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What Is the Difference Between Fast Carbs and Slow Carbs?

By Christa Miller ; Updated July 18, 2017

Although many diet plans claim that cutting carbs will help you lose weight, you shouldn’t assume that all carbs are bad for you, warns the Harvard School of Public Health. Carbs, which give your body fuel for its many functions, are as important for a healthy diet as are fats and proteins. But carbs aren’t all created equally and a well-balanced diet emphasizes carbs from the healthiest sources.

The Complication

Carbs were once classified as “simple” or “complex.” Complex carbohydrates such as brown rice and many vegetables were considered the healthier carbs and simple carbohydrates such as table sugar and fruit sugar were considered less healthy. However, that classification system has some limits because some complex carbs are less healthy and some simple carbs are an important part of your diet, according to the Harvard School of Public Health. For instance, French fries are in the complex carbs group but they are considered a junk food. Conversely, sugar from fruit is a simple carb,but fruit contains other important nutrients such as fiber, vitamins and minerals.

Slow vs. Fast Carbs

Carbs can also be split up into the categories “slow” and “fast.” Slow carbs raise your blood sugar levels at a gradual pace and give your body a steady stream of fuel. Brown rice, lentils, oatmeal, whole wheat bread, vegetables and fiber-rich fruits are examples of slow carbs. Fast carbs enter your body at a fast pace and cause your blood sugar to spike and dip and cause your body to produce large amounts of a hormone called insulin. Having fast carbs such as white bread, donuts and soda can increase your chances of having energy dips, becoming irritable and getting hungry quickly between meals, according to “Psychology Today.”

The Glycemic Index

A system known as the glycemic index helps classify carbs based on how fast and high they increase your blood sugar in contrast to pure glucose, according to the Harvard School of Public Health. Fast carbs are on the high end of the scale and slow carbs are on the low end. A score of 70 or higher designates a food as having a high glycemic index, whereas a food with a score of 55 or under has a low glycemic index. Having a diet that is high in high glycemic index foods, which will cause many blood sugar spikes and dips, may increase your risk of becoming overweight and developing diabetes and heart disease. A high glycemic index diet may also lead to ovulatory infertility in women, age-related eye problems and colorectal cancer. Ask your doctor whether she thinks you should use the glycemic index when planning your diet.


A healthy diet should include about 40 to 60 percent of calories from carbs, according to MedlinePlus. Whether or not you make carb choices based on the glycemic index, most of the carbs in your diet should be slow rather than fast. Let the U.S. Department of Agriculture's food group recommendations guide your decisions. For starters, make at least half of your grains whole grains – which means you should be getting about 3 to 4 oz. of whole grains a day – and aim for at least 1 and ½ to 2 cups of fruit and 2 to 3 cups of vegetables per day.

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