Up and Down the Ranks
Elizabeth Mayer-Davis says the glycemic index is bunk .
As you may recall from previous e-Alerts, the glycemic index (GI) is a scale that categorizes food items according to their effect on blood sugar levels. Low GI foods (such as meat and most fruits and vegetables) prompt a slow increase in blood sugar levels, while high GI foods (such as foods with added sugar, processed baked goods and starchy foods) produce a quick spike in blood sugar levels.
A steady intake of foods that rate high on the GI promotes a gradual insensitivity to insulin – the precursor of type 2 diabetes. But some nutritionists don’t agree that the GI is a useful indicator. Enter Ms. Mayer-Davis who recently led a study that found the GI to be an ineffective measure. According to an Associated Press article about the study, Mayer-Davis “says the use of the index should be ended altogether.”
When you make a claim as bold as that, you’d better have bold data to back it up.
Ms. Mayer-Davis and her team studied the diets and blood sugar levels for more than 1,250 adult subjects. The study lasted five years. Taken at face value, those two details promise the possibility of an exhaustive study. But in the end, it wasn’t exhaustive at all.
As reported in the February 2006 issue of the British Journal of Nutrition, the researchers asked subjects to fill out only two food frequency questionnaires: at the beginning of the study and again at the end. Blood sugar levels were also tested only at the beginning and end. Along the way, more than 440 subjects dropped out of the study.
When the dietary information was analyzed for glycemic index rankings and compared to the results of the blood sugar tests, researchers found no association between glycemic index levels and blood sugar levels.
And that’s it. Based on that, Ms. Mayer-Davis would advise us to ignore the “flawed” (as she puts it) glycemic index.
Pieces in a larger puzz le
Let’s take a quick look at another glycemic index study I told you about last year. This one comes from the Children’s Hospital in Boston.
Twenty-three obese subjects were divided into two groups. One group of 12 subjects followed a conventional low-fat, reduced calorie diet. The other group of 11 subjects followed a low-GI diet, avoiding starchy foods and consuming protein along with any carbohydrates they ate. In addition, subjects in the low-GI group were unrestricted in their calorie intake. Subjects in both groups were also urged to exercise regularly.
One year later, the low-fat subjects had lost a little more than six percent of their body weight on average, while low-GI diet subjects lost an average of nearly eight percent of their body weight. That difference may seem minor in whole numbers, but it’s a change of 33 percent.
More importantly, markers for heart disease were significantly improved by the low-GI diet. Low-fat dieters reduced their triglyceride levels by less than 20 percent on average. Subjects on the low-GI diet, however, reduced triglycerides by nearly 40 percent. And concentrations of plasminogen activator inhibitor 1 (a coagulation factor that increases blood clot risk) increased by more than 33 percent among the low-fat dieters, but DECREASED nearly 40 percent in the GI-diet group.
Which way is up?
Neither the Children’s Hospital study nor the Mayer-Davis study provides the last word on the usefulness of the glycemic index. Both have strengths and weaknesses, and both lay the ground word for future research.
In other words, to isolate the Mayer-Davis study and insist that it represents a final verdict is way off the mark.
In a press release posted by the University of South Carolina (Mayer-Davis is a researcher in the university’s Arnold School for Public Health) Ms. Mayer-Davis states: “Several recent studies show that dietary fiber is important to heart disease, diabetes and obesity. Typically, foods high in fiber have a relatively low Glycemic Index.”
The press release adds: “This means that, in some studies, the Glycemic Index may have been related to good health because of dietary fiber, not because of a unique characteristic of food called the Glycemic Index, Mayer-Davis says.”
Is it just me, or does that sound like a pretty good argument in SUPPORT of the glycemic index?
If you’d like to see what all the fuss is about, go to glycemicindex.com, where you can access a large database that lists foods and their GI rankings.
“Towards Understanding of Glycaemic Index and Glycaemic Load in Habitual Diet: Associations with Measures of Glycaemia in the Insulin Resistance Atherosclerosis Study” British Journal of Nutrition, Vol. 95, No. 2, February 2006, ingentaconnect.com
“Study Casts Doubt on Glycemic Index” John C. Drake, Associated Press, 3/1/06, ap.org
“Study Questions Glycemic Index as Diet Tool” University of South Carolina press release, 2/28/06, newswise.com