Eating Potatoes May Increase Risk for Type 2 Diabetes

News Author: Laurie Barclay, MD
CME Author: Désirée Lie, MD, MSEd

Disclosures
To earn CME credit, read the news brief along with the CME information that follows and answer the test questions.

Release Date: March 2, 2006; Valid for credit through March 2, 2007

Credits Available

Physicians - up to 0.25 AMA PRA Category 1 continuing medical education credits for physicians;
Family Physicians - up to 0.25 AAFP Prescribed continuing medical education credits for physicians

March 2, 2006 — Eating potatoes is associated with an increased risk for developing type 2 diabetes, according to the results of a prospective study reported in the February issue of the American Journal of Clinical Nutrition.

"The role of potatoes in a diet aimed at reducing the burden of chronic disease has been controversial," write Thomas L. Halton, MD, from Harvard Medical School and Brigham and Women's Hospital in Boston, Mass, and colleagues. "Potatoes, a high glycemic form of carbohydrate, are hypothesized to increase insulin resistance and risk of type 2 diabetes."

The investigators conducted a prospective study of 84 555 women, aged 34 to 59 years with no history of chronic disease, who were enrolled in the Nurses' Health Study. The women completed a validated food frequency questionnaire (FFQ) at baseline, and they were followed up for 20 years with repeated dietary assessments.

During the study, 4496 participants were diagnosed as having type 2 diabetes. After adjustment for age and dietary and nondietary factors, potato and french fry consumption were both positively associated with risk for type 2 diabetes. Comparing the highest and the lowest quintile of potato intake, the multivariate relative risk (RR) for diabetes was 1.14 (95% confidence interval [CI], 1.02 - 1.26; P for trend, .009). Comparing the highest and the lowest quintile of french fry intake, the multivariate RR was 1.21 (95% CI, 1.09 - 1.33; P for trend < .0001).

The RR for type 2 diabetes was 1.18 (95% CI, 1.03 - 1.35) for 1 daily serving of potatoes; 1.16 (95% CI, 1.05 - 1.29) for 2 weekly servings of french fries; and 1.30 (95% CI, 1.08 - 1.57) for substituting 1 serving of potatoes per day for 1 serving of whole grains per day.

In obese and sedentary women, the association between potato consumption and risk for type 2 diabetes was more pronounced. These subgroups were thought to be more likely to have underlying insulin resistance, which may exacerbate the adverse metabolic effects of higher glycemic carbohydrates.

"Our findings suggest a modest positive association between the consumption of potatoes and the risk of type 2 diabetes in women," the authors write. "This association was more pronounced when potatoes were substituted for whole grains."

A limitation of this study is the inability to completely separate the effects of potatoes and french fries from the effects of the overall Western dietary pattern.

"These data support a potential benefit from limiting the consumption of these foods in reducing the risk of type 2 diabetes," the authors conclude. "Substitution of these sources of carbohydrate with lower glycemic, high-fiber forms of carbohydrates such as whole grains should be encouraged."

The National Institutes of Health supported this study. The authors have disclosed no relevant financial relationships.

Am J Clin Nutr. 2006;83:284-290
Learning Objectives for This Educational Activity
Upon completion of this activity, participants will be able to:

* Describe the association between potato and french fry consumption and risk for type 2 diabetes in women.
* Describe the effect of obesity and whole grain consumption on risk for type 2 diabetes associated with potato and french fry consumption in women.

Clinical Context

According to the authors, in 1992 the US Department of Agriculture (USDA) Food Guide Pyramid included potatoes in the vegetable category and considered it a healthful food, and this was endorsed by the American Dietetic Association and the American Heart Association. However studies have shown a positive association between high potato consumption and subsequent risk for type 2 diabetes perhaps because of the high glycemic index of the rapidly absorbed starch in potatoes. The current authors hypothesized that substitution of potatoes with whole grains would attenuate this effect of potato consumption. They also theorized that obese persons with high insulin resistance would show a greater risk for type 2 diabetes associated with potato consumption.

The current trial is a prospective cohort study of women in the Nurses' Health Study to examine the association of potato and french fry consumption with risk for type 2 diabetes for 20 years in American women. The authors acknowledged that potato and french fry consumption were part of a Western pattern diet, and the association may be linked to this dietary pattern itself.
Study Highlights

* The study was initiated in 1976 and included 121 700 registered female nurses aged 35 to 55 years who completed a mailed questionnaire. 98% were white.
* Information on disease status and lifestyle was collected every 2 years.
* Diet was assessed using a semi-quantitative FFQ in 1980, 1984, 1986, 1990, 1994, and 1998.
* Women were excluded if they reported implausible daily caloric intake or left more than 10 food items blank, and if they had a history of diabetes, cancer, or cardiovascular disease.
* A total of 84 555 women remained and were followed up for 20 years.
* Participants reported the average frequency of consumption of foods with commonly used portion sizes for the previous year.
* A single potato serving was defined as 1 baked or 237 mL (1 cup) of mashed potatoes. A serving size of french fries was defined as 113 g (4 oz).
* The possible responses of potato and french fry consumption ranged from "never" or "less than 1 per month" to "6 or more times per day."
* When corrected for week to week variations, the correlation between the 1980 FFQ and the diet records were 0.66 for potatoes and 0.60 for french fries.
* All food consumption values were obtained from the Harvard University Food Composition Database, which was derived from USDA sources.
* Physical activity was documented as average number of hours spent per week in moderate or vigorous activity.
* Type 2 diabetes was defined as one of the following: (1) classic symptoms plus a fasting plasma glucose (FGP) level of 140 mg/dL or more or random glucose level of 200 mg/dL or more; (2) 2 or more elevated FPG on different occasions, including FPG of 140 mg/dL or more, or random glucose of 200 mg/dL or more or abnormal glucose tolerance test in the absence of symptoms; or (3) use of hypoglycemic agents.
* Frequency of potato or french fry consumption was divided into 5 quintiles.
* Researchers controlled for confounding variables, including smoking, physical activity, body mass index, use of hormone replacement, and family history of diabetes.
* At baseline, median potato consumption varied from 0.07 servings daily in the first quintile to 0.79 servings in the highest quintile. French fry consumption varied from 0 servings daily to 0.14 servings daily.
* Those who consumed potatoes and french fries had a higher dietary glycemic load and were more likely to consume red meat, refined grains, and had higher total caloric intake.
* For 1 597 429 person-years of follow-up, 4496 incident cases of type 2 diabetes were documented.
* For potatoes, the age-adjusted RR for type diabetes was 1.13 (P for trend .02) when the highest quintile of intake was compared with the lowest quintile. Adding body mass index as a variable increased the RR to 1.18 (P for trend .0003).
* The RR did not change after adjustment for family history, smoking, physical activity, and hormone therapy (1.17) and changed to 1.14 after correction for other dietary variables.
* The age-adjusted RR for french fry consumption between the highest and lowest quintile of intake was 1.65 (P for trend <.0001) attenuating to 1.25 (P for trend <.0001) after adjustment for smoking, family, physical activity, and hormone use. Adjustment for dietary factors changed the RR to 1.21 (P for trend <.0001).
* The RR for substituting 1 serving of potatoes for 1 serving of whole grains daily was 1.30 and the RR for substitution for 1 serving of refined grains was 1.22.
* The association between potato consumption and type 2 diabetes was statistically significant in obese but not nonobese women. This did not apply to french fry consumption.
* Red meat was significantly correlated with consumption of potatoes and french fries.

Pearls for Practice

* Consumption of potatoes and french fries in healthy women is associated with increased risk for type 2 diabetes.
* The risk for type 2 diabetes associated with potato consumption is present only in obese women and is more pronounced when substitution of potatoes for whole or refined grains occurs.