Home > Carbohydrates – GI, Sports Performance & Weight Loss

Carbohydrates – GI, Sports Performance & Weight Loss

By Shaun Wadham, Level One Triathlon Coach, Registered Fitness Leader

Carbohydrates are essential for optimal health and sports performance and they are also play a very important part in any weight loss program.  This article will help to answer any questions you may have in relation to this important dietary requirement, dispel the myths surrounding them and most importantly help you get the most out of your training sessions.

Carbohydrates provide the most readily available source of energy to fuel working muscles during exercise.  Insufficient carbohydrate in the diet results in decreased glycogen stores (stored carbohydrates) in the working muscles.  This in turn jeopardises exercise performance and causes fatigue.  As a general rule carbohydrates should make up approximately 60-70% of your daily food energy requirements, with lean protein and “good” fat making up the remainder in equal proportions.  Percentages are only a guide though.  The more accurate way to measure your carbohydrate requirements is by calculating your optimal intake in grams. The more you exercise the greater your carbohydrate needs.  The table below will give you an idea of the amount of carbohydrates you require during a day (if you are trying to maintain your current weight).  For example a female who weighs 55kgs and who exercises 1 hour per day requires approximately 275-330 grams of carbohydrates per day.

Up to 1hr per day of exercise 5-6 grams/kg of body weight
1-2 hours per day of exercise 6-8 grams/kg of body weight
2-4 hours per day of exercise 9-10 grams/kg of body weight
Greater than 4 hours per day of exercise 11-13grams/kg of body weight

What Type of Carbohydrates to Eat

The Glycemic Index (GI) is a ranking of food carbohydrates according to their impact on blood glucose levels. Foods with a low GI ranking (55 or less) release carbohydrates slowly into the bloodstream.  A high GI (>70) means that the carbohydrate is released quickly.  56-69 is intermediate.

Research indicates that, for optimal health a large portion of your carbohydrate foods should be low GI and the rest intermediate and high. This steers us away from the traditional held belief that all complex carbohydrates as opposed to foods containing simple carbohydrates (honey, jam, fruit) are released slowly into the blood and are therefore the best types of carbohydrates to consume.  This may still be the case with many but not all complex carbohydrates.

Low GI foods include multigrain breads traditional oats, pasta and Asian noodles, dairy products, most fruits, basmati rice, muesli, sweet potato, and legumes (lentils & beans).  High GI foods include white and whole meal bread, most breakfast cereals, most Australian rices, potato and honey. A more thorough listing can be obtained from a book called “The G.I Factor.”

Carbohydrates and Weight Loss

It is amazing how many people tell me they are on a high protein diet to lose weight and the first question I ask them is why.  If you want to lose weight and keep it off you need to reduce your overall food consumption slightly below your daily energy requirements but continue to follow a diet that is made up of carbohydrates, protein and fats in the percentages listed above and exercise regularly.

High protein/low carbohydrate diets will most probably leave you feeling tired and lethargic, especially when you are exercising.  It also has the added disadvantage of reducing your intake of grains, cereals and legumes, which are valuable sources of vitamin B and fibre.  Evidence suggests that grain consumption reduces the risk of certain cancers and heart disease and improves bowel health.

A high fat diet is not optimal for weight loss because, per gram, fat has twice as many calories than carbohydrates or protein.

The types of carbohydrates you eat are though important if your weight loss program is to be successful. High GI food produces a large rise in blood glucose levels, followed by a fairly rapid equal decline brought about by the release of insulin. (Insulin instructs muscle and liver cells to take up glucose for immediate use and storage).  Low blood glucose (amongst other things) stimulates us to eat.  A high GI meal therefore seems to lead to overeating and therefore weight gain.  Regular servings (snacks) of low GI foods throughout the day are the way to go to maintain blood glucose levels.

Carbohydrates before, during and after exercise

It is recommended that you choose food containing carbohydrates, which have a low GI rating before exercise to avoid large fluctuations in blood glucose.

It is recommended that you eat high GI carbohydrates immediately following exercise for optimal recovery
(i.e. about 60 grams of carbohydrates, which is the equivalent of 2 pieces of white toast and a bottle (600mls) of Gatorade or similar sports drink. The recommendation is 1 gram per kg of body mass in the first 30 min after exercise, repeated every 1-2 hours until regular meal patterns are resumed.

During training or events lasting longer than 1.5 hours it is important that you consume carbohydrates otherwise your body’s carbohydrate stores will run out and your muscles will be forced to use fat for energy.  This leads to a drop in the intensity you can exercise at.  This is commonly known as “hitting the wall.” Aim to consume approximately 0.8grams of carbohydrates per kg of body weight for each hour of exercise.  This is about a Gatorade and a sports gel per hour.  The type of carbohydrates you should consume should be a mixture of GI levels for quick and sustained release.  Consumption should take place, for example in a triathlon as soon as you get on the bike to allow time for the food to digest and get to muscles before they run out of carbohydrate energy.


So the message for daily eating is this.  A balanced approach incorporating mostly low GI carbohydrates, lean protein, healthy fats and plenty of fruit and vegetables will maintain variety and keep you healthy.  If you are trying to lose weight you need to reduce the food you eat whilst maintaining the proportions of carbohydrates, protein and fats detailed above.