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Let’s Talk About Calories

Why talk about calories?  Because they are used to define weight management goals and methods.  And I am going to suggest that they are getting too much attention and that they really don’t fit into the metabolic processes of the human body.  So let us start out by defining what a calorie is.

The calorie is a concept from physics.  It is the amount of heat necessary to raise one gram of water one degree Centigrade.  To further complicate the issue, what we refer to as a calorie is actually a kilocalorie:  one thousand calories.  How do they determine the caloric content of different foods?  They take the food, burn it up, and measure the amount of heat produced in the burning.  The amount of heat generated is the caloric content.

The focus on calorie content as a driver of weight loss or gain is actually something that’s only been around for about 50 years:  basically, medicine started thinking about it once obesity started increasing.  Physiology (a word that encompasses all the processes and reactions that describe how our bodies work) doesn’t work on calories.  When you look at the metabolism of carbohydrates, proteins, and fats, which we all had to learn in pre-med, calories don’t appear.  The unit of energy generated by the metabolism of these macronutrients is ATP, adenosine triphosphate.  It is STORED energy.  It becomes energy EXPENDED by losing a phosphate (called hydrolysis) and turning into ADP, adenosine diphosphate. 

So how much ATP equals one calorie (or Kcal)?  I looked it up on line, and got conflicting answers.  But I did find that one gram of glucose produces 4.1 kilocalories of stored energy.  Then I tried to find out how many molecules are in a gram of glucose but couldn’t find that out.  Instead, I got the number of molecules in 1.53 grams of glucose (there are 1.53180 moles, and one mole equals 6.02 x 10 to the 23 power molecules).  So an educated guess would be that one gram of glucose could equal one mole of glucose (maybe).  So a whole lot of molecules.

Now let’s go back to the adenosine triphosphate (ATP).  When one molecule of glucose goes through metabolism, it makes 38 molecules of ATP.  And when one molecule of ATP hydrolyzes into ADP, it releases 7.3 kilocalories of energy.   So going back to the 38 ATP made from one molecule of glucose, we can guess that hydrolyzing the 38 molecules would release 277.4 kilocalories.  Which is close to what I expend in an hour on the treadmill.  And that’s just one molecule of glucose.  Sorry to let you know this but exercising will not, by itself, make you lose weight.

The main point of this is that calories really have nothing to do with metabolism.  It’s a physics concept, nothing to do with physiology.  It’s quite unclear to me how the calorie reached such an exalted position in weight management.

So why did I confuse half of my audience with the details?  It’s because a major obesity myth is that a calorie is a calorie.  We say a calorie of donuts is the same as a calorie of steak which is the same as a calorie of butter.  It doesn’t matter what you eat as long as you keep the number of calories steady.  You go on a 1500-calorie diet and it doesn’t matter if you eat nothing but donuts as long as you keep to 1500 calories.  This is a very prevalent myth.  Even diabetes specialists are telling their patients that it’s OK to eat sugar.

So if the endocrinologists believe it, why do I say it is a myth?  Let’s start with the donuts.  Donuts are carbohydrates, and carbohydrates break down into sugar.  Even worse:  donuts are made of refined carbohydrates so they break down into sugar very easily, so easily that the breakdown begins with the saliva in your mouth, well before the donut reaches your stomach.  Further breakdown happens in the stomach, then it is even more easily (and rapidly) absorbed into your bloodstream in your small intestine. The rapid absorption makes your blood sugar (or glucose) level spike, followed by the release of insulin and a blood glucose drop which means that you are hungry long before it’s time for your next meal.

Proteins and fats are more complex structures and are not absorbed as rapidly or easily. They raise your blood glucose too but it takes longer AND they cause the release of satiety hormones like cholecystokinin and Peptide YY, hormones that tell you your stomach is full.  So you get full more easily and it takes longer to get hungry.  The donuts do not trigger satiety hormones; you can just go on eating them and eating them.  If you’re really sticking to that 1500-calorie diet, you can resist eating past your calorie limit,  but you will be hungry sooner.

Another example:  let’s compare eating a calorie of sugar to a calorie of olive oil and their effects on insulin secretion.  While both the sugar and the olive oil will increase the blood glucose level, the sugar will make the insulin level spike.  The olive oil will . . . not.

Another physics concept applied to weight loss is the First Law of Thermodynamics:  energy within a system cannot be created or destroyed or more simply:  energy in equals energy out.  In weight loss, it has been assumed that Calories In minus Calories Out equals Body Fat.  To prevent that body fat, we have to decrease the Calories In and increase the Calories Out.  But it doesn’t quite work that way.

One reason the Calories In/Calories out equation doesn’t work is because it looks at the body as one system within which energy must balance.  But the body is made up of multiple systems:  the heart has its own system, the liver has its own, the kidneys have their own, the brain, the muscles, the peripheral nerves, the adrenal glands, the thyroid and so on and so on.  Some of the systems work together, like the kidneys and the adrenal glands work together to control blood pressure, but the point is they are all separate systems in which ATP is produced and used.  Calories In and Out don’t have much effect on what’s going on in the individual systems. 

An astute member of my audience will point out that if you go on a 1500-calorie diet, you will lose weight.  And it’s true:  you will lose weight.  For about six months.  Then the weight loss begins to plateau and you stop losing weight as fast, then eventually you stop losing weight at all.  To get more weight loss, you have to decrease your calories even further which just makes you hungrier.

What is going on here?

It’s the Basal Metabolic Rate, the subject of my next blog.

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