We’ve churned through Atkins, South Beach, and The Zone and seen the rise and fall of countless other “miracle” diets. But as the nation’s collective waistline continues to swell, along with rates of heart disease and diabetes, many believe the solution lies in a decades-old system called the glycemic index. “It’s not glamorous, it doesn’t have any sizzle, but it works,” says Lucy Beale, a weight-loss coach in Utah and co-author of The Complete Idiot’s Guide to Glycemic Weight Loss (Penguin, 2005).
Created nearly 30 years ago, the glycemic index ranks carbohydrates on how much they raise blood sugar. It has been generating considerable buzz, with such celebrities as Bill and Hillary Clinton among its fans and TV commercials heralding it as the key to weight loss. At the same time, a chorus of critics has emerged questioning the index’s purported benefits and arguing that following it too strictly leads to an unhealthy diet.
Diabetes researchers in Canada invented the index in the late 1970s while testing the effect of starchy foods on blood sugar. When you eat carbohydrates, digestive enzymes break them down to glucose, which enters the blood and raises blood-sugar levels. The pancreas pumps out insulin, prompting cells to take in the glucose to either use as energy or convert to fat.
During the 1970s starch tests, the researchers discovered that—contrary to conventional wisdom at the time—not all carbs are created equal. Some, like Russet potatoes, speed through the digestive system and send blood sugar and insulin levels soaring and crashing fast; others, like lentils, metabolize far more slowly. Surprisingly, much maligned foods—like ice cream—actually spike insulin less than healthy-seeming ones like rice cakes.
In the glycemic index system, foods receive a score from zero to 100 based on how much and how quickly they raise blood sugar levels. Pure glucose scores a 100, while proteins and fats, which don’t impact blood sugar, get a zero. A score of 70 or higher qualifies as high glycemic; 56 to 69, medium; and 0 to 55, low. For years, the index didn’t spark much interest. But fast forward to 2006, and diet gurus and health experts have resurrected it, calling the low-glycemic or “slow carb” diet a healthier evolution of the low-carb fad.
“Part of the rationale of the low-carb diet is to reduce those radical spikes and ebbs in insulin,” says Thomas Wolever, MD, a professor in the Department of Nutritional Sciences at the University of Toronto and one of the pioneers of the index. “The GI is a way to do that without reducing the carb intake and without eating more fat and protein.” A growing body of research suggests that stabilizing blood-sugar and insulin levels not only lowers the risk for diabetes, but also fends off heart disease, certain cancers, and age-related macular degeneration. One Harvard study, for example, found that those who ate foods higher on the index had nearly twice the risk for a heart attack over a 10-year period. In another 2006 study, conducted at Tufts University, researchers followed 500 women between 53 and 73 years old and discovered that those who had eaten a high-GI diet during the previous decade were more than twice as likely to show early signs of macular degeneration.
Why should blood-sugar spikes contribute to these various diseases?
“There are some new theories that suggest when blood glucose is fluctuating, this puts a stress on cells and causes inflammation,” Wolever says. “And we know inflammation can be related to various chronic conditions.” Over the years, Wolever and Australian researcher Jenny Brand-Miller, PhD, a nutrition professor at the University of Sydney, have tested nearly 1,000 foods by feeding them to people and testing their blood glucose levels in the two hours afterward. The values are published in their book The New Glucose Revolution: The Authoritative Guide to the Glycemic Index (Marlowe and Co., 2003), the seminal tome that re-ignited worldwide interest in the index.
Even without precise GI numbers, Beale says people don’t need a secret decoder ring to figure out how foods rank on the index. Yes, a few surprising exceptions exist, like with ice cream and rice cakes. But as a general rule, more refined carbohydrates and finely milled breads and pastas tend to be higher glycemic, while coarser whole grains, high-fiber cereals, and legumes are lower. In essence, “Light and fluffy makes you puffy,” Beale says. She recommends roughly 85 percent of the day’s carbs come from low-glycemic sources.
Beale says low-glycemic foods help with weight loss because you digest them more slowly, which keeps you full longer. They also don’t trigger the same spikes in insulin and cortisol (a stress hormone) as higher glycemic foods. Elevated insulin and cortisol levels have been associated with both greater appetite and increased fat storage in the midsection. “Choosing low-glycemic foods keeps your waist thin,” says Beale, who maintains her “size 16 to size 6″ transformation with a low-GI diet.
A lotta hype?
Not everyone accepts such lofty claims, though. Many dietitians have criticized the diet, largely because many high-fat foods, including candy bars and pizza, are rated as low glycemic. Plus, despite the clinically proven disease-related benefits of the low-GI diet, research hasn’t yet concluded that it leads to weight loss.
Another tricky aspect of the glycemic index, according to Elizabeth Mayer-Davis, a diabetes researcher with the University of South Carolina, is that the glycemic value of a food can vary widely depending on what you eat with it. For instance, a sour food, like lemon, slows digestion of the food it accompanies, effectively lowering its glycemic ranking. How you cook a food also matters, as in the case of pasta (the longer you cook noodles, the higher the GI). Even the brand can affect GI ranking. With that much variability, Davis questions how useful the numbers are.
She also fears that people may shun foods that, though high glycemic, are rich in vitamins and antioxidants—choosing instead low-glycemic but nutritionally impoverished ones. “There are people who are thinking that perhaps they shouldn’t eat this fruit or that vegetable because of the glycemic index, and that is very unfortunate because there are so many other values to those foods,” she says. “I think it has been a real distraction to people trying to manage their weight or otherwise have a healthy diet.”
Controversy notwithstanding, the index continues to gain momentum. The New Glucose Revolution series has sold more than 2 million copies. Food manufacturers in Australia now include GI values in the nutrition label. And several nutrition bars, like Solo GI, Balance, and even Snickers Marathon, are specifically marketed as low glycemic.
So, is it a passing fad, or a lasting cure-all? Brand-Miller, who is slightly irritated by the hype the index is receiving in the US, insists it’s neither. Instead, she says, it offers a way to fine-tune a healthy diet filled with fruits, vegetables, and whole grains, as well as protein and healthy fats. By paying attention to the kinds of carbs we eat—while keeping in mind other commonsense nutritional guidelines—she believes people can go a long way toward preventing heart disease, diabetes, and other illnesses. If weight loss follows, that’s an added bonus. “Nobody, including me, ever said it was a magic bullet,” she says. “It’s one tool in the toolbox. It can be a helpful tool, but it’s not the only one.”