As the cold and flu season gets into full swing, seniors are more vulnerable to infections than younger people because their immune systems weaken with age. But a new study found that zinc supplements can boost blood levels of zinc in older people and strengthen their immune systems.

The double-blind, placebo-controlled trial involved nursing home residents 65 years and older who were moderately to severely deficient in zinc. Participants were given a daily multivitamin containing 5 milligrams of zinc as well as a 30 milligram zinc supplement, or only the multivitamin for three months.

The study, which was published in The American Journal of Clinical Nutrition, found that the concentration of zinc in blood serum rose 16 percent in those taking zinc supplements, while levels in the control group only rose 0.7 percent. In addition, zinc supplementation also improved the numbers and function of T-cells, a type of white blood cell that's a key component of the immune system.

"Our previous work showed that 30 percent of nursing home residents have low serum zinc levels and those with low serum zinc levels had a significantly higher incidence of pneumonia and morbidity from it," said the study's lead author Simin Nikbin Meydani, D.V.M., Ph.D. of Tufts University.

"Our new finding that serum zinc levels can be improved in older adults with zinc supplementation, and that this is associated with enhancement of T-cell numbers and function, strongly suggests that ensuring adequate zinc consumption by older adults could have a significant impact on reducing the incidence of and morbidity from infection, which is a major public health problem in older adults," Meydani said.

Although too much zinc can be harmful — the government sets 40 mg as the upper daily limit for adults — some researchers suspect that aging bodies don't absorb or use zinc as effectively as younger ones. In addition, while serum zinc levels are a commonly used measure to evaluate zinc deficiency, they might not accurately reflect the amount of zinc found on a cellular basis, and may be deficient even when serum zinc levels are normal.

The following four supplements can also boost your immunity against colds and flu:

Garlic: A British study found that people taking a daily garlic supplement containing allicin for 12 weeks reduced their chances of catching a cold by more than half when compared to those taking a placebo. In addition, those who caught colds recovered more quickly, and their chances of an infection following the cold were significantly reduced.

Probiotics: Probiotics, which are healthy bacteria found in yogurt, help the body fight viruses. An analysis of 10 studies found that people who took probiotic supplements reduced their chances of catching the common cold by 42 percent.

Vitamin D: Although vitamin D is vital for a well-functioning immune system, health experts estimate that about 75 percent of teens and adults are deficient. One randomized, double-blind study found that children who took 1,200 IU's of vitamin D every day lowered their risk of flu by 42 percent when compared to children who took a placebo. Some scientists speculate that higher doses might yield even better results. One German study found that adequate vitamin D increases immune function as much as fivefold.

Vitamin E: A placebo-controlled study from Tufts University found that people who took a 200 IU capsule of vitamin E each day had a significant reduction in upper respiratory infections over the course of the year-long study.