Why good energy balance is vital for female triathletes

Are you getting the right energy balance to support your endurance training? Most likely not. Here we explain the dangers of insufficient fuelling for female triathletes…

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It’s widely accepted that within the hierarchy of sports nutrition that energy balance/energy availability forms the foundations, and should be where the most attention is applied.


But one of the most common observations that I’ve encountered when working with female athletes is that they’re not eating enough to support their training. Yet, rarely do they ask, ‘Am I eating enough?’

This could be due to pressure to stay lean for performance gains. Or societal pressures that lead to dieting mindset and behaviours. While it may be true that leaner can equate to improved performance, there’s also a point where too few calories become detrimental, resulting in muscle loss, poor performance and even fat preservation – the exact opposite of what is trying to be achieved.

But what cannot be forgotten is that the number of hours’ training that a triathlete completes across a week requires a large amount of energy. And not just energy to support training and recovery, but also enough to support bodily functions and daily living.

This highlights the importance of understanding energy balance, the risks that low energy availability poses and how to put this into practice for both performance and long-term health protection.

What is Energy Balance?

Simply put: energy balance = energy (calories) in vs energy (calories) out.

Energy balance is the calories consumed through eating and drinking (‘energy in’) versus those that are burned through physical activity, metabolic function and digestion (‘energy out’ or Total Daily Energy Expenditure (TDEE)). Too few calories (deficit) results in weight loss; too many (surplus) results in weight gain. Weight maintenance is when calories in vs calories out are matched. TDEE comprises the following:

  1. Calories that are burned at rest or your basal/resting metabolic rate (BMR/RMR) – around 70% TDEE.
  2. Thermic effect of feeding (TEF) – the number of calories that the body burns when digesting food – around 10%.
  3. Calories burned through daily movement and activity – non-exercise activity thermogenesis (NEAT) – around 15%.
  4. Calories burned through planned exercise, such as running, swimming, cycling – around 5%.

As you can see, the majority are burned just by keeping us alive (BMR) rather than through exercise. So very quickly, too few calories can result in our metabolic processes being compromised, which is why it’s wise to think carefully about eating enough to support the exercise portion.

What is Low Energy Availability?

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Energy availability (EA) is the energy that you have to fuel your body after subtracting those calories that are burned through exercise. Low EA or energy deficiency is when you don’t have enough energy to support both your body’s functions and exercise.

The exact number of calories that is required to avoid this is difficult to ascertain but the recommendation is that female athletes should aim for energy availability of ~45 kcal/kg of fat-free mass (body mass less the fat).

So a 70kg female with 22% body fat will have 54.6kg fat-free mass. Her target therefore would be 45 x 54.6 = 2,457 calories. Any calories burned during exercise will need to be added on, so if she burns 500 calories during a run, her daily calorie requirements would be 2,957.

This may sound like a lot, and if she’s used to shooting for 1,500 calories a day this could be difficult for her to get her head around. Furthermore, in triathlon, where there may be more than one training session per day, or long brick sessions, for example, the calorie requirements are going to be that much higher, making meeting them a challenge in itself.

Insufficient calorie intake can be a result of failure to replace energy required when exercise levels are increased. Or it may occur due to dieting too hard (i.e. calories too low) or disordered eating behaviours, either as a result of negative body image or perceived performance improvements. Some of the signs and symptoms of low EA include, but not limited to: Poor performance in training/racing; low levels of concentration; fatigue; irregular or loss of periods; frequent injuries; bone fractures.

The Female Athlete Triad and RED-S

The Female Athlete Triad is defined by three conditions:

  1. Low EA, intentional or not, with or without and eating disorder.
  2. Menstrual irregularities or hypothalamic amenorrhea (miss at least three periods in a row). The long-term risks include compromised fertility, menopausal symptoms, decreased bone density and an increased risk of cardiovascular disorders due to resultant low oestrogen levels.
  3. Loss of bone mineral density, which could lead to osteoporosis.

The term Relative Energy Deficiency in Sport (RED-S) encompasses the wider health risks, which include reduced metabolic rate, protein synthesis and cardiovascular health, which occur across both genders as a result of low EA.

How to reduce the risks

Once you have an idea of how many calories you need in order to maintain good EA, you can begin to look at ways to match these requirements. Reaching a potentially high calorie requirement may be challenging, so let’s look at how we can practically do this.

Food and exercise logging. Firstly, understanding your current baseline from which to start is always going to be helpful. So logging your food and drink intake alongside exercise is the first step.

There are many apps, such as MyFitnessPal, where you can log your meals and integrate your training using your Garmin or Training Peaks, for example. Then once you’ve established a baseline you’re well-equipped to make adjustments or, if finances allow, seek assistance from a sports nutritionist or dietician.

Listen to your body. Log any symptoms such as fatigue, headaches, poor recovery and responding with dietary changes.

Track your monthly cycle. Many apps, such as WildAI, have excellent options for tracking sleep, energy levels, training and nutrition, which will provide an excellent picture of how you’re training and recovering.

Simple ways to eat more

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So, the logging reveals that you require more calories. Yes, this may seem daunting, especially if you struggle with large volumes of food, but there are some things you can try. First, let’s look at what to avoid.

Avoid extreme methods of dieting such as fasted training and low-carb diets. Renee McGregor, a leading sports dietician and RED-S expert, states that:

“Intermittent fasting is definitely not an approach any athlete should be taking and has a huge risk of developing RED-S. One thing that’s very clear from the science is that carbohydrate availability is critical, not only for performance but also training adaptation and bone health.

“Fasting puts unnecessary stress on the body, potentially raising the stress hormone, cortisol. If this becomes chronic, it switches off the hypothalamic pituitary response which then starts to shut down biological functions one by one.”

Practically speaking, this translates into not skipping breakfast and other meals, eating at regular intervals, and not excluding or reducing entire food groups, such as fats or carbs. Eat both at each meal and snack time.

Carbohydrates are the body’s primary fuel source. According to the International Sports Sciences Association (ISSA) endurance athletes should increase carb consumption up to 70% of total daily calories to support the high volume of glucose needed for that level of physical activity.

Don’t fear fats. Not only are they important for healthy hormones, cells and vitamin transportation, they’re also useful calories. Adding some more healthy fats to a meal or snack can be a simple method of upping calories without too much additional food volume. E.g. nut butter in a smoothie, olive oil on salad, butter on vegetables, fatty cuts of fish and avocados.

Having nutrients available whenever you need them requires some level of planning. Plan your daily meals and snacks, and this includes the fuel you will take on board before, during and after training (see highlight box, below).

Find the enjoyment

If you need to increase energy intake, do it slowly, gradually building up your intake. Although bringing it all together poses its challenges, doing so will enable you to perform at your best while protecting your long-term health.

With all the scientific recommendations in mind, it’s extremely important to listen to your body and find ways of fuelling which work well for you and that you can enjoy, to make it simple for you to stick to.

How to fuel

The below ISSA guidelines are in addition to regular meals:

1g carbs/kg body weight 2hrs prior
10 ounces fluid with electrolytes and 5% carbs every 20mins
1.5g carbs/kg body weight & 15-25g protein within first 30mins

Tracking your intake will help you understand which foods contain the nutrients required, what this may look like practically and where you could utilise energy bars, shakes, gels etc.


Top image credit: Getty Images