Why a Calorie isn’t Just a Calorie

Not All Calories Are Created Equal.

A calorie is a calorie, and weight loss or gain is only a simple mathematical equation: calorie intake minus calorie output equals weight gain/loss/neutrality depending on whether the resulting equation is positive/negative/zero. Right? Wrong. I’ve heard the concept before, often from intelligent professionals who work in the field of nutrition. This old concept of nutrition is simple, elegant, and ultimately, flawed. Let’s take a look the issue through a more rational lens.

Consider the following hypothetical scenario: three women of the same age, race, height/weight, body composition, physical output, etc., each consume 2000 calories daily for 30 days. Woman #1 consumes nothing but pure table sugar, woman #2 consumes nothing but lean chicken breast, and woman #3 consumes nothing but butter (2000 calories of butter is a mere 16 tablespoons, for those who’d care to indulge). After one month of their respective diets, will their body compositions be equal? Of course not. Why? The hormonal responses to carbohydrate, protein and fat are different. There is no shortage of clinical studies to prove that calories from lean chicken breast do not equal calories from table sugar. For instance, protein has been shown to provoke a greater thermic effect of food (TEF) than either carbohydrate or fat. Simply put, in digestion, a higher percentage of protein calories are expended as body heat vs carbohydrates or fats. This has actually led some nutritional scientists to suggest that the 4 calories per gram assumed for protein should actually be downgraded by 20% to 3.2 calories per gram. In another study, conducted by Kekwick and Pawan, three groups of people were put on calorically equal semi-starvation diets of 90út, 90% protein, or 90% carbohydrate. The outcomes, of course, were clearly not at all the same:

  • 1000cals at 90% fat = average weight LOSS of 0.9lbs per day
  • 1000cals at 90% protein = average weight LOSS of 0.6lbs per day
  • 1000cals at 90% carbohydrate = average weight GAIN of 0.24lbs per day

Clearly different calorie sources produce different results.

Another consideration in this debate reflects on the age-old adage “you are what you eat.” Consider the hypothetical scenario of two men of the same age, race, height/weight, body composition, physical output, etc., who eat identical meals for 30 days. The only difference is that one of the men has a malabsorption syndrome with chronic diarrhea. Will the body composition outcomes at the end of 30 days be the same? Of course not. Why? Just because it went in your mouth, it doesn’t mean that you absorbed it into your blood. The discoverer of the ‘calorie’, 19th century chemist Wilbur Atwater, determined the amount of calories in foods using a device known as a calorimeter. A calorimeter is basically an incinerator that measures the amount of heat (in calories) given off after incinerating a substance. Wilbur incinerated foods and came up with the calorie numbers for macronutrients we assume today (e.g., 9cal/g of fat, 4cal/g of carbohydrate and protein, etc.). It doesn’t take a physician to know that incineration does not equal human digestion. I think that it goes without saying that eating a fireplace log will certainly not provide the same number of calories as burning one! So that old adage? “You are not what you eat, but what you actually absorb” fits a little better.

So there you go. Don’t let anyone tell you that a calorie just a calorie. Calories are not all created equal because calories are digested, absorbed, and utilized differently depending on a great many factors. Things that affect calorie allocation—and that can be modified for fat-loss and muscle gain—include digestion/absorption, hormonal responses to foods, the protein/carbohydrate/fat ratios, and timing of calorie intake with respect to output.