Some important notes
For the scientists among our readership: Throughout the introduction and infographic, ‘calories’ — lowercase ‘c’, refers to kilocalories — or ‘Calories’. Over time, popular language has lost the big C/little c distinction.
Section 1: “Calorie counts are imprecise.”
In 1896, Wilbur O. Atwater, the father of food calories, sampled hundreds specimens of food products collected at the World’s Fair. Atwater calculated the caloric value of each food using bomb calorimetry, a very accurate and precise method for measuring total energy in any object. Interestingly, a wide range of total caloric values was found, even for single food types (i.e. apples) bred, picked, and stored identically. As a result, the Food and Agriculture Organization of the United Nations said in a statement: “Foods, being biological materials, exhibit variations in composition; therefore a database cannot accurately predict the composition of any given single sample of food.” Even more interesting: Atwater’s total caloric ranges were used to produce the kcal averages still in use on labels and in databases today. (They’re over 100 years old!) For a given food these values could be up to 50% off, as outlined in the infographic.
Bottom line: The trust many of us feel that calorie labels and nutrient databases are exact (or even accurate and reliable) may be misguided.
Section 2: “Calorie counts that reflect only what we’ll absorb.”
The averages of 4 kcals per gram of protein, 9 kcals per gram of fat, and 4 kcals per gram of carbohydrates — meant to reflect how much energy we actually absorb from food since these values are lower than the energy measured in the food — are Atwater general correction factors developed in 1897 and still in use today.
Subsequently, it’s been discovered that carbohydrates high in fiber have different correction factors, depending on the type of fiber (and even your gut bacteria / microbiota). It’s also been discovered that energy absorption from protein varies. Typical absorption from animal protein is higher than the general Atwater factor — for example, 4.36 kcals per gram of protein in eggs — and lower from most vegetables — generally 2.44 kcals per gram of protein. The revised absorption averages are called Atwater specific correction factors. Calories on food labels usually use the general factors while the USDA database uses the specific factors. Further complicating the question of absorption, a new correction factor was developed to take into account the energy burned through digestion of various macronutrients.
Livesey’s Net Metabolizable Energy values are:
- protein, 3.2 kcals per gram;
- fat, 8.9 kcals per gram;
- available carbohydrates, 3.8 kcals per gram; and
- fermentable carbohydrates, 1.9 kcals per gram.
Bottom line: The idea that a gram of any protein yields 4 kcal, a gram of any fat yields 9 kcal, and a gram of any carbohydrate yields 4 kcal is a gross oversimplification that could have significant implications when trying to control and balance calorie intake.
Section 5: “People aren’t great at eyeballing portion sizes.”
Research shows the people are generally terrible at estimating caloric intake. Even trained nutritionists underestimate calories in meals by an average of 30 percent. We often get the portion sizes wrong too. When trying to serve ourselves 1 tablespoon of, say, peanut butter, we often end up getting much more than an actual tablespoon. And it doesn’t just happen once in awhile… it happens most of the time.
This article was shared from Precision Nutrition