Ah, summer. It sure is a feel-good season.

The days are long... the nights are warm... and that means more time to spend with friends and family in the great outdoors.

And those sunny skies alone may be enough to make you feel cheerful -- in part because when your skin is exposed to UV rays from the sun, your body produces vitamin D (a.k.a. the "sunshine" vitamin), a proven mood-booster.

Now, getting TOO much sun on your skin can turn you as red as a lobster… and even up your risk of skin cancer.

But according to a new pair of studies, avoiding the sun completely isn't wise, because if you're not getting enough vitamin D, you're at a higher risk of two OTHER types of cancer: breast and colorectal.

In the first study, published in PLOS ONE, researchers pooled data of over 5,000 postmenopausal women who had their blood levels of vitamin D measured periodically over the course of about four years.

After the researchers crunched the numbers, they found that the higher the participants' blood levels of vitamin D, the LOWER their risk of developing breast cancer.

In fact, those with the highest blood levels of vitamin D (60 ng/mL) slashed their risk of breast cancer by 20 percent compared to those with the lowest levels (less than 20 ng/mL).

Now, the National Academy of Medicine currently recommends a blood level of only 20 ng/mL -- which means that if you're only getting enough D to meet that guideline, you could be a sitting duck for breast cancer!

What's more, a study published in the Journal of the National Cancer Institute found that the same holds true for colorectal cancer.

Compared to those whose blood levels of vitamin D were only high enough to meet the guideline, those with blood levels of 30 to 35 ng/mL had a 19 percent LOWER risk of colorectal cancer, and those with blood levels of 35 to 40 ng/mL had a 27 percent LOWER risk.

The theory is that vitamin D boosts immune system function by activating T cells that recognize and attack cancer cells, raising your body's natural defenses against cancer.

So, to protect yourself from breast and colorectal cancers, let the sun shine in!

I recommend at least 10 minutes of direct sunlight every day... without sunscreen, big funny hats, or long sleeves.

After that, cover up so you don't burn.

Supplementing with vitamin D3 (not the synthetic version, D2) is also a good idea. To reach a blood level of 60 ng/mL per day, take 4,000 to 6,000 IUs daily -- less if you also soak up some sun.

And you can also load up on D-rich foods including fatty fish (like salmon and mackerel), beef liver, egg yolks, and mushrooms.