Most of us have seen the statement “calories in vs. calories out”, typically in the context of weight loss. Is it really that simple? Short answer: No, far from it. Let's dive into the topic of energy balance and review key terms to shed light on the role of calories in the human body.
What is a calorie? A calorie is defined as the unit of energy needed to raise the temperature of 1 gram of water by 1 °C at atmospheric pressure. Simply put: It’s a unit of energy. Just like we measure length in meters, we measure energy in calories (we can also measure it in Joules, just like we can measure length in feet). So, whenever somebody says, "I ate x calories " or "I burned y calories," the number indicates how much energy they took up or expended.
*Important note: Colloquially we use the term "calories," although we actually talk about kilocalories (1 kcal =1000 cal). So, "I ate 500 calories for lunch" = 500 kilocalories.
Where does the concept "calories in vs calories out" come from?
This concept is derived from the First Law of Thermodynamics, which states that in a closed system energy cannot appear or disappear, it can only change its form. Applied to the human body, it leads to imply that we store the excess energy from food: energy in (food) - energy out (exercise) = energy stored. In theory, this equation implies that we will maintain our body weight if we spend the same amount of energy that we consume. In real life, however, this equation is more complicated.
Why “calories in vs. calories” does not apply to our bodies?
Human body is not a calorimeter: We do not simply “burn through” food the way a measuring device would – we interact with it. As we metabolize the food, we break down the chemical bonds that make up the food molecules via enzymatic and biochemical reactions.
A calorie is not a calorie is not a calorie: Calories from real vs. processed foods have a different metabolic value because of the chemical composition of these foods. Respectively, our body will interact with these foods in different ways: Starch is broken down initially already in the mouth, protein – in the stomach, fats – only in the small intestine. Therefore, 300 calories’ worth of nuts will be processed differently than 300 calories’ worth of Skittles.
Individual microbiome adds variability to the digestion. While the basic physiology of digestion is relatively standard, the microbiome population inside everyone is highly individual (we covered this topic in previous articles). The microbiome is not a single organ responsible for a discrete function like, say, liver. It is a collective of diverse bacteria, all of which partake in the energy balance by interacting with the food, modulating the break down and absorption of nutrients, and themselves consuming energy.
…along with many more physiological and environmental drivers. Finally, once the nutrients are absorbed in the bloodstream, they can be consumed in a multitude of ways depending on your body's requirements and its current state. This variability is regulated by hormones, which in turn are determined by sex, age, stress, sleep, exercise, body fat, and muscle mass – to name just a few factors. The "calories out" part of the equation is therefore variable: you could do the same workout on two different days, and the calories you "burned" during the workout will differ based on how well you slept, what you ate, time of day, etc.
The body is continuously learning and adapting. Our body is not static. It’s not like a bucket that you fill up and empty. The body learned to function on less or on more. Therefore, the interrelation among the abovementioned factors can shift during the lifespan.
Remember, you eat food, not calories. You are a dynamic living system, not a calorimeter.
So, when thinking about food, energy and weight loss, instead of asking yourself “how many calories am I eating”, ask yourself the following questions:
Am I eating real food? (e.g., my food consists of few ingredients, I can name them, and they occur in nature, not in the lab)
Am I taking care of my microbiome (e.g., by eating many varieties of plants-based, low-processed and/or fermented foods)
Is my body otherwise taken care of (enough sleep, low stress, sufficient exercise etc).