A calorie is a calorie, or is it?

We've all heard the old axiom "a calorie is a calorie."  This simple fact leads to a simple weight loss solution: Consume fewer calories than you burn.  Seems straightforward, right?  Unfortunately, in real life, and particularly in biology, things are rarely that simple.  The problem with the "a calorie is a calorie" concept is that while a calorie itself has a clear definition, (one calorie is the amount of energy necessary to heat 1 mL of water up 1 degree Celsius), the question of how one measures calories with regards to human metabolism is a tricky business. Understanding the limitations of the calorie system and the difference between "good" and "bad" calories can help you make informed decisions that will better help you reach your goals.

People often talk about "burning" calories as if there is some sort of internal calorie-powered steam turbine in each of us.  I think that most people realize, or at least I hope, that nothing is actually burning inside your body.  If it was, you would know.  It would probably hurt.  However, when scientists are assessing the caloric value of a food, they do just that, they burn it.  A defined sample of the material is ground up and burned in a device called a bomb calorimeter (shown at right). The calorimeter measures the amount of heat that is generated and calculates the number of calories in that food.

In contrast, in the human body large molecules are broken down into smaller components, which are then absorbed.  Absorption of nutrients happens primarily in the intestines.  Generally speaking, your body has a very difficult time absorbing large molecules, so if something isn't fully digested, it isn't available for use as energy by your body.  The Diabetes center at UCSF made a nice diagram to illustrate this process (below).

Let's say you have a delicious baked potato, with a dash of chives and a dollop of butter, of course.   Once you've finished enjoying masticating and ingesting that potato, it travels down to the stomach, where it experiences some preliminary digestion in the acidic environment of the stomach.  That breaks the potato down into medium sized molecules, which are represented in our illustration as "complex starch".  If you were to take a really close look at these complex starch molecules, they would look somewhat like a snowflake, branching and interconnected chains of starch forming all sorts of complex shapes.  These large structures are too big to be absorbed by the nutrient absorbing cells that line the intestines. In the intestines, enzymes called "amylases" break down these large complex starch molecules into their individual components, sugars like glucose. These individual sugar molecules are rapidly and efficiently absorbed by the intestines and pumped into your blood stream, where they travel throughout your body.

But what does all this mean when it comes to counting calories and understanding the difference between "good" and "bad" calories?  It is important to understand that in nature, like in life itself, nothing is truly free.  It takes energy to maintain all of the components of the digestive system: the digestive organs, the enzymes, the digestive cells, ion balances, sugar transportation, etc.  The pieces in this complex machine are constantly breaking down and in need of repair or replacement.  As a result, your body places most of its emphasis on processing foods and nutrients that are the easiest to digest and have the most caloric value.  It's the age old story of the lowest hanging fruit getting picked first.  Often times, things that are difficult to digest just don't get digested at all.  Think about the last time you had some corn on the cob or whole grain brown rice, notice anything afterward? Some undigested material perhaps?

What it all boils down to is this: Certain foods have much lower available calories than others, regardless of what the label says.  On the other hand, with some foods virtually all of the calories will be digested and absorbed by your body.   The difference is based on almost entirely on two things.  First, the source of the calories: Protein, fats, complex carbohydrates or simple carbohydrates (sugars).  Second, is the structural complexity of the food itself.  There is a huge difference between an ounce of whole grain wheat, and an ounce of whole grain wheat flour.

Because the wheat flour has already been processed into a fine powder, it is more efficiently digested by the enzymes in the gut and a greater percentage of the available calories are absorbed by your body.  This simple fact is why virtually every single diabetes treatment plan includes substituting whole grain foods for processed grain foods.  It is also the basis for their different values in the glycemic index, which measures the impact of different foods on the levels of glucose (sugar) in your blood.

The take home message here is simple: If you want to control your weight, the first step is to replace foods that are highly processed and rich in simple carbohydrates with foods that are less processed like whole grains, meats, eggs, fruit and vegetables.  It's a good dietary decision for people in general, but absolutely critical if you want to control your weight and body fat percentage.

References, Background Reading and Additional Information:


Books:


Good Calories, Bad Calories: Fats, Carbs, and the Controversial Science of Diet and Health
Gary Taubes


Nutrition: Science and Applications
Lori Smolin


The Biggest Loser Cookbook: More Than 125 Healthy, Delicious Recipes Adapted from NBC's Hit Show
Devin Alexander, Karen Kaplan and Bob Harper


Online Resources:


A calorie is Not a Calorie - An in-Depth Review


Carbohydrates - Wikipedia


The Glycemic Index - Wikipedia


The Science of Acne, click The Science of Acne.




Scientific Research Articles:


Association between Dietary Carbohydrates and Body Weight.  Ma, et al.  American Journal of Epidemiology. 2005


Whole Grain, Bran, and Germ Intake and Risk of Type 2 Diabetes: A Prospective Cohort Study and Systematic Review. Munter, et al. PLoS Medicine.  2007