Hi Charlotte, How accurate are “calories” as a metric for measuring energy consumption and managing your weight? Should I really trust the amount of calories indicated on food labels?
There are few layers to this so hopefully you stay with me all the way to arrive at the answer you’re after.
Calories can be an accurate way to measure the energy value of a food; this is distinct from measuring the energy the body obtains from this food. We can use a test called bomb calorimetry to precisely measure the energy value of food (it essentially involves burning the food and calculating the energy from it).
Now, the body isn’t perfect and it isn’t going to obtain all of the calories in a food. It’s also going to use up some energy digesting and absorbing foods and this can vary. The amount of time it takes to digest protein, fat, and carbohydrate varies, and fiber can be indigestible, further complicating things.
When it comes to food labels, the calories stated aren’t determined by bomb calorimetry, they’re estimated by the Atwater factors which I’ve mentioned in a previous piece about how to estimate the macros in your food. This is fine, but not perfect. In fact, food manufacturers are roughly allowed a 20% tolerance limit to what’s stated on the label versus lab test results.
Even then, calories can be overestimated - for nuts it can be as much as 30% - and there are also differences for individual amino acids and triglycerides; the building blocks of protein and fat respectively, which we explain in more detail here. So should you trust what’s printed on the label? I think so and they’re good enough if you want to use them as a tool for weight loss.
I say tool because it’s only one piece of the puzzle. You can use the calories on the back of a food pack as a guide. If you know how many calories you need to lose weight (remember this will also be an estimate) then putting the two together becomes simple math.
Humans are not a math equation though. Tracking results over time through measurements such as how you feel, stepping on the scale, taking photos, and working with a health professional (including understanding what your labs say), will help you understand better if the estimates you’re using are in the right place or if you need to adjust.
If you’re not trying to change your body weight, I don’t think calories are that useful. We eat food not calories, and they give no indication as to the healthfulness of a food.
So I’d say it’s less about assuming that calories are 100% accurate and can be applied with total confidence to your individual body, and more about understanding where the limitations lie and how calories could be used as part of the way to achieve your goals.
Assuming 100% accuracy from calorie measurements and the human body are only going to give you a headache, use them as a guide alongside other metrics and you’ll find they can be useful.
Written by: Dan Clarke, RNutr
Reviewed by: Charlotte Marie Werner, MS, RD, CDN
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