Medscape Medical News, a service that helps doctors keep current with the medical literature, recently reviewed two papers that purported to demonstrate risks or benefits of certain foods. It happens that the foods that were studied are some of my favorite comfort foods: potatoes, french fries, and chocolate milk. So, these studies were bound to get my attention.
Potatoes and french fries
The first study, Medscape described under the headline, “Eating Potatoes May Increase Risk for Type 2 Diabetes” was published in the February 2006 issue of the American Journal of Clinical Nutrition. The lead author, Thomas Halton, is a researcher at the Department of Nutrition at Harvard’s prestigious School of Public Health. Funding for the study came from the National Institutes of Health, a food-neutral source.
(Side note: It is always helpful to know where the money to fund a study comes from when you review a medical research paper.)
The authors point out in the introduction to the paper that white potatoes have been included in the “vegetable” category of the U.S. Department of Agriculture’s Food Guide Pyramid. Vegetables, of course, are foods we are encouraged to consume. But should we really consider consuming white potatoes in the same way we consume green and orange vegetables—the more, the better?
Glycemic index and glycemic load
White potatoes contain large amounts of easily absorbed starch and they have a high glycemic index (GI) and glycemic load (GL). Glycemic index is a measure of how much that food raises blood glucose compared with a standard carbohydrate, such as glucose or white bread. Glycemic load takes into account the amount of carbohydrate contained in the food in addition to its glycemic index. A number of scientific studies have shown an association between a high glycemic diet and the risk of type 2 diabetes.
The Nurses Health Study
The study utilized data from the Nurses’ Health Study which followed a large number U.S. nurses over a period of more than 20 years. Individuals who already had diabetes or other conditions in which diets might be altered were excluded from the study. Data from the remaining 84,555 (mostly white) women were analyzed, including data from a food frequency questionnaire that was filled out every four years. Individuals reported on the size and frequency of their potato and french fry consumption as well as consumption of a large number of other foods. Every two years, they reported their body weight, physical activity, smoking status, family history of diabetes, and whether or not they took postmenopausal hormones. They also reported whether or not they had been diagnosed with diabetes. Multiple statistical tests were performed to determine if there was an effect of potato consumption on the later development of diabetes.
Here is what they found. Women who ate the most potatoes and french fries had a higher glycemic load than women who ate the smallest amounts. They also ate more red meat, refined (as opposed to whole) grains, and had higher total calorie intake than the low potato consumption group. Hmmm…these were “meat and potato” people.
When comparing the high potato consumers with the lowest potato consumers, the relative risk (RR), adjusted for age, was 1.13 for potatoes and 1.65 for french fries. Adjusting the risk for other dietary factors minimally increased the potato RR to 1.13, but more significantly reduced the french fries RR to 1.25. When adjusted for body weight, the researchers found that the association between potato consumption and type 2 diabetes was statistically significant in obese, but not in non-obese women.
So what does this mean? First of all, it is important to remember that many obese people manifest resistance to the hormone insulin, a condition known as pre-diabetes. Left untreated, many prediabetic individuals will go on to develop full-blown type 2 diabetes. So the effect of potato consumption on the development of diabetes is occurring in individuals who are already at risk of developing the condition. Second, it is important to note that the reported risk is quite modest.
Understanding relative risk
Relative risk (RR) is the difference in risk of a certain outcome in one group compared to another group. For example, the relative risk of cigarette smokers getting lung cancer is 20 times higher than the risk in nonsmokers. The RR in this example is 20. Most scientists are skeptical of results when RRs are less than 3 and most would not believe a causal association if the RR is less than 2. In this study, the RRs ranged from 1.13 to 1.25, suggesting a weak association if one in fact really does exist.
The bottom line
I am not suggesting that potatoes or French fries be considered health foods. On the other hand, I do think it is a bit of a stretch to declare potato consumption a risk factor for type 2 diabetes based on this study’s results.
The second study appeared in Medscape Medical News under the banner headline “Chocolate Milk May Improve Recovery After Exercise.” It was published in the February 2006 issue of the International Journal of Sport Nutrition and Exercise Metabolism. The study was funded, in part, by The Dairy and Nutrition Council, an organization that must have been pleased with the way the results were presented. The study co-author, Joel M. Stager, Ph.D from Indiana University in Bloomington, was quoted in a news release about the study as saying “…chocolate milk is a strong alternative to other commercial sports drinks in helping athletes recover from strenuous, energy-depleting exercise.”
The researchers studied 9 male endurance cyclists on 3 separate days. They were randomly assigned to drink, after a bout of exercise to exhaustion, either chocolate milk, Gatorade, or Endurox, a sports drink with a carbohydrate content equivalent to the chocolate milk. Four hours after the first bout of exercise, the study subjects again cycled to exhaustion. Various parameters of exercise performance were measured, including lactic acid levels, VO2 max, time to exhaustion, and total work performed. Over the course of the 3 testing periods, each of the cyclists had a chance to drink each of the three study liquids.
Subjects were able to cycle 49% and 54% longer following ingestion of chocolate milk or Gatorade compared with Endurox. The total amount of work performed was 57% and 48% greater for chocolate milk and Gatorade compared with Endurox. To be fair, the authors did note that it was possible that the four hours of recovery time could have been too short to allow for complete digestion of the complex carbohydrates contained in Endurox.
The big “so what”
At this point, you may be thinking “what a waste of time and money” …So what if chocolate milk is as good as Gatorade in helping athletes perform well during a second bout of strenuous exercise. If that’s what you’re thinking, I have to agree. Not only did this study not demonstrate a chocolate milk advantage over a commonly consumed sports drink, Gatorade, the study may have disadvantaged the other sports drink, Endurox, by having a short recovery period. I am not sure how many sports events involve exercise to exhaustion followed four hours later by another bout of exercise to exhaustion. Further, in all my years of running 10Ks and marathons, I never once had a hankering for a great big glass of chocolate milk after the race. In fact, the thought of it makes me a bit queasy.
So, there you go. Potatoes, french fries, and chocolate milk—the good, the bad, and the ugly. These studies may not have helped us understand much about the value or risks of these comfort foods, but they have been very useful in furthering our understanding of what’s good and what’s not in medical study design and interpretation.