What are Carbohydrates?

See also: What is Fat?

Along with proteins and fats, carbohydrates are one of the fundamental macronutrient groups. They are the main sources of energy for the body. They include starches, sugars and plant fibres. They are so-called because, chemically, they contain only carbon, hydrogen and oxygen.

Carbohydrates are broken down inside the body to create glucose. Glucose is transported around the body by the blood, and is the primary source of energy for the brain, muscles, and other essential cells. Eating carbohydrates provides an easy source of energy for the body and prevents it from having to make glucose from protein or fats.

How the body uses carbohydrates

The body uses carbohydrates to provide fuel for muscles and organs, including the brain and central nervous system.

This fuel is used in the form of glucose, which is broken down in cells to provide energy. Excess glucose is stored in the liver as glycogen, or converted to fat and stored around the body.

Glucose is transported around the body by the blood, to reach organs and muscles that need fuel. The healthy body attempts to regulate glucose levels in the blood by using the hormones insulin and glucagon, which are produced by the pancreas.

  • Insulin lowers blood sugar levels by moving the glucose to various parts of the body and aiding its absorption into tissues such as fat.
  • Glucagon increases blood sugar levels by releasing glucose stored in the liver back into the bloodstream.

Blood sugar levels are usually kept under control if the pancreas and liver are healthy and functioning normally.

The Effect of Stress on Blood Sugar


When we are under stress, the hormones adrenalin and/or cortisol are released into the body.

These hormones raise blood sugar levels, giving the body a sudden boost of energy. This is also known as the ‘flight or fight’ response, providing the energy to run away or fight whatever has threatened us.


See our page: What is Stress? for more information.

The body can produce glucose from protein and fat, so some people have suggested that eating carbohydrates is not essential. Some weight-loss diets exclude or reduce carbohydrate intake, as a way to make the body convert its supply of fat to glucose and use it for energy.

However, there are three things that should also be considered.

  • First, carbohydrates are the most efficient way for the body to produce energy

    General nutritional advice is therefore to eat at least some each day, to avoid the body using protein for energy, which may mean breaking down muscle tissue. Using protein for fuel also has detrimental effects on the kidneys.

  • Second, studies have found that eating carbohydrates seem to have an effect on mental health.

    For example, people who eat diets that replace carbohydrates with fats tended to show higher levels of anger, depression and anxiety than those on a high-carbohydrate, low fat diet. In another study, women who had eaten no carbohydrates for a week showed lower cognitive function than women on a low-calorie diet with a ‘healthy’ amount of carbohydrates.

  • Third, carbohydrate-containing foods are often also rich in fibre.

    In fact, it is hard to get enough fibre in your diet without eating carbohydrates.

    Fibre helps you to feel full, and can therefore help with weight loss. Recent studies and advice suggest that excluding carbohydrates altogether can make it harder to lose weight, because of the effect of the fibre.


Types of Carbohydrate

There are two types of carbohydrates, simple and complex.

Simple Carbohydrates

Simple carbohydrates are also known as sugars. They have a simple molecular structure, with just one or two parts.

Because of their simple molecular structure, the body can process these sugars quickly. Eating them can therefore lead to an ‘energy spike’, or sudden rush of energy. This is followed by a ‘low’ once the process is complete and the simple carbohydrates have been used.

Foods that affect blood sugar quickly, like simple sugars, are said to have a high glycaemic index (GI). Processed and refined sugars tend to have a high GI. Naturally occurring complex carbohydrates have a lower GI.

Glycaemic Index (GI)


The glycaemic index of food is a measure of how quickly it causes glucose levels to rise in the blood.

Generally foods with a lower GI, which release glucose more slowly, are considered healthier. The body has to work harder to break these foods down, so you feel fuller for longer and burn more calories digesting and recovering glucose.

However, some foods with lower GI scores are also high in fat and salt, which may make them less healthy.

Example GI scores:

  • White Bread - 71
  • Wholemeal Bread - 49
  • Steamed White Rice - 98
  • Basmati Rice - 58
  • Milk Chocolate - 49
  • Dry Roasted Peanuts - 14
  • Whole Milk - 27
  • Skimmed Milk - 32
  • Refined sugar is a common source of simple carbohydrates in the modern diet.

    Many processed, packaged and fast foods contain simple carbohydrates. Sugar is used as a flavour enhancer in a lot of foods. Simple carbohydrates from added sugar have little or no nutritional value and are often described as ‘empty calories’. Most people can benefit from reducing their intake of these simple carbohydrates. If you buy processed and packaged foods, try to choose those with less added sugar, and reduce your consumption of sugary foods such as cakes, biscuits (cookies), sweets (candy) and regular (not diet) soft drinks.

    Simple carbohydrates are not always bad. They also exist naturally in foods that provide other nutritional benefits, including fruit, milk and other dairy products. Most fruits contain good levels of fibre, vitamins and micro-minerals as well as antioxidants. Milk and dairy products are good sources of protein and calcium, and most people agree that both fruit and dairy products are important to a well-balanced healthy diet.

    For more on sugar see our pages: What is Sugar? and Sugar and Diet.

    Complex Carbohydrates

    Complex carbohydrates are those with a more complex molecular structure with three or more parts. They include starch and cellulose, the material that helps to make plant cells rigid.

    The complex structure of these molecules means that it takes the body longer to break them down to produce glucose. Foods rich in complex carbohydrates often also contain valuable vitamins, minerals and fibre, all of which are vital to overall health and wellbeing.

    Foods containing complex carbohydrates are processed more slowly by the body, so they can provide sustained energy levels over longer periods of time than simple carbohydrates. The glycaemic index of foods rich in complex carbohydrates is therefore lower.

    Foods rich in complex carbohydrates include whole grains, wholemeal bread and wholegrain breakfast cereals, oats, pasta, rice (especially brown rice), potatoes, beans, lentils and chickpeas.

    Finding the Right Balance


    Consuming too many carbohydrates or the wrong type of carbohydrate can upset the management and balance of your body’s blood sugar levels. This can result in energy highs and lows and mood swings. This, in turn, can leave you feeling tired and irritated.

    However, consuming too few carbohydrates can leave you lacking energy, and losing muscle tone, because your body will try to make up the shortage of glucose from protein instead.

    It is important to find the right balance of carbohydrates in your diet, which may vary from person to person.

    If you often feel tired after lunch, you may want to try eating more protein and vegetables, and less carbohydrates. It may also be best to avoid alcohol and sugary drinks, especially outside mealtimes, as they are often high in sugar.


    How Much Carbohydrate Do We Need?

    There is no simple answer to this question.

    Different people have different ideas about carbohydrate consumption. However, as a general rule, it is worth aiming to get approximately half of your energy from carbohydrates, with as much as possible (ideally 90%) of this coming from complex carbohydrates and only 10% from refined and processed sugars (simple carbohydrates).

    A simple way to achieve a healthy well-balanced diet is to eat a variety of foods each day.

    Some people recommend that at each meal, your plate should be split so that a quarter is high protein food, such as meat, fish, eggs, and dairy produce (see our page What is Protein? for more information).

    A quarter of the plate should be complex carbohydrates, such as potatoes, pasta, rice, bread, and beans.

    The rest of the plate (half) should be made up of fresh vegetables, which will provide some carbohydrates and protein but also many vitamins and minerals.

    Finish your meal with a piece of fruit which will provide you with all the simple carbohydrates you need.

    What Happens if I Eat Too Many Carbohydrates?

    Eating too many carbohydrates is likely to lead to weight gain, because the body will store unused glucose for later.

    Usually, however, it is how the carbohydrates are prepared that will have the greatest effect on weight gain. For example, both chocolate and apples contain simple carbohydrates, but chocolate also contains fat, and apples contain fibre, vitamins and minerals. French fries, chips, crisps and roast potatoes are all prepared using fat and therefore contain fat. Baked and boiled potatoes are not cooked in fat. That is not to say that roast potatoes are bad for you, but they do contain more calories for the amount of carbohydrate.


    In Summary

    Carbohydrates contain the glucose that the body needs for energy.

    There are two main types of carbohydrates, simple and complex.

    More refined and simple carbohydrates are converted to glucose more quickly, which can cause peaks and troughs in blood sugar levels and result in variable energy levels. Refined or simple carbohydrates should therefore make up only about 10% of your daily carbohydrate intake.

    Complex carbohydrates, such as those found in starchy foods such as potatoes, bread, rice, whole grains, pasta and oats release glucose more slowly into the bloodstream. They therefore provide more stable and sustainable energy levels for the body.


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