Carbohydrate 101

With nutrition being a hot topic in the news almost every day, sometimes it’s useful to get back to basics. So with a series of posts, I thought I would give a brief overview of the main nutrients, and why you need all of them in your diet.

I’ll address the whole carbohydrate controversy (and low-carb diets) in another post, so for now will just stick to their functions in the body. Carbohydrates are the body’s main energy source. They are consumed in a variety of ways:

·         Simple carbohydrates – the main source of these is refined sugars, such sugars that are added to foods such as chocolate bars. If you’re trying to avoid it’s worth checking labels of any pre-prepared foods, since sugar is often added as a preservative and/or flavour enhancer. These are simple sugars such as glucose, but the most common naturally-occurring simple carbohydrate is fructose in most fruit. These are known as monosaccharides, since they are comprised of a single molecule.

·         Complex carbohydrates – these tend to come in the form of starch, in both natural foods such as bananas, potatoes and rice, as well as more refined foods such as pasta and breads. These are called polysaccharides, as they consist of chains of single molecules bound together. The body needs to break these chains down in order to absorb and use them for fuel. 

·         Indigestible carbohydrates – these tend to be labelled as “dietary fibre.” The body cannot use these directly, but they can be used by your gut bacteria to provide such benefits as increased absorption of nutrients, and reduction of blood cholesterol. Some of these fibres, such as inulin found in chicory, go under the name prebiotics (see my earlier post on gut bacteria). Many of these indigestible fibres occur naturally in plant cell walls (cellulose). When eaten, they absorb water, forming a paste which bulks up the bolus of food travelling down the gut. It is thought that eating foods high in dietary fibre can lead to greater feelings of fullness, which suggests that they are very useful if you’re trying to lose weight.¹

So how does the body use carbohydrates? The most obvious function is that of an energy source. While not the most energy-dense of nutrients (carbohydrates yield 4kCalories per gram, while fat yields 9), the body preferentially generally burns carbohydrate over fat. The energy source of skeletal muscles is called Adenosine Triphosphate (ATP), and this is created by several complex metabolic pathways within the cells. An illustration of how the body processes glucose is seen below. To explain would take several blog posts or a book chapter, and might well send you to sleep! Other functions include the synthesis of non-essential amino acids such as from the skeletons of carbohydrate, while pentose and ribose are used in the formation of DNA. This last point suggests that dietary carbohydrate might be pretty important! Another important function of carbohydrate is the formation of glycoproteins, which are vital in the formation of cell membranes and nerve cell sheaths.² It is also worth mentioning that while many human cells can use several fuels, glucose is the only fuel which the brain can use.

Whenever carbohydrates are discussed, the idea of Glycaemic Index (GI) is mentioned. This is basically the ability of the carbohydrate in question to raise blood sugar. Since more insulin is required to deal with higher GI foods, it is thought that these foods can cause increased body fat, since insulin can lead to fat storage. The upper-end standard of GI is white sugar which has a GI of 100. Generally simple carbohydrates have a higher GI, while more complex carbs are lower. However, several factors can affect the GI of a meal (since carbohydrates are generally not eaten alone): protein and fibre can slow down carbohydrate absorption blunting the insulin response, the ripeness of a food can affect how available the carbohydrate is. Cooking can break down cell walls and make carbohydrates more available meaning a higher GI, while some foods contain naturally-occurring enzyme inhibitors which can reduce absorption and reduce GI. As you can see, the GI of a food on its own is not necessarily the deciding factor as to how it will affect blood sugar levels.

So that is the basics on the incredibly complex subject of carbohydrates. If you would like me to discuss any aspect of carbs, or any other subject, do leave me a note in the comments.

References

1          Wanders, A. J. et al. The effects of bulking, viscous and gel-forming dietary fibres on satiation. Br J Nutr, 1-8, doi:S0007114512003145 [pii] 10.1017/S0007114512003145 (2012).

2.         Geissler C, Powers H, Human Nutrition, Elsevier Publishers, Edinburgh (2006 edition).