A carb is a carb is a carb?

"I was gaining weight very rapidly and read about the idea of restricting carbohydrates as an alternative to going hungry. I had a big appetite, so that was the only thing I would even consider."
- Dr Robert Atkins

In diet books the total calories in a food item is calculated by Fat * 9 + Protein * 4 + Carbs * 4 when each is measured in grams. This assumes the body is a perfect food processing machine that will convert the entire food into energy. But that's not true. Some foods digest quickly, some slowly, some perhaps not at all.

In 1980 at the University of Toronto, Dr David J Jenkins and colleagues decided to test real food on real people to see what effect each food had on blood sugar so they could give better advice to diabetics. They fed volunteers 50g of carbohydrate from one type of food and measured the blood glucose level over the next two hours. The total area under the glucose curve was called the Glycemic Index. Foods with a high GI are digested quickly and give a sugar rush or spike that is often associated with weight gain and high insulin demands for diabetics.

High GI: white bread, most white rice, corn flakes and most breakfast cereals
Medium GI: whole wheat products, basmati rice, sweet potato, baked potato
Low GI: most fruits and vegetable, legumes, beans, whole grains, nuts

The GI was quickly put to use in new diet plans like the South Beach Diet which my wife and I used and did lose weight (for a awhile). This was a refinement over the Atkins plan which banned all carbs completely.

But critics soon pointed out flaws with the GI tables.
1. The tests examined one food at a time. Not food combinations. It was later discovered that adding vinegar, for example, will lower the GI. The presence of fat or soluble dietary fiber can slow the gastric emptying rate, thus lowering the GI.

2. Each test required 50g of carbs in the food consumed. Watermelon, which is mostly water with a little sugar, has a high GI but there are only about 8g of carbs per 100g serving size. To get 50g carbs for the test, volunteers had to eat nearly 7 servings of watermelon. Compare this to white bread which is half carb by weight and just two slices equal 50g carbs.

To address this second point, Harvard University created a new standard called the Glycemic Load that takes serving size into account. The GL is the GI * (carbs per 100g) /100.  Under this system, watermelon gets a low GL score.

Under the GL system white bread is very high, bananas on the boarder of medium and low and carrots very low.

Bottom Line

A detailed list of GI and GL values can be found at http://www.mendosa.com/gilists.htm 

Foods with a high GL should be eaten sparingly. Fill up on low GL items.