Slow Digesting Carbs Vs. Fast Digesting Carbs | Which is Better?

By
Mike Kenler
July 22, 2021

What Are Carbohydrates? 

At its simplest form, a carbohydrate is made of sugar molecules. When we eat sugars or starches, our body breaks them down into glucose to carry out vital functions in every cell. 

There are three main macronutrients: carbohydrates, proteins, and fats. Carbs and proteins provide 4 calories per gram, whereas fats give us 9 calories per gram. 

In the fitness industry, carbs are always a hot topic. Many so-called “experts” like to make sweeping claims about all carbohydrates as a whole.

Most of these people are simply spreading misinformation like “carbs increase body fat” in order to sell their low carb diet plan.

Thankfully, there has been some resurrection of nuance in the carbohydrate discussion, as most people now recognize the difference between “good” and “bad” carbs.

In general, I tend to avoid using relative terminology when discussing foods. 

Instead, I am interested in learning about which carbs are most healthful—the ones that are proven to give us the most nutritional benefits. 

With that being said, let's explore the differences between slow digesting carbs and fast digesting carbs. 

What Is The Glycemic Index?

The Glycemic Index is a ranking of carbohydrates on a scale of 0-100. A high glycemic score (>70) indicates that the food is broken down very quickly in the body and causes a fast increase in blood glucose levels.

On the other hand, a low glycemic score (<55) means that the carbohydrate is broken down slowly and causes a more even rise in blood glucose levels.


Essentially, high glycemic foods are fast digesting carbs and low glycemic foods are slow digesting carbs. 

Fast Digesting Carbs

High glycemic, fast digesting carbs generally contain more simple carbohydrates and less fiber. Examples of foods on this list include soda, donuts, white bread, and pretzels. 

These fast digesting carbs are broken down extremely quickly, which causes our blood sugars to rise and fall very quickly, leaving us more hungry and tired afterwards. In other words, fast digesting carbs can give us a “sugar high” no matter how old we are.

While there is nothing wrong with occasionally enjoying simple, fast digesting carbs, we want the bulk of our calories to come from slow digesting carbs.


The Problem With Fast Digesting Carbs

Fast digesting carbs simply don’t fill us up. As a result, we are more likely to eat in excess throughout the day when we eat a meal of fast digesting carbs for breakfast.

In one study, twelve obese teenage boys were given either a high glycemic breakfast of instant oatmal (fast digesting carbs) or a medium glycemic breakfast of steel cut oats (slower digesting carbs). 

Despite eating the same amount of calories for breakfast, the boys who ate the fast digesting carbs went on to eat 53% more calories throughout the rest of the day when compared to the boys who ate the slow digesting carbs (1).

Clearly, eating fast digesting carbs on a daily basis can cause you to overeat without even realizing it. If you are looking to lose weight or achieve better blood sugar control, slow digesting carbs are the way to go.

Slow Digesting Carbs

Low glycemic, slow digesting carbs generally contain more complex starches, fiber, and health promoting antioxidants. Examples of carbohydrates on this list include nearly every vegetable, whole grains, legumes, and many fruits.

These slow digesting carbs are broken down much slower than high glycemic foods.

Therefore, slow digesting carbs can make us feel fuller for longer. We can achieve long lasting, steady energy instead of a quick boost of energy that makes us crash shortly after.

Health Benefits Of Slow Digesting Carbs

Fiber

One of the most important reasons for eating slow digesting carbs is their robust fiber content. 

According to the USDA, less than 3% of Americans eat the recommended daily intake of fiber of just 25-30 grams (2). This deficiency is a major issue in the United States, as dietary fiber consumption protects against cardiovascular disease, diabetes, obesity, and a number of other chronic diseases (3)

A plethora of studies demonstrate that a high-fiber meal of beans, whole grains and vegetables can blunt your immediate spike in blood sugars. 

However, it may be even more impressive that just one meal of beans or lentils in the morning can lower your blood sugar spikes throughout the day, even if you are eating fast digesting carbs for lunch (4).

This “second meal effect” demonstrates the power of fiber in controlling our blood sugars and curbing our appetites on a consistent basis.

Whole Foods

Slow digesting carbs also tend to be whole, unprocessed foods. For example, according to Harvard Medical School the glycemic index of a specialty grain bread is 53, whereas the glycemic index of white bread is 75. 

White bread is created by processing out much of the fiber and nutrients in the grains. In general, sticking to a diet of whole foods will provide the most health benefits. 

A simple tip—try to avoid nutrition facts labels with a laundry list of ingredients! 

Fruits and Vegetables

According to the World Health Organization, approximately 2.8 million deaths each year are linked to low fruit and vegetable consumption.

In addition, other researchers have found that if only half of the United States population were to eat just one more serving of fruits and veggies every day, an estimated 20,000 new cancer cases might be avoided (5).

These slow digesting carbs may be the key to preventing life or death situations. In other words, eat your fruits and veggies! 

The Bottom Line

A carbohydrate is a macronutrient. When we obsess about controlling our macronutrients, we stop treating food like food. 

White bread, potato chips and broccoli deliver the majority of their calories in the form of carbohydrates. However, as we now know, not all carbohydrates are alike. 

While we do want to focus on absorbing more slow digesting carbs, we can avoid searching up the glycemic index of every food before we eat.

Instead, we should aim to prioritize eating fruits, vegetables, legumes, whole grains and other unprocessed foods. In that way, we will be absorbing health promoting, slow digesting carbs on a daily basis.

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Mike Kenler
Director of Writing | Certified in Plant Based Nutrition at T. Colin Campbell Center for Nutrition Studies