Tell me what happens if you buy an un-seasonal food. All seasonal foods are not fresh. Because, the traders to keep them fresh use artificial methods of preservation. It results in robbing awaythe nutrients in the fruits. So don't take it for granted all seasonal foods are fresh. Foods that are unseasonal, but available in the market are not good for health.
This aspect has been specifically mentioned in the link article.
Regional healthy foods are seasonal, as they appear in particular time of year and disappear again, and of most of famous healthy foods which are available everywhere, are not fresh but still health benefits must be there.
