This is a guest post from the good folks at Examine.com, who do a great job providing objective research-based info on pretty much all areas of nutrition and supplementation. It’s an important topic that all-too-often is misunderstood by the general public. Thanks to Sol Orwell and his team for explaining the concept in an easy-to-understand manner and exploring its practical applications. For those interested in the technical aspects, I would recommend you read the paper by Thomas et al. A Mathematical Model of Weight Change with Adaptation.

Calories in versus calories out is one of the fundamental ‘laws’ of nutrition. Though it upholds both the law of thermodynamics and the conservation of mass, the calories in versus calories out theory doesn’t hold up when applied to the human body. If anything, it is a guiding statement based on a law.

When looking at a true closed system (humans don’t count), any possible conversions of energy can be predicted down to a single joule, with nearly 100% accuracy. This is because our equations for energy transfers in closed systems are very good.

When looking at an open system, like the human body, we also have a set of equations we can rely on. Unfortunately, they’re not quite as good as their closed-system counterparts. As an example, lets look at the most commonly used equation to determine caloric expenditure and your basal metabolic rate.

The Harris-Benedict formula for men:

88.362 + (13.397 x bodyweight in kg) + (4.799 x height in cm) – (5.677 x age in years)

The Harris-Benedict formula for women:

447.593 + (9.247 x bodyweight in kg) + (3.098 x height in cm) – (4.330 x age in years)

Don’t forget to multiply by the ‘activity’ factors. Use 1.2 if you are sedentary, 1.375 for light exercise, 1.55 for moderate, 1.725 for heavy, and 1.9 for intense exercise.

The above calculations are pretty good, since the parameters being measured (gender, weight, height, age, and activity level) account for a lot of variation in the metabolic rate between individuals.

Still, it’s not an absolute rule. There are many other factors that influence the metabolic rate that are not included in the Harris-Benedict equation. These include:

• Adipokine status, including leptin, adiponectin, and resistin.

• Thyroid hormone status

• Steroid hormone status, including estrogens and androgens, as well as DHEA.

• Mitochondrial efficiency and ‘uncoupling’, also known as thermogenesis.

The formula doesn’t include these factors because it is impractical to expect people filling out a BMR equation to know their mitochondrial efficiency.

Moreover, the activity levels are vague. What feels ‘intense’ to one person could be ‘moderate’ to another. A misinterpretation of activity levels for someone with a 2000kcal intake could result in a difference of up to 700kcal.

Calories in, as a concept, is fundamentally hard to predict. Differences in food absorption, whether due to genetics, nutrient co-ingestion or even the shape of the chyme in your stomach are hard to account for. The differences in nutrient partitioning after absorption, or whether the nutrients go to muscle, glycogen or fat are even harder to predict.

Don’t forget that calorie counts are not absolute. Though a food label may list “70kcal” as the caloric value for one serving size, a more accurate description is “70kcal, give or take a 5% margin of error.”

Does this mean I shouldn’t worry about calories?

No, no, and no. Despite the above complications, calories do matter in concept, and having even a rough grasp on caloric intake is invaluable. The only change that needs to happen is how you view your caloric intake.

Instead of deciding to eat 2,357 calories a day to maintain weight, eat “a little above 2,000 calories.” Rather than deciding to cut 500 calories from your daily diet to lose a pound of fat per week, aim to eat approximately 500 less calories per day, and track how your body responds.

At any point during the day, you should be able to accurately estimate your caloric intake, as well as how many calories you have left to eat that day. Instead of expecting a perfect caloric count, aim for +/- 200 calories of the true value.

A relaxed but consistent style of nutrient tracking means you’ll be hitting your goals and stressing less about it. Best of all, you’ll never feel constricted because your favorite food would put you over your limit by 50 calories. Just remember: being relaxed doesn’t mean you’re not determined.

Written by Kurtis Frank and Dr. Spencer Nadolsky. Both are directors at Examine.com, an independent and un-biased organization that focuses on evidence for supplementation and nutrition.