If you're frequently sick, fatigued, have back pain or muscle weakness, experience hair loss or depression, you may be surprised to learn that those problems can be symptoms of a vitamin D deficiency — something that becomes increasingly prevalent in the winter when exposure to sunlight dips dramatically for many.

But it's important to mention that even year-round, 42% of the American public is deficient.

Who is especially at risk for this shortage of D?

Mainly, people with darker skin. While people with light skin tone under the age of 65 can generally generate enough vitamin D with exposure to midday sun for 5 to 10 minutes several times a week, darker-skinned people may need up to three times longer exposure to produce the same level of the vitamin.

Deficiency is also a risk for people with digestive problems such as Crohn's disease, ulcerative colitis, and celiac disease, those who are obese, and those with chronic kidney or liver disease.

In addition, anyone taking steroids or medications to lower cholesterol, control seizures, or lose weight should ask their doctor for a blood test to check vitamin D levels.

We suggest you get at least 1,000 IU (under age 65) or 2,000 IU (65 and over) daily of D2 or D3 combined with vitamin K2.

Vitamin D2 comes from mushrooms (grown in UV light), fortified foods, and supplements. D3 comes from oily fish (salmon), fish oil, and supplements.

It's smart to get a blood test to check your level and take a vitamin D2/3 with K2 supplement as recommended by your doctor.