I want to describe an experiment that changed how I talk to patients, and that I wish everyone attempting weight loss knew about.
The study that should be famous
In 1992, researchers published a study in the New England Journal of Medicine of people who described themselves as “diet-resistant” — convinced they ate little yet could not lose weight. Many believed they had a metabolic defect. It is a group every doctor recognises.
The researchers measured actual energy intake using doubly labelled water, a technique that cannot be argued with, alongside the subjects’ own careful food diaries.
The results: these participants underreported their food intake by an average of 47% and overreported their physical activity by 51%. Their metabolic rates were normal. The defect was not in their metabolism.1
And here is the part that matters: they were not lying. They believed their diaries. The distortion happened before the writing did.
This finding has been replicated many times since, in many populations. Under-reporting of intake is essentially universal, running around 20% in normal-weight people and considerably higher in those with obesity. It is one of the most robust findings in nutrition research, and it is the reason self-reported dietary data has to be treated with such caution.2
Why our estimates are so bad
We do not count what does not feel like a meal. The handful of nuts while cooking, the last three chips off a child’s plate, the milk in four coffees, the tasting spoon. None of it registers as eating. All of it counts.
Portion estimation is genuinely difficult. Ask ten people to serve 100 g of pasta and you will get a fourfold spread. Restaurant portions have grown substantially over recent decades, and the calibration in our heads has not kept pace.
Liquid calories are nearly invisible. Juice, smoothies, speciality coffee and alcohol are poorly registered by appetite regulation and even more poorly registered by memory.
We remember intentions as actions. The gym membership feels like exercise. The plan to walk feels like walking. This is not dishonesty; it is ordinary human memory, which stores gist rather than detail.
And we are motivated. Recall is systematically biased toward the version of events we would prefer. Nobody is exempt, including me, including your doctor.
The exercise side of the ledger
The 51% overestimate of activity is, if anything, the more consequential error, because it licenses eating. People routinely believe a workout burned two or three times what it did, then eat accordingly.
The activity calculator on this site exists partly for this reason. Enter a realistic session and look at the number. Most people find it deflating — which is exactly the recalibration required.
What actually corrects it
The remedy is not more sincerity. It is measurement.
Self-monitoring is the single most consistently effective behaviour in the weight loss literature. Systematic review after systematic review finds that people who record what they eat lose more weight than those who do not, and the relationship is dose-dependent — more consistent recording, more weight lost.3 It outperforms most specific diets.
- Weigh food for two weeks, once. Not forever. Long enough to recalibrate your eye, which drifts and needs re-checking perhaps twice a year.
- Record everything, in the moment. Retrospective logging at bedtime reproduces exactly the memory distortion you are trying to defeat.
- Count the invisible categories deliberately: liquids, cooking tastes, leftovers off other people’s plates, and anything eaten standing up.
- Weigh yourself regularly. Daily or weekly weighing is consistently associated with better outcomes — not because the number is magic, but because it provides feedback that beliefs cannot override.4
- Trust the trend over the story. If intake is genuinely 1,600 calories and weight is stable for six weeks, intake is not 1,600 calories. The scale is not conspiring against you; it is the only honest witness in the room.
A word about how this is used
This evidence gets weaponised, and I want to head that off. “You are underreporting” is true of nearly everyone and is a useless thing to say to a patient in a tone of accusation. It is not a character flaw. It is a property of human cognition, no more shameful than an optical illusion.
The right response is not guilt. It is instrumentation — a scale, a food record, a calculator — because the entire point of a measuring device is that it does not care what you believe.
References
- Lichtman SW, Pestone M, Dowling H, et al. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. N Engl J Med. 1992;327(27):1893–1898. doi:10.1056/NEJM199212313272701 The doubly labelled water study showing 47% under-reporting of intake and 51% over-reporting of activity.
- Dhurandhar NV, Schoeller D, Brown AW, et al. Energy balance measurement: when something is not better than nothing. Int J Obes (Lond). 2015;39(7):1109–1113. doi:10.1038/ijo.2014.199 Review of the systematic inaccuracy of self-reported dietary intake data.
- Burke LE, Wang J, Sevick MA. Self-monitoring in weight loss: a systematic review of the literature. J Am Diet Assoc. 2011;111(1):92–102. doi:10.1016/j.jada.2010.10.008 Systematic review establishing the dose-dependent relationship between self-monitoring and weight loss.
- Zheng Y, Klem ML, Sereika SM, et al. Self-weighing in weight management: a systematic literature review. Obesity (Silver Spring). 2015;23(2):256–265. doi:10.1002/oby.20946 Evidence that regular self-weighing is associated with better weight outcomes.