You may not think about it often, but all the food you eat is broken down into something smaller. Some foods turn into amino or fatty acids, while there are other foods that turn into sugar. But it's not the white sugar you might put into your coffee. It's glucose, a sugar your body uses for energy.
While digestion is a complicated process that's not totally black and white, carbohydrates, as a general rule, turn into sugars, while protein and fat don't. There are some caveats and more minor details to this, but for the most part, that's how it works.
How Carbohydrate Digestion Works
To understand how or why carbohydrates turn into sugar, you need to understand how the digestion process works. Carbohydrates come in three major packages: sugars, starches and fibers. Fiber isn't digestible (which means it stays mostly in its full form and doesn't get converted into sugar), so forget about that for a minute and focus on the other two.
When you eat carbohydrates in the form of sugar or starches, your body's goal, as noted by the Cleveland Clinic, is to break them down into the simple sugar, glucose, which it can use for energy. The carbohydrates aren't really converted into glucose — they already contain the sugar in a more complex package. They just get broken down into smaller components so your body can actually grab and use the glucose for something else.
Think of it this way: You have a castle that you built out of blocks, but now you want to use those blocks to build a car instead. In order to access the individual blocks, you have to tear down the castle so you can use the blocks to build something new. That's how it works with carbohydrates. They always have the glucose there, but the body needs to figure out a way to access it.
So, when you eat sugars and starches, they get broken down by various enzymes, digestive juices and muscle movements along the length of your digestive tract and eventually make their way into the blood as glucose, which is the simplest form of sugar.
The Role of Glucose
But, glucose isn't there just to sweeten your blood. Your body uses that glucose to meet its energy demands. So, once the carbohydrates are broken down into sugar, your body does three major things with it:
- Uses what it needs for immediate energy
- Takes some of what's left over and converts it to glycogen (the storage form of glucose) in the liver and muscles until they reach capacity
- Takes any glucose that remains and converts it into fat, which is stored in unlimited amounts in the body
Depending on the amount of carbohydrates you eat, your body may do one or all of these things. If you haven't eaten for a while and you eat only a small amount of carbohydrates, there may be only enough to supply you with immediate energy. If you eat a large carbohydrate-rich meal shortly after another carbohydrate-rich meal, you may not need any glucose at the moment, so your body will jump right into the fat-storing part.
What About Protein and Fat?
Although protein and fat don't significantly raise your blood sugar after eating, they can affect it to a very minimal degree, so they're worth mentioning. Protein is made up of compounds called amino acids. When you eat protein-rich foods, your digestive system breaks them down into those individual amino acids and then uses those amino acids to make other proteins in the body. To understand this, you can refer back to the analogy about the blocks.
There are a total of 20 amino acids, and most of them are kept in their natural form and used where they're needed to build other proteins. But, according to a May 2013 report in Diabetes (the journal of the American Diabetes Association), there's one amino acid — leucine — that can be converted to glucose if your body wants to use it for that. However, the report mentioned that, in most cases, dietary protein doesn't get converted into sugar or raise your blood sugar after eating it.
Dietary fat has an even more minimal (or practically nonexistent) effect on blood sugar, according to the Joslin Diabetes Center. When you eat fat, it's broken down into fatty acids that move into the blood and carry out a variety of functions. If you eat more than your body needs, they group together and form triglycerides, which can circulate in your blood and get stored in your fat cells, increasing your body fat percentage.
Although fat isn't turned into sugar, carbohydrates (or sugars) can actually be converted into fat, which your body can store in unlimited amounts. This means that, if you eat more carbohydrates than your body needs, there's a potential that the carbohydrates get turned into fat and stored in your body's fat cells.
Foods That Turn Into Sugar
Now that you know which groups of foods get converted to — or broken down into — sugar, you're probably looking for a list of foods that contain a lot of those carbohydrates. Keep in mind that, while all carbohydrates (except fiber) get broken down into sugar, the effect that a particular carbohydrate-rich food has on your blood sugar depends on the whole package.
For example, some carbohydrates, like white sugar, are processed and completely stripped of fiber. These types of carbohydrates, which are classified as simple carbohydrates, get converted to sugar more quickly and raise your blood sugar to a greater degree.
Other carbohydrates might contain sugar and starch, but also a lot of fiber. Carbohydrates that fall into this category, which is called complex carbohydrates, do get broken down into sugar, but the process is slower, so it doesn't affect your blood sugar levels as much.
Read more: A Complete Guide to Complex Carbohydrates
The Harvard T.H. Chan School of Public Health provides a list of high-carbohydrate foods that are converted to sugar to some degree:
- Potatoes (white and sweet)
- Breakfast cereals
- Soda and fruit drinks
- White sugar
- Candy and desserts
- White and brown rice
- Other starchy vegetables (carrots, squash, pumpkin)
- Baked goods
- Diabetes: "Dietary Protein and the Blood Glucose Concentration"
- Diabetes Forecast: "How the Body Uses Carbohydrates, Proteins, and Fats"
- Joslin Diabetes Center: "5 Common Food Myths for People With Diabetes Debunked"
- Harvard T.H. Chan School of Public Health: "Carbohydrates and Blood Sugar"
- Kaiser Permanente: "How Our Bodies Turn Food Into Energy"
- American Diabetes Association: "Get to Know Carbs"
- Cleveland Clinic: "Carbohydrates"