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The role of carbohydrates in sports nutrition

Mar 1, 2020

Carbohydrates for runners

Before discussing the role of carbohydrates (CHO) in sport nutrition, let’s first understand some basic topics:

Aerobic and Anaerobic pathways:

1. High intensity, short duration exercise – Anaerobic production of ATP:

Only glucose and glycogen can be broken down for energy/fuel

2. Low intensity, long duration exercise – Aerobic production of ATP:

Can provide fuel by metabolising fat and protein

Most marathon runners use both pathways in a given event.

Metabolism of Glucose from CHO/Fats/Protein:

Firstly and most importantly, CHO is the chief energy nutrient.

Storing glucose as glycogen:

The liver stores about 1/3 of total body glycogen and releases glucose as needed (quick response, happens in minutes). Muscle cells can also store glucose as glycogen, about 2/3 and this is mainly used for muscle functioning.

Making glucose from protein:

Body protein can be converted to glucose to some extent, but protein has body functions of its own that no other nutrient can perform. Thus when a person does not consume enough CHO, body proteins are dismantled to make glucose in order to fuel these special cells. Only adequate dietary CHO can prevent this use of protein for energy and this is called protein-sparing action

Making Ketone Bodies from Fat Fragments:

With less CHO available for energy, more fat may be broken down, but not all the way to energy! Instead Ketone Bodies are formed. Muscles can use ketone bodies for energy, but when the production of Ketone bodies exceeds their use it can cause Ketosis (when the body begins to burn fat for energy because it does not have enough carbohydrates to burn.)

Converting Glucose to Fat:

It is important to note that body fat cannot be converted to glucose to any significant state, but glucose in excess is stored as fat.

High- and low blood sugar:

 Know the signs of hypoglycaemia (low blood sugar):

  • Weakness
  • Rapid heartbeat
  • Hunger
  • Anxiety
  • Trembling
  • Headaches
  • Nausea
  • Rapid and shallow breathing

Know the signs of hyperglycaemia (high blood sugar):

  • Dehydration
  • Weakness
  • Blurred vision
  • Fatigue
  • Muscle cramps
  • Dry mouth

CHO and sports nutrition (pre-race day and during an event)

 Pre-Race day/Carbo Loading:

For each athlete, their needs will be different and this is determined by your current weight, type of exercise/training and race-event that will take place. When planning a meal plan for an athlete, the focus is increased CHO (more than usual) with each meal (Breakfast, Snack, Lunch, Snack, Dinner, Snack).

When do we carbo load?

Usually if the event will be longer than 90min and if consumption of CHO during an event is restricted.

For how long do we carbo load?

Carbo loading may take place 1-3 days before an event. This is usually done whilst tapering to ensure you are not using up all the glucose stored as glycogen

During an event:

Again this is very individualised and needs to be calculated by a registered dietician according to your current weight, intensity, and type of sport as well as the practicality of consuming the CHO during the event. For every 45min-60min you need to take CHO to ensure continued energy supply and to ensure that your glycogen stores are not depleted, or as a runner would call it… hitting the wall. Choose low fibre foods, easily digested CHO foods. This will cause minimum bloating, cramping and limit the chances of an upset stomach.

Examples of CHO foods that can be consumed during a race:

  • Energy Gels
  • Energy Chews
  • Bananas
  • Potatoes
  • Sports Drink (take note, not an energy drink)
  • Pretzels
  • Sports Bars

Together with the consumption of CHO, plenty of water needs to be consumed during an event. Too much CHO (hyperglycaemia) leads to dehydration. The recommended guideline is that for every 15min of exercise, you need to consume 150ml of water.

Heste-Mari Viviers from SEMLI


SEMLI (Sport, Exercise Medicine & Lifestyle Institute) was established at the University of Pretoria in 2015, under Professor Martin Schwellnus. They enable people from all walks of life reach their full potential through health, wellness and physical activity, by providing clinical services, information and learnings backed by research evidence. Read more about Heste-Mari

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