Calculating calories in meals


Answer to Biochemistry Question N-02: (e) 500 Calories



                                      from zamber on flickr



Since usually we use large amounts of energy compared to the calorie unit used in Physics, the calorie unit used in Nutrition is different to the classical calorie definition used in Physics. While in Physics 1 calorie is defined as the quantity of energy necessary for increasing the temperature of one gram of water 1 Celsius degree at 1 atmosphere of pressure, the nutritional calorie correspond to 1000 this value (in a more accurate way, it correspond to what is called a large calorie, in contrast to the  classical “small” calorie of Physics, and should be abbreviated as Cal). It means, that 1 Nutritional calorie is equivalent to 1 kilocalorie in physics.


In the SI unit 1 physics’ calorie is equivalent to 4.184 joules, so a Nutritional calorie is equivalent to 4 184 Joules.



The body needs energy for:


1.- Mantaining Resting Metabolic Rate (RMR, former BMR)


2.- Heat production  (An important factor in heat production is the thermic effect of food, or diet induced thermogenesis, that may amount 5-10 %  of total energy expenditure/day)


3.- Physical activity



The human body obtain the energy it needs from the foods.


The major dietary energy sources:



4 cal/g  (nutritional calorie)


9 cal/g (nutritional calorie)


4 cal/g (nutritional calorie)



As you can see, fat is the most concentrated source of energy – weight for weight it provides just over twice as much as either protein or carbohydrate.


Alcohol provides almost as much energy as fat: 1 gram of alcohol can supply 7 cal/g . For some people alcoholic drinks form a large part of their energy intake. This can be harmful to health since a high alcohol consumption is a risk factor for several diseases.


The energy content of a food or drink depends on how many grams of carbohydrate, fat, protein and/or alcohol are present.


Since the source of energy in the diet has been implicated as a risk factor in certain diseases, it has been described a recommended distribution of calories in diet. The Acceptable Macronutrient Distribution Range (AMDR) has been defined as the range of intake for a particular energy source that is associated with reduced risk of chronic disease while providing intakes of essential nutrients (Dietary Guideline for Americans, Glossary . The caloric composition of diet should be approximately, 45-65 % from carbohydrates, 20-35 percent from fats and 10-35 % from proteins, but there are recommendations for specific groups.


Different websites show the caloric value of thousands of foods. This is one of those sites with several related links:



Anyway, it is important that you know the general principles described above.


USMLE released sample questions, that exemplify contents of the examination, have included questions about caloric calculation in past years (Ex: Question 45 in 2005 USMLE Step I Content Description and Sample Test, that also appears as Question 35 in the 2006 edition). Unfortunately, copyright issues do not allow us to reproduce that question here.



More information can be found in:


A classic:

Merrill, A.L.; Watts, B.K. :Energy values of food: Basis and derivations


Food composition and Nutrition Links from the USDA




2 thoughts on “Calculating calories in meals

  1. Pingback: Counting Calories in a Sandwich (Q:N-02) « The Biochemistry Questions Site

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