Ask Americans if they are vegetarian and about 2 in 100 say yes. Check what they ate on two days, and only half of them skipped meat both times

Headlines about vegetarians living longer, weighing less or having healthier hearts often come from big population surveys. Many of those surveys never asked anyone whether they were vegetarian. Instead, researchers looked at a day or two of food records and labelled anyone who reported no meat or fish as vegetarian. A new analysis, published on 27 October in the journal Scientifica, asks how much that shortcut changes the answer.

The team used the US National Health and Nutrition Examination Survey (NHANES) for 2007 to 2010, the only cycles that asked people directly: "Do you consider yourself to be a vegetarian?" The same people also completed two 24-hour dietary recalls, in which a trained interviewer walks through everything eaten the previous day. In total 15,489 participants had both recalls and a yes-or-no answer, and the figures were weighted to represent the US population.

The count swung widely. Going by the first food recall alone, 14.3% of people looked vegetarian, because they happened to report no meat, poultry or fish that day. Using both recall days brought it down to 4.0%. Asking people directly gave 2.2%. And requiring both, a yes answer plus no meat or fish on either day, left just 1.1%, about one in ninety. Put another way, half of the people who called themselves vegetarian reported some meat, poultry or fish across the two days: 212 of the 327 self-identified vegetarians in the raw sample.

That is not a finding that vegetarians are fibbing. NHANES never defined the word, so some people may use it loosely for mostly plant-based eating, and dietary recalls carry their own memory and reporting errors. Only two days were recorded, and the authors did not adjust for age, income or other factors. The practical point is for anyone reading diet research: before trusting a claim about vegetarians, check how the study decided who was one. A label built on a single day of eating can sweep in many occasional meat-eaters, which may blur or distort any link with health.