| Several approaches have been taken in an | | | | association between vitamin A supplementation |
| attempt to determine whether Vitamin A | | | | and incidence of bacterial infection. we are not |
| supplementation enhances immune response and | | | | aware of any long-term, randomized clinical trials |
| resistance or recovery from infection. In some | | | | that have evaluated the incidence of viral infection |
| investigations, researchers have attempted to | | | | in response to supplementation with vitamin A |
| correlate plasma concentrations of beta-carotene | | | | alone. However, several studies have examined |
| or retinol with immune response or susceptibility | | | | the possibility that supplementation with several |
| to infection. One limitation of this approach is | | | | multivitamins and or trace elements such as zinc, |
| related to the fact that plasma concentrations | | | | may alter susceptibility to infection. In general, the |
| may have depressed plasma retinol levels as a | | | | findings from these studies show no protection |
| result of disease. Therefore, it is not possible to | | | | from infection in association with vitamin intake, |
| establish whether low plasma retinol levels resulted | | | | but a slight decrease in the incidence of infection |
| in suppressed immune response or if plasma | | | | in those individuals consuming supplemental trace |
| retinol levels decreased in response to disease or | | | | elements such as zinc and selenium. At this time, |
| infection. Another approach used is to supplement | | | | the potential benefits of vitamin A |
| the diet with retinol precursors and examine | | | | supplementation for healthy well-nourished |
| immune response at a later time point. This | | | | individuals regarding susceptibility to infection |
| approach may be useful in examining the particular | | | | remain to be established. |
| aspects of immunity that may be altered by | | | | A high beta-carotene intake has also been |
| supplementation, but additional studies are | | | | associated with a reduced risk of cancer. Earlier |
| necessary to determine whether these effects | | | | epidemiological studies suggested a high natural |
| have clinical significance in terms of disease | | | | (fruits and vegetables) intake of beta-carotene |
| outcome. Vitamin A has been fairly well studied in | | | | was associated with reduced risk of cancer. |
| terms of its immunomodulatory effects, and we | | | | However, more recent studies have not |
| will review the evidence from randomized | | | | observed any benefit of beta-carotene intake on |
| controlled trials as well as potential mechanisms of | | | | incidence of cancer and two studies actually |
| action. | | | | observed an increased incidence of lung cancer in |
| Vitamin A supplementation may afford some | | | | those participants consuming beta-carotene |
| protection from infection in malnourished | | | | supplements. The presence of other carotenoids |
| individuals, but the potential benefits of | | | | in fruits and vegetables has been suggested to be |
| supplementation in normal wellnourished | | | | the protective factor in regards to cancer |
| individuals remain to be established. There is | | | | incidence in the early epidemiological studies based |
| evidence from several studies that suggests that | | | | on the findings from these recent studies, dietary |
| vitamin A deficiency is associated with depressed | | | | supplementation with high doses of synthetic |
| immune function and an impaired response to | | | | beta-carotene may be contraindicated for |
| influenza infection. Supplementation of vitamin A is | | | | smokers. As a reminder, it has been known for |
| associated with a reduction of mortality and | | | | some time that a high intake of vitamin A results |
| morbidity among certain populations. It appears | | | | in adverse effects (neurologic, dermatologic, |
| that populations suffering from malnutrition | | | | musculoskeletal, gastrointestinal, birth defects) and |
| may benefit from adequate or additional vitamin | | | | the results from the most recent studies suggest |
| A supplementation. However, it is less clear if | | | | a potential risk of high doses of synthetic |
| normal, healthy, well-nourished individuals will | | | | beta-carotene in certain populations. At this time it |
| benefit from additional supplementation with | | | | is probably safest to follow the National Cancer |
| respect to enhanced immunity. The results from | | | | Institute recommendations that suggest five or |
| several studies involving beta-carotene | | | | more servings of fruits and vegetables per day. |
| supplementation in the diet of healthy individuals | | | | Immune Effects and Exercise |
| suggest that certain aspects of innate immunity, | | | | We are currently aware of only one study that |
| such as NK cytotoxicity and monocyte production | | | | has examined whether vitamin A supplementation |
| of the cytokine TNFcx, are enhanced. It appears | | | | is associated with a reduced incidence of infection |
| that lymphocyte subsets or the lymphocyte | | | | in athletes. Several studies have shown that the |
| response to mitogens are not altered. In addition, | | | | risk of upper respiratory infection is increased |
| one study of healthy older individuals found that | | | | following competition in marathons or |
| vitamin A supplementation was associated with a | | | | ultramarathons However, vitamin A |
| reduction in the number of T lymphocytes. | | | | supplementation before marathon competition did |
| Whether these observed changes of immune | | | | not reduce the incidence of infection in the |
| function in response to supplementation actually | | | | postrace period. Therefore, to our knowledge, |
| result in reduced susceptibility to infection in | | | | vitamin A supplementation has not been |
| healthy individuals is not well established. The | | | | associated with enhanced resistance to infection in |
| results from one study demonstrated no | | | | healthy athletes. |