Most people think their diet is healthier than it is

"We found that only a small percentage of U.S. adults can accurately assess the healthfulness of their diet, and interestingly, it's mostly those who perceive their diet as poor who are able to accurately assess their diet," said Jessica Thomson, PhD, research epidemiologist with the U.S. Department of Agriculture's Agricultural Research Service in the Southeast Area, the study's lead author. "Additionally, most adults overrate the quality of their diet, sometimes to a substantial degree."

Thomson will present the findings online at NUTRITION 2022 LIVE ONLINE, the flagship annual meeting of the American Society for Nutrition held June 14-16.

The researchers wanted to find out whether a single, simple question could be used as a screening tool for nutrition studies -- to replace or complement the detailed dietary questionnaires commonly used in nutrition research. Previous studies have found that self-rated health is a strong predictor of morbidity and mortality, but there is scant research on whether self-rated diet quality is predictive of the actual quality of one's diet.

The study used data from the National Health and Nutrition Examination Survey, a nationally representative survey of U.S. adults conducted every two years. Participants were asked to complete detailed 24-hour dietary recall questionnaires and rate their diet as excellent, very good, good, fair or poor.

Researchers used the food recall questionnaires to score each participant's diet quality. Examples of foods ranked as healthier include fruits and vegetables, whole grains, healthy fats, lower-fat dairy products, seafood and plant proteins. Foods considered less healthy included refined grains and foods high in sodium, added sugars or saturated fats.

The study revealed significant disconnects between the researcher-calculated scores and how participants ranked their own diet. Out of over 9,700 participants, about 8,000 (roughly 85%) inaccurately assessed their diet quality. Of those, almost all (99%) overrated the healthfulness of their diet.

Surprisingly, accuracy was highest among those who rated their diet as poor, among whom the researcher's score matched the participant's rating 97% of the time. The proportion of participants who accurately assessed their diet quality ranged from 1%-18% in the other four rating categories.

Thomson said further research could help to elucidate what factors people consider when asked to assess their diet quality. For instance, it would be helpful to know whether people are aware of particular dietary recommendations and whether they take into consideration where their food is purchased or how it is prepared.

"It's difficult for us to say whether U.S. adults lack an accurate understanding of the components of a healthful versus unhealthful diet or whether adults perceive the healthfulness of their diet as they wish it to be -- that is, higher in quality than it actually is," said Thomson. "Until we have a better understanding of what individuals consider when assessing the healthfulness of their diet, it will be difficult to determine what knowledge and skills are necessary to improve self-assessment or perception of one's diet quality."