The main health benefit of locally grown foods is that they are fresher. Fruits and vegetables begin to lose their nutrients within 24 hours after they are harvested, so fresher produce is more nutritious. In addition, locally grown foods are harvested at their peak of maturity, when they are most nutrient-dense. At present, there is no officially agreed distance for what constitutes eating locally.
This leaves consumers to decide what “local” food means to them. For some people, anything that occurs within a 150-mile radius is considered local. For others, it means anything harvested in the same county. Most of the local products you'll find at farmers' markets have been picked up within 24 hours.
In addition to the advantage of freshness, producers who serve local customers are not limited by problems with harvesting, packaging, transportation and quality of shelf life. Instead, they have the freedom to select, cultivate and harvest their products. In this way, they can guarantee the highest qualities of freshness, nutrition and flavor. Rich Pirog, from the Leopold Center for Sustainable Agriculture, reports that the average fresh food travels 1,500 miles to reach our table.
That's a carbon footprint big enough to leave behind a little piece of garlic. Eating locally first means choosing foods that are grown and harvested close to where you live and then distributed over shorter distances than is often the case. This type of food consumption is the basis of the popular 100 mile diet, which promotes the purchase and consumption of food grown, manufactured, or produced within 100 miles of the consumer's home. For more information on the topic, check out this great reference from Harvard Medical School entitled Healthy and Sustainable Food.