|Unit of Weight||Approximate Calories||Approximate Kilojoules|
A pound of body fat equates to approximately 3500 calories. So if you have a calorie deficit of 500 calories (meaning that you burn 500 calories more than you eat each day) you would lose approximately one pound per week:
500 x 7 = 3,500
It's easy to see that a calorie deficit of 1000 calories would mean that you'd lose approximately two pounds per week. And that's a good number to remember, because two pounds a week is commonly accepted as the maximum rate of weight loss that is healthy. Losing weight too quickly has other disadvantages, too, and we'll be discussing this in the next topic.
As you will learn in the topic Calories in Protein, Fat and Carbohydrates, it is commonly said that a gram of fat contains 9 calories. But there are 454 grams in a pound, and 9 x 454 = 4086 calories, not 3500.
The reason for the discrepancy is that body fat, or adipose tissue, contains not only fat, but also other substances including protein, connective tissue, and water. The dietary fat referred to in the nutritional analysis of food is pure.
Looking at it another way, 3,500 / 454 = 7.7 calories. Thus a gram of body fat contains only 7.7 calories versus the 9 calories found in pure fat. It's easy to see that there should be a difference when you consider that body fat contains water, which has no calories.
Because of the differences in the two types of fat, it is appropriate to use the 3500 calories per pound figure when discussing fat "burned" by activity, and the 9 calories per gram figure when discussing the nutritional content of food.