Aug. 26, 2004 -- You've heard how people can shed pounds on those controversial but popular low-carb diets. So how do rodents, those treasured laboratory test animals used to predict human results, eat their way to less body fat and better health?
By having plenty of carbohydrates, as long as they're low in their glycemic value.
This glycemic index (GI) indicates how much and how quickly blood sugar will increase after eating a carbohydrate-containing food. High-GI foods cause higher and more sudden spikes in blood sugar and have been linked to an increased risk of obesity and diabetes. Low-GI foods cause lower, slower rises in blood sugar. These foods have been associated with lower body fat and lower weight.
A low-glycemic diet plan differs from a low-carb one in that it encourages eating many types of carbohydrates initially forbidden in diets such as Atkins or South Beach. These include fruits, legumes, and grain products like bread, pasta, and cereals.
In new research published in this week's The Lancet, Harvard scientists add to evidence on just how effective a carb-centric, low-GI diet can be. So what's different about this study?
Rats Offer Evidence Humans Haven't
"There have been nearly 100 studies suggesting beneficial effects of a low glycemic diet, but no health organization in the U.S. officially recognizes their role," says researcher David S. Ludwig, MD, PhD, director of the obesity program at Children's Hospital in Boston. "That's because these studies are often criticized because it's difficult to separate effects of GI index in foods from those of other things that go along with it, like fiber. You can't keep humans keep locked up for a year, controlling everything about their diets."
But you can do this with caged rodents, so his team fed two groups of rats and mice -- both with identical weights at the study's start -- a diet comprised of nearly 70% carbohydrates that was identical in every way but one.
"They consumed exactly the same [amounts of] protein, fats, carbohydrates, and fiber -- and we went even further by feeding them in a way to keep their body weights identical," Ludwig says. "They only difference was the type of [carbohydrate] they received, one with either a low- or high-glycemic index."
"The animals on the high-GI diet were gaining more weight with same amount of food, and we had to cut their food back increasingly over time to keep them at the same weight," he tells WebMD.
"But what was really interesting to us was that even though they maintained the same weight because they got less food, the high-GI group in both rats and mice doubled their body fat and had a reduction ... in muscle mass, which is exactly what you don't want.
"They also had increases in their blood sugars, insulin, lipids, and other disease risk factors, and their pancreas beta cells that make insulin looked like they were going through a scarring process. If continued, that suggests a high likelihood of getting diabetes."
When his team switched the diets midway through the study, and those high-GI-eating rodents were given the low-GI diets, these adverse changes reversed. Meanwhile, the rodents switched from the low- to the high-GI diets started to have the same problems with added body fat, less muscle mass, and signs of impending diabetes.
Processing: The Root of Problems?
What do this mean to you?
Scientifically, it suggests that a low-GI eating plan may be a factor in the amount of body fat and muscle mass a person has and their risk for diabetes. Eating low-GI carbohydrates may not only prevent, but actually treat obesity-related problems. Ludwig is recruiting for a human study on low-GI diets to confirm these rodent findings.
But it also adds more evidence that carbs aren't necessarily the enemy, and you should have them as part of a healthy diet, says Ludwig. "Just as it's too simplistic to think that all fats are bad when, in fact, some are very healthful, it's too simplistic to consider all carbohydrates unhealthful."
The key is to eat those with a low glycemic index -- usually, those in their least processed state. There's no need to calculate your GI index with on-the-web charts, says Ludwig. Instead, just follow that often-preached advice of eating as "whole" as possible.
Although some carbohydrates in their natural state, such as potatoes and carrots, have a high GI, what more typically dictates whether a food has a high or low glycemic index is in its degree of processing. Adding corn sweeteners and other sugars and refining whole grains to "white" ones often raises its GI value and the problems that result from it.
That explains why Raisin Bran may be high in fiber, but the added sweeteners classify it as a high-GI food. Processed white bread also has a high GI, but stone-ground breads don't. Conversely, pasta, legumes, and fruits that are to be avoided on low-carb diets typically have a low GI, says Ludwig.
High Praise for High Carbs
"The advice is simple," he tells WebMD. "We want people to have an abundant of fruits, vegetables, beans, and nuts. They shouldn't restrict carbohydrates, just reduce consumption of those that have been refined and have concentrated sugars. Pasta is good and has a low glycemic index, just like many other foods restricted on low-carb diets."
He cites the much-ballyhooed Mediterranean diet, rich in healthy fats and whole, low-GI carbohydrates, as an excellent eating plan -- "nutritious, delicious, varied, flexible, and one gets away from a nutritional extreme." Ludwig's research comes on the heels of a Tufts University study published earlier this month showing that middle-aged spread can be avoided by eating a high-carbohydrate diet that focuses on unprocessed foods.
"Now that everybody is talking about counting carbs, many people believe that carbohydrates are the enemy," says Katherine Tucker, PhD, of the school's Jean Mayer USDA Human Nutrition Research Center on Aging, who conducted the Tufts study. "But the truth is very simple: It's the type of carbs you eat that makes a difference. You need to eat more whole foods and less refined foods."