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Stimulate the intestine's immune system and slow the growth of infectious organisms in the intestine by regularly eating yogurt and other foods containing live cultures, or take a supplement containing 10 billion colony-forming units a day of acidophilus or bifidobacteria
Take advantage of the benefits of moderate exercise on immune function—but be careful about prolonged or intense exercise, which can temporarily increase your risk of infection
Keep your alcohol intake low or moderate to avoid damaging effects to your immune system
The immune system is an intricate network of specialized tissues, organs, cells, and chemicals. The lymph nodes, spleen, bone marrow, thymus gland, and tonsils all play a role, as do lymphocytes (specialized white blood cells), antibodies, and interferon.
Two types of immunity protect the body: innate and adaptive. Innate immunity is present at birth and provides the first barrier against microorganisms. The skin, mucus secretions, and the acidity of the stomach are examples of innate immunity that act as barriers to keep unwanted germs away from more vulnerable tissues.
Adaptive immunity is the second barrier to infection. It is acquired later in life, such as after an immunization or successfully fighting off an infection. The adaptive immune system retains a memory of all the invaders it has faced. This is why people usually get the measles only once, although they may be repeatedly exposed to the disease. Unfortunately some bugs—such as the viruses that cause the common cold—"disguise" themselves and must be fought off time and again by the immune system.
Symptoms of decreased immune function include frequent colds and flus, recurring parasitic infections, initially mild infections that become serious, opportunistic infections (infections by organisms that are usually well controlled by a healthy immune system, such as toxoplasmosis, cryptococcosis, and cytomegalovirus), and cancer.
Both excessive thinness and severe obesity are associated with impaired immune responses.1 Obesity increases the risk of infection, at least in hospitalized patients, according to preliminary research.2 However, these effects may not occur with mild to moderate obesity in otherwise healthy people, and attempts to lose weight through dietary restriction may actually be harmful to the immune system.3 The detrimental effects of both appear to be offset when people regularly perform aerobic exercise.4, 5
The effects of exercise on immune function depend on many factors, including frequency and intensity of exercise.6 Regular moderate physical activity has positive effects, at least on some measures of immunity, and has been shown to reduce risk of upper respiratory infection. However, very intense and prolonged exercise, such as running a marathon or overtraining, can, in the short term, actually increase the risk of developing infections.7 The positive effects of moderate exercise on immunity may also partly explain the apparent reduced susceptibility to cancer of physically active people.8
The immune system is suppressed during times of stress. Chronic mental and emotional stress can reduce immune function, but whether this effect is sufficient to increase the risk of infection or cancer is less clear.9, 10 Nevertheless, immune function has been increased by stress-reducing techniques such as relaxation exercises, biofeedback, and other approaches,11, 12 although not all studies have shown a significant effect.13
The effect of fats on the immune system is complex and only partially understood. Excessive intake of total dietary fat impairs immune response, but some types of fat may be neutral or even beneficial. For example, monounsaturated fats, as found in olive oil, appear to have no detrimental effect on the immune system in humans at reasonable dietary levels.
Research on the effect of the omega-3 fatty acids that are abundant in some fish, fish oils, and flaxseed oil is conflicting. Liquid diets containing omega-3 fatty acids used in hospitals for critically ill people have been shown to improve immune function and reduce infections. However, in one controlled study in healthy people, a low-fat diet improved or maintained immune function, but when fish was added to increase omega-3 fatty acid intake, immune function was significantly inhibited.
Supplementation with DHA (an omega-3 fatty acid found in fish oil) in healthy young men has been shown to decrease the activity of immune cells, such as natural killer (NK) cells, and to inhibit certain measures of inflammation in the test tube. The anti-inflammatory effects of DHA may be useful in the management of autoimmune disorders; however, such benefits need to be balanced with the potential for increased risk of infections. Other studies suggest that increased oxidative damage might be the reason for the negative effects on the immune system sometimes caused by fish oil, and that increased intake of antioxidants, such as vitamin E, could correct the problem.
As with omega-3 fatty acids, omega-6 fatty acids (as found in vegetable oils) have also produced conflicting effects on the immune system. Enriching a low-fat diet with omega-6 fatty acids did not impair immunity. However, diets high in omega-6 fatty acids have suppressed immunity in other reports.
In summary, low-fat diets with moderate levels of monounsaturated fat from olive oil appear least likely to compromise immune function and may provide small benefits. Conclusions about the desirability of diets high in either omega-3 or omega-6 fatty acid supplementation await further research.
Many studies, in both animals and humans, have demonstrated immune-stimulating effects from yogurt which contains live cultures, such as Lactobacillus acidophilus and other probiotics (friendly bacteria). The effects of probiotics observed in humans include increasing the activity of several types of white blood cells. In preliminary human studies, consumption of live probiotic-containing yogurt has been associated with a reduced incidence of several immune-related diseases, including cancer, infections of the stomach and intestines, and some allergic reactions.
Alcohol intake, including single episodes of moderate consumption, interferes with a wide variety of immune defenses. Alcohol's immune-suppressive effect may be one mechanism for the association between alcohol intake and certain cancers and infections. However, moderate alcohol consumption (up to three to four drinks per day) has been associated in preliminary studies with either no risk or a decreased risk for upper respiratory infections in young nonsmokers.
Our proprietary "Star-Rating" system was developed to help you easily understand the amount of scientific support behind each supplement in relation to a specific health condition. While there is no way to predict whether a vitamin, mineral, or herb will successfully treat or prevent associated health conditions, our unique ratings tell you how well these supplements are understood by some in the medical community, and whether studies have found them to be effective for other people.
For over a decade, our team has combed through thousands of research articles published in reputable journals. To help you make educated decisions, and to better understand controversial or confusing supplements, our medical experts have digested the science into these three easy-to-follow ratings. We hope this provides you with a helpful resource to make informed decisions towards your health and well-being.
3 StarsReliable and relatively consistent scientific data showing a substantial health benefit.
2 StarsContradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit.
1 StarFor an herb, supported by traditional use but minimal or no scientific evidence. For a supplement, little scientific support.
Most, but not all, double-blind studies have shown that elderly people have better immune function and reduced infection rates when taking a multiple vitamin-mineral formula. In one double-blind trial, supplements of 100 mcg per day of selenium and 20 mg per day of zinc, with or without additional vitamin C, vitamin E, and beta-carotene, reduced infections in elderly people, though vitamins without minerals had no effect. Burn victims have also experienced fewer infections after receiving trace mineral supplements in double-blind research. These studies suggest that trace minerals may be the most important micronutrients for enhancing immunity and preventing infections in the elderly.
Caution: Synthetic beta-carotene has been linked to increased risk of lung cancer in smokers. Until more is known, smokers should avoid all beta-carotene supplements.
Beta-carotene and other carotenoids have increased immune cell numbers and activity in animal and human research, an effect that appears to be separate from their role as precursors to vitamin A. Placebo-controlled research has shown positive benefits of beta-carotene supplements in increasing numbers of some white blood cells and enhancing cancer-fighting immune functions in healthy people at 25,000–100,000 IU per day.
In double-blind trials in the elderly, supplementation with 40,000–150,000 IU per day of beta-carotene has increased natural killer (NK) cell activity, but not several other measures of immunity.
Controlled research has found that 50,000 IU per day of beta-carotene boosted immunity in people with colon cancer but in not those with precancerous conditions in the colon. Beta-carotene has also prevented immune suppression from ultraviolet light exposure. Effects on immunodefiency in HIV-positive people have been inconsistent using beta-carotene.
Vitamin A plays an important role in immune system function and helps mucous membranes, including those in the lungs, resist invasion by microorganisms. However, most research shows that while vitamin A supplementation helps people prevent or treat infections in developing countries where deficiencies are common, little to no positive effect, and even slight adverse effects, have resulted from giving vitamin A supplements to people in countries where most people consume adequate amounts of vitamin A. Moreover, vitamin A supplementation during infections appears beneficial only in certain diseases. An analysis of trials revealed that vitamin A reduces mortality from measles and diarrhea, but not from pneumonia, in children living in developing countries. A double-blind trial of vitamin A supplementation in Tanzanian children with pneumonia confirmed its lack of effectiveness for this condition. In general, parents in the developed world should not give vitamin A supplements to children unless there is a reason to believe vitamin A deficiency is likely, such as the presence of a condition causing malabsorption (e.g., celiac disease). However, the American Academy of Pediatrics recommends that all children with measles be given short-term supplementation with high-dose vitamin A in cases of hospitalization, malnutrition, and other special circumstances determined by a doctor.
A combination of antioxidants vitamin A, vitamin C, and vitamin E significantly improved immune cell number and activity compared with placebo in a group of hospitalized elderly people. Daily intake of a 1,000 mg vitamin C plus 200 IU vitamin E for four months improved several measures of immune function in a preliminary study. To what extent immune-boosting combinations of antioxidants actually reduce the risk of infection remains unknown.
Vitamin C stimulates the immune system by both elevating interferon levels and enhancing the activity of certain immune cells. Two studies came to opposite conclusions about the ability of vitamin C to improve immune function in the elderly, and two other studies did not agree on whether vitamin C could protect people from hepatitis. However, a review of 20 double-blind studies concluded that while several grams of vitamin C per day has only a small effect in preventing colds, when taken at the onset of a cold, it does significantly reduce the duration of a cold. In controlled reports studying people doing heavy exercise, cold frequency was reduced an average of 50% with vitamin C supplements ranging from 600 to 1,000 mg per day. Thus, the overall effect of vitamin C on immune function is unclear, and its usefulness may vary according to the situation.
Animal and test tube studies show noni to have some immune-enhancing activity. Specifically, the polysaccharide component has been shown to increase the release of immune-enhancing compounds that activate white blood cells to destroy tumor cells. The usual recommendation is 4 ounces of noni juice 30 minutes before breakfast (effectiveness is thought to be best on an empty stomach). Human studies are needed to confirm the usefulness of noni.
A double-blind trial showed that 45 grams per day of whey protein increased blood glutathione levels in a group of HIV-infected people. Test tube and animal studies suggest that whey protein may improve some aspects of immune function.
Zinc supplements have been reported to increase immune function. This effect may be especially important in the elderly according to double-blind studies. Some doctors recommend zinc supplements for people with recurrent infections, suggesting 25 mg per day for adults and lower amounts for children (depending on body weight). However, too much zinc (300 mg per day) has been reported to impair immune function.
While zinc lozenges have been shown to be effective for reducing the symptoms and duration of the common cold in some controlled studies, it is not clear whether this effect is due to an enhancement of immune function or to the direct effect of zinc on the viruses themselves.
1. Chandra RK. Nutrition and the immune system: an introduction. Am J Clin Nutr 1997;66:460-3S [review].
2. Stallone DD. The influence of obesity and its treatment on the immune system. Nutr Rev 1994;52:37-50.
3. Nieman DC, Nehlsen-Cannarella SI, Henson DA, et al. Immune response to obesity and moderate weight loss. Int J Obes Relat Metab Disord 1996;20:353-60.
4. Nieman DC, Henson DA, Nehlsen-Cannarella SL. Influence of obesity on immune function. J Am Diet Assoc 1999;99:294-9.
5. Scanga CB, Verde TJ, Paolone AM, et al. Effects of weight loss and exercise training on natural killer cell activity in obese women. Med Sci Sports Exerc 1998;30:1666-71.
6. Nieman DC. Exercise immunology: practical applications. Int J Sports Med 1997;18:S91-100 [review].
7. Nieman DC. Exercise and resistance to infection. Can J Physiol Pharmacol 1998;76:573-80 [review].
8. Shephard RJ, Shek PN. Associations between physical activity and susceptibility to cancer: possible mechanisms. Sports Med 1998;26:293-315 [review].
9. Herbert TB, Cohen S. Stress and immunity in humans: a meta-analytic review. Psychosom Med 1993;55:364-79 [review].
10. Palmblad JE. Stress-related modulation of immunity: a review of human studies. Cancer Detect Prev Suppl 1987;1:57-64 [review].
11. Kemeny ME, Gruenewald TL. Psychoneuroimmunology update. Semin Gastrointest Dis 1999;10:20-9 [review].
12. Halley FM. Self-regulation of the immune system through biobehavioral strategies. Biofeedback Self Regul 1991;16:55-74 [review].
13. Whitehouse WG, Dinges DF, Orne EC, et al. Psychosocial and immune effects of self-hypnosis training for stress management throughout the first semester of medical school. Psychosom Med 1996;58:249-63.
Last Review: 06-08-2015
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