Many people think that tomatoes are a vegetable. They are actually classified as a fruit, even though they do not taste quite as sweet as other fruits. Tomatoes can be used in a variety of different ways -- on sandwiches, in salads or pasta, and even eaten plain or with salt. Tomatoes can be eaten fresh, or purchased canned, year-round. The nutrition varies slightly between the two different types.

Tomatoes are low in calories and perfect for those who are watching their weight. One medium-sized, fresh tomato has about 22 calories, according to the USDA National Nutrient Database. A 3.5-ounce can of tomatoes has just slightly more calories, with 32 calories. There are slightly more calories in canned tomatoes because they are more densely packed and contain less water content.

Dietary carbohydrates are an essential part of any person's diet; they provide the body with energy. Yet eating too many calories can lead to weight gain, no matter where they come from. In a fresh tomato, there are 4.8 grams of carbohydrates in a fresh tomato and about 7.6 grams carbohydrates in a 3.5-ounce can of tomatoes. Regular canned tomatoes don't have any additives, but stewed tomatoes sometimes have additives like sugar, which can increase the carbohydrate count even more.

There are only 6 milligrams of dietary sodium in a medium-sized tomato, according to the USDA National Nutrient Database. Tomatoes are naturally low in sodium. Yet canned foods often use sodium, or salt, as a preservative and flavoring agent. The USDA National Nutrient Database states there are 243 milligrams of sodium in 3.5 ounces of canned tomatoes, and 564 milligrams of sodium in 1 cup of stewed tomatoes. The American Heart Association states that the average American eats around 3,436 milligrams of sodium every day, when he should be consuming 1,500 milligrams or less, to protect heart health.

Antioxidants are cancer-fighting compounds that are naturally present in fruits and vegetables. Lycopene is the strongest antioxidant in tomatoes. According to the November-December issue of "Natural Product Radiance," eating whole fresh tomatoes, skin, pulp and small seeds included, delivers more antioxidants than tomatoes that are cooked down and processed without the skins. This is because much of the lycopene content is present in the skin of the tomato, which is most often removed during the canning process. However, the lycopene in cooked tomato products can be absorbed easier by the body, which may make these products more beneficial if you are looking to increase your lycopene intake. According to the Pennington Biomedical Research Center, 1 raw tomato contains 4,631 micrograms of lycopene while 1 cup of tomato puree contains 54,385 micrograms.

Canned tomatoes, and other canned goods, sometimes are contaminated with harmful agents such as botulism. The USDA Food Safety and Inspection Service advises that you never buy cans that are dented or bulging, as this could be a sign the product is contaminated, and you should always wash the tops of a can before opening them to prevent contamination from any bacteria that may be on lid. Fresh tomatoes may also become contaminated and it is usually with the salmonella or E-coli bacteria. When you select fresh tomatoes, store them unwashed. Before use, wash your hands and any area the tomato will touch, then wash the tomato under warm, running water. Gently rub the tomato skin with your hand but do not use detergents. The Texas Cooperative Extension explains that you should cut the green vine end off the tomato, as this is where bacteria like to hide.