Vitamin D is essential because it helps us absorb calcium from food, which helps us build strong bones, helps us grow and is great for teeth formation.
A deficiency in vitamin D can lead to a number of things, osteoporosis, depression, weak bones, the softening of bones, obesity, severe asthma in children, as well, a lack of vitamin D can lead to a higher risk of cardiovascular disease, so you want to make sure you're getting enough.
One good way of getting enough vitamin D is by taking supplements, this is especially good for busy people who just find it difficult to get vitamin D in other ways.
Another way is just to go outside and be in the sunlight, your body naturally produces vitamin D through being in the sun, the recommended exposure is 5 to 30 minutes about twice a week with 15 SPF or less.
The best dietary sources of vitamin D include fatty fish such as sardines and mackerel, cod liver oil and egg yolks.
Since vegans and strict vegetarian do not eat animal products, for them a really great idea is to take a vitamin D supplement, that way they are not at risk for a vitamin D deficiency.