What exactly makes vitamin D so important to our health? Quite simply, it directly and indirectly influences most of what happens in our bodies every second of every day.

To understand, you have to shift your thinking a bit. Vitamin D isn’t actually a vitamin. Rather, it’s a hormone precursor that our biological ancestors made from being in the sun. When exposed to UV rays, a chemical cousin of cholesterol in the skin converts to vitamin D, which travels to the liver and is changed to the prehormone calcidiol. Calcitriol (the actual hormone) attaches to more than 2,700 sites on the human genome, and it turns on more than 1,000 genes, prompting them to do their jobs.

Creighton University’s Robert P. Heaney, MD, points out that vitamin D is a key part of the biochemical machinery that opens up our entire genome, so cells can tap into the vast information it contains. In a remarkable feat of biology, individual cells synthesize calcitriol, which then turns around to regulate those cells’ activities. It’s these fundamental roles of vitamin D that affect our risk for so many different diseases. In fact, says Heaney, “Vitamin D probably affects every disease.”

The evidence is particularly strong when it comes to vitamin D’s role in resisting infection, maintaining bone and muscle, and reducing cancer risks.

1. Cold and Flu Protection

Is it a coincidence that the vast majority of cold and flu outbreaks occur during the winter, when people have less sun exposure and lower levels of vitamin D? Probably not. In 2009 researchers analyzed patterns of deaths and disease complications (typically pneumonia) during the influenza pandemic that raged through the United States in 1918 and 1919, killing at least one-half million people. The researchers reported that the fewest flu deaths and complications occurred in southern cities, where the sun shone brighter throughout the year and, presumably, people had higher vitamin D levels. In contrast, the most deaths occurred in northern cities, where there was less sun exposure.

Granted, this association doesn’t prove cause and effect, but it’s certainly suggestive, and other evidence does support the protective role of vitamin D. Over the 2008–2009 winter months, doctors gave 1,200 IU of vitamin D daily to Japanese school children. Compared with children getting placebos, those taking vitamin D were 42 percent less likely to contract the flu and 83 percent less likely to suffer asthma attacks.

The underlying mechanisms are now understood. Numerous immune compounds depend on vitamin D, including PCL-gamma1, a molecule that activates immune cells so they’re capable of fighting infections. In addition, lung cells are among those that secrete 1a-hydroxylase, an enzyme that converts inactive vitamin D to its active form, helping fight respiratory infections. The vitamin D then turns on genes involved in immunity and boosts levels of cathelicidin, a powerful germ-fighting compound.

2. Stronger Bones and Muscles

Vitamin D has long been recognized as essential for normal bone formation, largely because it is essential for calcium utilization. Numerous studies have shown that the majority of seniors hospitalized for hip fractures are deficient in vitamin D.

But the problem might not be just weak bones. Heike Bischoff-Ferrari, MD, of University Hospital in Zurich, and others have made the case that weak muscles lead to falls and broken bones. The argument has its merits. Vitamin D is needed for normal muscle production and strength, and a lack of the vitamin leads to muscle weakness, a reduced range of motion, and increased physical frailty. With each passing year, seniors are more likely to be affected by sarcopenia, the age-related loss of muscle, along with osteoporosis. After analyzing 20 studies, which included more than 44,000 patients, Bischoff-Ferrari wrote in Osteoporosis International that 1,800 to 4,000 IU of vitamin D could greatly reduce the risk of falls in seniors. In contrast, the IOM recommended only 600 to 800 IU daily.

Continue to Care2.com to find out the other benefits of taking Vitamin D.

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