I’m a firm believer in the laws of thermodynamics in regards to weight gain/loss.  If you are at an energy surplus (you consume more than you burn) you’ll gain weight and if you do the opposite, you lose.  That said, thousands of people rely on food labels and calorie counting apps to determine calories ingested and the science behind those numbers is about 130 years old and far less accurate than most folks realize.

First, some boring science stuff. Here’s how calories are calculated (taken from an article written by journalist Dina Spector)

At its most basic, a calorie is a measure of energy. One Calorie (equal to one kilocalorie, or 1,000 calories) is the amount of energy that is required to heat one kilogram of water 1 degree Celsius at sea level.

The energy content of food was traditionally measured using a bomb calorimetry. A sample of food, for example a small piece of a hot dog, is placed into a metal vessel called a bomb. The bomb is filled with oxygen and placed inside a container where it is surrounded by water.

Then, the sample is ignited by a current of electricity. The water chamber absorbs the heat that is released as the food sample burns. A thermometer measures the rise in temperature of the water.

Since a Calorie raises the temperature of 1 kilogram (1 liter) of water by 1 degree, the calorie count is found by calculating the change in temperature of the water multiplied by the volume of water.

Manufacturer’s do not use this method to calculate the amount of calories in their processed foods.  Instead they rely on in indirect method developed by Wilber Olin Atwater around 1900.  Mr. Atwater was a brilliant man and conducted hundreds of experiments with a respiration calorimeter to measure metabolism in human’s and animals. He started his experiments around 1896, a time in which malnutrition rather than obesity was a huge concern. His work had a huge influence on American life as people began to think of the “energy” in food as a calories based on burning the food and calculating our how much energy it produced. The Atwater system uses the average values of 4 Kcal/g for protein, 4 Kcal/g for carbohydrate, and 9 Kcal/g for fat. Alcohol is calculated at 7 Kcal/g. Over a hundred years has passed since these numbers came into practice and though scientists now know they aren’t as accurate as most consumers believe, a viable alternative has not been adopted.

Here’s a real world example of the Atwater System: The label on an energy bar that contains 10 g (10×4=40) of protein, 20 g (20×4=80) of carbohydrate and 9 g (9×9=81) of fat would read 201 (40+80+81) kcals or Calories.  However, the Nutrition Labeling and Education Act of 1990 (NLEA), which dictates what information is presented on food labels, allows manufacturer’s to be off by 20%.  This means the 201 calorie energy bar might be actually have anywhere from 160 to 240 calories.

In an example of how the Atwater system itself misleads consumers. scientists have shown that foods such as nuts have about 20% fewer calories than indicated by Atwater calculations.   This is the case with most natural foods: their calorie content is underestimated. Unfortunately, the calorie content of most processed, highly palatable “franken food” is overstated.

So we’ve established that the way calories are calculated isn’t necessarily accurate, but there is even more error in how calories are absorbed.  No matter how efficient your digestive tract is, you don’t absorb every calorie from the food you eat and the amount you do absorb varies depending on the type of macronutrient. Of the three macronutrients, carbohydrates are the most completely absorbed and protein the least. Research shows 98% of the calories in carbs are taken in and used by your body, 95% of the calories in fat, and only 92% of the calories from protein makes it past your digestive tract. So, you’re shaving off a few calories simply because digestion and absorption aren’t perfect.  The difference in the macronutrient absorption isn’t limited to percentage, there is also a difference in how quickly they are absorbed. Highly processed carbohydrates are swiftly absorbed into the bloodstream providing a fast shot of energy and a more pronounced release of insulin, the hormone that carries the sugar out of the bloodstream and into the body’s cells.  The liver can store some of the excess, but any that remains is stashed as fat.  So consuming large quantities of highly processed, hyper-palatable foods is the fastest way to create body fat.

With plant-based foods, the absorption issue is very unreliable and  there’s no insulin spike. Many vegetables and fruits have hardy cell walls that are tough to break down. If your digestive tract can’t crack open the cell walls, the inside of the plant cells, where all the calories are, can pass through your digestive tract without being absorbed. You see this mostly with raw foods. Cooking helps to break down stubborn plant cell walls so more of the calories inside are absorbed. That’s why eating more raw foods could theoretically be a strategy for weight loss. In addition the calorie load of carbohydrate rich items such as rice, bread and pasta can be slashed simply by cooking, chilling and reheating them. As starch molecules cool they form new structures that are harder to digest. Scientists in Sri Lanka discovered in 2015 they could  more than half the calories potentially absorbed from rice by adding coconut oil during cooking and then cooling the rice.

Genetics also plays a part in how calories effect someone. You’ve probably sensed this intuitively, but it’s true that some people have a harder time losing (or putting on) weight than others.  According to a study published in the Feb 2014 issue of “Current Opinion in Lipidology”, 40-60% of BMI variance can be accounted for by genetic factors. Some people’s intestines are 50% longer than others: those with shorter ones absorb fewer calories.

Your microbiome is another factor in regards to calorie absorption. Researchers have been studying the gut microbiota of rodents and found huge differences in their body composition based on their manipulated gut bacteria even when they were fed the same chow.  Researchers are also starting to isolate specific genes and bacteria in the gut that contribute to food absorption and obesity.

Our obsession with counting calories assumes both that all calories are equal and that all bodies absorb calories in the same way.  The experiments that Atwater conducted a century ago, without calculators or computers, have never been repeated even though our understanding of how bodies work is vastly improved. There is little funding or enthusiasm for industry to change, and the current system pretty much lets food manufacturer’s off the hook. They get to report the “calories’ and you manage your weight. In addition, a growing body of research shows that that when different people consume the same meal, the impact on each person’s blood sugar and fat formation will vary according to their genes lifestyles and unique mix of gut bacteria.   That said, here are a few guidelines to help your optimize body composition and nutrition:

• Avoid highly processed foods.  Eat stuff from a real plant, not an industrial plant.
• Realize that the 4-4-9 at the heart of the calorie counting system are an oversimplification.
• Assume the calorie count on package food is 20% understated.
• Eat foods in as natural state as possible.  Eat an orange instead of drinking orange juice.
• When trying to lose weight, don’t be obsessed with counting calories to the point of making poor food choices.   Eating a handful of nuts with a 170 calories is a better choice than a 100 calorie Oreo pack.

Eat for health and focus on making your calories count, not counting your calories.