Vitamin C has been touted as an immune booster that can prevent illnesses, such as the common cold. However, research has had mixed results on the true preventive qualities of vitamin C. Here are some studies that shed some new light.

Vitamin C and cold prevention

A recent review of placebo-controlled trials on vitamin C and common cold shows that vitamin C can reduce risk of the common cold in people under heavy physical stress, such as marathon runners and skiers. However, the rest of the population does not seem to benefit from vitamin C supplements when it comes to preventing colds. Researchers did note, however, that it may reduce the duration or severity of a cold.

Researchers reviewed 29 trials with 11,306 participants who either took vitamin C or a placebo. While they found that regular use of vitamin C had no effect on common cold prevention in the general population, an analysis of 31 studies covering 9,745 cases of colds, did show a small, but consistent effect of vitamin C in reducing the duration of symptoms. A regular dose of 1 g or more per day of vitamin C  reduced duration of colds by 8 percent in adults and 18 percent in kids.

A review of five randomized trials involving marathon runners, Swiss students at a skiing camp and Canadian soldiers on a winter exercise did show that vitamin C reduced their risks of catching a cold.  Another more recent trial looking at competitive teenage swimmers found that regular intake of vitamin C cut the risk of colds in half for men, but had no effect for women.

            [SLIDESHOW: 10 Natural Ways to Fight the Common Cold]

Can vitamin C increase risk for kidney stones?

Consuming high levels of vitamin C can double the risk of kidney stones in men compared to those who do not take high levels of the vitamin, according to a recent study. Although the new research does not say vitamin C causes kidney stones directly, large doses may be harmful for your body. Vitamin C may be elevating risk because the body breaks it down into oxelate, a part of the composition of kidney stones.

Researchers looked at 23,000 Swedish men between the ages of 45 and 79 in 1997 and followed them until 2009. Around 900 of the men took 1,000 milligrams of vitamin C, and 3 percent of those men developed kidney stones. Less than 2 percent of the rest of the group developed them. The men who received high doses of the vitamin had a 1.7 to 2.2 time greater risk of developing kidney stones. Researcher noted that there is no reason to take such high doses of vitamin C, and that multivitamins without such high levels of vitamin C do not cause kidney stones.

Can a deficiency cause fetal brain damage in pregnant women?

According to a study from November 2012, pregnant women with a vitamin C deficiency can cause damage to the baby’s brain. Unfortunately, once the damage has been done, taking vitamin C supplements will not correct the damage.