It’s common to have vitamin D levels so low that health problems develop. This nutrient is needed to make serotonin, so deficiency may lead to sadness and irritability. Vitamin D is also important for boosting immunity, so its deficiency can cause respiratory infections and cognitive decline. Even muscle and bone strength depend on it, so pain and injury is likely without enough. Spend 10–15 minutes outside each day to help the skin make enough vitamin D.