We all know or hear that we need carbohydrate in our diet, but why?  A "zero carb diet" is not only difficult to achieve, it's not sustainable because it's not great for your body.

In this section, you will find out what carbohydrates are, why we need them, what they do for us and where we find them in our foods.

Why do we need carbohydrates?

Carbohydrate is a nutrient that our body needs, it is an organic compound made up of carbon, hydrogen and oxygen. Carbohydrates are the main energy source for our body- they are the energy that gets used first (before protein, fat and alcohol). It is recommended that about 45-60% of our energy intake should come from carbohydrates.

They have important roles in our body including:

Clearly, carbohydrates are important for normal health, so don't cut them out of your diet.

Find out about the type of carbohydrates.

What happens when we eat carbohydrate in our food?

Let's start off with the carbohydrates that are absorbed in our small intestine.  These are 'glycaemic' carbohydrates - so called as they affect our blood glucose level.  These are either be simple sugars, or complex carbohydrates which can be digested (i.e. broken down by enzymes so they can be absorbed) in the small intestine. 

Some glycaemic carbohydrates can begin to break down in our mouth (e.g. starch in bread), as the enzymes needed to break the bonds between the sugar units are present in our saliva.  Most digestion occurs in the lumen of the small intestine.  

Some carbohydrates are resistant to the digestive enzymes, so are 'non-glycaemic' and pass on to the large intestine, where they provide the energy source for bacteria, through fermentation.  See our section on fibre.  There are 3 types of carbohydrates that are never digested in the small intestine: non-starch polysaccharide (NSP), resistant starch and resistant oligosaccharide. Together they have the function of 'dietary fibre'.   The fermentation process generates volatile fatty acids, for absorption as another energy source, and gas. If the small intestine is diseased, or digestion is blocked, then gas production can become excessive - stinky!  The intestine is eventually able to adapt to different loads of carbohydrate, but that does take some time.

Don't cut out carbs

Glucose is the sugar unit that our body cells use for energy.  We cannot make glucose out of other nutrients, so it is essential to eat carbohydrates.  Other food substances are not used directly and get converted into glucose quickly, and starch is readily digested and absorbed as glucose.  We can only store small amounts of carbohydrate as glycogen, in liver and muscle, for urgent use.  So we need a fairly steady supply of carbohydrate from our meals.  Volatile fatty acids, from fernmentation, also contribute energy, mainly to the large intestine itself, and to the liver.

The rate of digestion and absorption of glycemic carbohydrates vary.  From a health perspectivem the slower the carbohydrates break down, the better, avoiding a spike (rapid high rise) in blood glucose level, and consequent rises in insulin etc.  The "Glycemic Index" (GI) is a guide to how fast a carbohydrate is broken down to glucose.  Foods can be categorised into high GI, ie broken down and absorbed quickly, or low GI, ie broken down and absorbed slowly.

The GI of a food is affected by many factors such as the particle size of food, the fat and protein content of food, how the food was cooked, how long the food was chewed for in the mouth, metabolism rate and the rate of gastric emptying (the rate of which food leaves the stomach and enter the small intestine).  However, it is broadly possible to categorise certain foods as high GI (eg glucose, bread, mashed potato) and low GI (eg sucrose or table sugar, beans, mashed potato which has been left to cool).  It is thus possible to exchange some high GI foods for low GI equivalents, but this GI system is only a small part of defining a balanced diet.  Many important foods do not have equivalent alternatives, and the overall effect is less in the context of mixed meals. 

Now, we know that we do need carbohydrate in our lives. It's the most readily available energy source in our food (and drink).  The glucose we use mostly comes from digestion and metabolism of other carbohydrates. 

Don't be scared of carbohydrates - they're not bad for you - it's all about the amount of carbohydrate we eat in relation to our total calorie intake, and the type of carbohydrate (simple or complex) that determine how it affects our weight and energy levels.