Forty years ago, experts warned that vitamin D was toxic in amounts as low as 1,000 IUs daily. Now we know now better. In fact, in last week's Guide to Good Health, we learned that vitamin D may protect women against bacterial infections. And this week—it seems—we need more vitamin D to stay mentally sharp. That's according to a new study that that caught my attention.

Scientists looked at vitamin D levels in about 3,000 European men between the ages 40 and 79. They wanted to examine the connection between the amount of vitamin D found in the blood and mental function in adults.

According to NutraIngredients.com, researchers found that "higher blood levels of vitamin D were associated with better performance in tests of attention and speed of information processing." In fact, researchers found the strongest positive connection between vitamin D and mental performance in men over 60.

So for anyone out there who's over 50 and just not feeling mentally sharp, ask yourself this: Are you spending enough time in the sun each day? Most likely, you're not. Not surprisingly, as we age, many of us tend to spend less time outside.

But this is a shame…because just spending 30 minutes a day in the sun (without sunscreen) will give you 10,000 IUs of vitamin D. (Yes, you read that right, 10,000 IUs of vitamin D!) Make an early-morning walk around the block part of your daily routine!

If you can't quite pull that off, take 2000 IUs of a vitamin D supplement daily. In winter months, go for 4000 IUs daily.