There are four major classes of biomolecules - carbohydrates, proteins, nucleotides, and lipids. Carbohydrates, or saccharides, are the most abundant of the four.
Carbohydrates have several roles in living organisms, including energy transportation, as well as being structural components of plants and arthropods.
Carbohydrate derivatives are involved in fertilization, the immune system, the development of disease, and blood clotting.
Contents of this article:
Fast facts on carbohydrates
Here are some key points about carbohydrates. More detail and supporting information is in the main article.
- "Saccharides" is another word for "carbohydrates"
- Foods high in carbohydrates include bread, pasta, beans, potatoes, rice, and cereals
- One gram of carbohydrate contains approximately 4 kilocalories
- High glycemic index carbohydrates quickly enter the bloodstream as glucose
Four major biomolecule classes
People think of bread, pasta, and rice when they think of carbs, but all these foods are rich in carbs, too.
Carbohydrates (saccharides) - consist of carbon, hydrogen, and oxygen atoms. They are a major food source and a key form of energy for most organisms.
When combined together to form polymers (chains), carbohydrates can function as long-term food storage molecules, as protective membranes for organisms and cells, and as the main structural support for plants.
Lipids (fats) - Molecules consist of carbon, hydrogen, and oxygen atoms. They are the main constituents of membranes in all cells (cell walls), food storage molecules, intermediaries in signaling pathways, vitamins A, D, E, and K, and cholesterol.
Proteins - Molecules contain nitrogen, carbon, hydrogen, and oxygen. They act as biological catalysts (enzymes), form structural parts of organisms, participate in cell signaling and recognition, and act as molecules of immunity. Proteins can also be a source of fuel.
Nucleic acids (nucleotides) - DNA and RNA. These molecules carry genetic information, as well as forming structures within cells. They are involved in the storage of all heritable information of all organisms, as well as the conversion of this data into proteins.
Most organic matter on earth is made up of carbohydrates because they are involved in so many aspects of life.
What are saccharides?
"Saccharides" is another term used for "carbohydrates." They are sugars or starches. Saccharides consist of two basic compounds: aldehydes (double-bonded carbon and oxygen atoms, plus a hydrogen atom), and ketones (double-bonded carbon and oxygen atoms, plus two additional carbon atoms).
There are various types of saccharides, including monosaccharides, disaccharides, and polysaccharides.
This is the smallest possible sugar unit. Examples include glucose, galactose, or fructose. When we talk about blood sugar we are referring to glucose in the blood; glucose is a major source of energy for a cell.
In human nutrition, galactose can be found most readily in milk and dairy products, while fructose is found mostly in vegetables and fruit.
Disaccharides are two monosaccharide molecules bonded together. Examples of disaccharides include lactose, maltose, and sucrose. If you bond one glucose molecule with a fructose molecule, you get a sucrose molecule.
Sucrose is found in table sugar and is often formed as a result of photosynthesis (sunlight absorbed by chlorophyll reacting with other compounds in plants). If you bond one glucose molecule with a galactose molecule, you get lactose, which is commonly found in milk.
Polysaccharides are a chain of two or more monosaccharides. The chain may be branched (the molecule looks like a tree with branches and twigs) or unbranched (the molecule is a straight line). Polysaccharide molecule chains may be made up of hundreds or thousands of monosaccharides.
Types of polysaccharides
Different polysaccharides act as food stores in plants and animals. Polysaccharides also have structural roles in the plant cell wall and the tough outer skeleton of insects.
A polysaccharide that humans and animals store in the liver and muscles.
These are glucose polymers made up of amylose and amylopectin.
Starches are not water soluble. Humans and animals digest them using amylase enzymes. Rich sources of starches for humans include potatoes, rice, and wheat.
The structural constituents of plants are made mainly from the polysaccharide cellulose. Wood is mostly made of cellulose, while paper and cotton are almost pure cellulose.
Carbohydrates and nutrition
Bread, pasta, beans, potatoes, bran, rice, and cereals are carbohydrate-rich foods. Most carbohydrate-rich foods have a high starch content. Carbohydrates are the most common source of energy for most organisms, including humans.
Carbohydrates are not classed as essential nutrients for humans. We could get all our energy from fats and proteins if we had to. However, our brain requires carbohydrates, specifically glucose. Neurons cannot burn fat.
- One gram of carbohydrate contains approximately 4 kilocalories
- One gram of protein contains approximately 4 kilocalories
- One gram of fat contains approximately 9 kilocalories
Proteins are used in both forms of metabolism - anabolism (building and maintaining tissue and cells) and catabolism (breaking molecules down and releasing/producing energy). So, the consumption of protein cannot be calculated in the same way as fats or carbohydrates when measuring our body's energy needs. Not all carbohydrates are used as fuel (energy). A lot of dietary fiber is made of polysaccharides that our bodies do not digest.
Most health authorities around the world say that humans should obtain 40-65 percent of their energy needs from carbohydrates - and only 10 percent from simple carbohydrates (glucose and simple sugars).
High-carb vs. low-carb
Every couple of decades, some 'breakthrough' appears which tells people either to 'avoid all fats,' or 'avoid carbs.' Carbohydrates have been, and will continue to be, an essential part of any human dietary requirement.
The obesity explosion in most industrialized countries, and many developing countries, is a result of several contributory factors. One could easily argue for or against higher or lower carbohydrate intake, and give compelling examples, and convince most people either way. However, some factors have been present throughout the obesity explosion and should not be ignored:
- Less physical activity.
- Fewer hours sleep each night. A study published in the journal SLEEP identified an association with duration of sleep and obesity in both children and adults.
- Higher consumption of junk food.
- Higher consumption of food additives, coloring, taste enhancers, artificial emulsifiers, etc.
- More abstract mental stress due to work, mortgages, and other modern lifestyle factors. A study by scientists from the United States and Slovakia revealed that neuropeptide Y (NPY), a molecule that the body releases when stressed, can 'unlock' Y2 receptors in the body's fat cells, stimulating the cells to grow in size and number.
In rapidly developing countries, such as China, India, Brazil, and Mexico, obesity is rising as people's standards of living are changing. However, a few decades ago when their populations were leaner, carbohydrates made up a much higher proportion of their diets.
Those leaner people also consumed much less junk food, moved around more, tended to consume more natural foods, and slept more hours each night. Saying that a country's body weight problem is due to too much or too little of just one food component is too simplistic - it is a bit like saying that traffic problems in our cities are caused by badly synchronized traffic lights and nothing else.
Current diet promoters of either high or low carb regimes in North America, Western Europe, and Australasia have not really addressed those obesity contributory factors properly. Most of them promote their branded nutritional bars, powders, and wrapped products which have plenty of colorings, artificial sweeteners, emulsifiers, and other additives - basically, junk foods.
If consumers are still physically inactive and not sleeping properly, they may gain some temporary weight loss, but will most likely be back to square one within 3 to 4 years.
However, it is true that many carbohydrates present in processed foods and drinks tend to spike glucose and subsequently insulin production, leaving you hungry sooner than natural foods would.
The Mediterranean diet, with an abundance of carbohydrates from natural sources plus a normal amount of animal/fish protein, have a much lower impact on insulin requirements and subsequent health problems, compared with any other widespread Western diet.
Dramatically fluctuating insulin and blood glucose levels can have a long term effect on the eventual risk of developing obesity, type 2 diabetes, heart disease, and other conditions. However, for good health, we do require carbohydrates.
Carbohydrates that come from natural, unprocessed foods, such as fruit, vegetables, legumes, whole grains, and some cereals also contain essential vitamins, minerals, fiber, and key phytonutrients.
Blood sugar levels
Carbohydrates are broken down into glucose, which then enters the blood, raising blood sugar levels.
When we eat foods that include carbohydrates, our digestive system breaks some of them down into glucose. This glucose enters the blood, raising blood sugar (glucose) levels. When blood glucose levels rise, beta cells in the pancreas release insulin.
Insulin is a hormone that makes our cells absorb blood sugar for energy or storage. As the cells absorb the blood sugar, blood sugar levels start to drop.
When blood sugar levels drop below a certain point, alpha cells in the pancreas release glucagon. Glucagon is a hormone that makes the liver release glycogen - a sugar stored in the liver.
In short - insulin and glucagon help maintain regular levels of blood glucose for our cells, especially our brain cells. Insulin brings excess blood glucose levels down, while glucagon brings levels back up when they are too low.
If blood glucose levels are rising too rapidly and too often, the cells can eventually become faulty and not respond properly to insulin's "absorb blood energy and store" instruction; over time, they require a higher level of insulin to react - we call this insulin resistance.
Eventually, the beta cells in the pancreas wear out - because they have had to produce lots of insulin for many years - insulin production drops and, eventually, can stop altogether.
Insulin resistance leads to hypertension (high blood pressure), high blood fat levels (triglycerides), low levels of good cholesterol (high-density lipoproteins), weight gain, and other diseases. All these illnesses, together with insulin resistance, is called metabolic syndrome. Metabolic syndrome leads to type 2 diabetes.
If over the long-term, blood sugar levels can be controlled without large quantities of insulin being released, the chances of developing metabolic syndrome are considerably lower. Natural carbohydrates, such as those found in fruits and vegetables, legumes, whole grains, etc., tend to enter the bloodstream more slowly compared with the carbohydrates found in processed foods. Good sleep and regular exercise also help regulate blood sugar and hormone control.
Carbohydrates which quickly raise blood sugar are said to have a high glycemic index, while those that have a gentler effect on blood sugar levels have a lower glycemic index.
The glycemic index
Carbohydrates enter the bloodstream as glucose at different rates - high glycemic index (GI) carbohydrates enter the bloodstream as glucose rapidly, while low GI carbohydrates enter slowly because they take longer to digest and break down.
A meal with lower GI carbohydrates will raise your blood glucose levels more slowly, and over a longer period - this is better for long-term health and body weight control.
People who are relatively physically inactive (sedentary), and don't sleep at least 7 hours every night are especially vulnerable to the long-term detrimental effects of regular consumption of high GI carbohydrates.
Low GI carbohydrates have the following benefits:
- Individuals are less likely to put on weight.
- Low glycemic diets may be better for weight loss. A diet of foods less likely to spike blood sugar levels helps dieters lose more weight, according to a systematic review from Australia.
- Better diabetes control.
- Blood cholesterol levels will most likely remain healthy.
- Heart disease risk is lower.
- It will take longer to become hungry after a meal.
- Physical endurance will improve.
How can I switch to a low GI lifestyle?
Cutting out processed food is an important part of switching to a low GI lifestyle.
- If eating cereals for breakfast, switch to oats, barley, or bran. Make sure the oats are as natural as possible; milling or grinding can increase their GI dramatically.
- If eating bread, only consume wholegrain bread.
- Eat plenty of fresh fruit and vegetables.
- Substitute fruit juice for fresh, whole fruit.
- Eat rice with the husk still there (brown rice).
- Choose whole grain pasta.
- Eat plenty of salads.
- Cut out all junk foods, processed foods, foods with too many additives.
How processing affects the glycemic index of carbohydrates
Milling and grinding of foods always raise their glycemic index. Unfortunately, the processes often eliminate other nutrients, such as minerals, vitamins, and dietary fibers, leaving what is often no more than starchy endosperm (the inner part of the seed/grain, mainly starch).
Slow carbs matter much more than low carbs. A well-balanced diet consisting of good quality foods is as important as physical activity and adequate sleep.
If you are overweight and you want to lose weight, focusing on slow carbs is useful. A well-balanced and nutritional diet, such as the Mediterranean diet, plus healthy sleep and plenty of physical activity, is much more likely to lead to long-term success and good physical and mental health.