We may be more vulnerable to colds in the winter because there’s less sunlight, and we’re absorbing less vitamin D. People with decreased levels of D have weaker immune systems. That’s because D kick-starts our body’s immune response. But it’s not like you can pop some D when you feel a cold coming on, to beat it. You need to increase your levels before you’re sick. So either take 2,000 International Units in a supplement daily – of course talk to your doctor first – or get 10 minutes of daylight exposure every day.