Nutrition is an ever-changing science with one consistent message: If you are above a healthy body weight, lose weight. The World Health Organization, the National Academy of Sciences, and many other organizations consider weight loss to be the first line of action for decreasing the incidence of diseases such as high blood pressure, cardiovascular disease, and diabetes. In 2007, the American Diabetes Association stated in its clinical practice recommendations that for people who already have Type 2 diabetes, even moderate weight loss (on the order of 5% of body weight) can improve insulin sensitivity, lower fasting blood glucose levels, and reduce the need for medication.
Yet, while the message promoting weight reduction for those who are overweight is quite clear, there is no consensus on the best way to achieve weight reduction, even among health professionals. There is evidence that supports high-protein, low-carbohydrate diets, and there is evidence to support low-protein, high-carbohydrate diets. But no matter what approach is recommended, there is one thing that is clear: To reduce body weight, you must reduce the number of calories consumed.
According to the North American Association for the Study of Obesity and the National Heart, Lung, and Blood Institute’s Obesity Education Initiative, for weight loss to occur, caloric intake should be reduced by 500–1,000 calories per day from what a person is currently consuming. Eating 500 calories less per day will result in a weight loss of 1 pound per week, while eating 1,000 calories less per day will result in a weight loss of 2 pounds per week. However, it’s important not to cut back on calories too much. In general, for weight reduction, the Obesity Education Initiative recommends that most women consume at least 1,000–1,200 calories per day, and that men and women who weigh more than 165 pounds consume 1,200–1,500 calories per day.
What are calories?
Technically, a calorie is a unit of energy. One calorie from food is the amount of energy needed to raise the temperature of 1 gram of water, at 15°C, by 1°C. In more practical terms, calories in food provide the energy needed for humans (and other animals) to breathe, eat, sleep, walk, and perform other activities. The number of calories a person needs to maintain his body as is (with no weight gain or loss) depends on his basal metabolic rate, or the amount of energy he expends at complete rest, as well as the number of calories he burns through activity. To find out approximately how many calories you need each day, see “How Many Calories Do You Need?”
Once you know how many calories you need each day to maintain your body mass — or to bring about weight loss or weight gain — you need to know the calorie values of the foods you eat. Scientists use what’s called a bomb calorimeter to determine the calorie composition of foods. This device consists of a closed container in which a weighed food sample is burned in an oxygen atmosphere by ignition with an electric spark. The container is immersed in a known volume of water, and the rise in the temperature of the water is noted to determine how much heat is generated.
Energy values of food can also be determined using chemical tests and analyses of recipes to estimate a product’s digestible components. This is how the values on food labels are arrived at, because burning food overestimates the amount of energy the human digestive system can extract from the food.