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When you buy canned fruits and vegetables, you're getting lower quality food that lacks in nutrition and is usually loaded with salt and preservatives. On a scale of healthiness and freshness, fresh fruits and vegetables are best, followed by frozen foods and then canned. Last on the list, canned items are slowly becoming a thing of the past.
When foods are canned, they go through a cooking process that destroys up to a full half of the vitamins A, C, riboflavin and thiamin. And sitting on shelves in storage, the deterioration continues with the foods losing an additional 5% to 20% of these vitamins. The remaining vitamins only decrease their values slightly.
The thing to remember is that everything depends on the time between the harvesting and the canning and freeing process. Generally, the vegetables are picked immediately and taken to canning or freezing divisions when their nutrient contact is at its peak. How the food is canned affects the nutrient value also. Vegetables boiled for longer than necessary and in large amounts of water lose much of their nutritional value as compared to those only lightly steamed.
Another real concern with canned foods is the presence of Bisphenol A - a chemical used in plastics and also a part of the lining of virtually all food and beverage cans. While originally determined safe for humans in small amounts, recent studies have found that people are absorbing much higher amounts than previously thought. This can have dangerous effects on developing fetuses, small children (especially those who use plastic bottles and sippy cups), and in general can have possible effects on obesity, the thyroid and reproductive organs. The US National Institute of Health has expressed concerns about neurological effects, also. In general, it's just another reason for canned foods to be avoided.
When we pick fresh vegetables or fruit at the farm, they are always more nutritious than canned or frozen – this is a fact. If you cannot buy fresh, at least buy frozen.