Feeling tired, achy or struggling to lose weight? Low levels of vitamin D, aka the “sunshine vitamin,” could be the culprit (yes, even in the summer). Vitamin D deficiency is one of the most common—and most overlooked—nutrient shortfalls in the country, and women in midlife are especially vulnerable. The good news? Once you know what to look for, it’s surprisingly simple to turn things around. Vitamin D is often called the sunshine vitamin because our bodies make it when sunlight strikes your skin.