Q: I'm really confused about carbohydrates. I've been hearing so much about the glycemic index and glycemic load, but what do these terms really mean?
A: Carbohydrates have been controversial for decades. Many people have blamed bread, pasta, and potatoes for weight gain or diabetes. The indictment has not held up well, given that people who eat generous amounts of carbohydrates (many Asian cultures and vegetarians, for example) are the thinnest people on the planet, with enviably low diabetes rates. Not to mention the fact that carbohydrates have less than half the calories found in fats —4 calories per gram for carbs, versus 9 for fats. Carbohydrates power our movements, our thoughts, and myriad other functions by providing glucose—blood sugar. So how could they pose any sort of problem? There are legitimate issues about carbohydrates. The first problem is that carbs often act as a Trojan horse; hiding inside that seductive cookie or croissant may be enough butter or margarine to pack on the pounds. And carbohydrate-containing foods vary dramatically. A liter of cola and a bean burrito both contain a great deal of carbohydrates, but they have vastly different effects on the body.
Here are two markers for rating carbohydrate-containing foods:
Refined versus unrefined. When the outer (bran) layer and inner germ layer are removed from grains, the result is white bread, white pasta, or white rice, with most of the beneficial fiber gone. Unrefined (whole) grains keep the fiber intact.
High GI versus low GI. In 1981, David Jenkins, MD, PhD, DSc, from the University of Toronto invented the glycemic index to distinguish foods that release sugars into the bloodstream rapidly from those that release sugars more gradually. The test is done by feeding a measured amount of a test food to volunteers and tracking their blood sugar over the next two hours. Low-GI foods can help people with diabetes maintain healthful blood sugar levels. They may also help reduce triglycerides (blood fats). And for people who have bouts of low blood sugar (hypoglycemia), lower-GI foods can steady the blood sugar roller-coaster rides. But which foods are high-GI and which are low-GI? If you look online, you will find all manner of glycemic index charts, often with contradictory numbers. The most reliable site, glycemicindex.com, is from the University of Sydney. And to simplify things, here are some of the main high-GI foods, with their low-GI alternatives:
Wheat breads (white or whole wheat) have a high GI, while rye and pumpernickel are lower.
White potatoes have a high GI, while yams and sweet potatoes are lower. Most cold cereals have a high GI; bran cereals and oatmeal are lower.
Sugar itself is higher than most fruits, which, despite their sweetness, have an unexpectedly low GI. Some foods are wonderfully low-GI—beans and green leafy vegetables, for example. And pasta is low-GI, believe it or not, even though it is made from wheat flour, just like wheat bread. The explanation is that bread is made with yeast, which creates air pockets as it rises. During digestion, stomach acid and digestive enzymes can enter these air pockets and quickly break the bread apart. Pasta—yes, even white pasta—is denser, so it digests much more slowly.
Q: What's the difference between glycemic index and glycemic load?
A: The concept of glycemic load is that if you have only a little bit of a high-GI food, it will not perturb your blood sugar. But if you eat more of it, the effect is more pronounced. The glycemic load is calculated by multiplying the glycemic index of a food by the amount you're eating. In other words, it means that a food's effect on blood sugar depends not just on its GI, but on how much of it you actually put on your plate. So here's the bottom line: as a group, carbohydrate-containing foods are healthful energy sources that are not responsible for weight problems. And it's easy to pick out the best choices. The healthiest ones are those that are not impregnated with fat, keep their natural fiber, and are low-GI. That means most whole grains, legumes, vegetables, and fruits.