Everyone should have their vitamin D level checked at least once a year. If they are deficient their doctor can recommend a supplement to increase their level. Even though vitamin D is supplemented in milk, many people do not drink enough to get the amount they need. The other main source of vitamin D is from sunlight. Since people in northern countries have less sun exposure during the year they are especially prone to deficiency. Widespread sunscreen use has contributed to low levels of vitamin D also.
In learning about nutrition, you will need to choose between two approaches. One approach bases its findings entirely on studies of the physical body. The other also includes the mind or spirit as a factor. It says that we transmute, to some degree, the physical substances we take in--in accord with our mental or spiritual state.
Have a doctor check your blood for levels of various minerals and vitamins in your blood stream. Ask for a complete check. That way, if you find you are deficient in any one thing, or a few things, you can immediately start taking steps to change your diet so you stay well and healthy.