Question

What is the difference between necessary and essential nutrients?

I don't think any difference. essential nutrients is the term used more frequently by health care professionals. Se the two sites below:

http://en.wikipedia.org/wiki/Essential_nutrient

http://www.tropicana.com/TRP_HealthProfessionals/OrangeJuiceNutrition/OJN_NecessaryNutrients.cfm