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. When we are healthy, our bodies 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.

This is a simplified version, but blood sugar levels are usually kept under control if the pancreas and liver are healthy and functioning normally.

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.

There is also a fourth factor to consider, which is that we are all individuals—and more importantly, we all have highly individual gut bacteria or microbiomes.

This means that we digest fat and carbohydrate differently from each other. What works for one person may not work for another.

One study compared groups on high-fat, low-carbohydrate diets and low-fat, high-carbohydrate diets to examine whether they lost weight. On average, there was no clear effect of either diet. However, when the researchers looked at individuals within the study, they found that those averages masked some striking weight losses within both groups.

In other words, some people are more suited to eating more fats, and others to eating more carbohydrates. Cutting out and demonising whole food groups is unlikely to be an effective dietary strategy in the long term.

See our page on Understanding and Improving Your Gut Microbiome for more.

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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 in some people. This is followed by a ‘low’ once the process is complete and the simple carbohydrates have been used. Many foods and drinks that are highly processed, or which contain very few other nutrients, are high in simple carbohydrates. These may therefore cause this ‘spike’ in sugar levels.

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 soft drinks.

For more on sugar and managing and reducing blood glucose spikes, see our pages: What is Sugar? and Complex Carbohydrates, Sugar and Diet.

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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, and the lattice within which they are contained, means that it takes the body slightly 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 tend to be more complex generally. They often contain fibre as well as carbohydrates, and this slows down the processing of the carbohydrate by the body. This reduces the damaging ‘glucose spike’ that is invariably seen with simple carbohydrates.

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.

Refined Carbohydrates

There is one other group of carbohydrates worth mentioning, not least because these have become more or less ubiquitous in our diets.

Refined carbohydrates are carbohydrates that have had most of their other nutrients removed through processing. They include foods like white bread, white rice, and breakfast cereals. Many of these have vitamins and minerals added back (a practice called ‘fortifying’), but it is not possible to add back the fibre.

This absence of fibre has many effects, including on our general gut health (and for more about this, see our page on Fibre). One of those effects is to change the speed at which carbohydrates can be absorbed from food. Without fibre, and the ‘food matrix’ that originally surrounded the carbohydrate, it is much easier for the body to access. This can result in much bigger, faster spikes in blood glucose than you might expect for the taste and nature of the food (there is more about this in our page on Complex Carbohydrates, Sugar and Diet).

It is therefore a good idea to try to reduce the volume of refined carbohydrates that you eat, and instead replace them with less processed alternatives.

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How Much Carbohydrate Do We Need?

There is no simple answer to this question.

Different people have different ideas about carbohydrate consumption. As should already be clear, our different metabolisms and gut microbiome also mean that the answer is effectively different for all of us. What is certainly clear is that we could all do with reducing the amount of refined or processed carbohydrates we consume, replacing them with more complex and less processed carbohydrates. We should also aim to eat more fruit and vegetables in their whole form, rather than turned into juice or smoothies.

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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. You should therefore reduce or avoid processed foods that contain mostly refined or simple carbohydrates. Instead, try to get your carbohydrates from sources such as vegetables, including potatoes, root vegetables and beans, and whole grain foods, where possible.

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