For years, doctors assumed that “a carb is a carb,” that it didn’t matter if sugar came from an apple or a candy bar, the effects on blood sugar and insulin levels were the same. This didn’t seem right to some doctors, so in the 1980s scientists set out to see if some carbohydrates affect blood sugar and insulin levels differently from others even if they deliver the same amount of sugar into the bloodstream.
For each food they studied, they measured the amount of carbohydrate that the digestive system breaks down to sugar—the “available” carbohydrate. Then they fed volunteers whatever amount of each food it took to provide 50 grams of available carbohydrate. Sure enough they found that some carbohydrates raise blood sugar and insulin levels more than others even though the amount of sugar that eventually enters the bloodstream is the same. For example, 50 grams of sugar provided by white bread raises blood sugar levels twice as much as 50 grams of sugar delivered by carrots.
How high the blood sugar rises after eating a carbohydrate—whether fruit, vegetables, bread, potatoes or rice– depends on how fast the digestive system breaks it down to sugar. Cell membranes and cellulose encase the sugar in fruits and vegetables, which protects them from digestive enzymes. The fiber in fruits and vegetables acts like a sponge, soaking up sugar and slowing its digestion. The digestive system ultimately overcomes these barriers and sugar eventually enters the bloodstream, but the process takes time. The slower sugar enters the bloodstream, the less it raises blood sugar and insulin levels, again, even though the amount of sugar that ultimately enters the bloodstream is the same.
To compare the differences in the way various foods affect blood sugar scientists developed a measurement called the glycemic index. It reflects how much 50 grams of available carbohydrate in a food raises blood sugar compared with 50 grams of pure glucose.
Here is a list of glycemic indexes and loads of some common foods.
| Food | Glycemic Index | Typical Serving | Glycemic Load |
|---|---|---|---|
| Tomatoes | 15 | 1 Medium Size | less than 15 |
| Carrots | 50 | ! Medium Size | Less than 15 |
| Apple | 56 | 1 Medium Size | 78 |
| Table Sugar | 94 | 1 teaspoon | 35 |
| White Bread | 100 | 1-1 oz Slice | 100 |
While the glycemic index proved its point, it has limited value as a guide to eating. The problem is that the amount of a food it takes to provide 50 grams of sugar bears no relationship to the amounts people actually eat. For example, to consume 50 grams of sugar in carrots, you have to eat 7 full-size carrots. Scientists later developed a more useful measurement, the glycemic load, which takes into account amounts of food people typically eat. Here is a more complete list of glycemic loads of common foods.
If you come to understand the difference between glycemic index and glycemic load you may find that you were avoiding a lot of foods you could be enjoying. For example, you might worry about adding a teaspoon of sugar to your coffee. Indeed, table sugar has a glycemic index as high as that of white bread–50 grams of table sugar will raise your blood sugar as much as 50 grams of white bread. However, the glycemic load of a teaspoon of table sugar is only 30 percent of that of a slice of white bread. It’s simply not as big as a slice of bread. It weighs 4 grams compared to 26 grams for a slice of bread.
Sugar isn’t poison. You don’t have to worry about a little of it in your coffee, sprinkled on fruit or even in some candies. Dark chocolate contains sugar but not much. The glycemic load of two squares of Ghirardelli’s dark chocolate is less than half of that of a slice of white bread.
No matter how sweet something is, as long as you don’t eat more than an amount you could wrap your fingers around, it won’t raise your blood sugar as much as a slice of white bread would.
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