Health, Nutrition, Podcast, Science

Triathlon Nutrition: Calories, Carbs, Fats, and Proteins – part 1 | EP#94

 January 18, 2018

By  Mikael Eriksson

Triathlon Nutrition: Calories, Carbs, Fats, and Proteins - part 1 | EP#94

A research-based review of nutrition, including overall energy intake, energy balance, and macronutrients (fat, protein and carbs) for endurance athletes. Part one of two. 

Discuss this episode!

  • Let's discuss this episode and the topic in general. Post any comments or questions in the comments at the bottom of the shownotes. Join the discussion here!

In this Episode you'll learn about:

  • Energy balance
  • Energy availability (you may be cutting yourself short even if intake equals energy expenditure)
  • How the body uses different ratios of carbs and fats to fuel energy performances
  • Why you need to be a carb AND fat burner!
  • Carbohydrate for endurance athletes

Sponsored by:

Precision Hydration

Precision Hydration
One size doesn't fit all when it comes to hydration. Take PH's free Triathlon Sweat Test to get personalised hydration advice tailored to what you're training for. Use the discount code THATTRIATHLONSHOW to get 15% off electrolyte supplements.

Ventum BIkes

The ultimate triathlon bikes, using technology from fighter jets and Formula One race cars. Easier to build, pack, and travel with than any other tri bike. Get 110% of the value of your old bike towards the purchase of a new Ventum bike.


Research base

3:20 - 

  • A lot of this discussion is based on the Nutrition and Athletic Performance position statement written jointly by the Academy of Nutrition and Dietetics, the Dietitians of Canada and the American College of Sports Medicine. It was published in Medicine, Sports and Exercise Journal in 2016.  
  • There is one author representing each of the three entities and they include some of the biggest names in the sports nutrition scientific community. 
  • In addition to the three main authors: Louise Burke, Kelly Urban and Travis Thomas, there is a long list of reviewers for the statement who are all experts in the field of nutrition for endurance sports and athletes. 

How much should you eat/how big should your energy intake be overall?

5:07 - 

  • Simple answer: unless you want to lose weight, you have to take in as much energy as you expend. This is called being in energy balance. 
  • Energy balance equation:
    • You need to get in as much energy as your total energy expenditure.
    • Your total energy expenditure = your basal metabolic rate (how much energy you would burn just laying in bed all day) + thermic effect of food (the energy your body uses to process the food/nutrition you consume) + thermic effect of activity (any activity you do e.g. walking, training for triathlon etc).
  • Important to note that basal metabolic rate isn't the same as resting metabolic rate. Resting metabolic rate is roughly 10% higher than basal metabolic rate and is the usual measurement taking during in-body measurements as it's easier to measure.
  • For sedentary individuals, resting metabolic rate is usually 60-80% of daily energy expenditure.
  • For elite endurance athletes, resting metabolic rate can be between 38-47% of their daily energy expenditure.
  • Elite athletes have a higher resting metabolic rate than sedentary individuals, and they burn a lot of energy during training. For these athletes, the energy expended in physical activity is almost 50% of their daily energy expenditure and the thermic effect of food is pretty small in comparison to the other two components.
  • Most age group triathletes naturally fall somewhere between sedentary individuals and elite athletes - so somewhere in the middle of that range of 70-40% of your total energy expenditure being your resting metabolic rate.
  • For more reading about energy requirements, check out the publication Energy requirements of the athlete: Assessment and evidence of energy efficiency by Manore and colleagues from Oregon State University.

Energy availability

8:55 -

  • A lesser known concept than energy balance (balancing your energy expenditure with your energy intake) is energy availability.
  • It's related, but it's different because it defines the minimum energy intake requirements that you need for optimal health and function.
  • It is defined as: your energy intake, minus the energy that you expend in exercise, divided by your fat free mass.
  • NB - Fat free mass is your overall weight once you've removed the fat percentage - e.g. if your fat percentage is 10% and your weight is 70kg, 7kg of your weight is fat so your fat free mass would be 63kg
  • For example: your energy intake is 2,500 calories and you expand 500 calories through exercise, leaving 2,000 calories. Then you divide that 2,000 calories by your fat free mass (e.g. 63kg as above) = 32 kcal per kg fat free mass

Energy deficiency

10:45 -

  • Energy availability has been primarily studied in females as it originally came from studying the female athlete triad: disordered eating, menstrual dysfunction and poor bone density/bone health.
  • The female athlete triad is one potential consequence of insufficient energy availability.
  • More recently this concept has been extended to include both male and females, now called relative energy deficiency in sport (RED-S).
  • RED-S negatively impacts all factors of the female athlete triad as well as more general hormonal, metabolic, hematological, growth and development, psychological, cardiovascular, gastrointestinal and immunological systems and function.
  • It has been found that an energy availability of 45 kcal per kilogram of fat free mass is sufficient for optimal health and function.
  • However, going below 30 kilo calories per kilogram of fat free mass has been associated with RED-S and the dysfunctions noted in the female athlete triad.
  • It is important to consider that if you are chronically trying to cut your caloric intake, then your resting metabolic rate will fall and you will have to consume even less calories to be an energy balance, but that will then negatively impact your energy availability.
  • If you are trying to lose weight, you will need a caloric deficit, but it is recommended only to have a deficit of 200-300 calories per day. More info to come in a follow up episode on body composition.

Macronutrients for fuelling endurance performance

15:35 - 

  • The primary sources of fuel for the working muscles are carbohydrates, which come in the form of glycogen in your muscles and liver, and fat, which comes in many different forms in different places in the body.
  • Carbohydrates can be oxidized ("burnt" as fuel) aerobically. And it can also be oxidized anaerobically, which happens more at higher intensities of endurance exercise.
  • For further listening about aerobic processes versus anaerobic processes, listen to Episode 71 of this podcast: ​Threshold Confusion: Aerobic, Anaerobic, Lactate, Functional - Help! | EP#71​​​
  • Fats are only oxidised aerobically - meaning their contribution is higher during lower intensity exercise.
  • It is important to note that there's no such thing as only doing aerobic work or only doing anaerobic work, it's always a mixture, but the proportion or contribution of the different energy systems varies with intensity.
  • From the position statement about these energy systems and the utilisation of different fuel: "an athlete's a skeletal muscle has a remarkable plasticity to respond quickly to mechanical loading and nutrient availability, resulting in condition specific metabolic and functional adaptations. These adaptations influence performance nutrition recommendations with the overarching goals that energy systems should be trained to provide the most economical support for the fuel demands of an event while others strategies should achieve appropriate substrate availability during the event itself."
  • Key points: we need to train our energy systems in accordance with our goal events so that we get to the most economical nutrition support from them. We also need to make sure that we have enough energy substrates availability (fat and carbs in the body).
  • Since fat is abundant in the body it's mostly carbohydrate that we need to make sure we have available to us during events. Carb stores are limited which is why we must supplement during long distance events, to ensure we don't run out.

Should we rely on carbs or fats for fuelling endurance sports?

19:04 - 

  • Both! We should train to be excellent at burning or oxidising carbs and fat - they're not mutually exclusive.
  • At any given time in an endurance event you are using both, and the ratio of fats and carbs that are used depends on the intensity as well as some other factors, (e.g. training, nutrient availability).
  • Basically, the higher the intensity the higher the ratio of carbs to fat.
  • Important to note: there are no other energy substrates besides carbohydrates that can be used to provide energy anaerobically.
  • As discussed in EP71 of this podcast, once you get to the anaerobic threshold, every additional bit of energy that your body requires is going to be fuelled anaerobically (i.e. carbs only)
  • Even in a flat Ironman, where you may think that you're going at a steady, slower pace than your anaerobic threshold, you will have spikes in intensity which start depleting carb resources.
  • Carb stores in the body are limited to roughly 2000 calories. So for long events especially, you need to use fat for fuel to not burn through those carbs stores as quickly as you would if you couldn't use fat.
  • Also, you need to take in additional carbs during the event to make sure your glycogen stores don't run low - which would lead to a bonk.
  • Carb stores running low leads to increased fatigue, reduced performance and impaired skill and concentration as well as increased perception of effort. This is true even if you can still keep moving by using fat as fuel.
  • The big advantage of fat as fuel is that fat stores are abundant.
  • The big advantage of carbohydrates is that they can provide energy anaerobically, and they provide aerobic energy more efficiently than fat.

Energy utilisation

23:36 - 

  • Muscles use energy in the form of ATP.
  • In aerobic energy processes, ATP is formed when carbs or fat are oxidised in the presence of oxygen.
  • You get more ATP when your muscles are oxidising carbs compared to fat - roughly a 10% difference.
    • If you oxidise carbs you get 5.1 kcal per litre of oxygen
    • If you oxidise fat you get 4.6 kcal per litre of oxygen
  • As you approach your anaerobic threshold your muscles cannot take up any more oxygen than they already are. At this point, the more oxygen is used to oxidise fat, the less ATP you will get, since any given amount of oxygen produces more ATP when oxidising carbs than fat. 
  • Essentially, the more carbs you burn the more ATP you get, and the more power output you can produce, because your muscles can produce more energy in the first place.
  • All this said, it bears repeating: to ensure you don't run out of carb stores you also want to efficiently burn fat.
  • The energy efficiency process of getting 10% more ATP from carbs versus fat cannot be trained. It is biochemistry equation - you will always get the same amount of calories per litre of oxygen for carbs, and fat, respectively.
  • Therefore, you need to be excellent at oxidizing both carbs and fats and your nutrition and your training should take that into account.
  • An example study to illustrate this from 2014, published in the International Journal of Sports Medicine, is called: Improved gross efficiency during long duration submaximal cycling following a short term high carbohydrate diet
  • Gross efficiency here means, if your body is producing 100 calories of energy, then around about 20 of those calories actually go to producing performance (pedaling, running and so on).
  • For example, if you're using a bike power meter and you're pedaling produces 250 watts, then 750 watts is being created at the same time but goes to all sorts of different processes in the body (e.g. heat generation), so gross efficiency is only 20%.
  • This study examined gross efficiency in 15 trained male cyclists that did 3 x 2 h tests at sub maximal exercise intensity (60 percent of maximal one-minute power). Participants consumed an equal amount of energy, 4,000 kilocalories per day, in the three days preceding each test. It was a crossover design so the diet was rotated so all athletes tried all different options: a high carbohydrate diet with 70 percent carbs and 20 percent fat; a low carbohydrate diet with 70 percent fat and 20 percent carbs; a moderate carbohydrate diet with 45 percent carbs, 45 percent fats and the rest protein.
  • They measured gross efficiency in the lab, as well as blood lactate and heart rate on regular intervals during the 2-hour tests.
  • Results: gross efficiency was significantly greater following the high carbohydrate diet (20.4%) compared to the moderate or low carbohydrate diets (19.6%), so a four percent improvement in efficiency on the high carbohydrate diet.
  • Heart rate responses were significantly lower in the high carbohydrate conditions compared to the low conditions.
  • Diet had no effect on blood glucose or lactate levels.


30:10 - 

  • For further reading: Louise Burke, Jeukendrup et al, 2011 - Carbohydrates for training and competition.
  • Daily carbohydrate intake guidelines for different levels of training:
    • If you do light exercise (low intensity, skill based activity) the recommended daily carb intake is 3-5 g per kg body weight
    • If you do a moderate amount exercise (roughly 1 h exercise per day) the recommended daily carb intake is 5-7g per kg body weight
    • If you do a high amount exercise (1-3 hrs per day, incl. moderate and high intensity) the recommended daily carb intake is 6-10g per kg body weight
    • If you do a very high amount exercise (4-5 hrs per day, incl. moderate and high intensity) the recommended daily carb intake is 8-12g per kg body weight
  • Example: Female, 50 kg weight, at the low end of the moderate exercise (5-7 g carbs / kg / day) spectrum. She wants to consume 5g/kg which is 250 g of carbs. This would mean 1,000 calories per day coming from carbs (since 1 g of carbs equals 4 calories).
  • Note that it is important to focus on the quality of carbs, just as it is for fats and protein. There are lots of ways to get 1,000 calories of healthy carbs easily. E.g. from fruit and vegetables and fiber rich carbs. 
  • Grains are considered unhealthy in most of today's mainstream media, but aren't unhealthy if you have the right nutrient timing (more on this in a future episode). For example, after a long or hard workout, pasta bolognaise is a perfect recovery meal
  • Unprocessed, healthy carbs should be the majority of carbs consumed.

Training low

  • Training on a low availability of carbs (e.g. wake up and train on no breakfast, or no carbs at least) has been recently coming into popularity as a way to potentially benefit endurance performance.
  • The reasoning behind this is that training with low glycogen stores  may increase maximal mitochondrial enzyme activities, mitochondrial content and increase rates of a fat oxidation.
  • Research described in the position statement suggests that training low is more effective when you have intentionally depleted your glycogen stores from a previous session. This can be achieved by doing a workout in the morning (long/hard enough to deplete glycogen stores) and then avoid any high-carb content recovery nutrition. Focus on proteins and fats in between that first and the next workout.
  • This has been shown to be more effective in causing the positive metabolic adaptations than just training on an empty stomach in the morning.
  • In the Louise Burke article Fueling strategies to optimize performance: training high or training low?, she concludes that "despite increasing the muscle adaptive response and reducing the reliance on carbohydrates utilization during exercise, there is not yet clear evidence that these strategies enhance exercise performance". In other words, you get the metabolic adaptations but it's not know if this translates to faster times in races.
  • Another key publication is by Trent Stellingworth: Contemporary nutrition approaches to optimize elite marathon performance. This article comes to the same conclusion as Louise Burke's review, but includes some more relatively positive anecdotal evidence (I know...) from East African marathon runners.
  • My personal guess that we will soon see research showing performance benefits from strategic use of 'training low'.

Links and resources mentioned

Connect with host Mikael Eriksson


Hi! I'm your host Mikael,

I am a full-time triathlon coach and an ambitious age-group triathlete. My goal is podium at the Finnish national championships within the next few years.

I first started the website Scientific Triathlon in autumn 2015 as a passion project to share my learnings with a larger triathlon audience. Later on, in early 2017 I started the podcast That Triathlon Show. 

I sincerely want you to contact me to

  • Send me feedback
  • Give constructive critic​ism 
  • Request topics and guests for the podcast
  • Send me your triathlon-related questions 
  • Tell me that you've rated and reviewed That Triathlon Show so I can give you a shout-out on the show and tell you how much it means to me!
Subscribe to That Triathlon Show and never miss an episode!


Let's discuss this episode and the topic in general. Post any comments or questions in the comments below. I'll be here to reply and take an active part in the conversation, so don't be shy! 

Mikael Eriksson

I am a full-time triathlon coach, founder of Scientific Triathlon, and host of the top-rated podcast That Triathlon Show. I am from Finland but live in Lisbon, Portugal.

Please contact me if you have feedback on the podcast or want to make suggestions for improvement or send in a question for a Q&A episode.

If you are a long-time listener and appreciate the value the podcast brings, please consider taking a couple of minutes for leaving a rating and review on iTunes/Apple Podcasts, or wherever else you can think of leaving a rating and review.

{"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}

Explore our products and services