Download The Remarkable Calorie

The Remarkable Calorie
By Carole A. Conn, Ph.D., R.D. & Len Kravitz, Ph.D.
Introduction
Energy represents the ability to do work. Playing sports, running around, and even walking to class are
examples of activities in which foods are being converted to chemical energy in the muscle cells and then
transformed into mechanical energy for the physical exercise or movement. In the United States, the most
common term used to express energy is the calorie.
The number of calories is listed on the label of any energy bar you pick up. Broccoli has calories even
though it has no label telling you how many. Somewhere along the line most people learned that calories
are used by the body for energy and if you eat too many you get fat and if you eat none you eventually
starve. But have you ever wondered just what a calorie is, how it got into your food, and how your body
goes about using it? This article will review these aspects of the remarkable calorie.
Just what is a calorie anyway?
A calorie is a measure of energy. It is defined as the heat energy required to raise the temperature of
one gram of water by one degree Celsius. The energy used in physical activity and the energy stored in
foods is actually given in kilocalories (the heat energy required to raise the temperature of one kilogram
of water by one degree Celsius). Often kilocalories are referred to as kcals or as large calories or as
Calories, where the capital ‘C’ indicates kilocalories. However, because a calorie is such a small unit of
energy the word ‘calorie’ to define a small calorie is mainly used in scientific literature. Most of the time
‘calorie’ spelled with the small ‘c’ actually refers to the kilocalories provided in food and used during
exercise. In this article, we follow the common custom and use calorie to refer to kilocalorie. (The prefix
“kilo” means 1,000, so one kilocalorie equals 1,000 calories).
Why do foods have calories?
Foods have calories because foods come either from plants or from animals that have eaten plants. It is
actually the plants that create the primary molecules in food that contain the energy quantified as
calories. Green plants create these molecules from carbon dioxide and water by capturing energy from
the sun in a process called photosynthesis. The green plant pigment chlorophyll absorbs energy from the
sun which is then converted to chemical energy (food) in the bonds that link the carbon from carbon
dioxide (CO2) to water (H2O), creating glucose (C6H12O6) and releasing oxygen (O2) into the atmosphere.
From glucose and other carbohydrates, plants can create other molecules that contain captured energy;
these are fats and proteins. Humans can use carbohydrates to synthesize most fatty acids, fats, nonessential amino acids and proteins just like the plants. However, the primary source of all calories are
carbohydrates created by plants from carbon dioxide and water by capturing the energy of the sun.
Why do foods have different calorie levels?
There are six classes of nutrients in foods: carbohydrates, fats, proteins, vitamins, minerals and water.
Only the carbohydrates, fats and proteins can provide energy. Because these three classes are consumed
in large quantities in the range of 50 to 500 grams per day, they are called macronutrients. In contrast,
the micronutrient classes of vitamins and minerals need to be consumed in very small quantities of 1 to
100 milligrams per day. Vitamins, minerals and water provide no calories but they are essential in our
ability to use the calories stored in the macronutrients.
Most foods are mixtures of some or all of the six classes of nutrients, and different foods contain different
amounts of each class. For example, butter contains a lot of fat, a little protein, vitamins, minerals and
water, but very little carbohydrate. Meat contains a lot of protein and water, some fat, vitamins and
minerals, and little or no carbohydrate, while whole wheat bread contains a lot of carbohydrate, a little
protein and fat, many vitamins and minerals, but not much water. So part of the reason that foods have
different calorie levels is that a usual serving of each food contains different amounts of the three classes
of the energy-providing nutrients: carbohydrates, proteins and fats.
Another part of the reason that foods have different calorie levels is that the energy-providing nutrients
supply different amounts of energy per gram. Fats supply the most energy at 9 calories per gram.
Carbohydrates and proteins each provide 4 calories per gram for use as energy in the body. We know this
because of the careful work of W. O. Atwater and his colleagues done in the late 1800’s. These scientists
pioneered the analysis of the nutrient classes in foods and the differing ability of each macronutrient
class to supply energy (Merrill and Watt, 1973). From their work, we know that more calories will come
from the peanut butter, which is higher in fat content, than from the jelly, which contains more
carbohydrate, on your ‘P B and J.’
How do the calories in food become available for use by the body?
The energy stored in the carbohydrates, fats and proteins in foods becomes available to the body when
the energy stored in the chemical bonds of the macronutrients has been transformed into the highenergy phosphate bonds that are usable in all of the chemical reactions that occur in the body
(metabolism!). The main molecule that carries these high-energy bonds is adenosine triphosphate (ATP).
The transformation of food in the mouth to ATP in the muscle involves digestion, absorption and cellular
respiration. Digestion results in the breakdown of carbohydrates into the simple sugars called glucose.
Proteins in food are broken down to amino acids and dietary fats to fatty acids and glycerol. These small
molecules are absorbed by the cells lining the intestines, passed into the bloodstream and then circulate
in the blood until they enter the cells of the rest of the body. Creation of ATP from cellular respiration of
glucose, fatty acids and amino acids occurs within each cell. ATP is composed of high-energy bonds
which, when split with the help of enzymes, releases energy for use by the muscles for movement, by
the liver for protein synthesis, by the brain for neural transmission and by all of the body’s
metabolic systems that need energy. So, it is important to emphasize that the energy that is released
during the breakdown of food is not directly used for exercise, but to manufacture ATP. ATP is often
referred to as a high-energy compound that is stored in small amounts of the tissues.
Questions next page 
Questions
1.
What is the definition of energy? What types of activities require energy? (Think of some of your
own too!)
2.
All foods have calories. What will happen if you eat enough calories? What will happen if you eat
too many calories?
3.
What is the definition of a calorie (be as detailed as you can)?
4.
What is the difference between a Calorie (capital C) and a calorie (lowercase c)?
5.
If we trace the energy (calories) we get from eating food all the way back to where it came from
originally, where will we end up?
6.
How do plants convert sunlight energy into chemical (food) energy? (Name the process).
7.
What are the six classes of nutrients in foods?
8.
Which three classes can NOT provide energy (they have no calories!)?
9.
Which three classes DO provide energy (calories)? What are these called?
10.
Why do different foods contain different calorie levels?
11.
Which part of your ‘PB & J’ has more calories, the PB or the J?
12.
What is the term used for all of the chemical reactions that occur in your body?
13.
What is the name of the molecule that carries high-energy bonds for the cell to use? Please write
the full name as well as the abbreviation.
14.
What types of processes in the body (and cells) is ATP used for?
15.
Please bring in a ‘Nutrition Facts’ label from a food item. Below write down the name of the food
item and how many calories it has. Also write down the ‘calories from fat.’