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Eating potatoes causes high blood pressure! Or maybe it doesn't, and the real problem is that studies of foods are inherently unreliable, except for generating headlines.

Potatoes are the newest nutritional villain, or at least one study tries to convince us that they are.

Three researchers at Brigham and Women's Hospital in Boston and the Harvard School of Public Health recently published a study that claims that eliminating one serving of potatoes per day lowers the risk of high blood pressure by 7 percent.

The study doesn't say that eliminating one serving of potatoes per day will lower your blood pressure by 7 percent. It says that if everybody in the universe eliminated on average on serving of potatoes per day then 7 percent fewer people would be diagnosed with high blood pressure (although potato chips may be OK, the study says). And actually, the study found that eating potatoes is vaguely related to having higher blood pressure, not that not-eating potatoes lowers it. Right off the bat, it's obvious that many journalists are misreporting the study's findings. But let's take a look at what this study really says and why studies of single foods are almost universally misunderstood.

187,453 People Can't Be Right

The researchers analyzed data collected on an ongoing basis from 62, 175 women in the Nurses Health Study, 88,475 women in the Nurses Health Study II, and 36,803 men in the Health Professionals Follow-Up Study who were free of high blood pressure at the beginning of the period analyzed. These health professionals have been followed for as long as 30 years to track the relationships between lifestyle choices and health outcomes. In all three studies, volunteers filled out a survey with questions about their consumption of 130 different foods, including potatoes, every four years. No actual measurements of foods eaten were made.

This study relied solely on what harried doctors and nurses could remember about what they had eaten in the previous four years. That would seem to be a problem, but it's standard methodology for food studies, since it's easy and cheap for researchers to collect the data. Also, with over 100,000 respondents to the survey, there isn't as much of a concern about errors in the way the data are collected.

Potatoes Are Evil Unless They Are Chips, Apparently

The researchers took the results of the food questionnaires and compared them against just one item in the participants' health records. They looked at the relationship between potato consumption and being diagnosed with high blood pressure. Participants were asked:

  • Three questions about their consumption of baked, boiled, or mashed potatoes, giving them an opportunity to estimate having eaten zero to six or more servings per day of one potato or one cup each over the preceding year.
  • One question about their consumption of French fries, giving them an opportunity to estimate having eaten zero to six or more servings per day of 4 oz (112 grams) each over the preceding year.
  • One question about their consumption of potato chips (crisps), giving them an opportunity to estimate having eaten zero to six or more servings of 1 oz (28 grams, one small bag) each over the preceding year.

Following the old adage, "If you torture the data, it will confess," researchers analyzed their data in every possible way to try to find some kind of relationship between eating potatoes and high blood pressure. 

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