The glycemic index is a measure of how quickly and how high your blood sugar rises after eating food.
But it’s not nearly as important as a less famous measure called the glycemic load. And the glycemic load is something worth knowing about.
When you eat any food- especially carbohydrate and to some extent protein- your blood sugar goes up. (It hardly goes up at all when you eat fat.) In response to the rise in blood sugar, your pancreas secretes insulin, whose job is to act as a traffic cop and escort the excess sugar out of the bloodstream and into the cells where- in an ideal world- it can be used for fuel. Blood sugar (and insulin) both gradually go back down to pre-eating levels, and in a few hours you repeat the whole process.
Problem is, this is anything but a perfect world.
We overeat high sugar carbohydrates, which quickly drives our blood sugar up into the stratosphere. The pancreas sends out a ton of insulin in an attempt to lower blood sugar, but most of us are pretty sedentary, so our muscle cells aren’t interested in taking it in. Insulin knocks on the doors of the muscle cell walls and the cells say, “sorry dude, we don’t need any sugar, our guy’s going to be sitting at the computer all day, go somewhere else”. (Sugar winds up going to the fat cells, which are far more welcoming.) Meanwhile both blood sugar and insulin have been raised, setting you up for hypertension, fat storage, hunger, cravings and mood crashes when your sugar eventually does fall.
Not a great situation by any means.
To measure the effect of food on blood sugar, scientists came up with the idea of the glycemic index. Using pure glucose as a standard (with an index of 100), they tested 50 gram portions of digestible carbohydrate and measured how quickly and how high blood sugar rose in reaction to eating them. By eating low-glycemic index foods you presumably could avoid the blood sugar roller coaster.
But there are two big problems with using the glycemic index as a guide to eating.
One, it only applies to a food eaten alone- in other words, a banana, not a banana with peanut butter.
Two, and more important- the glycemic index doesn’t take into account portion size.
The glycemic index of 50 grams of spaghetti is “moderate”, but no one eats 50 grams of spaghetti- at least they don’t at the Olive Garden, or at any home cooked Italian meal I’ve ever seen!
And the glycemic index of 50 grams of carrots is “high” but no one eats 50 grams of carrots (there’s 3 grams of carbohydrate in a carrot).
Glycemic load takes into account portion size. Carrots- which have a high glycemic index- actually have a very low glycemic load. Spaghetti- which has a moderate index- has a very high glycemic load.
Glycemic load is all you need to pay attention to, because it tells you what’s going to happen to your blood sugar in the real world. (Even then, it still refers to food eaten alone. Add some fat to your carbs– peanut butter on an apple, for example– and you’ve just lowered the glycemic load.)
Glycemic index is a pretty useless indicator of anything, but glycemic load is meaningful. (You can find a definitive of glycemic index and glycemic load of food here.) Bottom line: eat as little sugar as possible and go easy on the foods that turn into sugar quickly- like cereals, breads, pastas and anything white (except chicken, cauliflower and mushrooms!)
When it comes to sugar one thing is very clear: Less is more and zero is better.