The sun, not diet, is where most of your Vitamin D comes from. So even if you are eating a well balanced diet, you are unlikely to meet your vitamin D requirements.
Some people are really at risk for Vitamin D deficiency
Pregnant and breastfeeding mums
People with dark skin or those that cover their skin when going outdoors
People who spend most of their time indoors like babies, teens with too much screen time, office dwellers
People over the age of 65 - their skin has lost some of its ability to make Vitamin D.
Vitamin D deficiency, if left untreated, can result in rickets (permanent deformaties to the bones, weakened muscles and reduced growth - see pic below), fits and osteomalacia. Vitamin D may also have a role to play in making us happy, lowering blood pressure, increasing muscle strength and acting as an anti inflammatory.