Carbohydrates are generally regarded as a major source of energy in our diets.
In the body these nutrients are metabolized into glucose – the form in which it is transported in the blood throughout the body. The cells then use glucose as their primary source of energy.
The brain, in particular, uses only glucose as its fuel, and it must be supplied in the correct amount on a constant basis.
There are three major types:
* Complex carbohydrates such as starch which is present in cereals, whole grains, potatoes, legumes etc.
* Simple carbohydrates such as sugar. Sugars occur naturally in fruits and vegetables. This is fortunate as it makes the food palatable, and they also contain important nutrients such as phytochemicals, and antioxidants.
Our tendency towards a sweet tooth has been our undoing, however, as we have learned of ways to add massive amounts of sugar to just about everything we eat. White table sugar, corn syrup, fructose and even honey are problematic additives to foods.
* Fibre. This is indigestible carbohydrate present in foods of plant origin. It serves a very important role in the body.
Ultimately all carbohydrates must be broken down into glucose to supply energy to the body.
The conversion process is under various regulatory control mechanisms as the blood glucose levels need to be carefully regulated. The liver and pancreas play important roles in this regard.
When food is digested blood sugar levels rise. In response to this the pancreas releases the hormone insulin. The role of insulin is to cause increased uptake of glucose from the blood into the cells.
In the liver, insulin also stimulates the conversion of glucose into glycogen which is a storage form of glucose thus stabilizing the blood glucose levels.
Glycogen levels in the liver can supply glucose for only 24 hours of fasting. If starvation continues for longer periods fat is mobilized as an energy source for the muscles, but the brain has to have a supply of glucose as its only source of energy.
As a result the body starts to break down tissue protein into amino acids which are converted into glucose in the liver – a process known as gluconeogenesis (‘the production of new glucose’).
This cannot be allowed to continue for very long because one’s body starts breaking down its tissues such as muscles, heart and liver which is not a good idea,
Another, short-term, side effect is ketosis where by-products of the process, called ketones, are produced. These lead to symptoms such as headaches, nausea, weakness, dizziness, sleep problems and fatigue.
One cannot decide which carbohydrates are likely to elevate ones blood glucose levels (and therefore cause a spike in insulin secretion) based on whether they are complex or not. Other factors have to be taken into account as well.
Currently the way of judging a foodstuff’s impact on insulin secretion is to look at the glycaemic index (GI).
Some of the factors that influence the GI value are as follows:
* Processed grains have a higher GI than whole grains
* Foods that are rich in fibre tend to have lower GI values than foods which have more digestible carbohydrates. However, if the fibre-rich foods, especially grains, are finely ground the sugar is released more readily.
* Some starches are more readily digested and therefore raise the blood sugar levels more quickly e.g. potatoes have a higher GI and sweet potatoes a lower GI!
* Fatty foods tend to have lower GI values.
* The riper fruits and vegetables are the higher the sugar content and the higher the GI value.
Many tables are available where the GI values of foods are recorded to enable you to make sensible food choices. One such source is available from the University of Sydney in Australia at www.glycemicindex.com. A GI score of above 70 is considered to be high, between 55 and 70 medium and below 55 low.
More recently the concept of glycaemic load has been proposed. This is to take into account how much digestible carbohydrate is actually available in a particular foodstuff, not simply how digestible it is.
Watermelon, for example, has a high GI, but its glycaemic load is low because it consists mainly of water.
The glycaemic load is calculated by multiplying the GI by the amount of carbohydrate present in a serving. A glycaemic load of 20 or above is high, 11 to 19 is medium and 10 and below is low.
Carbohydrates are important for cellular recognition and communication. For this function they are usually attached to proteins. These types of molecules are called glycoproteins.
They perform a number of different functions in the body:
* Many proteins that occur on the surface of cell membranes have short carbohydrate chains attached to them. They are thought to play an important role on cell-to-cell recognition, for example in immune responses, blood clotting, fertilization of ova, and self-non-self recognition.
* Carbohydrates are also important components of mucins which are glycoproteins that are secreted by the respiratory and digestive tracts to protect their surfaces.
* Connective tissue is also rich in structural glycoproteins such as collagens, and are important for sound bone formation.
* Several hormones are glycoproteins, e.g. follicle-stimulating hormone (FSH which stimulates the maturation of ovarian follicles), luteinizing hormone (LH which stimulates secretion of sex steroids from the gonads in both sexes) and erythropoietin (a hormone which stimulates the production of red blood cells by stem cells in bone marrow).
In order for the body to function optimally all the necessary glycoproteins need to be in place. If they are faulty – missing any of the sugars or amino acids that make them up - communication breaks down and ill health and disease follow. In other words, the communication network is sabotaged.
Fibre (also known as roughage) refers to the parts of plant-based foods pass through the digestive tract without being digested and absorbed.
This is still a pivotal role for maintaining health and is ignored at your peril. In fact, it is sometimes called the ‘essential non-nutrient’.
Modern diets have become more and more refined, and are generally not supplying us with adequate amounts of dietary fibre.
There are two fibre groups:
* Soluble fibre
Soluble fibre dissolves in water in the bowel. Gums and pectins are found in this group.
Soluble fibre delays gastric emptying, thereby improving digestion. It also slows absorption of glucose into the bloodstream, thereby reducing the risk of insulin spiking.
It also assists in controlling cholesterol levels by inhibiting its absorption from the gastrointestinal tract and prevents obesity from developing.
* Insoluble fibre
Insoluble fibre which does not dissolve in water. Cellulose, hemicellulose and lignin are all insoluble fibres.
Insoluble fibre plays the role of the broom which sweeps the digestive tract clean by increasing stool bulk and therefore preventing constipation, diveticulosis and haemorrhoids.
Another important benefit of this is the reduced risk of developing colon cancer.
Good sources of insoluble fibre in the diet are legumes, whole grains, fruits, vegetables, nuts and seeds.
Adults should take in between 25 and 35 grams of fibre per day. Most adults do not get more than 15 grams per day, so pay attention to your diet and see how you are doing.
Increasing fibre intake should be undertaken slowly to prevent bloating and discomfort, and increased gas. These symptoms settle as your system gets used to the new state of affairs.
Fluid intake (preferably water) must be increased at the same time – the 6-8 glasses a day really becomes important as ones daily fibre intake increases.
If you feel that you are unable to increase your fibre through your diet, good supplements are also effective.
Carbohydrates should form 45% - 65% of our daily intake of calories.
They are essential for our survival because:
* They are a primary source of fuel and energy
* They are vital for intestinal health and proper elimination of waste
* They protect the body against using up protein as an energy source, leading to wasting of muscles
* They are fundamental components of the intercellular
communication networks that keep all the complex organs and cells
operating effectively as a unit.
To see where carbohydrates fit into the ideal diet, click on our link to Nutrition and Food