Organic products have no significant nutritional advantage over conventional foods, even though consumers can pay more for them, a new study finds.

The findings suggest that a key reason behind why many people buy organic products — a $28.6 billion market last year — may not be borne out by the science.

The four-year project began when two doctors wondered what advice they should give their families and patients about whether to buy organic or conventional foods.

It became much larger than we expected,” says Crystal Smith-Spangler, a primary care doctor at Stanford University and lead author on the study appearing Tuesday in the journal Annals of Internal Medicine.

Eventually, 12 researchers looked at 240 studies conducted from 1966 to 2011 covering nutrient and contaminant levels in foods. The findings: