Good Carbs, Bad Carbs - Is this Helpful?

Are There Really Good Carbs and Bad Carbs?

mixed carb breakfast
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If you pay attention to popular nutrition news, you have probably heard that there are "good carbs" and "bad carbs". On the other hand, many low-carb diets don't pay attention to this distinction, instead focusing on the amount of carbohydrate being consumed. So, which is the correct way to look the carbohydrates we eat?

The Bottom Line Synopsis

There are different ways of categorizing "good" and "bad" carbs, with some overlap and some conflicts between them. Within the amount of carbohydrate that works best for your body, eat a variety of nutritious whole foods, and don't worry much about the glycemic index or what some authority thinks is "good" or "bad".
You can be sure if you do this that the carbohydrate you consume will be "good".

Is There Such Thing as a Good Carb?

This may surprise you: nutrition textbooks do not usually talk about good and bad carbs at all. This is strictly a popular concept that has sprung up in recent years. Partly this is due to the recent popularization of identifying foods as their primary macronutrient: suddenly bread is a carb, steak is a protein, and olive oil is a fat. I have written about how I find this trend to be more confusing than clarifying, particularly in the case of carbohydrates. But the labels are here to stay, at least for now. (I even sometimes lapse into this way of talking about food.)

Foods can have carbohydrate, fat, or protein, but most foods are a combination of the three. A carbohydrate molecule is either sugar, starch, or fiber. Starches are long chains of sugars (almost always glucose).
These molecules do not contain other nutrients, or any moral baggage.

So, What is a "Good Carb" vs a "Bad Carb"?

Since there is no scientific meaning, writers are essentially free to make up their own definitions of good and bad carb foods, and they have. One popular medical Web site states that it has to do with the amount of fiber in the food (spinach good, watermelon bad?).
Other authors say it has to do with whether or not a food is in its whole state or has been refined (potatoes and parsnips good; white rice and candy bad). Still others talk about the glycemic index of the food as being the defining factor: how much the food is likely to raise blood sugar (cherries and grapefruit good; potatoes and parsnips bad).

These ways of categorizing tend to overlap a fair amount -- after all, donuts and cookies are going to fall onto the "bad" side of the scale no matter how you are defining it.

Does it Really Matter?

Purely from a nutritional standpoint, the source of carbohydrate certainly matters, even more so if you are restricting carbs in your diet. It is definitely a good idea to go for foods that contain more nutrients, other factors (including carbohydrate) being equal. Whole foods not only usually contain more vitamins and minerals, but often more antioxidants and other phytonutrients, huge numbers of which have probably not been discovered yet.

However, you don't have to spend a lot of carbs to get high-nutrient foods.
Non-starchy vegetables, low-sugar fruits, and nuts are some of the best nutritional bargains around.

Making Your Carbs Count: Low-Carb, High Nutrition

What About the Glycemic Index?

A lot of popular diets use the glycemic index to help dieters decide which foods to choose, and it makes sense that foods that raise blood sugar more would be good to stay away from. However, the data on the value of doing this is quite mixed. There are a number of reasons for this. One is simply that eating lower glycemic foods generally means staying away from processed and other less healthy foods. This brings about the question: "What would happen if the effect of the glycemic index was compared using people eating healthy diets?"

A study published in the December 2014 Journal of the American Medical Association looked carefully at whether eating carbohydrates lower on the glycemic index improved health markers above and beyond the effect of simply eating healthy foods. They assigned people to eat either a standard amount of carbohydrate (58% of calories) or slightly under the recommendation at 40% of calories (what we might call "moderate-carb", but not really "low-carb"), and also to getting their carbs either from low-glycemic or high-glycemic (but still healthy) sources. It turned out that the amount of carbohydrate had an effect on some of the outcomes, but the glycemic index did not. When the participants were eating the lower-carb diet, they had modest improvements in triglycerides, HDL cholesterol, and blood pressure. But the glycemic index of the foods showed no effects.*

In a New York Times article about the study, Dr. Frank Sacks, the lead investigator, is quoted as saying that "unless someone has diabetes and must monitor their blood sugar levels, people who are already following a healthful diet do not need to worry about the blood-sugar impact of one type of fruit or grain versus another".

I have a mixed reaction to this. First, I would assert that people should not wait until they actually are diagnosed with diabetes to be concerned about their blood sugar, particularly if they are on the diabetes spectrum as a surprising percentage of Americans, often unknowingly, are. Also, food choices that are clearly less glycemic are often so because they have more resistant starch and other more slowly-digested carbohydrate, which can have positive health effects. For example, grinding grains into flour always causes a greater blood sugar rise, partly because when grains are eaten whole and intact some of the starch passes through to the colon (is "resistant" to digestion). But you don't have to know the glycemic index numbers to understand this.

On the whole, I think the evidence is that paying attention to the glycemic index is not very helpful. I think that the glycemic index itself is a very flawed measure, partially due to the large amount of individual variation in response to a food. If you want to maximize the healthfulness of your diet, there are a lot more important things to pay attention to.

*Extra notes on the study for the science-minded: This was a cross-over study, with each subject doing at least two of the diets for five weeks each. It is always more powerful when people are compared to themselves rather than other random people. Also, all the food was given to the participants, insuring a much higher level of compliance, and the diet was aimed at weight maintenance, so that weight loss would not be a factor in the results. The participants were overweight and at somewhat greater risk for heart disease the average, but not diabetic.


Sacks, Frank, et al. "Effects of High vs Low Glycemic Index of Dietary Carbohydrate on Cardiovascular Disease Risk Factors and Insulin Sensitivity" Journal of the American Medical Association. 2014;312(23):2531-2541.

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