Carbohydrates ~ What are they?
Carbohydrates
Now, what are carbohydrates? Is sugar a carbohydrate? Yep. What about starch? Well, starch is also part of the carbohydrates. Fibre? Fibre is also part of the carbohydrates, but a special kind as we’ll see. We’ll start by looking at the chemistry of carbohydrates. So carbohydrates are built from small building blocks called monosaccharides. And the main monosaccharide is glucose.
Chemistry of Carbohydrates
Carbohydrates are one of the three macronutrients in our eating regimen other than fat and protein. So let’s talk about what carbohydrates are and what is their chemical composition?
Carbohydrates can be isolated into simple carbohydrates, alluded as sugars, and complex carbohydrates. But for now, we will concentrate on the simple carbohydrates, which can be separated into monosaccharides and disaccharides.
The monosaccharides are glucose, fructose and galactose. And the disaccharides are composed of these three monosaccharides:
- So we have maltose, which is composed of two molecules of glucose.
- We have sucrose, which is composed of a molecule of glucose linked to fructose,
- and we have lactose, which is composed of glucose linked to galactose
All these mono and disaccharides exist in our diet except galactose which is only present in our diet as part of lactose. The monosaccharides can be directly absorbed into our bloodstream, so they don’t require any digestion. However, the disaccharides need to be broken down to the individual monosaccharides. This process is part of normal digestion and is called hydrolysis.
So we can break down the disaccharides into the individual monosaccharides through hydrolysis. And as an example we can have sucrose, which is an important part of our diet, it is table sugar, and it can undergo hydrolysis to yield the individual monosaccharides glucose and fructose. Through that process, we can utilize the energy that is available in the disaccharides by forming the monosaccharides which can subsequently be absorbed into the bloodstream and used as an energy source.
Carbohydrate Content of Foods
As you can see from the table above, many foods are high in carbohydrates. The predominant sources of carbohydrate in most people’s diet are starchy foods such as wheat, corn, rice, cassava and potatoes. The raw forms of these foods (e.g. whole wheat, brown rice, etc) also contain substantial amounts of fibre, which is mostly lost during processing. Numerous nourishments are wealthy in carbohydrates because of their high sugar content, which is available normally (as in organic products) or included during processing.
- Meat contains only very small amounts of carbohydrate in the form of glycogen.
- More information about the composition of foods and food composition tables can be found here and here.
*Please note that the carbohydrate content of food shown on the food label is calculated differently in different parts of the world, in particular when it relates to dietary fibre. For instance, in the USA the carbohydrate content is the amount left over after taking into account the amounts of water, protein, fat, alcohol and ash (minerals). The carbohydrate content is thus obtained by subtraction.
On nutritional labels in Europe, carbohydrate is defined as “available carbohydrate,” which does not include dietary fibre. Available carbohydrate is obtained by adding up the sugars and starches in the sample.
Carbohydrates Digestion
Dietary carbohydrates require digestion in the first place before they can be absorbed. And those include starch, a polymer of glucose and the three disaccharides sucrose, maltose and lactose.
Let’s start in the mouth. We know that we produce saliva, and saliva contains an enzyme called amylase and amylase can break down starch, to some degree. Because as soon as you swallow the food and it enters the stomach, the salivary amylase is quickly inactivated by the acidity of the stomach and no further digestion is taking place.
And digestion doesn’t restart again until the food reaches the duodenum, the first part of the small intestine, where the pancreatic juice is added to the food.
The pancreatic juice contains an enzyme called pancreatic amylase. That’s also a starch degrading enzyme and through its action, starch - the polymer of glucose, is broken down into much smaller units, ultimately maltose, the disaccharide of two glucose subunits.
So we end up with maltose, sucrose and lactose as the disaccharides that need further digestion before they can be absorbed. And the body has the appropriate enzymes to be able to break down those three disaccharides to the monosaccharides. So we have an enzyme called lactase to break down lactose. We have an enzyme named sucrase to break down sucrose and we have an enzyme called maltase to break down maltose. So in the end, we end up with the three monosaccharides: glucose predominantly, galactose and fructose.
Carbohydrates Absorption
So let’s follow those three monosaccharides. They are produced in the small intestine and they need to be absorbed into the bloodstream. And this happens after uptake by the intestinal cells, the so-called enterocytes. So the part of the enterocytes that borders the interior of the small intestine has special transporters that allow these cells to take up the monosaccharides.
On the other side of the cell, there is the bloodstream. So there is another set of transporters to transport the monosaccharides from the interior of the cell to the bloodstream. And what is important to realize is that once they enter the bloodstream, the monosaccharides immediately go through the portal vein to the liver. So they are not distributed across the body.
They first go to the liver so the liver can decide on what happens to these monosaccharides. And that means, most of the fructose is cleared or almost all of it is cleared, so no fructose reaches the tissues outside the liver. Whereas for glucose substantial amounts of glucose continue to go to the rest of the body.
Now, what about some of the non-digestible carbohydrates or dietary fibre?
The non-digestible carbohydrates also called dietary fibre. The definition of dietary fibre is that it is not subject to digestion to our normal digestive enzymes. And that separates it from the digestible carbohydrates that we just discussed.
That doesn’t mean there is no digestion at all because depending on the type of fibre there can be some digestion taking place in the large intestine or colon. And that happens through the microbiota, the bacteria that reside in the colon and we call this fermentation.
Now depending on the type of bacteria present you get different products of fermentation, but an important class of products are the so-called short-chain fatty acids.
But as we can recognize, as many of us have experienced when we eat too much fibre at a single meal, there are other gasses produced that give rise to flatulence or gas production.
Now let’s zoom in a little bit more on these microbiotas and specifically on some of these products. So we talked about short-chain fatty acids, they include acetate, propionate and butyrate. And they’re believed to be at least partially responsible for the beneficial effects of fibre. So fibre reaches the colon, is fermented, produces the short-chain fatty acids and they are absorbed in the cells locally, but also they travel further away, for example, arriving at the liver and have beneficial outcomes somewhere else.
On top of that, we produce other gases. Methane is one of them, hydrogen sulfide is a very foul-smelling one and there are other gases that we produce.
Nowadays there is a lot of interest in the role of dietary fibre and especially in the role of the gut microbiota. Because we are increasingly realizing that a lot of the beneficial effects of dietary foods and dietary nutrients, may be mediated through changes in the gut microbiota.
So in short, this is how dietary carbohydrates are digested. That essentially we require several enzymes, mainly amylases and various disaccharidases, that break down the starch and the disaccharides into monosaccharides. That the non-digestible carbohydrates or dietary fibre end up in the colon where it is subject to digestion or fermentation by the bacteria that reside there and that can give rise to various products including short-chain fatty acids.
Carbohydrate Metabolism
One of the main nutrients in our diet is carbohydrate. Previously we’ve looked at how carbohydrate is digested in the GI tract and absorbed into the bloodstream. Now, we’ll look in more detail at what happens to carbohydrate, the monosaccharides once they’re absorbed into the bloodstream. How are they distributed across the body and how is that regulated?
Consider the situation after a meal. After a meal, your blood glucose level will go up and that increase in blood glucose level will trigger the pancreas to release a hormone called insulin.
Insulin has two major actions:
- It stimulates the uptake of glucose into tissues,
- and it stimulates the conversion of the glucose into the storage form of glucose which is called glycogen.
The consequence of those two actions is that blood glucose levels go down. After a while, the decrease in blood glucose level will trigger the production of another hormone by the pancreas which is called glucagon.
Glucagon does the exact opposite of insulin. So it promotes the breakdown of glycogen, and thereby it contributes glucose to the bloodstream. It helps maintain blood sugar levels during periods of fasting when no food is coming into the body. As a result, the blood glucose level is maintained and the cycle restarts again.
Let’s discuss in a little bit more detail the storage form of glucose called glycogen and there are two organs in the body where we store glycogen. The first one is the liver, and glycogen is a polymer of glucose that resembles starch. It is formed after a meal when blood glucose levels are high, but it is broken down during fasting when you don’t eat when you need to maintain your blood sugar levels. That process of glycogen break-down is triggered by glucagon. It leads to the release of glucose units into the bloodstream and therefore maintenance of the stable blood sugar level.
What about glycogen in the muscle?
Glycogen in the muscle functions very differently. Glycogen in the muscle cannot contribute to the maintenance of blood sugar level and is used locally, inside the muscle where it is stored and it provides energy for that particular muscle.
Particularly during high-intensity exercise, if you would start running up the stairs, the glycogen that is stored in your thigh muscles would be used as an energy source. It would be broken down to glucose and the glucose would be immediately used in the muscle cell to provide energy. So, we can store excess glucose, we can store excess carbohydrates in the form of glycogen in muscle and liver.
But what if these stores are filled up? What if we over-consume carbohydrates? But these two stores are already filled up. Then we can convert glucose into fat, we can convert carbohydrate into fat. The body has that ability, which means that if you overeat on sugar, you can become obese, very easily. The inverse can’t occur, which means we cannot convert fat into carbohydrate!
That is a serious limitation that especially has its impact during fasting because it causes our body to break down very valuable tissues to provide glucose when we are for instance, in a state of fasting or famine. But overall the point that I want to make today is that glucose metabolism is carefully regulated with the main aim to maintain stable blood sugar levels. And the reason for this is that the body and the brain can only function if it gets a sufficient amount of glucose. So the body has developed an intricate mechanism that involves certain hormones, insulin, glucagon but also others to maintain the blood sugar level. We can store glucose in the form of glycogen in the liver and muscle and we can convert carbohydrate into fat if we consume too much of it.
Carbohydrates and Health
Some people would like you to believe that we should avoid carbohydrates as much as possible, but is there any basis for that? Well, there are major concerns about a certain type of carbohydrate and that is sugar. So we’ll look at sugar in a little bit more detail. And how much sugar are we eating?
Sugar has a bad rep, some people say it’s responsible for the current obesity epidemic. But is that justified? Of course, the sugar industry denies that we consume too much sugar and you would wonder why?
Now, what can be done to lower sugar consumption? That is not so easy. Is there a job for artificial or non-nutritive sweeteners? The aspartame, sucralose. Some people are waging a crusade against artificial sweeteners because they believe they cause all kinds of nasty diseases. But is there any basis for this? But carbohydrates are not all bad, some carbohydrates are really good for you!
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