There is so much confusion when it comes to carbs. We live in a world with conflicting nutrition advice, and carbohydrates are at the front and center of nutrition controversy.
Not all carbohydrates – or all carbohydrate-rich foods – are equal in terms of the nutrition they provide or how the body uses them. There are differences in types of carbohydrates, how they are digested and absorbed, and how the body uses them.
Sucrose is a type of simple carbohydrate called sugar. Specifically, sucrose is a disaccharide, meaning it’s made of two monosaccharides; in the case of sucrose, it’s made of the monosaccharide glucose and the monosaccharide fructose. Other disaccharides include maltose, 2 glucose molecules fused, and lactose, glucose, and galactose fused.
Individual sugar units can combine into more complex, often branched, polysaccharides, typically consisting of hundreds or thousands of glucose molecules fused.
These carbohydrates are found in grains and vegetables and can be broadly categorized into digestible and indigestible types. Digestible polysaccharides include starch and glycogen. Starch is the storage form of glucose found in plant tissues, and glycogen is the storage form of glucose in the muscle & liver in humans and animals.
But Fun Fact about glycogen – when you eat meat, you don’t consume glycogen because it has already been broken down when the animal died. Indigestible polysaccharides mainly consist of fiber, which only comes from plants. Cellulose is an example of a fiber composed entirely of glucose molecules. But unlike the glucose found in starch or glycogen, the glucose molecules in cellulose are fused to make it impossible for our gut enzymes to break them down.
Certain foods contain two types of fiber, both of which are important for you. Insoluble fiber is a type of polysaccharide that cannot be broken down in our bodies. It can’t be absorbed into your bloodstream, providing 0 calories. It passes unchanged through our digestive tract, and I’ll later explain why this is so important. On the other hand, soluble fiber dissolves during digestion, turning into a gel-like substance. It’s not digestible, but our gut bacteria can break down some components in the colon, and what those bacteria do with it becomes very important.
For example, gut bacteria metabolize soluble fiber into short-chain fatty acids like butyrate, which get absorbed into the bloodstream. And that’s a good thing because butyrate has anti-inflammatory properties. But let’s for a second, say you eat a carrot, which, yes, contains fiber, but it also contains starch.
You’re crunching on that carrot, and the saliva in your mouth starts to break down the starch molecules into smaller starch molecules because of an enzyme called…salivary amylase. If you eat a cracker and chew it long enough before swallowing, it tastes sweet on your tongue because the starch gets broken down to maltose.
So after that, the carrot goes into your stomach, where you have some acid, which deactivates the salivary amylase enzyme mixed in with those little carrot chunks. Then it makes its way to the first section of your small intestine, called the duodenum. Most of the starch is broken down into disaccharides there….maltose. Why? Because the pancreas secretes an enzyme here called pancreatic amylase.
You also have other enzymes located in the small intestine that break down the disaccharides into sugar’s simplest form, monosaccharides, mainly glucose and fructose, and lactose if you consume milk products. The monosaccharides get absorbed from the intestine into the blood, which immediately goes to your liver through your portal vein. The liver will convert fructose and galactose into glucose.
The human body is very efficient when it comes to absorbing sugars. The body uses glucose to provide energy for all sorts of functions, and that’s why the body carefully monitors blood glucose levels. It’s called glucose homeostasis, controlled by the pancreas and liver.
The pancreas secretes two important hormones, insulin, and glucagon when controlling blood glucose levels. When blood glucose increases, such as after eating carbohydrates, insulin is released from the pancreas. Insulin lowers blood glucose by stimulating glucose uptake by tissues, including converting extra glucose into glycogen in the liver and muscles. When blood glucose is low, glucagon is released from the pancreas, which has the exact opposite effects of insulin to raise blood glucose levels.
Doctor Mike Hansen, MD
Internal Medicine | Pulmonary Disease | Critical Care Medicine
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