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Slow Carbs vs. Fast Carbs

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Slow Carbs vs. Fast Carbs
By Susan McQuillan MS, RDN, CDN
Reviewed by QualityHealth’s Medical Advisory Board

Mention the word “carbs” and most people think of white breads, sugars, and pasta—the foods we’re frequently told to avoid when we read about diet, nutrition, weight loss, and blood sugar control. However, not all carbohydrates are created equal.

In broad, general terms, carbohydrates can be broken down into two basic types: slow and fast. These labels refer to the rate at which these foods are digested and absorbed into the bloodstream. Slow carbs tend to be high in fiber and are more slowly absorbed, so they have a less dramatic effect on blood sugar levels. Fast carbs are absorbed very quickly, and cause the higher spikes in blood sugar that we want to avoid.

How is this measured?

The rate at which carb-rich foods are absorbed can be measured by a tool known as the glycemic index (GI). On a scale up to 100, the glycemic index ranks foods by how much they raise blood sugar, once absorbed. The lower the number, the less effect a food will have on your blood sugar. Only carbohydrates are measured on the glycemic index because fat and protein do not directly affect blood sugar levels.

Slow carbs (the ones you want to eat!)

Slow carbs are those foods with a low glycemic index (55 or below), such as most vegetables, legumes, most fruits, and whole-grain foods. They include:

  • Apples
  • Beans
  • Lentils
  • Oranges
  • Pears
  • Rice, wild or brown
  • Yogurt, plain
  • Whole-grain breads

Fast carbs (the ones you want to avoid!)

Fast carbs are those with a high glycemic index (70 or above), such as white bread, white potatoes, most processed foods and snacks, and other refined foods that are low in fiber or fat, or high in sugar. They include:

  • Doughnuts
  • Honey
  • Potato, baked
  • Potato, French fries
  • Potatoes, mashed
  • Rice, short-grain or sticky white
  • White breads

Some carb-rich foods are considered “medium” on the GI scale, with a value between 56 and 69. Some examples are bananas, oats, pineapple, sweet corn, long-grain white rice and white table sugar.

Although individual carb-rich foods are each assigned an independent GI value, the GI can change when that food is cooked, processed, or eaten with other types of food at the same meal. A low GI food eaten at the same time as a high GI food can balance out the effects of the higher ranked food. Even your own metabolism can affect the GI value of foods. Also, some foods may have a high GI, but when they are eaten in standard portion sizes, they will not cause notable spikes in blood sugar.

Another scale, that takes this concept a step further, is known as the Glycemic Load (GL). The GL uses GI values to calculate the specific number of carbs in a standard portion size of a carb-rich food. On many tables that list the GI of different foods (see link, below), you will also see the GL values. A GL below 10 is considered low and a GL of 20 is high.

“Overall, it is best to control your blood sugar by choosing mostly low glycemic, or slow foods,” says Marina Chaparro, RDN, CED, MPH, Spokesperson for the Academy of Nutrition and Dietetics. “And it is equally important to limit your portion sizes, and eat combinations of foods from at least two or three different food groups that supply a variety of nutrients at each meal.”

For those diagnosed with prediabetes or diabetes, this is an essential practice for maintaining good blood sugar control. With or without diabetes, it is simply good preventative health care. Overeating high-glycemic foods that cause chronic spikes in blood sugar will not only increase your risk of developing type 2 diabetes, but can also contribute to weight problems, high blood pressure, high blood fats, inflammatory conditions, heart disease, and stroke.

For a more complete list of glycemic index values, see this article in the American Diabetes Association journal Diabetes Care.

Marina Chaparro, RDN, CED, MPH, reviewed this article.

Sources

Chaparro, Marina RDN, CED, MPH. Email to author, September 11, 2015.

Foster-Powell K, Holt SHA, Brand-Miller JC. “International table of glycemic index and glycemic load values.” American Journal of Clinical Nutrition. January 2002;76(1):5-56.

Larsson S, Akesson A, Wolk A. “Sweetened beverage consumption is associated with increased risk of stroke in women and men.” The Journal of Nutrition. June 1, 2014;144(6):856-860.

Te Morenga LA, Howatson AJ, Jones RM, Mann J. “Dietary sugars and cardiometabolic risk:systematic review and meta-analysis of randomized controlled trials of the effects on blood pressure and lipids.” American Journal of Clinical Nutrition. July 2014;100(1):65-79.

American Diabetes Association website. Glycemic Index and Diabetes. Reviewed October 2, 2013. Edited May 14, 2014. Accessed September 6, 2015.