Your immune system is a network that works to keep harmful substances, such as viruses, bacteria and chemicals, from entering and triggering disease in the body. While a strong immune system lowers your risk for health problems and enhances healing once you have them, poor immune function increases your risk for illness and can slow or prevent healing. A healthy diet, limited in certain foods, may guard against these risks.
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Red meat is a top source of saturated fat, which may increase inflammation in your body -- a common way your body reacts to harmful substances, injuries and disease. For boosted immune function, a review in the November 2013 issue of the "American Society for Nutrition" recommends switching red meat out of your diet in exchange for another source of protein. Choose oily fish for protein instead. Oily fish, such as salmon, mackerel and halibut, provide omega-3 fatty acids -- essential fats with anti-inflammatory properties.
Fried foods, such as potato chips, french fries and fried pastries, are additional rich sources of saturated fat. Many also contain trans-fats, which can increase your LDL, or "bad" cholesterol and reduce your HDL, or "good" cholesterol, according to the American Heart Association, increasing your risk for heart disease significantly. For heightened benefits, replace fried foods in your diet with moderate amounts of healthy fat sources, such as nuts, seeds, avocados and vegetable oils.
Added sugars contribute sweet flavor and calories, but few nutrients, to foods. Consuming 8 tablespoons of sugar, or the amount found in one 12-ounce can of soda, can reduce your white blood cells' ability to destroy germs by 40 percent, according to the Environmental Law Center of the United Kingdom. This immune-suppressing effect may begin within 30 minutes of ingestion and last for up to 5 hours. Other particularly sugar-rich foods include candy, colorful cereals, pancake syrup, jelly, jam, frosting, frozen desserts and commercially-prepared cakes, cookies, pastries and pies. Healthier alternatives include unsweetened apple sauce, all-fruit frozen bars, tea sweetened with stevia and low-sugar bran muffins.
Refined grains, such as white flour, instant rice and enriched pasta, contain few nutrients and little fiber compared to the natural whole grains they derive from. The May 2010 issue of "Brain, Behavior and Immunity" reports that increasing your fiber intake strengthens your immune function, so swap white bread and other refined foods out for 100 percent whole grain alternatives. Nutritious options include oats, barley, wild rice, brown rice and air-popped popcorn.