Understanding Carbohydrates

Understanding carbohydrates  

What are carbohydrates?

Carbohydrates are a macronutrient along with protein and fat. They are essential for a vast amount of cellular activity such as brain and nervous system function and energy production, which is important when you’re an athlete in need of all possible sources of energy. 

A quick summary of the importance of carbohydrates

  • They provide the most readily available energy source for our bodies to fuel endurance sports and activities. Our body requires less oxygen to burn carbohydrates so this makes them a highly efficient fuel source too.
  • Glucose is the main carbohydrates in our diet used for energy, brain function and the nervous system function. Running low on this fuel source can result in you becoming lethargic and disoriented. A lapse in concentrating or performing even simple tasks can become more challenging.
  • The body uses carbohydrates to metabolise fats. So, ensure those levels are topped up to keep those fat furnaces going.
  • Complex carbohydrates are slower absorbing and nutrient rich and should be consumed more than the simple carbohydrates in your overall diet.
  • Carbohydrates can be ingested before your activity to get you fuelled and ready; during an activity to sustain your efforts, and after an activity to replenish used glycogen stores, boost immune system and energy levels.

 carb word written in bread rolls and bread buns

Different types of Carbohydrates

All sugars have an impact on blood sugar level and these levels can be influenced by the amount and structure of the carbohydrates consumed. Carbohydrates are comprised of carbon (C), hydrogen (H) and oxygen (O) bonded together in a ratio of 2:1 of H to O atoms (like water).

Simple Carbohydrates

Monosaccharides are carbohydrates made of one sugar unit. They include glucose, fructose, and galactose. 

Glucose is the most important source of energy in organisms. It can be absorbed directly into the blood and moves around the body as blood sugar.

Fructose found in fruits, honey and vegetables is the sweetest of all carbohydrates and is almost twice as sweet as table sugar (sucrose). It too is digested into the blood but takes a little longer to breakdown, therefore resulting in lower insulin spikes than glucose.

Galactose (ose = milk sugar) is another monosaccharide and when combined with a glucose molecule it forms the milk sugar, lactose.

Disaccharides are carbohydrates made up of two sugar molecules. Sucrose, which is table sugar is made of glucose and fructose molecules. Lactose, as mentioned above is made of galactose and glucose and is found in milk from mammals. Maltose or malt sugar is made of two glucose molecules and is the main carbohydrate in beer!

Complex Carbohydrates

Oligosaccharides are made up of between 3 and 10 units of sugar and examples can include raffinose, stachyose, and verbascose. Examples include some fruits, whole grains, legumes, and some cruciferous vegetables. Do note that we don’t really digest this stuff well, so it tends to zip through our digestive system where it is later fermented before exiting our system.

Polysaccharides have more than 10 sugar units. Some examples are starch made of amylose and amylopectin, fibre made of cellulose and chitin, and glycogen or animal starch. Major sources of starches are rice, wheat, potatoes, and maize and are the storage forms within plants.

Fibre, made up of glucose units, is considered a non-digestible carbohydrate due to their bonding. They are a structured form of CHO from plants, and they are broken down by fermentation and bacteria in the large intestine. Soluble fibre will dissolve in water and form a thick gooey substance in the tummy, which later breaks down in the large intestine yielding a few calories worth of energy. It can interfere with the absorption of cholesterol and fat!

Insoluble fibre won’t dissolve in water to form that thick gooey substance and it won’t be broken down either so it will pass through us yielding zero calories. This form of fibre is super important to help bulk up and improve the movement of our stools. It’s a great digestive system cleaner. Both forms of fibre can affect our satiety levels so if you are wanting to feel full sooner then look out for foods that are higher in fibre.

carbohydrate digestion in intestines with images of fresh vegetables and fruit on chopping board

Digestion of Carbohydrates

Digestion starts in the mouth with the act of chewing, called mastication. Food is mechanically broken down by grinding it into smaller pieces (bolus) making it easier to swallow. Saliva is used to lubricate the bolus to help the food travel down the oesophagus.

The pancreas and salivary glands release the saliva which also contains Amylase. An enzyme used to break down the multiple sugar unit that hold carbohydrates together. About 5% of starches are broken down in the mouth and the rest in the stomach. 

Eventually food is broken down into a semi fluid (chyme) and enters the small intestine. Here the pancreas releases pancreatic juice, which includes pancreatic amylase, which helps break down the carbohydrates into shorter chains. 

Lactase breaks lactose into galactose and glucose. Maltase breaks down into two units of maltose. The digestive enzyme sucrase breaks sucrose into glucose and fructose. Monosaccharides (carbohydrates in their simple states) are transported to the liver where they are stored and ready for use. The liver is the first destination for fructose, galactose, and glucose.  

Different carbohydrates are oxidised at different rates. It has been shown that exogenous carbohydrate oxidation rates don’t exceed 1g/min (60g/hr) with a single carbohydrate type. This is important as it highlights the potential upper limit of carbohydrate intakes when exercising. It’s also one of the reasons we use more than one type of carbohydrate in our drink mixes – so you can surpass this 60g of carbohydrates per hour limitation. In essence, there is a correlation with higher intakes of carbohydrates and better performance.

The Glycaemic Index

The Glycaemic index (GI) refers to the rate at which carbohydrates are digested. Once carbohydrates are digested, they are then taken into the bloodstream. This then causes a rise in blood sugar and insulin levels. The rates at which carbohydrates can cause a rise in blood sugar levels can be slow, medium, or fast depending on the type of carbohydrates consumed. Similarly, carbohydrates can be categorised into fast, medium, or slow energy releasing with the slow releasing raising blood sugar levels slower than fast releasing.  

This index ranks carbohydrate-based foods on a scale of 0 – 100. Glucose with a score of 100 is the benchmark upon which all carbohydrate-based foods are ranked. Glucose is rapidly digested, absorbed and metabolised as are all carbohydrates with a high GI score. This in turn can result in greater fluctuations in blood sugar levels but also the quickest, short-term available energy source. 

Lower GI scores results in slower and smaller changes in blood sugar levels. This can also help with feeling full for longer.

Processed foods tend to have a higher GI count as most of their nutrients (protein and fibre) have been stripped away during processing. You can use this to your advantage in some cases. For example, if you are considering a meal before an exercise session, then white bread sandwiches with jam will give you a nice combination of different releasing sugars with minimal fibre, thus effectively fuelling you in prep for your exercise. Some athletes for example, start their race mornings with white bread sandwiches, or a bowl of white pasta or white rice. Yes, these are processed and don’t carry a lot of overfall wholesome nutrition, however they do a high energy to weight ratio with little fibre. This means you can take onboard more fuel at that meal and worry less about fibre filling you up prematurely or causing you to make a visit to portaloo on course.

Fruit is interesting as it is a complex carbohydrate, yet as it ripens the sugars break down to simple sugars. A banana is a great example. When firm and yellow, it holds a more complex carbohydrate structure and takes longer to digest. As the banana ripens, it gets softer and brown. This is partly due to the sugars breaking down. So, if you are passing an aid station during your marathon, for example, and they have bananas, then grab the browner one as it will be easier to digest whilst on the move. They will taste sweeter, and texture will be easier to get down. Remember, the simple sugars (riper banana) break down, gets absorbed and enters the blood quickly, thus raising your blood sugar a lot quicker than the less ripened option. You’ll have a quicker source of energy and it’s a great option if you need an instant boost!

Similarly, when cooking some vegetables, such as roasting carrots, the starches breakdown into sugars making them sweeter. 

If carbohydrates are consumed with protein, fats and/or fibre their rate of digestion and absorption rate is slowed down. In turn, this has a slower impact on blood sugar levels. Adding peanut butter to that sandwich for example, is a great way to up your protein intake, which aids muscle repair. It will also slow down the snacks digestion rate, but you are now getting a backup fuel source, which may be used later … and tastes good!

Fructose has a GI of 23, sucrose 65, and maltodextrin 105-180. We have taken these absorption rates into consideration when formulating our drink mixes. This ensures you get a combination of fast, moderate, and slow releasing energy, which covers your energy requirements whether you need an instant boost or something a little more gradual.

In summary, the factors that affect the digestion rates of a carbohydrate on the Glycaemic index are:

  • The type of sugar it is made of, for example, glucose versus fructose.
  • If it is a starchy food and whether it consists of amylose and amylopectin. Amylose makes foods harder to digest, thus giving that food source a lower GI number.
  • The ripeness of fruit. The riper the higher the GI and quicker it is digested.
  • Processed foods usually have a higher GI value then wholesome natural foods.
  • The longer you cook food, the more you breakdown sugars. This can result in a higher GI number for that food. 

Storage of glucose

It’s helpful to be aware of the storage of glucose in the body and how when you run out of stores, you’re likely to experience “bonking” or hitting the wall. Total glycogen supply is roughly 1,800 to 2,600 calories per person or enough fuel for 90 to 120 minutes of continuous, vigorous activity. 

The release and storage of glycogen occurs throughout the day, and this is how we maintain a fine balance of blood sugar and energy levels. This helps avoid us experiencing the hypoglycaemic effect i.e., ultra-low levels of blood glucose.

You can find glycogen in all our cells, but the main storage areas are muscles and the liver, and the body works hard to maintain optimal storage levels in these energy warehouses. The brain can use around 400 or more calories a day of glucose, and this is primarily sourced from the liver’s store of glycogen. Our glycogen stores along with our training status determine the length of time we can exercise continuously for.

Experienced athletes have come to realise they need to consume enough carbohydrates daily to adequately replace the muscle glycogen that is used as fuel during workouts. Depleted levels of carbohydrates can result in the athlete feeling sluggish, unmotivated, experience lapses in concentration, have slower recovery times, find it harder to maintain their usual or higher pace and subsequent training days can also feel harder.


woman triathlete on bicycle racing on road 

Carbohydrates and Athletic Performance 

Any sport that involves an effort above 40 percent of VO2max is going to use carbohydrate as a fuel source. Spiriting 10-20 seconds may be the only exception, as phosocreatine would be your fuel source. However, you’d still need carbohydrates to replenish those stores too.

Yes, you could use fat as a fuel source but there are two considerations with this. Firstly, your body requires more oxygen and effort to burn fat and secondly, fat can’t oxidise quickly enough and at the rate you need it to sustain a high level of output.

The amount of carbohydrates someone needs is based upon their total energy output in the day. A reasonable baseline is around 40 percent of calories in the day coming from carbohydrates. It tends to be higher for endurance athletes. Paying attention to the timing and type of carbohydrates ingested can optimise energy production and spare glycogen reserves of the body. 

An important consideration for athletes is the use of protein as a source of energy. Limiting this happening is important, as the body will break down the muscle cells in order to fuel itself energy, and this can significantly impact athletic performance. 

Keeping the liver and muscle glycogen stores topped up with daily nutrition will encourage the replenishment of the glycogen stores and spare the utilization of proteins and reserved glycogen stores. 

We can maximize our glycogen stores using loading schemes. This is of particular interest to endurance athletes and those going for long or multi competition days in other sports.

The type, timing, and the amount of fuel you take on board can greatly impact your performance adventures. Carbohydrates are the backbone of fuelling smart, however, protein and fat play crucial roles, too.

Although eating a well-balanced diet won’t guarantee success, poor eating habits will limit your potential. Here are some fuelling guidelines to help you design your nutrition strategy before during and after exercise. 

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