"Calories by eye": why we almost always miss the target — and what does BMI have to do with it

We are used to thinking that we can “by eye” distinguish 200 kcal from 500 kcal and estimate how full we will be from a donut, a handful of nuts or a glass of soda. A study in Nutrients checked how people with different body mass index (BMI) perceive portion sizes, calorie content and “satiety” of popular snacks and drinks. It turned out that almost everyone is wrong, and there is no significant difference between BMI groups - the type of product and gender differences in perception are more important.

Background of the study

The background for this study was the growing role of snacks and drinks in daily dietary energy and the well-described “portion size effect”: the larger the portion served, the more people eat – often without noticing it. Against the backdrop of a global rise in obesity and widespread access to ultra-processed foods, the question is not only what we eat, but also how we visually assess the “size” and caloric content of portions. An error in eye measurement of tens or hundreds of kilocalories, repeated day after day, can imperceptibly shift the energy balance towards a surplus. This is why the accuracy of portion and energy perception is considered a behavioral target for the prevention and treatment of obesity.

However, empirical data on who makes mistakes and how remain contradictory. Some studies document systematic overestimation of the caloric content of “unhealthy” snacks, while others underestimate liquid calories and small but energy-dense portions like nuts and chocolate. Gender differences in eating behavior and possible “cognitive traps” of visual assessments are discussed, but until now it was unclear whether such distortions are related to body mass index (BMI) or are universal for most people, regardless of weight.

The authors of the Nutrients study set out to test whether BMI influences portion size perception, calorie estimates, and expected satiety for a range of common snacks and beverages. The study was built around standardized product images and comparisons of subjective assessments between normal-weight, overweight, and obese groups, as well as between men and women. This design allows us to separate the effect of weight itself from general perceptual errors and to clarify which components of food choice require the most “retraining” in practical work with patients.

The practical interest in this topic is obvious: if people with different BMIs really did have different error profiles, this would open the way to targeted educational strategies - from "calibrating" basic portions to emphasizing control over drinks and snacks. If BMI does not play a role, then the focus shifts to mass, universal interventions - improving "calorie literacy", visual cues, and restructuring the food environment. The results of this article clarify the debate about the role of BMI in portion perception and suggest where exactly to look for application points for preventing overeating.

What exactly was checked?

• Estimate portion size by eye (scale 1-10).
• Calorie content estimate (in kcal).
• Expected satiety and subjective “usefulness” of the product.

The study was based on an online survey that featured color photographs of 15 common foods (from baby carrots and nuts to donuts, cookies, hot dogs, and sugary drinks). Participants rated several items, and the authors compared responses by BMI, gender, and age using standard nonparametric statistical tests.

Who participated

• Adults 18-77 years old, about two hundred people.
• There are significantly more women than men.
• The average BMI is in the "overweight" range.
• BMI groups: normal, overweight, obese.

The bottom line is mundane: the ability to “guess” caloric content is low for most people and is weakly related to BMI. However, there are noticeable differences between men and women in how “large” portions of ultra-processed snacks (cookies with cream, doughnuts, hot dogs, etc.) seem.

Key Results

• No significant differences were found between the BMI groups for most metrics.
• The general trend is to overestimate calorie content (people tend to inflate the number more often than underestimate it).
• For drinks and ultra-processed snacks, women are more likely to rate portion sizes as "larger."
• Age has virtually no effect on the accuracy of estimates.
• Expected satiety often differs from actual calorie content.

Photos are misleading: a visually small but energy-dense portion seems “lighter,” and impressive-looking food often contains fewer calories than expected.

Where were the most noticeable mistakes

• A handful of nuts - small volume, high energy content; chronic underestimation of grams.
• Half a chocolate bar - the visual size is consistently deceiving, the ratings "wander".
• Donuts and cookies with cream are an overestimation of both calories and “satiety”.
• A glass of strong alcohol - calories are "not visible", the answers are contradictory.
• Sweet drinks - some participants underestimate the contribution of "drinking" calories.

The lack of large differences in BMI suggests that overeating is not due to a “special distortion of the eye” in obese people, but to universal traps – packaging, marketing, snacking habits and an overabundance of ultra-processed foods around us.

What does this mean for practice?

• Relying on intuition when counting calories is risky - even educated adults make mistakes.
• The visual “largeness” of a portion does not equal caloric content, and even less so satiety.
• Planning snacks and calibrating portions is more important than the abstract “eat less.”
• Gender differences in perception should be taken into account in recommendations and training materials.

Simple tricks help to "fasten" the eye to reality. A few training sessions with kitchen scales - and you will begin to recognize your portions without a calculator.

Practical advice "for every day"

• Calibrate basic portions: 15-20 g nuts ≈ ~100-120 kcal; 45-50 g chocolate ≈ ~250-280 kcal.
• Don't count the packages, but the grams: a small package often contains 2-3 "food" servings.
• Change the structure of snacks: more whole foods (vegetables, fruits) + protein.
• Keep "anchors" handy: A pair of "reference" plates/glasses makes it easy to keep track.
• Check your satiety after 15-20 minutes: the brain needs time to “see” what you’ve eaten.

It is also important to remember the limitations: online design, self-reporting of height/weight, assessment by photo instead of real food. These factors can "blur" the accuracy of conclusions and skew the sample.

Limitations of the study

• Unrepresentative sample (predominance of women, many participants with higher education).
• Subjective answers without clinical verification.
• Photos instead of real food and familiar surroundings.
• Cross-section is about associations, not causality.

The bottom line: we all have a "missing sight" on calories, and it's not so much about BMI as it is about universal distortions of perception and an environment that encourages snacking. The good news is that the eye can be trained: a few weeks of conscious practice usually reduces misses and helps you eat exactly as much as planned.

Source: Durma AC et al. Differences in the Perception of Snacks and Beverages Portion Sizes Depending on Body Mass Index. Nutrients 2025;17(13):2123. https://doi.org/10.3390/nu17132123