Carbohydrates are a big and very important part of our daily diets, but also a source of confusion for many people. There are very many contradictory opinions on carbohydrates and how large of a part in your diet they should play. Some people say you should eat a lot of them, while others say you should eat a low carb diet. All these strategies have their merits, but might not be for everyone. Your basic strategy on the amount of carbohydrates depends on what type of a person you are, what your goals are and also what your activity levels are. So what might be the “right” carbohydrate strategy for one person, might not be the best one for another person.
So what actually are carbohydrates and what do they do? Carbohydrates are macromolecules with various combinations of carbon, oxygen, and hydrogen. Together with proteins and fats, they form one of the three main nutrient types in the human diet. Unlike the other two however, no carbohydrate type is considered essential, as the human body itself can manufacture any type of carbohydrate that it needs.
There are two main roles of carbohydrates in organisms: to serve as storage of energy, and in some organisms carbohydrates also serve a structural role and provide structure (for example cellulose in plants, chitin in arthropods). There are also other roles that carbohydrates play. One type of carbohydrate, ribose, also forms part of coenzymes and can be found in the structure of RNA. Another carbohydrate, deoxyribose, forms part of DNA. Carbohydrates also participate in things like fertilization, blood clotting, development and also play a role in the immune system. Furthermore, scientific studies also show that an adequate supply of carbohydrates (glucose) has a positive effect on the brain.
Most carbohydrates in our diets come from plants, although milk (comes from different types of mammals) also contains lactose (which is a carbohydrate). In terms of energy, a carbohydrate is about 4 calories per gram.
Carbohydrates can be divided into 4 chemical groupings (with each grouping containing several types of carbohydrates):
These are the simplest type of carbohydrates and cannot be broken down (hydrolyzed) into smaller types of carbohydrates.
- glucose – main source of energy for a cell
These are composed of two monosaccharide molecules bonded together.
- sucrose – also known as table sugar, composed of fructose + glucose
- lactose – found in milk, composed of galactose + glucose
- maltose – two units of glucose joined through an α-1,4 bond
- cellulobiose – two units of glucose joined through a β-1,4 bond
These are repeating units of monosaccharides joined through glycosidic bonds. Usually contain three to nine units.
- inulin oligosaccharides
These are made up of chains of many monosaccharides. They can be branched or unbranched (straight chained). These act as storage of energy in plants (starch) and animals (glycogen). In plants, they can also serve a structural function (cellulose, pectin). They also perform a structural function in some insects (chitin).
- starch – main way of storing glucose in plants
- glycogen – very similar structure as amylopectin in starches, but instead found in animals
So what purpose do carbohydrates serve and where do they come from?
All these different carbohydrates come from different sources and have different purposes. Glucose serves as the main source of energy for the body. Living organisms have developed several ways of storing glucose in order to be able to use it later. This is where the polysaccharides come in. In many organisms, glucose molecules are bonded together to form long-chains strung together. In plants this is in the form of starches, while in animals glycogen serves this purpose.
You probably know several types of carbohydrates under a different name. Monosaccharides and disaccharides are known in everyday language as sugars. There are many types of monosaccharides, with the most important being glucose, followed by fructose and galactose.
The main source of glucose are fruits and vegetables. When it is present in the body, it is sometimes referred to as “blood sugar”. It can sometimes be ingested into the human body as individual units, but very often the glucose in the body is a result of different metabolic processes breaking down bigger chains of carbohydrates. Glucose is the main source of energy for living organisms and the easiest to metabolize. It is the basic fuel for the human body and its proper functioning. Glucose can be broken down to form adenosine triphosphate (ATP), which is sometimes called the body’s “currency of energy” and basically is the energy that is used to spark different processes in the cells.
The main source of fructose are fruits. Fructose, however unlike glucose, has to be processed in the liver first before it is able to be processed by the body. Galactose does not occur alone in nature, but can be isolated through the breakdown of other carbohydrates. Galactose is one of the building blocks of lactose, which is found in milk. You probably never thought of milk as being composed of sugar, but in fact it is!
There are also other important carbohydrates. Sucrose, which is commonly known as table sugar, comes mainly from refining or processing sugar cane and sugar beets. When it enters the small intestine, it is split into its two constituent parts, fructose and glucose. Starch is the main energy storage mechanism for plants. It is made up of long chains of glucose strung together. These chains of glucose are very easy for the human body to digest and break down. Cooking and chewing can break down the cellulose walls and then different enzymes go to work in order to release the individual glucose in the chains. Humans can get starch from bread (wheat), corn, rice, cereals, grains, roots, different vegetables and things like beans or peas.
Another type of carbohydrate are fibers. Fibers usually cannot be digested by the human body (or only partially), but do serve an important purpose. They help in the prevention of certain diseases, but also aid in the digestive process. There are two types: soluble and insoluble. Sources of soluble fiber are peas, beans, lentils, oats, and also barley. Sources of insoluble fibers are things like whole grains. Whole grains are grain products that still have all parts of the grain seed – bran, germ, and endosperm.
To illustrate a bit of what happens in the human body once it ingests different carbohydrates, let’s take the example of milk. Milk contains lactose, which is composed of galactose and glucose. Once lactose is in the human digestive system, an enzyme called lactase goes to work on it in order to break it down. However only a percentage of the human population has this enzyme, mostly the ones descended from Europeans (where early on cattle breeding was an important way of sustenance). In other parts of the world, people do not have this enzyme and so cannot process lactose. These people are referred to as lactose intolerant.
In fact, not all carbohydrates can be processed by the human body. The processing of carbohydrates depends on the types of enzymes that are present in the human body. For example some types of animals can process certain types of carbohydrates, which humans can’t. As was illustrated on the example of lactose, even within human populations there are differences on which carbohydrates can be processed and which can’t. All this is a result of evolution.
As mentioned before, glucose is the main source of energy for the human body. Sometimes, the body cannot use all the glucose in it and it also needs to keep some glucose as a reserve for later use. What happens is that it stores this extra glucose as glycogen. Glycogen is the main storage mechanism of glucose for animals, sort of like starch is for plants. In this phase, it is when an enzyme known as insulin comes into play. Insulin is an enzyme that is released when glucose levels in the body are too high. It is released from the pancreas and acts on the hepatocyte (a type of cell in the liver) in order to stimulate the action of several types of enzymes. These enzymes then go to work binding all the loose glucose into glycogen chains for storage and later use. These are then stored in the liver. When glucose is needed later on, different types of enzymes (the main one being glucagon) go to work breaking down these glycogen chains and freeing individual glucose. Muscle cells also store glycogen, but this glycogen is not released for use by other parts of the body, but only serves for the internal use of these muscles.
However glycogen storage is limited in the human body. Once this is filled, all the remaining excess glucose can be converted into fatty acids and stored as fat in the body. The storage of glucose as fat has an unlimited capacity. This is why the amount of carbohydrates you eat has a direct impact on your fat levels.
Strategies for fueling yourself up with carbohydrates
In order to discuss what type of strategy you should adopt in terms of taking in carbohydrates, we need to clarify some terminology that is floating around and often used by nutritionists.
Nutritionists usually classify carbohydrates into simple and complex. These two terms don’t have an agreed upon meaning and one nutritionist’s definition can differ from the definition of another one. However the easy way to think of these two terms is to think of simple carbohydrates as being all those that are in mono- or disaccharides and the complex carbs are the ones that are classed as polysaccharides. So anything that an average person on the street calls “sugar” is a simple carbohydrate (they usually taste sweet as well), while most of the other ones are complex.
One often also comes across talk of “good” and “bad” carbs. Bad carbs are usually from all the refined and processed foods (such as white flour and refined sugar) that contain what is called “empty” calories, that is a high amount of calories, but no other things such as vitamins, minerals and other good stuff. Eating too many of them can spike up triglyceride (fat) and cholesterol levels in the body. Good carbs come from foods that also give a lot of other things (such as vitamins) besides just the carbs. Good carbs are also often complex carbs, which take a longer time to be broken down in the body.
There are three basic tools that can help you in determining what types of foods to eat. The most well-known one is called the glycemic index. It is based on a score of 0 to 100. Pure glucose gets a score of 100, so all the foods are measured relative to pure glucose. The glycemic index measures how fast glucose levels rise inside the body after the ingestion of a particular food. High glycemic foods often spike up the glucose levels fast. This means these carbohydrates break down quickly and release their glucose into the bloodstream really fast. Low glycemic foods are the ones with carbohydrates that break down slower and their glucose is released more gradually into the bloodstream.
Foods that contain “bad” carbs are usually foods with a high GI score. For example refined foods made of white flour have high GI scores. Eating too many high GI foods can lead to weight gain, but with a lot of fat gained in the process. The glycemic load is another score that can be useful to look at when choosing what types of food to eat. It measures the amount of food that will raise a person’s blood glucose levels. There is also the insulin index, which measures the insulin response to foods.
The actual strategy towards carbohydrates depends on your goals, whether it is to gain weight, lose weight, support your athletic endeavors…etc. The recommendations for the average person vary from 45% to around 75% of the total calories in your diet being composed of carbohydrates (with the Institute of Medicine recommending 45-65% and the WHO and FAO recommending 55-75%). However the actual amounts should also take into account your goals (lose weight, gain weight) and activity levels (for example long exercise and long-distance running). So if you are trying to gain weight, having a large number of carbohydrates in your diet may help you. Good sources of carbohydrates for someone trying to gain weight are brown rice and sweet potatoes. On the other hand, for someone trying to lose weight, you might try to cut down on the amount of carbohydrates. The strategy once again is different for active people. Carbohydrates are basically the fuel for the body and so for active people who do long and demanding exercise or activities (such as long distance running), they might need to also take in more carbohydrates, before, during and after exercise.
So remember to eat those carbohydrates. They give your body energy!