Why would one high-carb food have a different GL than another? Why does white rice, for instance, have a higher GL than, say, honey? It has to do with the way nature constructed them. Carbohydrates consist of starches and sugars. Starch — think of starchy foods like beans and potatoes — is made up of sugar molecules bound together in long chains. When you eat a carbohydrate- rich food, your body converts those starches and sugars into glucose, or blood sugar.
Some starches, like those in white rice, are extremely easy for the body to convert, and therefore blood sugar levels rise like a hot temper after you eat them. Others, like those in beans, take a lot more work to break down, so blood sugar levels simmer rather than explode.
Four factors determine how fast the body breaks down carbohydrate.
The Type of Starch — or Why to Avoid Sticky Rice
Remember, starches are made of sugar molecules chained together. Some chains have straight edges, while others are branched. The straightedged type, called amylose, is harder for your body to break down and turn into blood sugar. The branched type, called amylopectin, is much easier to break down because there are so many places for the enzymes that break down starch to get at it. Think of a tree with lots of branches — there are a lot more spots for birds to land on it compared to a simple post.
White potatoes are very high in amylopectin, the branched kind of sugar chain, which is why they raise your blood sugar in a jiffy. Peas and lentils are high in amylose, the straight kind, so they’re converted to blood sugar at a snail’s pace. The more amylose a food contains, the slower it will be digested and converted into blood sugar. Take rice, for instance. Some types contain more amylose than others. In general, the softer and stickier the rice is after cooking, the lower its amylose content; this is why “sticky rice” is dastardly to your blood sugar. The firmer the rice, the higher the amylose and the harder it is for your body to turn into blood sugar quickly — making brown rice a better choice. Some genetic variants of rice — such as some sold in Australia, for example — are particularly high in amylose (as much as 25 percent), but unfortunately, most of the rice we eat is low in amylose and thus has a high GL.
The Type of Sugar — or Why Fruit Is A-Okay
Sugar is the molecule that makes up carbohydrates, but there is more than one kind. There’s table sugar (sucrose) as well as the kind found in fruits and grains (fructose), the kind in milk (lactose), and the kind in malted barley (maltose).
The sugar in milk and fruit tends to be absorbed more slowly than other sugars since it needs to be converted into glucose by the liver first, which is why these foods are gentle to your blood sugar.
Ironically, table sugar, which is half fructose and half glucose, is turned into blood sugar more slowly than some starches, like bread or potatoes. That doesn’t make sugar good for you, of course. One reason is that fructose, especially in the amounts contained in packaged foods loaded with high-fructose corn syrup, raise triglycerides, blood fats that increase the risk of heart attack. (Fruit, by contrast, contains a little fructose plus plenty of water, fiber, and nutrients.) The other reason is that sugar packs a lot of carbohydrate calories in a small package.
That’s why one 32-ounce (1-liter) cola drink contains a whopping 400 calories — and will send your blood sugar soaring.
Processing — or Why to Avoid White Flour
All starch, whether it’s made of straight or branched chains, is composed of crystals, which don’t dissolve in cold water. Think of a grain of rice or a piece of raw potato — put it in water and it stays the same. But heat breaks down those crystals so the starch can dissolve in water — a little like a snowflake that comes in from the cold. When you cook a starchy food, it absorbs water and becomes easier to digest.
The more overcooked rice or pasta is, the faster it makes your blood sugar rise. When starch is heated and then cooled, it can return, in part, to its crystal form; that’s why hot potatoes have a high GL, while potato salad’s is slightly lower. Just make it with olive oil instead of mayo to keep it healthier.
Have you ever noticed that some wheat breads are as smooth as white bread, while others have crunchy kernels in them? Those kernels take a long time for your body to break down. So do any whole, intact grains, such as wheatberries (small kernels of wheat, delicious in salads). Modern commercial flour, on the other hand — especially white flour — is extremely easy for the body to turn into blood sugar, which is why we suggest throughout this book that you choose whole grains that are still intact and foods such as beans, lentils, and wheatberries instead of those made from white flour. (Unfortunately, we’re surrounded by white-flour foods. You’ll need to make a conscious effort to cut back.)
Modern manufacturing also allows grains to be turned into highly processed forms such as cornflakes or puffed corn snacks, which tend to have higher GLs than grains left intact, like popcorn, or those milled in an old-fashioned manner, like coarse, stone-ground whole wheat flour used in stoneground wheat bread.