Calories in Food - How do we know how many calories food contains?

In this video I'd like to explain how we know how many calories are in different types of food. In other words, how is the energy in food measured? First, let's review what a calorie is. When we talk about how many calories a food has, we are actually talking about kilocalories. A kilocalorie is a thousand calories. When you see the word calorie with a capital C, this means kilocalories, whereas a small c would just be one calorie. If we were to talk about foods in terms of one calorie at a time, the numbers would be huge. Most people do not bother with this distinction, so just be aware that we are actually working in thousand small calorie units when measuring calories. I'll clear that up more in a bit and the links in the description will provide a thorough overview. One calorie is defined as the quantity of heat necessary to raise one kilogram, or 1 liter, of water 1° Celsius. That's 1000 grams. So, this calorie is actually a kilogram calorie hence, kilocalorie, as I explained. This is usually abbreviated k-c-a-l (kcal). A single calorie, as I said, with a small c, is the amount of heat it takes to raise ONE GRAM of water 1 degree Celsius. That's the same as 1 milliliter. The small calorie is sometimes called a g-calorie, where g stands for gram. Current US Dietary Reference Intakes define 1 cal as 4.186 Joules and 1 kilocalorie as 4.186 kilojoules. I'll provide an explanation of a joule in the description below. To measure the energy from foods, foods are burned inside a special device using a process called direct calorimetry. The device used is called a bomb calorimeter. This is a metal container inside which the food sample is burned in a sealed and pressurized pure oxygen environment. The food is burned inside the reaction chamber which is ignited by an electrified fuse running through the top of the chamber. This ignites the oxygen and food inside the chamber, which, as it burns, gives off heat which is absorbed into the surrounding tank. The reaction chamber that the food is burned inside is surrounded by a water bath. This water bath is well-insulated so that the outside environment cannot change its temperature. So the heat from the food in the reaction chamber also raises the temperature of the water to some extent, which is recorded by a highly accurate thermometer. The change in temperature of the water, once the food is combusted, is the heat of combustion or the thermal energy of the food, which gives the caloric content. Using this method, the average caloric content of a wide variety of foods has been determined. These average caloric values are then used to derive an average caloric value for different types of macronutrients, and this is how we decided average caloric value for proteins, carbohydrates, and fats. Now, if we were only to consider the thermal energy of the macronutrients, the numbers would be higher than the ones we are used to hearing. For instance, proteins are generally 5.65 Calories per gram. This is higher than the 4 grams you've probably been taught. See, the thermal energy is the gross energy value of the food. We also have to consider the net energy value for humans. As I described how calories were measured, you may have been thinking that your body does not ignite food and burn it to ashes. You're right! The fact is that we are not capable of deriving ALL of the energy from food. Digestion is not one hundred percent efficient so we cannot digest one hundred percent of the foods we eat in order to extract energy from them. The efficiency of digestion for different foods is called the coefficient of digestibility. This is usually given as a percentage value. For example, meats and fish, on average, have 97% digestibility. Also, once the nutrients are absorbed, some of the energy is lost in the process of metabolism. This is especially true of protein, since the body cannot oxidize the nitrogen component, the nitrogen must be combined with hydrogen to form urea, which is excreted in the urine. This loss of hydrogen, to deal with the nitrogen, represents a loss of energy, which averages about 19% of a protein molecule's energy lost. Because of all this energy loss, the gross energy value, as measured with direct calorimetry, and the net energy we humans can derive, will differ. Well not all lipids have the same amount of energy. A gram of pork or beef fat, on average, yields about 9.5 kilocalories gross. And 9.5 is also the average for lipids from fish or eggs. Butter fat yields about 9.27 Calories and dairy fat gives about 9.25. The average for lipids from vegetables and fruits is about 9.3. All of this makes the average heat of combustion for all fats about 9.4 kilocalories per gram. The net energy for humans is the same. We usually round this down, so you can consider the net Calories from fats to be 9 Calories per gram. Carbohydrates give much less energy per gram than fats. Gross energy from glucose is 3.74 kcal per gram, and 4.2 for starch. The average for carbohydrate is given as 4.2 kcal per gram gross energy, and the net energy in humans is the same. The actual amount of energy from any carbohydrate varies depending on the shape of the molecule. The calories from carbohydrate is usually rounded to 4 calories per gram. Energy from protein can depend on the amount of nitrogen the protein contains, as well as the digestibility of the food. The higher the nitrogen content, the lower the amount of energy that can be derived through human metabolism. Proteins from meat, eggs, beans, and corn have about 16% nitrogen. Protein from nuts and seeds, and most grains (cereals) have a higher nitrogen content of around 18.9%. Protein from milk has a lower nitrogen content of about 15.7%. The average heat of combustion for protein is 5.65 kcal per gram. The average net energy for humans is given as 4.2 Calories per gram, the same as for carbohydrate. This, again, is usually rounded to 4 Calories per gram. Alcohol actually has higher Calories than most realize. Alcohol, or ethanol, yields around 7 kcal per gram through direct calorimetry. The net energy for humans is usually the same. If, however, a large amount of alcohol is consumed, it is possible for less energy to be available to the body, which may be due to damage to the mitochondria in the liver cells.