You look at a yogurt container and see a clear number: 150 calories per serving. It looks like mathematical certainty — you add up all your meals for the day and you know exactly how much energy you consumed. Unfortunately, reality is much more complicated. The number on the label doesn't actually tell you how much energy your body will really get from the food — it tells you how much energy would be released if you burned it in a laboratory furnace. And your body is not a furnace.
There's a fundamental difference between "burning in a furnace" and "digesting in the stomach," and it's called bioavailability. It's precisely this concept that explains why two people can eat the exact same meal and their bodies end up extracting completely different amounts of usable energy from it.
What is gross energy and why do food labels rely on it
The values on food labels are calculated using a method that comes from chemistry, not physiology. A food sample is burned in a device called a calorimeter, and the amount of heat released during combustion is measured. This "gross energy" is then converted using general coefficients (typically 4 calories per gram of protein and carbohydrates, 9 per gram of fat), and the result is printed on the package.
The problem is that the human body doesn't work like a combustion chamber. Before energy from food reaches the bloodstream, it has to go through digestion, absorption in the intestines, and metabolic processing in the liver and cells. Each of these steps either "steals" some of the energy or prevents it from being absorbed at all.
Bioavailability: what determines how much energy the body actually gets
Many factors affect how much energy the body actually extracts from food. Two of the most significant are the way the food is prepared and the thermic effect of food.
How food is prepared
Cooking, blending, baking, or chopping changes the physical structure of food. Heat treatment and mechanical processing break down the cell walls of plant foods as well as connective tissue in meat, which makes it significantly easier for the body to access the nutrients inside.
- Raw vs. cooked vegetables: The body gets considerably more usable energy from cooked or blended sweet potatoes than from the same amount of raw, crunchy pieces, because with raw food the digestive system has to work its way through tough cell walls, without always succeeding completely.
- Blending fruit: A smoothie is absorbed faster and differently than a whole piece of fruit, even with the same composition — the fiber is partially broken down and the sugars enter the bloodstream at a different rate.
- Grinding grains and legumes: Finely ground or long-cooked grains are more digestible than whole, hard varieties, where the body either has to "wait" longer for the nutrients or some of them pass through undigested.
Thermic effect of food (TEF)
The second key factor is the energy the body spends on digesting, absorbing, and metabolizing the food itself — known as the thermic effect of food (TEF).
- Protein: Processing protein is the most energy-intensive — the body spends up to 30% of its caloric value on digesting and metabolizing protein. That means out of 100 calories in a chicken breast, you end up with significantly less "net" energy.
- Carbohydrates: Digesting carbohydrates costs the body less energy than protein does; for simple sugars, this cost is even lower than for complex, fiber-rich varieties.
- Fat: Fat is the easiest to digest, and processing it costs the body the least energy of all the macronutrients — which is why the fewest of its calories are "lost".
The result is clear: a protein-rich meal actually delivers a lower net caloric value than what the label shows, while a meal made mostly of fat and simple sugars comes much closer to its "on-paper" number.
Why two people eating the same food don't lose weight at the same rate
This leads to an important conclusion: two people can eat an identical 500-calorie meal — say, the same portion of pasta with chicken — and their bodies will extract different amounts of truly usable energy from it. The difference comes from a combination of macronutrient ratios, preparation method, individual microbiome, digestion speed, and many other physiological variables that differ from person to person.
This is also why two people with the same diet and the same starting weight can experience different weight-loss progress, even though on paper they're "doing the same thing." The calorie count on the label is just a rough starting point, not a final verdict.
Does this mean counting calories is pointless?
No — but it does mean that chasing perfect precision is futile and needlessly stressful. Trying to figure out exactly how many calories digestion "stole" from a specific portion of broccoli is practically impossible and completely pointless in everyday life. It's far more important to track trends over time and respond to what your body is actually doing — that is, your weight, measurements, and energy levels — than to chase a perfect number on your plate.
This is exactly the principle behind the approach used by FitAid. The app doesn't pretend that the calorie value you log is the absolute truth. Instead, its AI coach works with weekly data on food intake and weight trends and dynamically adjusts recommendations for each individual based on real-world results. If your weight is dropping more slowly or quickly than the calorie deficit you set would suggest, the system takes that into account and adjusts your recommended intake — regardless of whether the discrepancy is caused by bioavailability, the thermic effect of food, or some other individual factor. This sidesteps the whole problem of label inaccuracy: it doesn't matter why the estimate and reality diverge — what matters is that the app adapts to the data from your own body.
Practical tips for working with this even without perfect precision
- Track the trend, not a single day. Both weight and intake fluctuate, but the average over a week or two gives you a much clearer picture.
- Make sure you get enough protein, even knowing that the body will "spend" part of its caloric value on digestion — that's actually one of the reasons protein helps with weight loss.
- Don't be afraid to cook and adjust your food to match your goals — if you need more energy, cooked and blended food will deliver it more efficiently than raw food.
- Let the data speak for you. If your results differ from what you expected, adjust your plan based on your actual weight trend, not the theoretical number on the label.
The label gives you a good starting point, but your gut and the results that show up on the scale over the weeks always have the final say. Counting calories down to a tenth of a percent will never be possible — and thanks to tools that adapt to real-world data, it doesn't need to be.