–Observational study links potatoes to high blood pressure
Depending on your position in the dietary wars, potatoes are now either a newly-confirmed nutritional villain or an innocent victim of overzealous and misapplied epidemiology.
A new study, published in The BMJ, is the first to establish a direct link between potato consumption and blood pressure. One motivation to perform the study, say the authors, was the recent inclusion of potatoes in U.S. government guidelines for healthy meals. Potatoes were included because they contain a large amount of potassium. But the authors wrote that potatoes, with their high starch content, can increase glycemic load. Another potential harm is weight gain (although the authors adjusted for weight changes in their analyses).
The new study combined data from three large observational studies (the Nurses’ Health Study, the Nurses’ Health Study II, and the Health Professionals Follow-up Study). In their main analysis, the researchers compared people who ate at least four servings of any type of potatoes each week with people who had less than one serving each week. After adjusting for differences — including the entirely unsurprising finding that the potato eaters were less active and more likely to smoke — the researchers found potato consumption independently associated with about 11% higher risk of hypertension.
The association was stronger in women than in men. Although french fries were associated with an increased risk among both women and men, there was no significant association for potato chips among either men or women. Men who ate potato chips actually had a lower numerical risk of hypertension.
One overall weakness of the study is that potato consumption was measured using a dietary questionnaire, the reliability of which has been questioned. This may also help explain the differences between men and women in the study.
The authors estimated that cutting one potato serving a week could lead to a 7% reduction in the risk of hypertension. The study findings, they wrote, “have potentially important public health ramifications, as they do not support a potential benefit from the inclusion of potatoes as vegetables in government food programs but instead support a harmful effect that is consistent with adverse effects of high carbohydrate intakes seen in controlled feeding studies.”
But an accompanying editorial by Australian authors questioned whether the study adds anything significant to our knowledge. Mark Harris, MBBS, and Rachel Laws, PhD, decried the fact that we are “constantly assailed by media reports of studies and (sometimes conflicting) recommendations about the risk posed by specific dietary foods.” But they questioned the focus on individual foods or nutrients. “The overall pattern of foods offered in school lunch programmes may be more important than the isolated inclusion or exclusion of potatoes,” they wrote.
Their skepticism was echoed by John Ioannidis, MD, of Stanford University, who recently wrote a powerful indictment of the dangers of epidemiological studies in nutrition. Asked to comment on the paper, Ioannidis said that it was “a strong study in that data from three cohorts are used, but the results are not convincing to me. Nutritional observational effects of small magnitude are very tenuous. Personally, I will continue to eat both potatoes and vegetables and not worry about how much of each (within logical range).”
Franz Messerli sent the following comment after the story was originally published.
Early in his career, when he was still living in the Netherlands, van Gogh painted a large canvas called the Potato Eaters which is now prominently displayed in the van Gogh Museum in Amsterdam. When looking at these 6 starving peasants one doesn’t get the clinical impression that any one of them had hypertension. Clearly, when regular caloric intake borders on starvation, a once daily high glycemic load such as may have occured with a potato meal most likely has no effect on blood pressure.
However, when such a glycemic load is superimposed several times daily on subjects with latent insulin resistance as is present in many people in the US today, it may well cause prolonged and excessive postprandial hyperglycemia. This in turn has been documented to increase the risk of oxidative stress, endothelial dysfunction, and inflammation. Little surprise that these mechanisms in concert accelerate the pathogenesis of essential hypertension.