Women who eat the most potatoes also appear to have a higher risk of gestational diabetes — a rare but dangerous condition that can affect pregnant women and their babies alike.
A new study finds that women who ate the most potatoes — baked, fried, mashed or boiled — had a higher risk of gestational diabetes. But if women just ate more whole grains, beans or vegetables instead of a serving or two, their risk appears to fall back to normal.
The researchers at the National Institutes of Health are not saying that potatoes are dangerous and they say their study doesn’t actually prove that eating more potatoes causes gestational diabetes. They say it just suggests that women should vary their diet more — something most nutritionists advise, anyway.
“Women who consumed more potatoes before pregnancy had higher rates of developing gestational diabetes mellitus,” Cuilin Zhang and colleagues at the National Institute of Child Health and Human Development and Harvard University wrote in the British Medical Journal.
“Substitution of two servings a week of total potatoes with other vegetables, legumes, and whole grain foods was significantly associated with a 9-12 percent lower risk of gestational diabetes.”
The Centers for Disease Control and Prevention estimates that 5 percent to 9 percent of pregnant women develop gestational diabetes, the type that shows up for the first time during pregnancy. That's why pregnant women are supposed to take a urine test to look for high glucose levels on every doctor visit.
About half of women who have had gestational diabetes develop full-fledged Type 2 diabetes in the months to years after their child's birth.
Related: Making the New Eating Guidelines Work for You
If untreated, the mother's high blood sugar can make the fetus grow too large, leading to C-sections and early deliveries. It also can trigger a potentially life-threatening condition called preeclampsia. It even increases the baby's risk of becoming obese in childhood.
Zhang and her team were curious about a growing body of research that suggests something called glycemic index of foods can affect diabetes risk.
They studied 15,000 women taking part in the larger Nurse’s Health Study. Volunteers fill out diaries of what they eat and how often and they get regular physical exams.
Over the 10-year study, 854 women developed gestational diabetes.
“We asked each participant how often, on average, they had consumed baked, boiled, or mashed potatoes or French fries during the previous year,” they said.
Women who ate one serving of potatoes a week were 21 percent more likely to develop gestational diabetes than women who ate less. Those who ate five or more servings a week — not a lot by most U.S. dietary standards — were 62 percent more likely to develop it, they found.
That’s not a giant increase in absolute risk. A 62 rise in risk takes the absolute risk of gestational diabetes from 5 percent to 8 percent.
Related: Can't We Eat Butter and Salt?
But potatoes are a big part of the U.S. diet.
“Potatoes are the third most commonly consumed food crop in the world, after rice and wheat,” the researchers wrote.
“In United States, about 35 percent of women of reproductive age (that is, aged 19-50) consume potatoes daily, accounting for 8 percent of daily total energy intake,” they added.
They are nutritious, but have some drawbacks.
For one thing, fried potatoes carry a lot of fat, and most potato dishes are a vehicle for butter, cheese, cream, bacon and other unhealthful extras. Plus they are starchy.
“Though potatoes are rich in vitamin C, potassium, dietary fiber, and some phytochemicals, unlike other vegetables they can have detrimental effects on glucose metabolism because they contain large amounts of rapidly absorbable starch,” Zhang’s team wrote.
Other studies have linked high potato intake to higher levels of blood sugar and a higher risk of diabetes.
Now, it’s possible that people who eat a lot of potatoes have poor diets in general, so the researchers tried to account for this. They noted that women who ate more potatoes tended to weigh more at the start of the study and to smoke. But the risk remained clear even when this was factored in.
“Potatoes are a food with a high glycemic index because of the large amount of starch that is absorbed rapidly after ingestion,” they wrote.
The sharp rise in blood sugar after eating a serving of potatoes can stress the beta cells in the pancreas that produce insulin. It’s the constant stress on these pancreatic cells that causes type-2 and gestational diabetes.
“In contrast, other vegetables, legumes, and whole grain foods typically have a low glycemic index, in addition to their richness in vitamins, minerals, fiber, and bioactive phytochemicals; therefore, substitution of potatoes with these foods could lower the risk of gestational diabetes,” the researchers concluded.