Caloric values for foodstuffs are often given for the raw or uncooked food. However, cooking a food can significantly alter its nutritional profile and the number of calories present in the same quantity by weight. If you are watching your calories for purposes of weight loss or weight control, it is important to consider how the method of cooking your food may alter its caloric content.
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To use the example of chicken -- breast meat only, no skin -- you can see the difference in caloric and nutritional values with different modes of cooking. According to the USDA nutrient database, 100 g of raw chicken contains 114 calories -- in a single chicken breast with no skin or bones this equates to roughly 270 calories. When the same piece of chicken is fried, its caloric value increases to 187 calories per 100 g, or 322 calories per breast. If roasted, this chicken has 165 calories per 100 g; stewed, there are 151 calories in every 100 g.
Cooking in Oil
Cooking a food in oil, butter or another fat will add to the foods caloric content. This happens because you are adding calories to the food through the addition of fats, batter or breading. The concentration of calories in fats and oils is very high, so even a small weight amount of oil or fat used in cooking can have a significant impact on the caloric value of your food. Using the example of the chicken breast, frying the raw breast adds 73 calories per 100 g -- a 64 percent increase.
When food is grilled or broiled, fat and water from the food typically drip and drain away from the food so that they are not consumed in the cooked foodstuff. MayoClinic.com recommends baking, braising, grilling, broiling, poaching, roasting and steaming as healthy cooking methods which do not add calories through cooking fat. Returning to the chicken example, roasted chicken has 165 calories per 100 g; stewed chicken has 151 calories per cooked 100 g. This is a smaller caloric increase from the raw chicken than is seen with frying.
You might wonder why, even when foods are cooked without additional fat, their caloric values per 100 g nevertheless increase slightly. One answer to this question is that water present in the raw food is often lost during cooking, and this increases the density and therefore the caloric value of the cooked food. For example, 100 g of raw chicken contains 75.8 g of water and 21.2 g of protein. When this chicken meat is cooked through roasting, water is lost so that there is 65.3 g of water and 30 g of protein present in every 100 g of roasted meat.