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Let’s Talk About Carbohydrates And Blood Sugar Levels

Nuala Mcbride


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Let’s Talk About Carbohydrates And Blood Sugar Levels

Let’s Talk About Carbohydrates And Blood Sugar Levels

Carbohydrates are one of the three main macronutrients, along with protein and fats. They occur naturally in many foods such as fruits, vegetables, grains, milk, nuts, pulses and seeds. Carbohydrates are extremely important for one’s diet as they are the main source of food for the body.

When we talk about carbohydrates, there are three main types – sugars, starch and fibre.

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  • Sugars are the simplest form of carbohydrates.
  • Starch is sometimes termed a complex carbohydrate as it consists of multiple sugar units bond together (more on this later).
  • Fibre is also termed a complex carbohydrate. It is really important is the diet for two key reasons: bowel movements and slowing the absorption of starch (i.e. carbohydrates).

What are carbohydrates?

A carbohydrate is constructed from a chain of carbon and hydrogen atoms. The chemical formula is (CH20)n (where n is the number of carbon atoms in the chain) (Mann and Truswell 2012).  There are three types of carbohydrate molecules:

  • Monosaccharides (e.g. glucose, galactose, fructose)
  • Disaccharides (e.g. lactose, maltose, sucrose) – two monosaccharides joined together.
  • Polysaccharides (e.g. starch, fibre or cellulose) – multiple monosaccharides joined together. 

The polysaccharides (starch & glycogen (stored carbohydrate)) are broken down into disaccharides by the enzyme amylase, then glucose by the enzyme maltase. The disaccharides are broken down into monosaccharides by enzymes called maltase, sucrase, and lactase. These are transported through the wall of the small intestine into the portal vein which takes them to the liver. This is illustrated below.

The glucose in the blood signals for insulin to be released (Davis 2014), which allows the glucose to be used as energy. Whereas, fructose enters the enterocyte system and is stored in the liver as glycogen (which is essentially back up energy). Once the liver has enough stored it is converted into triglycerides (Davis 2014).

Glycemic Index (GI)

To understand the true effects of foods on blood sugar levels the Glycemic Index (GI) was created. This looks at the extent to which food increases blood sugar levels in comparison to glucose (Davis, 2014). Glucose is used as a comparison as it’s the smallest unit of carbohydrate and therefore rapidly absorbed. It has a GI of 100, whereas white bread has a GI of 75 ((Atkinson, Foster-Powell and Brand-Miller, 2008). This shows that white bread affects the blood sugar levels in nearly the same way as a pure sugar molecule. A high GI means that the white bread is rapidly digested and absorbed causing a spike in the blood sugar level. 

Why does white bread have a high GI?

Bread - Lets talk about carbohydrates and blood sugar levels
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White bread is mainly composed of starches providing the high carbohydrate composition. In chemical terms a starch is made of polysaccharides, which are multiple glucose monosaccharide molecules joined together (Mann and Truswell, 2012). White bread is rapidly broken down by the body, as it nowadays contains highly gelatinised starch, low fibre content and porous structure (Miller and Stanner, 2016). One study showed that 80% of the starches in white bread were broken down into oligosaccharides (short chain carbohydrates, usually 3-9 in a chain) by the salvia in the mouth (Freitas and Le Feunteun, 2019). Therefore, once they reach the stomach they are already partially broken down, meaning further digestion by the enzyme amylase is faster.

In the stomach, these oligosaccharides are broken down into glucose monosaccharides units – which is the simplest unit of carbohydrate. They are then absorbed through the intestinal wall into the bloodstream, causing a spike in blood sugar levels. The hormone insulin is then released from the pancreas to move the glucose out of the blood and into the cells, where it’s used for energy (Mann and Truswell, 2012).

What about candy bars?

Chocolates - Lets Talk About Carbohydrates And Blood Sugar Levels
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It is a similar case with the  mars bar and kitkat. These are predominantly composed of refined sugars called sucrose (which is classic table sugar). Sucrose is a disaccharide made of two monosaccharides – glucose and fructose (Mann and Truswell, 2012). These are rapidly digested and absorbed into the bloodstream causing a peak in blood sugar levels.

Why is a high blood sugar level bad?

Over time the rise and fall of these blood sugar levels can have a negative effect on the body. This is because every time there is a spike in blood sugar levels the hormone insulin is released by the pancreas. Insulin is one of the key hormones that metabolism depends on (Perlmutter and Loberg, 2016). The higher the blood glucose level, the more insulin is released. Over time, this can have negative effects on the pancreas producing insulin and potentially cause diabetes or other diseases.

So, what carbohydrates should I be eating?

Here’s my top tips to make sure you’re eating the right types of carbohydrates:

  • Focus on fruits and vegetables for your daily carbohydrate intake. A lot of people forget that there’s carbohydrates in vegetables and fruits!
  • Always choose whole grain. Look for whole grain pasta, whole grain bread etc
  • Limit ‘added sugars’. These are the worse type of carbohydrate!

Read more about ways to improve your diet.


If you’re still feeling a like unsure about where to start with carbohydrates, why not book a Nutritional Analysis Consultation with me?



Atkinson, F., Foster-Powell, K. and Brand-Miller, J., 2008. International Tables of Glycemic Index and Glycemic Load Values: 2008. Diabetes Care, 31(12), pp.2281-2283.

Davis, W., 2014. Wheat Belly. 2nd ed. London: HarperThorsons.

Freitas, D. and Le Feunteun, S., 2019. Oro-gastro-intestinal digestion of starch in white bread, wheat-based and gluten-free pasta: Unveiling the contribution of human salivary α-amylase. Food Chemistry, 274, pp.566-573.

Mann, J. and Truswell, S., 2012. Essentials Of Human Nutrition. 4th ed. Oxford: Oxford University Press.

Miller, R. and Stanner, S., 2016. A summary of evidence on the digestion, absorption and metabolism of white bread carbohydrates. British Nutrition Foundation, [online] Available at: <https://www.fob.uk.com/wp-content/uploads/2016/12/BNF-White-bread-carbohydrate-report-FINAL.pdf> [Accessed 13 April 2020].

Perlmutter, M. and Loberg, K., 2016. The Grain Brain Whole Life Plan. Boston: Little Brown & Company.


Hi! I'm Nuala

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