Weekly Column_120 / August 16, 2012

Good and Bad Carbs

By Charles Platkin, PhD

In fact, the Centers for Disease Control and Prevention (CDC) found calorie intake increased markedly between 1971 and 2000 (22% for women and 8% for men), mainly from eating more carbohydrates (e.g., rice, bread and pasta). The CDC holds accountable the consumption of pizza, salty snacks and takeout foods, as well as bigger portions.

If carbs are so bad, then why were we told to eat more in the first place? Recent diet books have been emphasizing the “good” carbs, with astonishing success. But how do we know which are the right carbs to eat? And is this really the healthier path in the first place?

The concept of “good” versus “bad” carbs actually comes from the outdated concept of complex (starches) versus simple carbs (sugars). The idea was that smaller, simple sugars digest quickly, and the longer-chain complex carbs take longer to digest and therefore keep you full. However, a complex carbohydrate can be refined (e.g., white bread, white rice, and pasta), which strips away much of the good stuff like fiber, vitamins, and minerals. At the same time, there are many simple carbohydrates that are unrefined (e.g., fruits and vegetables) and therefore still contain fiber, vitamins and minerals. So dividing carbs this way is too simplistic to guide your food choices.

That’s where the glycemic index (GI) comes in. Created as a research tool to determine the actual effect carbohydrates have on blood sugar, GI measures how quickly a food that contains carbohydrate raises a fasting person’s blood sugar levels and subsequent insulin in two hours. (Insulin is the hormone that activates cells to absorb sugar from the bloodstream, thus reducing blood sugar levels.)

“The glycemic index cuts across some traditionally accepted boundaries of ‘healthy’ and ‘unhealthy’ foods. Some starches with a healthy reputation are surprisingly high, and some foods that you wouldn’t think are nutritious have a low GI,” explains Susan Roberts, Ph.D., of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston. High GI foods include white bread, white potatoes, pasta, doughnuts, and bagels, but also watermelon and carrots. Low GI includes most vegetables, low-fat yogurt, lentils, and peanuts, but also whole milk and pound cake.

Why is it important to know a food’s GI value?

Foods with a high GI value raise the body’s blood sugar levels very quickly, which signals a corresponding rapid release of insulin into the bloodstream; this quick rush of insulin then lowers blood sugars levels. “When your body takes in carbohydrates, or any food, there is a mechanism that switches from the feeling of being ‘famished’ to being ‘fed,'” explains David Ludwig, M.D., Ph.D., director of the Obesity Center at Boston Children’s Hospital and Harvard Medical School.

“High GI foods create a slower and more difficult transition from the fasted to fed state and studies have shown that this causes hunger,” says Ludwig. “Typically, foods that are high on the glycemic index are highly processed foods, and we are not physiologically programmed to handle this.”

In contrast, low GI meals cause a slower release of sugars into the bloodstream. So instead of spikes of insulin, a steady, moderate insulin release allows for the sensible metabolism of sugars both into and out of their storage.

Experts disagree about the use of glycemic index and its benefits. “There aren’t any conclusive studies. The majority of, but not all, one-day studies show low GI foods suppress hunger, but no relevant long-term studies have been completed,” says Christine Pelkman, Ph.D., professor of nutrition at the University at Buffalo in New York. “Not only that, there’s no proof that following a low GI diet will aid in weight loss.”

GI is based on blood sugar response after fasting, and in real life, we eat several times a day. Your earlier meal or snack can affect your current meal’s GI. Many other variables affect a food’s GI too: fiber or fat content, acidity, food combinations, preparation method, and even ripeness.

And it’s not easy to follow a low GI diet. For starters, GI is based on a 50-gram carbohydrate portion of food. This is nowhere near the size of a typical serving of many foods. For example, carrots have a high GI (92), which is much higher than many other vegetables. But this is a little misleading because a 50-gram carbohydrate portion of carrots is about one and a half pounds, much more than most people would consume.

To take the serving size into account, the glycemic load (GL) was introduced in 1997. The GL of a portion of food is equal to that food’s GI value multiplied by the number of carbohydrate grams in the portion of food, divided by 100. For example, carrots (GI of 92) take on a GL of between 3 and 4, once portion size is taken into account. Another example is Shredded Wheat with a GI of 95 and a GL of 36. (For GI/GL values: www.glycemicindex.com)

Both the GI and GL fail to account for calories. Take ice cream for example — a half cup has a GI of 42 (16g carbs) and a GL of 7, making it seem almost healthy when it’s loaded with calories and fat! When people are trying to lose weight, study after study shows a calorie deficit and satiety (not being hungry) are what’s needed. “If you look at the GI/GL all on its own, it might not lead to a healthier or more satisfying (less hunger) diet,” says Pelkman. In her study, she found no difference in hunger levels between participants fed a high GI and GL diet for six weeks. “Soda has a lower GI than carrots, so if you were just using GI, you would choose soda over carrots — these are not choices that would lead to weight loss,” cautions Pelkman.

Another problem is the fact that we rarely eat carbohydrates alone, which means other hormones besides insulin work to satisfy feelings of hunger. “We need to focus on the net effect of food intake and satiety,” says Pelkman.

Many nutrition experts advise eating a diet rich in whole grains, legumes, vegetables, fruit, and nuts, de-emphasizing processed foods. But if you look at foods such as whole wheat bread compared to white bread, they are both high glycemic foods.

“You can’t just say that any foods that are whole wheat are low on the GI,” says Pelkman. “Eating whole grains and fiber doesn’t necessarily keep you from getting hungry.” The research out there shows that foods high in fiber promote weight loss, NOT the fiber itself. Foods high in fiber also tend to be bulky foods that have lower calorie densities. “You need to look for low energy, low density foods that will satisfy you,” says Pelkman.

Glycemic index and glycemic load should be used for what they are — tools. They can help you choose healthier carbohydrates, but should not be the only gauge you’re using to evaluate your foods. When faced with two high GI foods, choose the item that offers more nutrients and more bulk for fewer calories — this is often the food that is the least processed.

Eat your carbs in combination with fat and/or protein to decrease the impact on blood sugar (e.g., toast with peanut butter, or rice and beans).

Finally, keep this in mind. It’s not just the GI that defines a “good” or “bad” carb. Good carbs also carry other nutrients or phytochemicals, have lower total calorie content, and are eaten with other nutrient-dense foods.

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