by David C. Nieman, DrPH
Tratto da Vegetarian Nutrition & Health Letter Vol.3 n.1, January 2000, pubblicato dalla Loma Linda University (Traduzione in italiano).
Dr. David C. Nieman is a professor of Health and Exercise Science at Appalachian State University in Boone, N. Carolina.
The Incredible Immune System
From birth, we are exposed to a continuous onslaught of bacteria, viruses, and other disease-causing organisms. Without an effective shield, each of us would soon succumb to infectious disease and cancer. In the battle with microbial invaders, we protect ourselves with a complex array of defensive measures collectively identified as the immune system.
The immune system is remarkably adaptive and is able to generate an enormous variety of cells and molecules capable of recognizing and eliminating a limitless variety of foreign invaders. There are two functional divisions in the immune system:
Innate immunity is the basic resistance to disease that we are born with, acting as a first line of defense. It includes barriers to infection such as the skin, mucous membranes, and body temperature. It also includes special chemicals such as interferon, and cells such as natural killer cells and neutrophils that can engulf, kill, and digest whole microorganisms.
Acquired immunity, also called specific immunity, produces a specific reaction to each infectious agent which is then "memorized" by the immune system. An acquired immune response is activated when the innate immune system fails to effectively combat an invading pathogen. The acquired immune system includes special cells called B- and T-lymphocytes, which produce a large variety of specialized chemicals called antibodies and cytokines. T-cells can also engage in direct cell-on-cell warfare.
The Immune Cells Depend on the Nutrients We Eat
In the growing fetus and in the early months of life, nutrition impacts the development of the immune system. It has long been known that malnourished children have a high risk of severe and life-threatening infections . Lack of protein and calories adversely affects virtually all components of the immune system [1-3]. Anorexia nervosa has also been linked to impaired immunity .
Throughout life, nutrients are also necessary for the immune response . Zinc, iron, copper, selenium, vitamins A, B6, C, and E all have critical roles in the maintenance of optimum immune function [1-3]. Most of these nutrients are best obtained from fruits, vegetables, whole grains, nuts, and seeds.
Current investigation has centered on the role of specific nutrient deficiencies and nutrient supplements on immune function in a wide variety of human subjects including the elderly, children and adults from developing countries, patients infected with human immunodeficiency virus (HIV), patients with eating disorders, and healthy adults.
Antioxidants and Immune Function
Immune cells can be damaged by exposure to oxygen -a process called oxidation, which produces highly damaging compounds called free radicals. Adequate amounts of antioxidants such as vitamins C and E, as well as various phytochemicals, can help prevent this damage . The balance between dietary antioxidants and oxidants (compounds that cause oxidative damage) is important in immune cell function . Studies have found that diets rich in antioxidant nutrients are linked to a reduced incidence of cancer. This link may depend in part on the antioxidant related boost in immunity.
Nutrition and Immunity in the Elderly
The need for optimal intake of nutrients and antioxidants to support a healthy immune system is especially critical for older people[6-11]. Immune deficiency associated with old age (sometimes called immunesenescence) is partly responsible for the afflictions of old age. In particular, some people experience a decrease in the number of T-cells. Elderly persons are more susceptible to many infections, autoimmune disorders and cancers, compared with younger adults. Free-radical formation increases in old age and contributes, at least in part, to this phenomenon.
Two studies have shown that daily supplementation with low-to-moderate doses of certain nutrients improves immune function in healthy elderly individuals [12, 13], Long-term supplements of beta carotene (50 mg every other day for 10-12 years) was linked to enhanced activity of natural killer cells in healthy elderly menu. Vitamin E supplements have been consistently associated with improved immunity in older people even when they had normal vitamin E status [7, 14]. Evidence from animal and human studies indicates that vitamin E plays an important role in maintaining the immune system. Even a marginal vitamin E deficiency impairs the immune response, while supplementation with higher-than-recommended levels enhances immunity . The current RDA for vitamin E may not be enough to maintain optimal immune function in older individuals.
Do Vegetarians Enjoy Enhanced Immunity?
Very few studies have compared immune function in vegetarians with that of nonvegetarians. One study of 22 vegetarians and 22 nonvegetarians did not find any difference in activity of natural killer cells . Similarly, in a study of eight male athletes consuming either a lacto-ovo vegetarian diet or a meat-rich diet, there were no differences in natural killer cell activity or T-cell function . In a study of 80 women with varying dietary habits, no link between animal product intake and immunity could be established . Nor did a recent study at Loma Linda University comparing 25 vegans and 20 nonvegetarians, find any difference in natural killer cell function and T-cell function .
These findings are surprising, given that studies do show a link between plant constituents like antioxidants-which are higher in vegetarian diets-and immune function. Compared to nonvegetarians, vegetarians typically have a higher intake of fruit and vegetables, antioxidant nutrients, and phytochemicals, all of which are important for adequate immune function. Vegetarians also eat more soy products, which may have some positive effects on immunity. Although findings are still speculative, soybean isoflavones have been shown in laboratory studies to stimulate natural killer cell activity [21, 22]. In mice, the isoflavone called daidzein was shown to enhance cell-mediated immunity .
Vegetarians also have higher intakes of plant sterols (phytosterols) which may also improve immune function . Finally, vegetarians also have lower levels of certain compounds like iron that can act as pro-oxidants.
Several explanations are possible for the lack of demonstrated benefits of a vegetarian diet. Subjects studied thus far have been young-to-middle aged adults, but the benefits to the immune system from a plant-based diet may emerge only during the stress of old age. Although the theory is still debated, some nutrition immunologists believe that nutrient supplementation beyond what can be obtained from the diet is necessary to optimize immune function, especially in old age [9, 10, 14]. However, this recommendation awaits the results of several ongoing studies looking at both effectiveness and safety of nutrient supplements. There are potential health problems associated with very high intakes of some vitamins and minerals, and dietary excess of one nutrient may have a detrimental effect on the absorption and utilization of another.
The data currently available strongly suggest that elderly vegetarians who have consumed a wide variety of plant foods most of their lives should experience enhanced immune function, compared to nonvegetarians. At this time, further research is warranted to evaluate immune function in elderly vegetarians and non vegetarians.
Vegetarian Diets and the Immune System: Room for Improvement?
It's a surprise that studies haven't found enhanced immunity among vegetarians, despite their higher intake of antioxidants and perhaps other important compounds. Is it possible that some vegetarian diets lack important promoters of a healthy immune system? The answer might be "yes."
Compared to omnivores, vegetarians tend to have low intakes of zinc, vitamin B12, and omega-3 fatty acids, all of which have been shown to be important for a healthy immune system.
In one study, vitamin B12 supplements increased natural-killer cell activity in people who suffered from B12 deficiency . Of course, vitamin B12 deficiency is rare even among vegans, but both vegans and lacto-ovo vegetarians tend to have lower serum vitamin B12 levels compared to omnivores. In older people, vitamin B12 supplements may be necessary to maintain normal blood levels. It's not clear whether low but-normal vitamin B12 levels affect immunity.
Vegetarians also have lower zinc intakes than omnivores, although their serum levels are similar . However, serum zinc levels may not be the best indicator of status. In a study of elderly subjects in Rome, subjects were given supplements of zinc, zinc plus vitamin A, or a placebo. Cell-mediated immunity was enhanced in those who received just the zinc supplements .
Finally, because of their effects on the hormone-like compounds prostaglandins and on cell regulators called cytokines, omega-3 fatty acids may enhance immunity . Vegetarian diets are often low in omega-3 fats.
Bottom line: A vegetarian diet, because of its high content of antioxidants and perhaps because of the inclusion of foods like soybeans, may offer some immunity-enhancing benefits. But many vegetarians may need to alter their diet to make it as healthful as possible. To boost your immune system, make sure you are eating plenty of zinc-rich foods and be sure to include daily sources of omega-3 fats in your diet. Finally, vegans, and perhaps even lacto-ovo vegetarians, should be certain that they are meeting daily requirements for vitamin B12.
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