Do potatoes cause high blood pressure?

You might have seen news reports recently about a study connecting potatoes with high blood pressure, which is of course one of the main risk factors for stroke (see for example ABC News). But whether you’re a spud lover or a tater hater, it’s usually best not to jump to any conclusions based on one scientific paper.

Every week brings new papers reaching new conclusions about whether certain foods are good and bad, and it’s easy to think science can’t make up its mind. But the general dietary advice, to eat a balanced diet with plenty of fruits and vegetables, hasn’t changed much for decades.

As nutritionist Dr Rachel Laws from Deakin University, who co-wrote an editorial in the same journal that published the article, said, “The overall pattern of eating is far more important to focus on than looking at individual foods.”

However, if you’re really interested in a particular research finding, then it’s also good to try to dig beneath the headlines. It can sometimes be difficult to find the paper—although a good news report, like the ABC article, will link to it—and often the full text is behind a paywall. But you should at least be able to find the abstract, which gives a summary of the research.

If you manage to dig up the potato paper, called Potato intake and incidence of hypertension: results from three prospective US cohort studies, published in the British Medical Journal, you’ll find that it doesn’t necessarily prove what the headlines say it does.

The biggest issue is unfortunately a rather technical one, so please bear with me here. The researchers report a “hazard ratio” of 1.11 for eating four or more servings per week of baked, boiled or mashed potatoes. This means an increase in the prevalence of high blood pressure of 0.11, or eleven per cent.

However, this is a statistical calculation based on the data they analysed, and doesn’t necessarily match the real rate, whatever that may be. More precisely, the researchers reported that they’re 95% confident the real rate is between 0.96 and 1.28.

So it could just as easily be 1.00, which would mean potatoes don’t affect high blood pressure at all. In this case we normally say the result isn’t statistically significant, or it’s a null result. If you like, you could say the paper didn’t actually show potatoes increase the rate of high blood pressure.

Their results were slightly stronger for four or more servings per week of French fries/hot chips (hazard ratio of 1.17, but 95% confident it’s between 1.07 and 1.27), but that probably wouldn’t have made headlines. And there are other findings that raise one’s eyebrows too.

For instance, they were surprised that potato chips/crisps seemed to reduce the rate of high blood pressure by 3 per cent (hazard ratio of 0.97, 95% confident it’s between 0.87 and 1.08, so technically another null result). And the risks seemed to be greater for women than for men (and men got greater benefit from the chips).

Although it’s possible to come up with plausible explanations for these findings, it’s just as easy—maybe easier—to say it’s a fluke.

To be fair, these kinds of studies are very difficult to perform. Ideally you’d want to get a few thousand people, randomly divided into two groups where the only difference is one group eats a certain amount of potatoes per week and the other group doesn’t, and then follow them over many years. But you’re unlikely to get the funding to do that just for potatoes.

Instead, what the researchers did was look back on three large studies that followed people over a number of years, asking them questions about their diet and blood pressure. But when you analyse past data in this way, it’s always going to be possible to find spurious correlations. That’s especially if you start dividing them into sub-groups, such as men vs women, or boiled potatoes vs chips.

Added to that is the fact that people aren’t very good at recalling what they ate. And the follow-ups were every four years. Could you remember how often you’ve eaten potatoes, French fries and chips over the past four years?

Unfortunately, it’s the only data we have. And it did actually achieve the stated aim of the research project, which was to test whether potatoes reduced the rate of high blood pressure.

That’s right: apparently, in 2010 potatoes were removed from school lunch programs in the United States, to much opposition from the providers of those programs. Their lobbying led to the ban being lifted in 2015, with the reason given being that potatoes contain potassium. And a diet rich in potassium is known to lower blood pressure.

So although I don’t believe the study demonstrated that potatoes increase your blood pressure, it’s quite clear that it didn’t show potatoes reduce blood pressure. Which effectively makes the lobbyists’ arguments invalid.

For those of us who aren’t lobbyists or US Congressmen, the message is not to worry that your potato consumption will lead to high blood pressure.

The wider message though is to be sceptical of dietary science news. But be warned: make sure you’re sceptical of all such reports, not just those you disagree with.

It can be a lot of work, but as a bonus here’s a quick rule of thumb: if you see a news report where the headline is a question, the answer is usually “No”.

That goes for this article too.