Nutrition and the Immune System

Thursday, January 15, 2009 | By Ilana Katz MS, RD, LD
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Winter has a nickname: cold and flu season. While cold weather may be inescapable in winter, not everyone has to get sick. The human body has an innate mechanism that protects against the microorganisms that cause colds and influenza: the immune system. It involves the thymus gland and its production of T-cells, the combat force against bacteria, viruses and other foreign substances. 

The other component of immunity is a reserve of antioxidants, functioning as the body’s soldiers of defense against internal cell damage from free radicals. Free radicals are unstable byproducts of the body’s cells’ use of oxygen to produce energy. The increase in free radicals generated during strenuous exercise may degrade the immune system. Because antioxidants are available in our food source, they have become a nutrition icon, particularly amongst athletes. On hearing about these metabolic busybodies, athletes are often curious to know whether they need to include antioxidants as dietary supplements to boost immunity. 

Oxidative Stress and Antioxidants

Free radical formation is enhanced by oxidants, the toxic byproducts of metabolism and exercise, as well as cigarette smoke, pollution, and even sunlight exposure (radiation). Oxidative stress occurs when the number of free radicals produced exceeds the body’s defenses against them. This is especially harmful because their radical movement within cells damages and eventually kills the cells they inhibit.

Antioxidants are micronutrients available in both food and in supplemental form (vitamins and minerals) that neutralize oxidants and thus reduce free radical formation. Moreover, research suggests a strong correlation between most cancers and a high volume of free radicals that have not been neutralized by antioxidants. Thus, diets low in antioxidants increase the risk of developing cancer and, conversely, diets high in antioxidants may be substantially protective. 

Antioxidant nutrients include many vitamins and minerals, such as vitamin C, vitamin E, selenium and beta-carotene. The best sources of vitamin C include citrus fruit, strawberries, bok choy, green and red peppers, brussels sprouts and broccoli. The best sources of vitamin E include safflower, corn and canola oils, wheat germ, soybeans and sunflower seeds. The best sources of beta-carotene are all fruits and vegetables that have an orange color, such as sweet potatoes or yams, carrots, mangos, and apricots, as well as spinach, fortified milk and beef liver. The best sources of selenium are seafood, organ meats, other animal meats, and grains and vegetables, as long as the soil they were grown in is rich in nutrients. 

Fat and Immunity

In addition to the cancer risks, free radicals and oxidative stress also increase the risk of cardiovascular disease and atherosclerosis (hardening of the arteries) and weaken the immune system. Preventive measures must go beyond simply taking antioxidants. Cancer and heart disease have a positive relationship to higher-fat diets and particularly diets high in saturated fat and trans fatty acids. Preventive measures against colds and flu require some fat to aid in the absorption certain antioxidants, and other fat-soluble vitamins (vitamins A, D, E, and K). Ideally, therefore, fat, an established necessity, should account for less than 30 percent of total calories, and less than 7 percent of total calories should come from saturated fats.

Good fats include omega-3 fatty acids, normally found in fish oils, which positively affect immune function. The two major omega-3 fatty acids, eicosapentanoic acid (EPA) and docosahexanoic acid (DHA), both decrease the inflammation response by increasing the production of T-cells, and DHA reduces the incidence of natural cell death. Alpha-lipoic acid leads to regeneration of antioxidants like vitamins C and E, enhancing their overall antioxidant effect. Foods high in omega-3’s and alpha-lipoic acid include nuts and seeds (walnuts, soybeans), leafy green vegetables, grains, and vegetable oil (corn, safflower, cottonseed, sesame, sunflower and flaxseed oil). In particular, fatty fishes such as mackerel, salmon, anchovy, herring, sardines and tuna offer the most available EPA and DHA to the immune system. 

Protein and Immunity

Glutamine and arginine are amino acids (protein building blocks) that play a role in strengthening the immune system. Glutamine is a major component of the intestinal wall of the gastrointestinal tract and its presence prevents infectious organisms from migrating from the gastrointestinal tract into the bloodstream. Arginine contributes to the production of nitric oxide, which functions in killing many infectious microorganisms. Glutamine and Arginine are both non-essential amino acids, meaning the body produces them naturally; however, an increase in complete proteins, such as egg whites, lean beef, chicken, and turkey can also help enhance immunity, as long as the fat is kept low.

Micronutrients and Immunity

A slight deficiency in any one nutrient can weaken immunity. Getting essential nutrients from food rather than in supplemental form is recommended because micronutrients are more bioavailable from food, meaning the vitamins and minerals in food are more absorbable. In fact, many supplemental forms of nutrients have less than 40 percent absorption. Unfortunately, though, the “typical” American diet is considered deficient in a variety of nutrients including calcium, iron, vitamin A, and vitamin C. In addition, the recommended daily allowance (RDA) from the American Dietetic Association for many nutrients can potentially be below what is optimal to support an efficient immune system in the active person. In this case, vitamin and mineral supplements may aid in immunity by protecting against micronutrient deficiencies. 

To combat the risk of deficiency, ensure your diet includes: 

  • Iron: Iron exists in two forms—heme and nonheme. Heme iron is bioavailable from animal tissue. Nonheme iron comes from animal tissues as well as plant tissues and various fortified breads and cereals. The body absorbs heme iron much more efficiently than nonheme iron; thus, animal sources such as chicken, lean beef and liver are the best sources of iron. Spinach is also a great source of bioavailable iron. 
  • Calcium: Calcium is most absorbable from dairy products such as milk and cheese. Other sources include broccoli, black-eyed peas and sardines. Be aware, though, that although antacids contain calcium as one of the main ingredients, they are not a reliable form of calcium. Antacids decrease the absorption of some other vitamins and minerals. The amount of antacids required to produce an appropriate amount of absorbable calcium would negatively reduce the absorption of iron, Vitamin C and vitamin D to possible deficiency levels. 
  • Vitamin A: Beta carotene is the precursor for vitamin A, so again, orange colored plant foods are especially effective for a strong immune system.

Summary of Recommendations for Enhanced Immunity 

  • Limit saturated and high fat sources. Replace the red meats (higher in fat) with servings of fish, particularly oily fish such as salmon, tuna and mackerel. These fishes are all high in omega-3 fatty acids, which, as noted, have natural anti-inflammatory properties that enhance immunity. 
  • Cook with olive oil, being rich in mono-saturated fats, versus other oils, which have unfavorable types of fats for the immune system. Avoid excessive use of margarine. Though most margarines are unsaturated in their fat content they are artificially prepared and are thus higher in trans fats. 
  • Eat more fruits and vegetables. Green leafy vegetables in particular are very rich in antioxidants. Add more servings of other fruits and vegetables to your diet, as they are rich sources of antioxidants (vitamin C, vitamin A, vitamin E, etc.).
  • By following these recommendations you can achieve optimal immune function without supplementation. As an added benefit, this nutritious energy will not only decrease the risk of cancer, but will also decrease the risk for heart disease, especially if adequate fiber (25-35 grams per day) is consumed. 


Maintaining a good nutritional status and adequate micronutrient stores in the body is essential for mounting an effective immune response to opportunistic infections. With sound eating habits and proper nutritional planning, supplements become less necessary and never should be used in lieu of eating properly. In addition, by reducing exposure to environmental factors that promote the production of free radicals you can further ensure that you are doing everything possible to lessen the effect of free radicals.

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