Search for Health Information You Need

Diet and Immunity

Although vitamins and minerals alone cannot cure or prevent an infection, extra doses of certain nutrients do seem to reduce the severity of some infectious diseases. It goes without saying that a long-lasting vitamin deficiency is an invitation to disease. A vigorous immune system is all that stands between us and perpetual infection while dietary nutrients are known to have a significant impact on the status of our immune system. Some experts even suggest that the weakened immunity that accompanies old age is to certain extent a result of poor nutrition.

Several recent studies suggest that when it comes to certain nutrients, more than the official recommended dietary allowances (RDA) may be required in order to maintain our immune system in top condition. After all, the recommended dietary allowances were established to prevent deficiency diseases, not to promote the immune system. On the other hand, however, too much of certain nutrients can actually weaken immunity. Amid this uncertainty, a few vitamins and minerals really stand out, being regarded as especially important for a strong, healthy immune system.

Vitamin A

Carotenoids, namely beta-carotene, are the most important form of vitamin A. The best dietary sources of carotenoids and, thus also of vitamin A, are fruits and vegetables, especially dark leafy vegetables, carrots, sweet potatoes, squash, dried apricots, cantaloupe melon, peas and red peppers. Although deficiency in vitamin A can increase the vulnerability to infection, overdosing is known to be harmful. Unlike water soluble vitamins, vitamin A, being fat-soluble, is stored in your body fat where it can build up to dangerous levels.

Carotenoids help to regulate three types of immune cells: T and B lymphocytes, natural killer cells, and macrophages. Insufficient intake of vitamin A can reduce the size of the thymus gland and thus decrease the production of the disease-fighting white blood cells, i.e. lymphocytes, reduce the production of antibodies that fight infection and impair the ability of the respiratory tract to push bacteria out of the system. In addition to being a vital component of the immune system, vitamin A is essential for regulating growth and development and maintaining good vision.

Several studies show that patients with pneumonia, measles or those who have a long-lasting fever may benefit from increased intake of vitamin A above the RDA. On the other hand, there is evidence to suggest that the body reduces vitamin A circulation as part of its response to infection. If this was the case, then increasing vitamin A intake during infection would be counterproductive as long as the patient was not deficient in vitamin A prior to getting sick. There is also some evidence that too much beta-carotene may impair the immune system, specifically when it comes to increasing the risk of developing lung cancer in smokers. Therefore, smokers should limit their beta-carotene intake to the amount found in a standard multivitamin tablet.

Vitamin B complex

Deficiencies of certain B vitamins have been proven to lower our resistance to infection. It is mostly vitamins B2, B6, B12, pantothenic acid (B5) and folic acid (B9) that have the greatest impact. Oral contraceptives can reduce the supply of B vitamins, namely B2, B6, B12 and folic acid (B9). However, eating diet rich in the B vitamins, such as whole unprocessed foods, should be enough to overcome this effect. A deficiency in vitamin B12 is the most common among all B vitamin deficiencies. Although most people get enough of this vitamin from their diet, around 25% of older adults lose their ability to adequately absorb the naturally-occurring vitamin B12. For these individuals supplementation with fortified foods or vitamin pills is recommended.

Vitamin C

There is sufficient scientific evidence to show that vitamin C can affect our immune response. It does so by boosting the bacteria-killing ability of immune cells called macrophages and neutrophils. Some recent studies also suggest that vitamin C stimulates production of interferon, which in turn helps to improve the thymus gland’s ability to produce infection-fighting lymphocytes. Recommended dietary allowance of vitamin C for adults is just under 100mg, except for smokers and lactating women who need more. Megadoses of vitamin C (one to two grams per day) have been found to deliver only marginal additional benefits, if any at all. Vitamin C is water soluble, hence its excess amounts are being rapidly flushed from the body to prevent overdosing. However, excessive doses can in some individuals cause bloating, diarrhea, heartburn, nausea and vomiting. It is not recommended to take more than one gram (1000mg) of vitamin C per day.

Vitamin E

Vitamin E, alongside vitamin C, is highly valued for its antioxidant effects. However, its value as an immunity-booster is debatable. Studies show that megadoses of vitamin E can suppress the function of B cells, an important component of the immune system. Although many people take vitamin E supplements, it is rare to be deficient in this vitamin. Nuts (especially almonds), seeds (especially sunflower seeds), dark green leafy vegetables, liver, and some vegetable oils (especially sunflower oil) are rich sources of vitamin E (d-alpha-tocopherol). The tolerable upper intake level of vitamin E is 1000mg per day for adults (versus RDA of 15mg a day). This level can be reached only by taking supplements. Consuming greater amounts of vitamin E increases the risk of bleeding, due to its blood-thinning effects.


Iron deficiency is very common, especially among menstruating women, who lose iron with the blood in their periods. Without sufficient amount of iron in the body, the number of antibodies in the immune system is decreased so that neutrophils and macrophages, the system’s natural killer cells, are unable to eliminate harmful bacteria as effectively as they should. Iron deficiency may not only lower the number of white blood cells but also reduce their ability to fight infection.

However, too much iron may also be harmful. Excess iron interferes with immune responses and limits the number of cells that grow into lymphocytes. In addition, iron overload can lead to disorder called hemosiderosis. Hence, it seems that both, iron deficiency and iron overload may impair your immune response. The recommended dietary allowance for men and postmenopausal women is 8mg a day whereas women in their childbearing years need 18mg of iron a day. You should take iron supplements only to correct a deficiency as intake above the RDA will not prop up your immune system.


Although deficiency in selenium is associated with an increased risk of certain types of cancer, its role in combating infections is unclear. When levels of selenium are too low, the body cannot make sufficient amount of antibodies to fight disease. Although a moderate increase in intake is known to boost immune response, an overdose of selenium can do just the opposite. The recommended dietary allowance is 55mcg for adults (men and women alike) and 60mcg for pregnant women whereas doses above 400mcg per day are considered unsafe. Warning signs of overdose include hair loss and nail damage.


Zinc is an essential nutrient involved in numerous aspects of cellular metabolism. The recommended dietary allowance for zinc is a daily intake of 8mg for women and 11mg for men and pregnant women. Deficiency in zinc impairs the immune response in multiple ways including shrinking the size of the thymus and thus reducing the production of lymphocytes, impairing the function of lymphocytes and the germ-killing cells macrophages, decreasing antibody response and preventing wounds from healing properly.

Zinc deficiency can be most commonly found in the elderly, alcoholics, dieters, infants, pregnant women and people with burns or infections. Zinc is abundant in meat, seafood, eggs, dairy products, beans and tree nuts, hence most people do not need to be deficient in this mineral. However, when it comes to zinc supplements, more is definitely not better. In fact, exceeding the RDA may be harmful. The tolerable upper intake level for zinc is 40mg per day for adults. Doses of 90mg to 100mg per day taken over a long period of time can reduce immunity, diminish levels of “good” HDL cholesterol and cause anemia.


Strong evidence suggests that imbalances in amino acids, the building blocks of proteins, can weaken our immune response. Some studies found that the use of supplemental L-arginine can lead to an increase in the number of lymphocytes and shorten hospital stays after major operations. However, most healthy individuals do not need to use L-arginine supplements as their body produces sufficient amounts of this essential amino acid.


Severe protein insufficiency is known to reduce the size, weight and functionality of the thymus gland. The thymus produces the white blood cells that make antibodies, the role of which is to kill invading microorganisms such as viruses and bacteria. If the gland shrinks, the body’s ability to fight infection decreases. Protein deficiency is uncommon in Western countries except among those with certain illnesses and some elderly people who may find it too difficult to prepare nutritious meals. In fact, most people eat more protein than they actually need and, therefore, deficiency is rarely to blame for a malfunctioning immune system.

When Extra Nutrients May Be Needed

The elderly people are particularly vulnerable to infectious diseases. They get sick more easily and take much longer to recover than younger adults. As a result, infections happen to be among the leading causes of death in the elderly. Although it seems natural that the immune system declines with age, an inappropriate diet can make matters worse. Ideally, the elderly should prop up their immune systems by eating nutrient-rich foods. Unfortunately, however, many elderly people neglect cooking and drinking enough fluids or take prescription drugs which interfere with absorption of nutrients from food and are, therefore, often deficient in essential nutrients such as zinc, iron and vitamins A, B6 and C needed for maintaining robust immune system. Correcting these deficiencies with supplements can successfully strengthen their immune response.