I did a google search and came up with these answers, hope they help.
The term calorie as it applies to food, is a bit removed from the strict scientific definition. Physicists and chemists use the word calorie to describe the amount of energy required to heat 1 gram of water 1 degree Celsius. Nutritionists, on the other hand, say a calorie is the amount of food that would have to be burned to heat 1 kilogram of water one degree Celsius. So, a food calorie is technically a kilocalorie.
Calories Provide Energy for the Body
In the body, food is not burned by fire, but is digested, which, like burning, is a form of oxidation. Food molecules are broken down and in the process, energy is released. Food energy is used for tissue growth and repair, metabolic functions, fighting infection and for muscle movement.
Not All Calories Are Equally Nutritious
Vegetables are nutrient rich. Cookies are nutrient poor.Calories impart energy for body functions, but the body also requires nutrients. Protein, vitamins and minerals are the building blocks that the body uses to grow, maintain and heal its tissues. Certain foods, like butter or ice cream, contain a lot of calories, but relatively few nutrients. The calories in these foods are nutrient poor. Other foods, like fat-free cottage cheese and spinach, offer a lot of nutrients with relatively few calories. These calories are nutrient rich.
Unused Calories Are Stored as Body Fat
Too much butter or too many green beans--either will result in extra body fat.Whether a calorie comes from sugar, fat or protein, it delivers the same amount of energy--precisely one calorie. The body uses the energy the same way, no matter the source. Caloric energy that is not used for muscle movement or other body functions is stored as body fat. The amount of body fat is determined only by the number of calories, not by the food source of the calories. So, 500 extra calories from broccoli will result in the same amount of stored body fat as 500 extra calories from ice cream.
The Type of Food Affects Calorie Absorption
The type of food does not determine the ultimate fate of a calorie, but the molecular structure of food does affect how quickly calories are absorbed. Simple molecules like table sugar are easy to digest and will go into the bloodstream almost immediately. Therefore, a sugar calorie must be used almost immediately or be turned to body fat. Conversely, a large protein molecule is more difficult to digest. So calories from protein enter the bloodstream more slowly and can be used over a longer period of time before being stored as body fat. Nondigestible dietary fiber like that in whole grains, vegetables and fruits also slows calorie absorption in the digestive system.
Body Fat Stores Energy, Not Nutrients
Fuits, vegetables and nuts, all filled with nutrients, are good weight-loss food.Extra fat that accumulates on the body is not accompanied by a full complement of nutrients. In other words, body fat is a nutrient-poor calorie source. When you reduce calorie consumption to make your body use its own fat for energy, be sure to eat foods that are rich in protein, vitamins and minerals to make up for the nutrients not found in body fat.
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