Vitamin D is a fat-soluble vitamin in a family of compounds that includes vitamins D1, D2, and D3. It can affect as many as 2,000 genes in the body. But signs that you're low on the nutrient are a bit quieter. If you notice one or more of these issues, head to your physician to get your blood tested. But don't reach for supplements quite yet—you'll want to make sure it's a vitamin D deficiency you're dealing with first since excessive intake of the nutrient can be linked to its own health concerns.)
Muscle Weakness
Decreased muscular size can be the result of not enough vitamin D in muscle and nerve tissue, says Kimberly Mueller, R.D., owner of Fuel Factor Nutrition Coaching. So if you notice that you can't eek out the...