Potatoes Villainized by Questionable Science

April 4, 2006
Questionable conclusions and reckless reporting have painted potatoes as dietary 'bad guys.' Dr. Anthony begs to differ.

"Your ears are so dirty; you can grow potatoes in them!" Years ago that line was used to goad kids into bathing. It was a harmless, even humorous lie. But a not-so-harmless lie, one that could actually be dangerous: A recent study from the departments of nutrition and epidemiology in the Harvard School of Public Health linked consumption of potatoes with type 2 diabetes. As if the Atkins fad weren't misleading enough, consumers are being influenced to join another game of connect the dots joining potatoes to diabetes.

The study included 86,455 participants from the Nurses' Health Study who filled out food frequency questionnaires periodically over the 20 years from 1980 to 2000. During that time, 4496 of the participants (about 5.4 percent) developed type 2 diabetes. After calculating multivariate relative risk (RR) comparing different levels of potato and French fry consumption, the authors found a modest association with the development of type 2 diabetes in women, notably obese women. The authors suggested that, based on these data, we should eat fewer potatoes and French fries, and substitute lower-glycemic, high-fiber carbohydrates such as whole grains.

Are potatoes dangerous? That was the headline that flashed across the screen, if you watched CNN immediately after the report surfaced.

To answer that, we have to know the strength of the association. The study divided intake of potatoes and French fries into five quintiles, lowest to highest. Potato consumption ranged from 0.07 servings per day to 0.79 in the highest. French fry intake ranged from 0 to 0.14 servings per day.

The authors then expressed data in round numbers and gave an average relative risk of developing type 2 diabetes. After adjusting for potentially confounding variables (BMI, family history of diabetes, smoking, postmenopausal hormone use, physical activity, trans fat consumption, the ratio of polyunsaturated fat to saturated fat, cereal fiber and total calories), the RR for developing type 2 diabetes was 1.18 for consuming 1 serving of potatoes per day and 1.16 for eating 2 servings of French fries per week.

That equated to a nearly 20 percent increased risk of developing type 2 diabetes, just by eating potatoes on a regular basis - modest, but still pretty scary.

One thing that stands out about the results of this study is how much more "dangerous" French fries are than potatoes, even after controlling for trans fats that would have been in the frying oil. Eating two servings of French fries per week is allegedly as "bad" as eating fresh potatoes every day.

Tuber the Terrible? Not so, if you dig beneath the surface to get the real dirt on this venerable vegetable. Photo courtesy of healthypotato.com.

But the reporting is flawed. Presumably, the subjects ate about five times as much potatoes as French fries, and this was said to be similar to the consumption of potatoes and French fries for the general U.S. population during the years 1980-2000. Yet a quick check of the USDA website reveals that during those years, consumption of frozen potatoes (the vast majority of which would be used to make French fries) went from 39.4 to 57.6, and average of 48.5 pounds per capita per year, while fresh potatoes went from 51.12 to 47.14, an average of 49.1 pounds. That's about the same, certainly not five times greater.

Also ignored: subjects who ate either more potatoes or more French fries also ate more red meat, more refined grains and more total calories.

Another insurmountable flaw: The significance of the association between potatoes and type 2 diabetes disappeared when subjects were not obese.

These are just three of numerous fundamental failures of the study that were "conveniently" ignored. So we have to ask: Was the study really an indictment on the fast-food pattern of eating and not of potatoes? The authors state the following: "White potatoes and French fries are large components of a ‘Western pattern' diet.

"This dietary pattern is characterized by high consumption of red meat, refined grains, processed meat, high-fat dairy products, desserts, high-sugar drinks, and eggs, as well as French fries and potatoes. A Western pattern previously predicted a risk of type 2 diabetes. Thus, we cannot completely separate the effects of potatoes and French fries from the effects of the overall Western dietary pattern."

The paper to which the authors of the Harvard study refer is a study of dietary patterns and risk for type 2 diabetes in men, reported in Annals of Internal Medicine, 2002. In the paper, which followed 42,504 male health professionals for a period of 12 years, the "Western dietary pattern" was associated with a modest increase in type 2 diabetes (a relative risk of 1.59).

However, the "Western dietary pattern" was characterized by a "high consumption of red meat, processed meat, refined grains, French fries, high-fat dairy products, sweets and desserts, high-sugar drinks and eggs." The correlation of potatoes with the Western pattern of eating was low - the same as nuts - and the association with type 2 diabetes was nonexistent. The only individual foods that had a significant positive association with type 2 diabetes were red meat and refined grains.

To say the effects of French fries cannot be completely separated from the effects of the overall Western dietary pattern may be true in this case. But to say the same of potatoes in general is to make an absolutely spurious connection, to say the least.

Why might eating potatoes and French fries increase your risk of developing type 2 diabetes? According to the authors, it's because potatoes have a high glycemic index. Yet French fries, which are presumably more "dangerous" than potatoes (even after accounting for trans fats) have a lower glycemic index than fresh potatoes.

The difference between French fries and fresh potatoes becomes even more glaring when you look at what how we have eaten potatoes during the entire time that fast foods have influenced our eating. To do that, we have to go back to data on potatoes from 1960. Frozen potato consumption in 1960 was a paltry 7.6 pounds per person per year; we hadn't cranked up the fryers to full force yet and were consuming fresh potatoes at a rate of 81 pounds per person, per year - about double the amount we now consume.

During that same time, other components of the "Western dietary pattern" sharply increased: high-sugar beverages (particularly those with high-fructose corn syrup and which has a low glycemic index), refined grains, cheese and pizza. The latter has a low glycemic index, especially if you load it with high-caloric meat toppings.

Most Americans now eat more calories, and move a lot less, than did our grandparents. And the quality of fast food isn't up to par with prudent dietary recommendations, which include eating more fruits and vegetables and whole grains. Of course, all this has a powerful impact on obesity and type 2 diabetes.

But this does not explain the study's attempt to implicate fresh potatoes as part of the fast-food pattern of eating when clearly they are not.

Fresh potatoes are as close to French fries as whole grains are to doughnuts. In the twisted logic of the Harvard study reporters, all potatoes must be bad foods, all potatoes because if they're not, then glycemic index doesn't make sense as a reason for obesity and type 2 diabetes. But if Harvard went by the facts, the Harvard pyramid - which has potatoes at the top - would be rendered useless.

As Darrell Huff and Irving Geis wrote in their best seller, How to Lie With Statistics, "If you torture the data long enough, it will confess." Excuse me; I think my potatoes are nearly done.

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