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).
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