New research, published this week in The BMJ, puts a worrying new slant on a global staple – the humble potato. Researchers link increased potato intake to high blood pressure in adults. The study is only observational, but the findings are likely to spark debate.

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Potatoes may be common, but are they healthy?

Thanks to the potato’s hardy nature, the calorie-packed tuber has become an incredibly cheap and useful global product.

Potatoes are grown in 125 countries and all 50 U.S. states. There are 4,000 different varieties, and the global crop exceeds 300 million metric tons.

There are some nutritional benefits to the potato, but, as recent research appears to demonstrate, there is no such thing as a free lunch.

U.S. government food programs initially restricted starchy vegetables, like the potato, to one cup a week, but in recent years, the potato has returned to U.S. government healthy meals programs.

The justification for their inclusion relies on their high potassium content, which is associated with lower blood pressure.

Despite the well-documented effect of potassium on blood pressure, the long-term effects of a potato-heavy diet on blood pressure has not previously been studied.

Scientists at Brigham and Women’s Hospital and Harvard Medical School, both in Massachusetts, investigated links between potato consumption and blood pressure for the first time.

The team looked at potatoes in all their forms – mashed, boiled, chips, and fries. Data were taken from three large American studies, spanning more than 20 years and charting the diets of 187,453 men and women.

The team controlled for numerous factors, including weight, smoking status, level of physical activity, and current dietary habits.

After controlling for these factors, the team found that eating four or more servings of mashed, baked, or boiled potatoes per week was associated with an increased risk of high blood pressure, or hypertension, when compared with less than one serving per week. The effect was not found in men.

On further investigation, the team found that replacing one serving of potatoes per week with a portion of non-starchy vegetables was associated with a significant drop in blood pressure.

Also, an increased intake of French fries was associated with increased hypertension in both men and women. More surprisingly, those who consumed higher amounts of potato chips did not display an increased risk for hypertension and, in fact, the men who ate more chips showed a reduced hypertension risk.

Potatoes have a higher glycemic index than other vegetables, and this can trigger a spike in blood sugar just following a meal. Increased sugar in the blood – known as hyperglycemia – has previously been associated with oxidative stress, endothelial dysfunction, and inflammation, all of which could help explain a higher hypertension risk.

It is worth noting that the current study was observational, so causation can not be proven at this stage. Additionally, self-reporting of diet is always open to errors – no one has perfect recall. And, of course, not everyone is wholly honest when it comes to a diet-based questionnaire.

However, if the results are backed up by future studies, the researchers believe they “have potentially important public health ramifications” because “they do not support a potential benefit from the inclusion of potatoes as vegetables in government food programs.”

The study is accompanied by an editorial, written by Prof. M. F. Harris and R. A. Laws, from the University of New South Wales, Sydney, Australia, titled “Dietary patterns matter more than isolated food items, and that’s what we should be studying.”

The authors put emphasis on the importance of the whole diet, rather than specific food types. For example, the glycemic index of a potato varies between varieties and also depends on how it is cooked – fried potatoes are not all equal, the length of time they are cooked for and the type of oil they are cooked in can make a difference, for instance.

Added to that, the amount of protein and fiber eaten at the same sitting affect the overall glycemic index of the whole meal. The authors of the editorial conclude:

We will continue to rely on prospective cohort studies, but those that examine associations between various dietary patterns and risk of disease provide more useful insights for both policy makers and practitioners than does a focus on individual foods or nutrients.”

Because hypertension increases the risk of heart disease and stroke and affects an estimated 70 million American adults, no doubt, the potato-blood pressure debate will continue.

Learn more about diet, hypertension, and gestational diabetes.