the system leaves the whole body susceptible to infection and illness.
And lifestyle and environment can cause small breakdowns in the system all the time. Smoke, air quality, sunlight and poor diet can all contribute to a weakened immune system, particularly in the form of free radicals. These highly reactive molecules with unpaired electrons can break down cells, leaving them vulnerable to invading viruses and bacteria.
Antioxidants, a type of chemical found in plants, help neutralize free radicals and protect cells, thus bolstering the immune system. Antioxidants often give plants their color and can also include vitamins C, A, and E. Experts estimate that there are many more antioxidants that are as yet unidentified.
And studies have shown that some of the protective vitamins and minerals, when given alone, do not benefit people as much as getting those nutrients from their diets.
"I don't say any one food is going to protect you," said Dawn Jackson Blatner, a registered dietician and author of "The Flexitarian Diet." "There is a crazy thing that happens, and it is called synergy. ... You start putting multiple [foods] together, the effects are multiplied and it is shocking."
The nutrients in food are not meant to be consumed in a vacuum. The context in which the body encounters the healthy minerals and molecules is almost as important as the nutrients themselves, which may be why chicken soup is such a popular home remedy.
"It's the recipes that have the magic, just the way the foods are combined," said Dr. John La Puma, author of "Chef MD's Big Book of Culinary Medicine." "If you just eat one food and expect it to act like a drug, you're out of luck."
But prevention is not the name of the game for most of these foods in targeting a specific illness. Once ill, the expert advice boils down to fluids, rest and eating a variety of healthy food.
"Nothing is a magic bullet, as far as making a major impact in making you feel better as quickly as possible," Neville said.
Some dietitians do recommend taking a daily multivitamin because, as Neville pointed out, people do not typically eat as many fruits and vegetables as they should -- around two to three cups per day.
Over all, maintaining a diverse diet, rich in fruits, vegetables, whole grains and lean protein, is the body's best defense against viral invaders.
Following are some foods to focus on to help boost your immune system when feeling under the weather.
Yogurt can be delicious layered with fruit in a parfait or as a cooling side for a dish of spicy food, but it also packs a healthy dose of good bacteria that can protect the body against harmful bacteria and infections.
"They're like little soldiers, lining the intestinal tract to fend off invading germs," said Blatner, also an ADA spokeswoman.
These little soldiers -- the good bacteria -- are called probiotics, and studies have shown eating yogurts rich in them can lead to an improved immune response by increasing the body's white blood cell count. Probiotics are found in yogurts with live or active cultures -- the lactobacillus and bifidobacterium strains.
Traditional kefir is similar to yogurt but cultured with special kefir grains, so it contains slightly different bacteria. Originally from the Middle East, kefir has a sour, refreshing taste and is slightly effervescent from the carbon dioxide produced during fermentation. Unlike the bacteria in yogurt, which are transient and pass through the system over time, the bacteria in kefir are capable of colonizing in the intestinal tract. Kefir also contains good yeasts that help fight off pathogenic yeasts in the body.
This rich, flavorful spice has been used for centuries as part of Ayurvedic and traditional Chinese medicines, in addition to being used for cooking. Turmeric is found in every yellow curry, and its golden color is the result of curcumin, a polyphenol with strong cold and flu-fighting properties.
Although the mechanism is unclear, a 2008 study published in Biochemical and Biophysical Research Communications found that curcumin prevents some immune cells from responding to stimulants and so has modulating and anti-inflammatory effects. Other studies have also shown the immune-boosting properties of curcumin in turmeric, however these have not been confirmed in humans.
Turmeric is found naturally as the rhizome part of the turmeric plant and it looks very similar to ginger. The powdered spice is made by boiling, drying and grinding the root. The powder has antiseptic qualities when applied topically and often is used on cuts, burns and bruises.
Garlic may be the wunderkind of the plant world, its properties ranging from medicinal to mystical to culinary. Ancient Egyptians considered garlic holy and used it as currency. Indeed, the pungent smell is a small price to pay for the health benefits garlic can confer.
"Garlic has been a miracle food for everything," Neville said.
Much of the immune-boosting properties of garlic come from its sulfur-containing compounds, which also give the bulb its aroma, particularly one called allicin. These compounds are effective against bacterial, viral, fungal and parasitic infections. They also enhance the immune system and have anti-tumor and antioxidant features, which help guard cells from everyday wear and tear.
Americans are growing increasingly aware of the powerful properties of garlic. According to an article in the Journal of Nutrition, garlic is the second most used supplement in the United States.
In some parts of the world, particularly the Balkans, garlic is considered so powerful it is thought to guard against vampires and witches. In 1994, a group of scientists decided to test the protective effect of garlic against vampires using leeches as a stand-in for the blood-sucking monsters. They offered their leeches two arms, one bare and the other covered in a garlic paste.
Unfortunately, in two out of three cases the leeches showed an obvious preference to the garlicky arm, attaching to it in 14.9 seconds, compared to 44.9 seconds for the bare arm. In an article published in the Journal of the Norwegian Medical Association, the researchers concluded that garlic may attract vampires rather than repel them and that restrictions on garlic use might be considered in order to avoid Balkan-like developments in Norway.
Oregano is an herb whose name is derived from the ancient Greek word meaning "joy of the mountains." And it is joyful indeed to think that your spaghetti sauce or pizza, flavored with this bold, peppery herb, can help keep you free from infections.
"Herbs and spices are incredibly potent antioxidants," Blatner said. "In terms of herbs, [oregano] is the highest in antioxidant compounds."
The antioxidant activity in oregano is due to its high content of phenolic acids and flavonoids, color compounds that are also anti-inflammatory. When eaten, oregano can protect against the common cold, influenza, fevers and indigestion.
But oregano is rarely eaten alone, and the combination of the herb and other foods may contribute to its disease-fighting abilities.
"It could be a synergistic effect," said Mary Beth Kavenagh, an instructor in the department of nutrition at Case Western Reserve University. She also pointed out that oregano is often eaten with immune-boosting garlic and tomatoes, which contain vitamin C, beta carotene and leutine, all of which benefit the body.
Topically, oregano has antimicrobial properties, guarding against bacteria. Scientists have plans to tap this property by using oregano to create thin wraps for covering fresh food to protect it from spoiling.
Bell peppers are part of the nightshade family and originated in South America before spreading to Europe and the rest of the world. Bell peppers are both low in calories and dense in nutrients. They are a good source of phytochemicals as well as beta carotenes and vitamin C.
In fact, gram for gram, red bell peppers have twice the vitamin C of most vitamin C-containing fruits and vegetables, Blatner said, including oranges.
Linus Pauling, one of the most influential scientists of the 20th century, was an advocate of megadoses of supplemental vitamin C to prevent colds. Whether vitamin C is effective at preventing a viral infection that will cause a cold is under debate and hasn't been fully proved or disproved.
But research has gone far enough to show that increasing vitamin C intake can reduce the length of time cold symptoms last as well as reduce the severity of those symptoms.
And experts are not huge proponents of supplemental vitamin C.
"The best way to get vitamin C is through food," La Puma said.
The FDA recommends getting about 90 milligrams of vitamin C each day, which is easily obtainable through daily meals. A half cup of raw red bell pepper contains 142 milligrams of vitamin C.
Vitamin C is known to maintain the skin, which is the body's first line of defense against microbes and viruses of all kinds. Vitamin C may also help to increase white blood cell count as well as antibody production.
Tea is the most widely consumed beverage in the world, after water, and to great effect. Tea is rich in polyphenols -- plant antioxidants -- as well as a number of other chemicals that can help protect the body against cold or flu.
Green tea has undergone minimal oxidation during drying and processing, and it has been subject to many scientific studies. Some of the more convincing studies highlight a compound called epigallocatechin gallate, or EGCG, a powerful antioxidant and anti-cancer agent. EGCGs have been shown to inhibit the growth of cancer cells without harming healthy tissues.
Tea can also be physically beneficial.
"Some of the helpfulness of tea is the fact that it's warm and therefore kind of soothing," La Puma said.
The soothing, steamy effect can apply to any warm drink or soup, as well, including chicken soup.
And Neville said some studies have shown that EGCGs can inhibit a virus' ability to replicate, which may offer an offensive strategy for preventing a cold, as well as improving the body's overall immune response.
Pumpkins are good for more than a lighted jack-o-lantern on the front porch. Their rich, orange flesh is packed with beta carotene, a nutrient that the body breaks down to make vitamin A.
Vitamin A helps the proteins that regulate cell-to-cell communication, which is the foundation of the immune system. Vitamin A also aids in cancer prevention, because cell-to-cell communication breakdown is one of the primary causes of cancer.
Research suggests that vitamin A may help keep the respiratory system healthy, Blatner said, which can be particularly helpful when you have a cold or the flu.
"The good news is we're in beta carotene season now," Kavenagh said, referring to the abundant orange fall vegetables such as squashes, carrots and sweet potatoes, all good sources of the nutrient. And the more intense the color, the higher the levels of beta carotene.
But experts caution against too much vitamin A. Because it is fat-soluble, excess vitamin A can be stored in the body's fat cells and large quantities can be toxic, Blatner said. Eating beta-carotene-rich foods should provide the FDA recommended nine milligrams each day and may be safer than taking a vitamin A supplement directly.
Perfumed and flavorful, the word 'ginger' comes from the Sanskrit word meaning 'horn shaped,' referring to the root's branched structure.
While it can be sweet, ginger also has some heat from a compound called gingerol, a relative of capsaicin, the compound that gives chili peppers their zing and heat. When it is dried, ginger contains less gingerol and more shoagol, an anti-inflammatory agent.
Ginger is often recommended as a tea or a bath for those with a cold or flu because it is helpful in increasing sweat production, which may help us get rid of germs and "sweat out" toxins.
"It might be an old wife's remedy, but people do swear by it," Blatner said.
Ginger has also been shown to reduce nausea and vomiting, making it a very useful food to have around when you have the flu.
Oysters are widely thought to be one of nature's most potent aphrodisiacs. This fact probably has to do with their high zinc content, which is necessary for testosterone production, one of the most important hormones behind the human sex drive for both men and women.
Oysters may or may not give you a boost in bed, but there is no doubt that zinc is very good at protecting the body against colds and flu.
Zinc functions in more enzymatic reactions than any other mineral, making it indispensable to the immune system, which is involved in reactions and signaling all the time. In particular, zinc enhances the function of helper T cells, which are important in identifying foreign antigens and alerting other cells of the immune system to invaders. Even a mild zinc deficiency can have adverse effects on the immune system, particularly in children and the elderly.
But too much zinc can be toxic and will inhibit immune function. The FDA recommends about 11 milligrams of zinc per day.
Lean meats such as beef, chicken and seafood are the best sources of zinc. Nonmeat eaters may have a harder time getting enough of the mineral because zinc in plant proteins is not as available for use in the body as zinc from animal proteins. Some options for getting zinc into a vegetarian diet include pumpkin seeds, beans and mineral-fortified cereals.
Broccoli, with its intense green color and dense florets, just looks healthy. Derived from the Latin word for "branch" or "arm," broccoli belongs to the family of cruciferous vegetables which includes cabbage, brussels sprouts, cauliflower and turnip.
"These would, for sure, be foods with extremely potent antioxidant compounds to help fight disease," Blatner said.
In the winter, when some of the more exotic fruits may be of lesser quality than the summer, broccoli and other cruciferous vegetables are an excellent -- and cheap -- source of vitamins A, C and E. In addition, broccoli is high in glucosinolates which stimulate the body's immune system.
Broccoli is a doubly powerful food because of its high concentration of sulforaphanes, which are potent anti-cancer agents.
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