You Say Potato – Part I

Let’s say you really need to find reliable information about the best diet – for high blood pressure – or heart disease – or diabetes. Where do you go? Do you go to a website sponsored by Big Pharma that wants to sell you pills to fix your problem?

Or, do you want to treat the cause?   

Welcome to the NutritionFacts podcast – with the latest peer-reviewed research on the best ways to eat healthy – and live longer.

Today – we turn a laser-like focus to that round tuber – we call the potato. Are they good for us? Bad for us? Here’s our first story.

The trouble for white potatoes began in 2006, when the Harvard Nurses’ Health Study, which had followed the diets and diseases of tens of thousands of women for 20 years, found that greater potato intake was associated with a greater likelihood of getting type 2 diabetes. Okay, but wait. Of the hundred or so pounds of potatoes Americans eat every year, most are in the deep-fried form of potato chips or French fries. What happened when they looked specifically at mashed or baked potatoes?  They found the same link with diabetes. Okay, but what might potato eaters eat more of? I’ll give you a hint by rephrasing that as: what might meat-and-potatoes people eat more of? Indeed, people who ate more potatoes ate more meat, and we know that animal protein on its own is associated with increased diabetes risk. But the researchers tried to statistically adjust for that and still found increased risk with potatoes.

Well, what do people put on baked and mashed potatoes? Butter and sour cream. Again, the researchers tried to adjust for other dietary factors like these, as well as effectively looking at the ratio between plant and animal fats, and whether potato-eaters drank more soda, or maybe skimped on other vegetables. And still there seemed to be this potato/diabetes association.

Okay, but that was just one study. By 2015, Harvard researchers had also looked into other cohorts, including the all-male Health Professionals Follow-up Study to complement the all-female Nurses studies, and they continued to find a small increased diabetes risk associated with baked, boiled, or mashed potatoes––though French fries do indeed appear nearly five times worse. The authors concluded that potatoes are considered to be a healthful vegetable in dietary guidelines; however, the current findings cast serious doubts on that classification. Walt Willett, the chair of Harvard’s nutrition department at the time, went a step further, suggesting potatoes should be siloed up there with candy.

A meta-analysis of potato consumption and risk of type 2 diabetes published in 2018 combined all six of the prospective studies that have been done to date, and they found about a 20 percent increase in diabetes risk associated with each serving of potatoes a day, concluding long-term high consumption of potato may be strongly associated with increased risk of diabetes. But, again, the great majority of this potato consumption was fried, and we know there are all sorts of nasty things like advanced glycation end products in deep fried foods. The researchers weren’t able to assess French fries versus non-fried potatoes. Even just three servings of fries a week is associated with nearly 20 percent greater risk of type 2 diabetes, whereas there was only a tiny associated risk with potatoes in general, and that included the fries mixed in.

The world’s largest manufacturer of frozen French fries took issue with this conclusion. Laying claim to one in three fries eaten on planet Earth to the tune of billions of dollars, they have the money to fund reviews like this one to cast doubt on the science. But they do have an actual point. Observational studies can never prove cause-and-effect, and maybe potato consumption, even baked potato consumption, may just be a marker for an unhealthy diet in general. As much as researchers try to adjust for these other factors, as the journal of the Potato Association of America (American Journal of Potato Research) is quick to remind us, it’s not possible to completely separate the effects of potatoes and French fries from the effects of the overall crappy Standard American Diet.

If only there was a country where potato consumption was associated with a healthy diet. If potato consumption was still associated with diabetes there, then that would be concerning.  Here we go…a seventh study, but this time out of Iran, where not only is most of their potato consumption from boiled potatoes, but those who eat potatoes had the healthiest diets and ate the most whole plant foods—fruits, vegetables, whole grains, and beans. And though the researchers tried to tease out those other dietary factors, those eating the most boiled potatoes had only half the odds of developing diabetes. This supports the notion that it may be hard to completely separate out just the potatoes. The bottom line, this systematic review concluded, is that we really don’t have convincing evidence to date that the intake of potatoes in general is linked to type 2 diabetes, but we should still probably hold the fries.

In our next story, we ask – do potato eaters live longer or shorter lives than non-potato eaters?

Potato intake and the incidence of hypertension. Harvard researchers followed the diets and diseases of more than 100,000 men and women for decades, and found that those who ate potatoes on most days—even just baked, boiled, or mashed, not just French fries and potato chips—appeared to be at higher risk of developing high blood pressure. Okay, but what do people put on potatoes? Salt, not to mention butter; so, maybe the potatoes are just innocent bystanders. Maybe…but the researchers made attempts to tease out the effects of salt and saturated fat, and there still seemed to be a link between potato consumption and high blood pressure.

Maybe potato eaters are just meat-and-potatoes people. After all, these same Harvard researchers found that meat, including poultry alone, appeared associated with an increased risk of hypertension, and the same with even a moderate amount of canned tuna. So, in the potato study, they were careful to try to factor out any effects from the consumption of all types of animal flesh. Yet they still found an increased potato risk, and got concerned that the association of potato intake with hypertension could be a critical public health problem. We had assumed potatoes might actually decrease high blood pressure given their high potassium content, but they found evidence of the opposite effect.

Two similar studies performed in Mediterranean Europe did not find any association between potato consumption and high blood pressure, though. Perhaps this is because they don’t smother their potatoes in butter and sour cream in that neck of the woods, and instead eat potatoes with other vegetables. Now, the Harvard folks tried to control for the bad salty and fatty dietary components associated with eating potatoes in the West, just like these researchers tried to factor out all the extra vegetables, but you can’t control for everything.

A primary reason we care about blood pressure is because we care about the consequences. In two studies done in Sweden, where they primarily eat their potatoes boiled, no evidence was found that potato consumption was associated with the risk of major cardiovascular disease, and no relationship was found between potato consumption and risk of premature death found in Southern Italy either. In the U.S., however, potato consumption was associated with increased mortality: a whopping 65 percent increased risk of dying from heart disease, a 26 percent increased fatal stroke risk, a 50 percent increased risk of dying from cancer, and increased risk of dying from all causes put together. However, this all disappeared after adjustment for confounding factors. In other words, it wasn’t the potatoes at all. Potato eaters must just smoke or drink more, or eat more saturated fat, or something. Once you control for all these other factors, the link between potatoes and death disappears.

This was confirmed in the NIH-AARP study, the largest such study of diet and health in human history. If you just separate out the potatoes, researchers find they are not associated with increased risk of death, with the possible exception of French fries, which are associated with an increased risk of dying from cancer. Put all the studies together—20 in all—and no significant association has been found between potato consumption and mortality, though again fried potatoes may be the exception. Even just twice a week, fries may double one’s risk of dying prematurely, independently of other factors; but the consumption of unfried potatoes seemed to be neutral.

You know, it’s funny. I’ve done a bunch of videos on how all plant foods are not created equal, talking about healthy vs. unhealthy plant-based diets. To this end, researchers created not just an overall plant-based diet index—just scoring plant vs. animal foods—but also a healthy plant-based diet index (hPDI) and an unhealthy plant-based diet index (uPDI). The healthy index puts a greater emphasis on whole plant foods, whereas the unhealthy index scores how much low-quality plant foods you’re eating, grouping potatoes along with soda, cake, and Wonder Bread. Then, when you run the numbers, the more plant-based you eat the longer you live, the lower your risk of cardiovascular disease. In other words, more plant foods and less animal foods are associated with a significantly lower risk of dying prematurely. This benefit was limited, though, to those eating the healthier plant food diets, but they were surprised that those eating all that processed plant-based crap didn’t live significantly shorter lives. Now maybe, that’s just because they were eating fewer animal products, and that’s really the primary determinant of lifespan here, or maybe the lack of an association between less healthy plant-based diets and mortality outcomes is because potatoes were kind of coming to the rescue. And indeed, higher intake of potatoes did appear protective; so, given these conflicting findings, future studies may consider just resigning fried potatoes to the unhealthy list.

Now, in terms of mortality, fried potatoes may not be as bad as fried meat—fried chicken and fried fish—but that’s not really saying much.  The French fry death data gave the industry trade group Potatoes USA a bit of a chip on their shoulder, reminding readers that observational studies can only prove correlation, not causation, to which the authors replied, “our data add to the pressing public health calls to limit fried potato consumption.” French fries may be so bad for you that it wouldn’t be ethical to do an interventional study and randomize people to eat them.

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