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Why so many nutrition studies are "fundamentally and fatally flawed"

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What did you eat for lunch yesterday? Did you sprinkle nuts or dressing on your salad? Did you snack afterward? Exactly how many potato chips did you eat?

Chances are you probably don't remember. And yet, a lot of nutrition research today relies on just that kind of information: people's self-reporting from memory. Researchers ask people to recall the foods they consumed the previous day, keep a food diary, or check off from a list the foods they ate in the last day, week, month, or year.

It shouldn't come as a surprise, then, that researchers argue today in the journal Mayo Clinic Proceedings that this type of data is "fundamentally and fatally flawed" and "essentially meaningless."

They explain that "memory-based dietary assessment methods" — usually interviews, questionnaires, and surveys — are the dominant data collection method in many national nutrition surveillance and government-funded studies on food and obesity in the US. These studies inform things such as national food guidelines and food policy directions.

And yet, they argue, this data doesn't actually give an accurate picture of what people ate. For example, this table from the paper shows us that over the 39-year history of the National Health and Nutrition Examination Survey — which is based on self-reported food intake — the alleged number of calories consumed by 67.3 percent of women was not "physiologically plausible" given their body mass index (a measure of obesity).

Percentage of women whose reported food intake was implausible, organized by body mass index (BMI) in the National Health and Nutrition Examination Survey (1971–2010).

Mayo Clinic Proceedings

This may be because people lie about what they eat, offering up answers that are more socially acceptable. Or it may be a failure of memory. The authors draw on social science and neuroscience studies to explain why that's highly likely:

"The use of [memory-based surveys] requires faith in the belief that human perception, memory, and recall are accurate and reliable instruments for the generation of scientific data. Nevertheless, more than 80 years of research demonstrates that this belief is patently false."

While the researchers argue that future scientific funding should not be invested in memory-based studies, they offer little by way of an alternative.

This may be because the problem of getting an accurate read on what people ate is challenging the entire field, food policy researcher Marion Nestle explained in this Atlantic piece:

"We consider finding out what people eat the greatest intellectual challenge in the field of nutrition today. Why? We have no nice way of saying this. Whether consciously or unconsciously, most people cannot or do not give accurate information about what they eat. When it comes to dietary intake, pretty much everyone forgets or dissembles. This problem makes surveys of dietary intake exceedingly difficult to conduct and to interpret."

Nestle suggests synthesizing the results of many self-reports in a group to get some accurate average. 

Another
study, which also calls for an end to subjective self-reporting in nutrition studies, suggests the field embrace alternatives like digital photography, chewing and swallowing monitors, and wrist motion detectors that track "plate-to-mouth motion."