The energy on the swim, bike run. What is the best energy source?

The energy on the swim, bike run. What is the best energy source?

By: Miriam van Reijen | Team ASWComments: 0

During (high-intensity) exercise, two processes make it more difficult for the body to take in energy. First of all: blood flow to the GI system is reduced. This means it is more difficult to process and break down nutrients. Fat, fiber, protein and complex carbohydrates require more processing before your body can absorb them. On the other hand, simple carbohydrates (sugars) require little processing and are thus more easily absorbed. A second process to keep in mind is that during high-intensity exercise, oxygen (uptake) is a limiting factor for us to go faster. The purpose of the inhaled oxygen is to be transported to the muscles where it is needed most. You don’t want too much oxygen to be needed to burn the nutrients you take in. And again, carbohydrates are the winner, because burning carbohydrates requires less oxygen than burning fats or protein. A somewhat technical explanation. But just remember: whatever you choose to eat or drink during exercise. Carbohydrates are king!


Sports drinks come in three varieties: 1) sports drinks with only electrolytes and no added carbohydrates (sports waters or hypotonic drinks) 2) isotonic drinks and 3) hypertonic drinks. The isotonic drinks are the ones you want to use during exercise. They are designed to have the same concentration and osmolality as your body fluid and can thus be quickly absorbed. Don’t use high energy drinks during training or race. The higher concentration of carbohydrates requires your body to bring fluids to your stomach to absorb the drink, while you want the fluids to go to your muscles, to transport nutrients and oxygen.

Another important note is to be aware of electrolyte tablets. They provide you with electrolytes. That means minerals such as magnesium, sodium and potassium. However, they don’t contain any carbohydrates and thus contain no energy.

When? Sports drink is the preferred energy source when you race or train in hot conditions. It will give you both plenty of fluid and energy at the same time. On the bike, when you can carry enough bottles, sports drinks are a good way to fuel yourself.

How much: During lower intensity exercise you can aim for 500-750 ml of isotonic sports drink per hour. This will provide you with 30-45 grams of carbohydrate. During higher intensity exercise (and races) aim for 750-1000 ml per hour.


Gels can be seen as a concentrated form of sports drinks.  They are very useful when your access to sports nutrition is limited, you have to be self-provided, or you want an intake of a more concentrated form of energy. Gels are either isotonic (such that you don’t have to drink water at the same time) or hypertonic (than you do want to add water to make the gel less concentrated). A gel will always state on the package how much and if you have to drink water with it. Sometimes gels come with extra benefits. Such as added sodium, potassium, amino acids or caffeine.

When you start using gels for the first time, buy a whole range of different brands and try them out. It may take a while before you find a gel that you like, both in taste and consistency.

When: Due to the small size, gels are a great energy source during a run. Be sure to add fluids at aid stations if needed. Gels with caffeine are ideal just before the swim in a triathlon, on the bike just before attacking a hill, or during the run.

How much: During lower intensity exercise you should aim for 1.5-2 gels per hour, they will provide you with 37-50 grams of carbohydrate. During higher intensity exercise (and races) aim for 2-3 gels per hour.

Energybars and solids foods

Energy bars and solid foods are less convenient during high-intensity exercise when oxygen and blood flow to the GI system is limited. However, during easy runs or rides, you can choose to eat some solid foods. Make sure to still look for carbohydrate-based foods such as raisins, (ripe) bananas, wine gums or white bread with jam/honey. If your run or ride is longer than 2 hours, you might want to add a little bit of protein to reduce muscle damage and faster your recovery after your training.

About Miriam van Reijen: Miriam is a Sports Nutrition Expert and Pro-Triathlete. She is advising (elite) athletes on a healthy diet & lifestyle to optimize their performance and to maintain/aim for, a healthy weight. She is writing semi-scientific articles about running, nutrition, training and health for municipality Amsterdam, Intersport Losse Veter, Triathlonsport, Runnersworld, Sportgericht, Santé, Match Magazine and author of the book 'Hardloperskookboek'.

Miriam van Reijen (1983) studied Development Economics at the Wageningen University and Human Movement Sciences at the VriJe Universiteit of Amsterdam where she finalized her PhD in January 2019. 
She took up running on the age of 24 and since then has medalled in marathon (pb 2h41), duathlon (National Dutch Champion 2017 and bronze medal at the World Championship at 2017) and Triathlon (bronze medal at National Long Distance and a 4th place at the European Championships, both in 2019).
Miriam lives, trains and works in Amsterdam with her boyfriend and two cats.

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