This week’s post is about the glycemic index (GI).  Many people have heard of the GI, but they are not sure what it means, or how to apply it for their own benefit.  Several lines of scientific evidence have shown that individuals who followed a low-GI diet over many years significantly lowered their risk of developing type 2 diabetes, obesity, coronary heart disease and even certain kinds of cancer, compared with those who did not.

The glycemic index was developed at the University of Toronto (my daughter’s alma mater) to provide researchers with a way to compare the rates at which various foods, primarily carbohydrates, raise blood sugar.  Because the GI measures how quickly a fixed portion of food is absorbed into the bloodstream, it therefore correlates with the amount of insulin required.  As we have discussed, the faster you absorb a food, the more insulin you need to catch it and escort it to the cells.  So if your goal is to conserve insulin to the greatest extent possible, the glycemic index provides a way to compare foods, and to select ones with lower ratings.  I would arbitrarily rate a low glycemic index value as below 40, a high GI value as above 70, and 40-70 as mid-range.

Let’s look at some actual ratings at http://www.glycemicedge.com/glycemic-index-chart/.  In the “beans & vegetables” category, most beans are rated low.  No beans are rated higher than 48.  Baked beans are rated that high only because they are prepared with a lot of sugar.  So it isn’t the beans themselves that raise the glycemic index to 48.

Most green (broccoli, celery) and white (cauliflower, mushrooms) vegetables are rated below 25.  Red, yellow and orange vegetables are generally in or near the mid-range.  Instant mashed potatoes are a whopping 74.  Why?  Mashing potatoes begins the mechanical process of breaking down food in preparation for its absorption.  Any time we decrease our body’s work of digestion, we increase the rate of absorption.  The more broken down a food prior to eating it, the less work the stomach has to do, and so the more rapidly the food is absorbed. 

In the breads category, whole-grain breads are the lowest (40) and white breads range around 60-70.  French baguettes are extremely high (94).  Most cereals are in the 70-80 range, except for a few whole-grain products in the 50’s.  Rice chex are 89.  Among grains, instant rice is rated as 87 (consider it pre-digested).  Most pasta is in the low mid-range.  Why is steamed brown rice rated at 50, and boiled brown rice at 72?  I suspect that the increased mechanical action of boiling causes more fiber to break down during the cooking process.  I’d be interested to hear from other readers with ideas about this. 

Crackers (67-82) and cookies (vanilla wafers and graham crackers-mid 70’s) are high because they are generally made from white flour with little or no fiber to decrease absorption.  Oatmeal cookies (55) are better.  Most fruits are in the 30-50 range, with a few higher and a few lower. 

It is very interesting to note that the section with the highest glycemic index values overall is “Snacks & Chips,” including poptarts (72) and french fries (75), neither of which, by the way, would be classified as a snack or chip by me.  I thought poptarts were marketed as a breakfast substitute.  More truth in advertising.   Substitutes are not food.  That makes me wonder if perhaps it is the types of snacks, more than snacking itself, that contribute to our obesity epidemic.  Drinks, predictably, range from tomato juice (38) to gatorade (78).  Overall, the lowest glycemic index sections are dairy, beans, and vegetables.  The worst surprises on the list are the tofu frozen dessert (115) and pretzels (83).  The best surprise is the dairy section in general, and yogurt in particular (14).  Here is more evidence that, in fact, Grains are not Beans are not Fruit are not Vegetables: All Carbohydrates are not Created Equal .

The GI depends not only on the particular food being consumed, nor on associated foods eaten at the same time, but on preparation methods as well.  We can manipulate, to a certain extent, the rate at which we absorb various foods, even those with a higher GI.  Basically, whatever increases the work of breaking down food will lower the GI and, conversely, whatever decreases the work of breaking down food will raise the GI.  The longer it takes to break down food, whether mechanically or chemically, the longer it takes to absorb that food.  It takes time to tease apart the strands of a fiber-rich food to get at what’s inside.  This is why whole-grain breads, with higher amounts of fiber, generally have a lower GI value than white breads.  But, as usual, the buyer must beware.  Many brown breads are treated with enzymes that break down fiber and soften the crust.  This has the effect of raising the GI to levels comparable to those of white bread.  So don’t assume simply that switching to brown-colored bread is worth it.  First, check to be sure that the first ingredient is whole-grain flour.  Secondly, check to see if there are 3 grams of fiber per serving.  It’s no guarantee, but it certainly improves your chances of getting what you think you are paying for.

What else lowers the GI?  Fiber, fat and protein all slow gastric emptying (which means that they make a meal sit in the stomach longer), which reduces a food’s GI.  What else?  Acids like vinegar and lemon juice appear to lower gastric emptying.  And, according to one study, alcohol (though not beer), reduced the GI of a meal by 15%.  

So how best to use this information?  Compare the glycemic indexes of the foods you are considering, and choose the lower.  This approach is similar to that of  “Eat This, Not That,” by David Zinczenko, editor-in-chief of Men’s Health magazine and author of the Abs Diet series, and Matt Goulding, the magazine’s food and nutrition editor.  If you compare the GI values of the recommendations, you will see that in virtually all cases, a lower GI food is recommended as the preferred choice.  It’s not just about calories. 

What are the limitations of the glycemic index?  First, it measures only glucose, and not fructose, a major cause of insulin resistance, high triglycerides, and non-alcoholic hepatitis.  (See Fructose, Fiber, and High Fructose Corn Syrup .)  Secondly, the GI measures the intake of a 50-gram load of a food, even if the usual serving of that food is much smaller.  Despite these two major limitations, it is useful for our purposes.