Berry, Brie, and Almond Salad

Dietary Quality: Garbage In, Garbage Out

“Food diaries are important for people trying to improve their dietary health,
but the information in them is garbage.”
— Professor John P. Foreyt

With a new study of the healthfulness of American diets making headlines, it’s worth remembering that all of these studies are based on what people say they eat. A distinguished team of researchers have published an analysis of the changes in American dietary quality over the last ten years in the new issue of JAMA Internal Medicine. The headlines are all about how diets have improved, except among the poor. The fly in the ointment is the reliability of the self-reported dietary information that is the foundation for this study.

Do people lie about what they eat? Might they misremember?

In a word, yes. Dietary habits have long been associated with virtue, going all the way back to ancient dietary doctrines of most major religions. Over the last decade, advice to “eat healthy” has rained down torrentially. Anyone who is paying attention knows that First Lady Michelle Obama wants them to eat more healthfully.

What’s more, validation studies have quantified substantial holes in the reliability of various dietary self-reports. The authors of a pooled analysis published this summer conclude: “Clearly, improvements in our current dietary assessment methods are desirable.”

So this study of self-reported dietary habits simply proves that economically advantaged people are reporting better dietary habits now than they were ten years ago. Disadvantaged people, not so much. Maybe the better reports represent real change. Maybe it’s just grade inflation for people who are listening to the First Lady.

But it’s important to stick with the facts — especially if you call yourself a scientist.

Click here to read the study and here to read coverage from the Associated Press. Click here to read the pooled analysis of dietary self-reports.

Berry, Brie & Almond Salad, photograph © AmazingAlmonds / flickr

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9 Responses to “Dietary Quality: Garbage In, Garbage Out”

  1. October 15, 2014 at 11:30 am, Allen Browne said:

    Yup – we need a little more rigor in the “science.”

    • October 15, 2014 at 5:43 pm, Ted said:

      Thanks, Allen!

  2. October 17, 2014 at 7:12 pm, sabri said:

    lots of things are difficult to measure. but that doesn’t mean their measurement is necessarily useless for inference. it also depends on how the error is distributed and how well it can be quantified.

    • October 17, 2014 at 7:34 pm, Ted said:

      I agree that measuring dietary intake is far from useless. But self-reports are not an objective, reliable measurement. This analysis is worth reading:

  3. October 18, 2014 at 9:06 pm, sabri said:

    Self-report can be adequately objective if sources of nondifferential and differential error have been appropriately adjusted for, allowing less biased estimation of absolute intake and measures of association.

  4. October 19, 2014 at 7:00 pm, sabri said:

    The letter is misleading. In fact they are only inadequate if their shortcomings are not accounted for statistically. Some researchers appreciate this more than others, and it is unfair to base generalizations on the data itself rather than the quality of its analysis and interpretation.

    • October 19, 2014 at 8:18 pm, Ted said:

      We will have to agree to disagree. The body of work that the authors of that letter have produced on the subject of measuring dietary intake and expenditure is worth studying before you blithely dismiss their analysis as “misleading.”

  5. October 20, 2014 at 12:31 am, sabri said:

    Agree to disagree. If time allowed this would make for a good debate.