Burning food seems not be accurate for estimating calories in fiber or alochol, but I'm wondering how well it works for other carbs/proteins/fats. Any pointers to literature are appreciated.
The original method used to determine the number of kcals in a given food directly measured the energy it produced.The food was placed in a sealed container surrounded by water--an apparatus known as a bomb calorimeter. The food was completely burned and the resulting rise in water temperature was measured. This method is not frequently used today.
According to the National Data Lab (NDL), most of the calorie values in the USDA and industry food tables are based on an indirect calorie estimation made using the so-called Atwater system. In this system, calories are not determined directly by burning the foods. Instead, the total caloric value is calculated by adding up the calories provided by the energy-containing nutrients: protein, carbohydrate, fat and alcohol. Because carbohydrates contain some fiber that is not digested and utilized by the body, the fiber component is usually subtracted from the total carbohydrate before calculating the calories. The Atwater system uses the average values of 4 Kcal/g for protein, 4 Kcal/g for carbohydrate, and 9 Kcal/g for fat. Alcohol is calculated at 7 Kcal/g. (These numbers were originally determined by burning and then averaging.) Thus the label on an energy bar that contains 10 g of protein, 20 g of carbohydrate and 9 g of fat would read 201 kcals or Calories. A complete discussion of this subject and the calories contained in more than 6,000 foods may be found on the National Data Lab web site at http://www.nal.usda.gov/fnic/foodcomp/. At this site you can also download the food database to a handheld computer. Another online tool that allows the user to total the calorie content of several foods is the Nutrition Analysis Tool at http://www.nat.uiuc.edu.
How Do Food Manufacturers Calculate the Calorie Count of Packaged Foods?, Scientific American, 2003.
But it is not prudent to again assume this it. It varies
Calorie Calculations in the United States
In the U.S., there are six accepted methods. The two most frequently used are the 4-4-9 formula and the Atwater method.
4-4-9. In the U.S., most manufacturers use the 4-4-9 method, which assumes that each gram of protein contributes 4 Calories to the caloric total, each gram of carbohydrates contributes 4 Calories, and each gram of fat contributes 9 Calories.
Atwater. The USDA SR database, in contrast, commonly uses the Atwater method. The Atwater method uses more precise figures based on food type when assigning Calories values per gram to protein, carbohydrate, and fat. Find the Atwater table here.
4-4-9 adjusted for non-digestible carbohydrates and sugar alcohols. (Total carbohydrates less non-digestible carbs and sugar alcohols.) For soluble non-digestible carbohydrates, a factor of 2 Calories per gram (rather than 4) is used, and sugar alcohols use specific factors listed in No. 6 below.
Specific food factors approved by the FDA.
Bomb calorimetry. This process involves burning a food item to see how much heat it releases, which is directly convertible to Calories since, as we know, one Calorie equals the amount of energy required to heat one kilogram of water by one degree Celsius. Note the adjustment for Calories from protein in the CFR.
General factors for caloric value of sugar alcohols: Isomalt = 2.0 Calories per gram, lactitol = 2.0 Calories per gram, xylitol = 2.4 Calories per gram, maltitol = 2.1 Calories per gram, sorbitol = 2.6 Calories per gram, hydrogenated starch hydrolysates = 3.0 Calories per gram, mannitol = 1.6 Calories per gram, and erythritol = 0 Calories per gram.
For more information, see the U.S. food labeling regulations here.
Calorie Calculations in the European Union
The declared values in the nutrition table are average values and must be based on:
Total value. A calculation from the known or actual average values of the ingredients used. Known data. A calculation from generally established and accepted data. Estimates. (The same concept as 4-4-9 and 4-4-9-7) The energy value to be declared shall be calculated using the following conversion factors:
carbohydrate (except polyols): 17 kJ/g (4 Cal/g) polyols: 10 kJ/g (2,4 Cal/g) protein: 17 kJ/g (4 Cal/g) fat: 37 kJ/g (9 Cal/g) salatrims: 25 kJ/g (6 Cal/g) alcohol (ethanol), 29 kJ/g (7 Cal/g) organic acid: 13 kJ/g (3 Cal/g) fibre: 8 kJ/g (2 Cal/g) erythritol: 0 kJ/g (0 Cal/g)
For more information, see the EU food labeling regulations here.
The overall accuracies of such measruemnts with a bomb calorimeter is actually a frequent question:
The calorific content is measured with a device known as the bomb calorimeter. A sample of food is placed in an airtight chamber - the 'bomb' - which is filled with pure oxygen and then placed in a tank of water. The food is ignited by an electric spark so it completely burns up. The temperature increase in the water is measured and the actual energy content of the food can then be calculated, either in old-fashioned calories or more modern joules. This method is not completely accurate, as it is rather crude when compared to the way the human body uses food. For example, proteins are completely burned up in the bomb calorimeter, whereas in the human body some of them would be used not for energy but for the production of things like skin, hair, mucus and muscle tissue. Incidentally, the subject is well covered in most biology textbooks for A-level and above, as well as in the occasional Open University programme on television.
The four sources of food energy - protein, fat, carbohydrate and alcohol - yield 4, 9, 3.75 and 7 calories per gram respectively. The calorie value of a food is usually estimated by multiplying the protein, fat, carbohydrate and alcohol content by the appropriate factors. Many food manufacturers do not carry out chemical analyses but instead estimate the calorie content using values for ingredients derived from tables published by HMSO. Such calculations are normally within 10 per cent of the actual value. How do food companies work out the number of calories in their products?
More detail can be found in this Food energy – methods of analysis and conversion factors, FAO FOOD AND NUTRITION PAPER, 77, Report of a technical workshop Rome, 3–6 December 2002.