Is the Glycemic Index Making You Fat?
Article by Alan Aragon, MS
What do a Twix candy bar, a Pizza Hut supreme pie, and a Betty Crocker chocolate cake have in common? Answer: They're all "low glycemic" foods. And according to many nutrition experts, that qualifies them as healthy and also great diet fare. For example, NutriSystem has pegged its marketing campaign on the science of the "glycemic advantage," which the company claims is the key to losing weight while allowing you to eat the foods you love.
What is all this glycemic science? A bit of background: The glycemic index ranks foods based on the impact they have on blood sugar. So the higher a food's GI, the higher it elevates the amount of glucose coursing through your veins. The idea is that this overload of glucose leads to wild swings in blood sugar, which ultimately causes you to crave more carbs. As a result, you overeat or, at the very least, feel deprived from denying yourself, say promoters of the glycemic index. What's more, they argue that high blood sugar triggers the release of insulin, a hormone that helps lower blood-glucose levels but also promotes fat storage.
This has led to the differentiation between good carbs and bad carbs. The good carbs are said to be low glycemic, meaning they break down slowly, which keeps blood sugar and insulin levels more stable and holds hunger at bay; the bad carbs are high glycemic and do the opposite.
All of which sounds smart, but don't put Pizza Hut on your speed dial just yet. Turns out, the science of the GI isn't that simple—in fact, it's even a little sketchy.
The First Flaw
Let's say you decide to eat based on GI. So given a choice, you'll choose the foods with the lowest GIs. Now consider these facts: 1. Both pound cake and soda have a lower GI than watermelon. 2. Chocolate ice cream has a lower GI than a parsnip. 3. The GI of a Twix bar is lower than that of all the foods just mentioned. According to this analysis, you should opt for a Twix over a slice of watermelon. A soda would be better, too. Intuitively, of course, that doesn't make sense. After all, per serving, the watermelon is lower in calories and higher in essential nutrients than the Twix. And, well, it's fruit—not junk.
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What gives? It's simple: GI doesn't compare real-world portion sizes. Rather, a food’s GI is determined by giving people an amount that provides 50 g of digestible carbohydrates, which include starch and sugar but not fiber. This is the amount of carbs in about three-quarters of a king-size Twix. However, you'd have to eat 5 cups of diced watermelon to match that number—not exactly an apples-to-apples comparison. So even though eating a lot of watermelon may raise blood sugar dramatically, a single serving of the fruit has significantly less sugar than a candy bar. Your takeaway A food's ranking on the GI doesn't necessarily indicate whether it's a good or bad choice. As a general rule, whole foods—such as produce—are superior to their processed counterparts, regardless of where the items fall on the GI.
The Fitness Factor
Another surprise: A food’s GI isn't a set number. University of Toronto scientists found that the value can vary by 23 to 54% from person to person. What's more, it can differ within the same person. Scientists at Syracuse University discovered that a single, intense weight-training session reduces the effect of a high-sugar drink on blood glucose by 15% for 12 hours. Exercise uses the glucose stored in your muscles. And to replenish those stores after a workout, your body starts shuttling more of the glucose from your bloodstream to your muscles, where it's packed away for future use. This helps reduce blood-glucose levels quickly, even after a high-sugar meal. Consider it another reason to strength-train: That extra muscle gives you a larger storage area for glucose. Your takeaway The more active you are and the more muscle you build, the less you need to worry about how foods affect your blood sugar. This is also why the best time to eat fast-absorbing carbs is just before, during, and right after your workout.
The Low-Glycemic Loopholes
Look on the back of a king-size Twix package and you'll find that it has 46 g of sugar. Why, then, is it a low-glycemic food? Three reasons:
1. Not all sugar causes spikes in blood glucose. Here's why: The nutrients you eat that have the greatest impact on blood sugar are glucose and starch. But most sweeteners, such as sucrose (table sugar) and high-fructose corn syrup, are only about half glucose. The rest is primarily fructose, a sugar that has just a tiny effect on blood sugar. So only a portion of the sweetener in a Twix bar is high glycemic. Your takeaway Don't be fooled into thinking low-glycemic junk food isn't still junk. Remember, eating highly sweetened snacks is an easy way to consume the excess calories that make you plump.
2. Fat lowers a food's GI. That's because it slows the absorption of glucose into the bloodstream (as does fiber). For instance, UK researchers found that adding full-fat Cheddar cheese to a baked potato reduced its GI. With a Twix bar, you're eating a combination of sugar and fat, which means that blood sugar won't rise as high after you've finished it. Fiber also slows glucose absorption: That's why bread made from whole grain flour has a lower GI than the kind made from refined flour. Your takeaway Go ahead, have some fat. Spreading a pat of butter on a slice of whole wheat bread or tossing some nuts into your cereal will ensure that your blood sugar rises at a steadier rate afterward. Keep in mind, though, that this is not a license to overindulge. Total calories is the most important factor in managing weight.
3. GI is relative. High-glycemic foods are those with GI values of 70 and up; medium-GI foods fall between 56 and 69; low-GI foods are 55 and below. However, within that low-glycemic category, for instance, there's broccoli, with a GI of zero, and macaroni, with a GI of 47. A serving of broccoli contains just 4 g of digestible carbs and 31 calories, yet it shares the same classification with this pasta, which delivers 49 g of carbs and 221 calories. Your takeaway “Low glycemic” doesn't necessarily mean “low carb.” The reality is that the most dominant factor affecting how much a food raises blood sugar is the total amount of digestible carbs you eat. So when you're trying to lose weight, limit higher amounts of carbohydrates—more than 40 g—to the hours around a workout. The rest of the time, cap your carb intake at 40 g per meal and 20 g per snack.
The Final Word
Unless you're having just a soda, you're usually eating a mix of nutrients. That is, most meals with fast-absorbing carbs also include protein, fiber, and/or fat. (If yours don't, they should.) This makes GI an unreliable tool, as fat and fiber both lower blood-sugar response after eating.
You don't need the GI to make wise choices. Just get the majority of your calories from whole foods: meat, fish, fruit, vegetables, dairy, and whole grains. This automatically eliminates the junk and provides a filling diet rich in vitamins and minerals and other healthful nutrients, which will keep you lean and healthy for life.
Source: Alan Aragon, MS