Nutrition, Podcast, Science and Physiology

Carbohydrates – science and practice with Tim Podlogar, PhD | EP#354

 September 5, 2022

By  Bernardo Gonçalves


Tim Podlogar - That Triathlon Show

Tim Podlogar, PhD, is a Research Fellow at the University of Brimingham, Assistant Professor at the University of Primorska (Slovenia), and nutritionist for the Bora Hansgrohe World Tour cycling team. One of Tim's main areas of expertise is the science and practice of carbohydrates in endurance sports, and this is the topic for today's interview. 

In this episode you'll learn about:

  • Carbohydrates before, during, and after competition
  • When and why to use glucose and fructose
  • Is 120 g/h carbohydrate feeding superior to the commonly recommended 90 g/h?
  • Training in a glycogen-depleted state
  • Reverse engineering your carbohydrate intake by looking at upcoming workout demands
  • Continuous blood-glucose monitoring
  • Training the gut

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Tim's background

03:37 -

  • I have a PhD in Exercise Metabolism from the University of Birmingham.
  • I currently work at the University, looking at how heat stress affects exogenous carbohydrate oxidation.
  • I am an assistant professor at one of the Slovenian Universities. I am also a nutritionist for Bora-Hansgrohe Cycling Team.

Carbohydrate recommendations


05:11 -

  • Typically, amateur athletes start their races early. So, we need to consider what to recommend to them.
  • For example, Vuelta is taking place. Races start at 13h, which is much later than for some amateur races.
  • On the day, the aim is to fill up both glycogen stores: liver and muscle.
  • It is essential to distinguish between liver and muscle stores because we will not use muscle glycogen stores at night.
  • However, the brain and vital organs are working, so we can reduce liver glycogen stores overnight.
  • The morning nutrition is to replenish liver glycogen stores.
  • We have done a study in which we gave the same breakfast concerning quantities but different types of carbohydrates. (glucose or glucose and fructose)
  • This difference is because fructose first needs to go to the liver.
  • As it goes first to the liver, it takes care of those liver glycogen stores. We found out that time to exhaustion was higher for people that ate glucose and fructose.
  • In practical terms, it means something like rice or porridge oats with some sources of fructose (honey, jam, juice or anything like this)
  • The next topic is the type of carbohydrates (complex or simple)
  • If we start early, there is not much time to digest the food properly.
  • We want "easy-to-digest" foods (low fibre, low fat).
  • For example, rice is better than porridge oats.
  • On the other hand, if you start at 13h, gastrointestinal comfort will not be a problem; porridge oats can also be a good choice.
  • Porridge has more fibre and fat, so the release of carbohydrates will be slower, but you will not be as hungry in the morning as you would with rice.
  • The addition of protein and fat will depend on the race. If it is a one-day race, carbs are the only thing that matters in the morning.
  • We want protein for multiple-day stage races because we need to think about recovery and are not fresh in the morning.

Short stage races

10:34 -

  • The first thing is to get the carbohydrates right. (high carbohydrate intake)
  • If we have some time to digest, we add protein and fat.
  • If you have problems with consuming too much food, you should reduce the amount of fat and protein.

Sources of fructose

12:07 -

  • Some fruits have a lot of fibre, which we do not want in the morning.
  • We also need to check the fructose content of specific fruits because sometimes, fruits are not high in fructose at all.
  • For example, grapes could be high in fructose but have skin, so they have plenty of fibre. I usually have rice with pure fructose.
  • On the top, I add Nesquick powder, which is sugar.
  • It is acceptable for one-day races but could be too dull for multiple-stage races.

Analysing the effect of fructose in the morning

13:33 -

  • The study we did was in Slovenia, so we had limitations with the mechanisms we could investigate. We did not access liver glycogen.
  • You would need an RMI for this.
  • We saw improvements 7min in time-to-exhaustion in tests of 2h.
  • The breakfast was 2 g/kg of carbohydrates. The intensity was the lactate threshold.
  • There was no change in metabolism during exercise. The oxidation of the total amounts of carbohydrates was not at the highest rate at the beginning of the exercise.
  • Often, you see that we have higher glycogen stores at the start of exercise so that you will oxidise more carbohydrates for the two hours to achieve the same results.
  • However, in this case, the carbohydrate oxidation remained constant throughout the session.
  • It means the body uses the carbohydrates stored to prolong exercise rather than burning more carbs and less fat.

Muscle glycogen stores

15:45 -

  • Some people recommend multiple days of glycogen loading.
  • I always go with only one day, usually meaning that athletes train the day before (1-2h spin).
  • We know that the more glycogen you store, the more you use it.
  • Therefore, I do not see a point in loading two days in advance and using so much glycogen the day before the race.
  • You can do the session early in the morning and then use this stimulus to replenish glycogen afterwards.
  • You eat 10-12 g/kg of carbs daily with moderate protein and low fat.
  • There is time to get the levels high.
  • Even professional athletes do not have two days between stages; they usually recover successfully in one day.
  • We can deplete glycogen stores in 1h30-2h. 
  • When we do glycogen depletion protocols in the lab, people can work for 1h30-2h30 and then have to reduce intensity.
  • They get depleted and tired.
  • You want glycogen stores topped up in any session with more than 90 minutes.

Glycogen loading for short events (10k run)

18:32 -

  • Maybe not loading, but a high carbohydrate intake (8 g/kg of carbs).
  • You still want many carbs in the body.
  • We can use carbs more efficiently than fat to gain a few seconds.
  • At the same time, you do not want to gain extra weight due to water retention because it can reduce exercise economy.
  • Some people could feel different when they do glycogen loading, so this is more personal.

Sources of carbohydrates to consider

20:23 -

  • I am a supporter of Haribo gummies. I think they are the best source of carbs to do glycogen. 
  • If you want to push for high intakes of carbohydrates, you want simple carbohydrates.
  • Juices would be okay, and the worst would be to eat whole grains, try to be healthy, and eat potatoes.
  • I recommend having regular meals with some protein and moderate fat and eating primarily simple sugars. (it is okay, you will not get diabetes in one day)

During the competing

22:01 -

  • It is a challenging question because there is some evidence that people can oxidise more than 90g/h.
  • Therefore, there is a trend to promote higher carb intakes.
  • The first rule is to get the glycogen stores up. Then, you focus on eating during exercise.
  • Low glycogen stores are a factor that influences fatigue. Ingestion of carbs during exercise cannot save muscle glycogen stores. (it saves liver glycogen stores to maintain blood sugar stable)
  • During exercise, it will depend on energy expenditure.
  • If you are doing an event with 120 W, the energy expenditure is much lower than 280 W.
  • For most people, going 90g/h makes sense. For female athletes, 60 g/h is what I recommend.
  • I practice with my athletes increasing that intake over time.
  • If you are riding for 5-6 hours, you have full glycogen stores initially, and the muscle is not ready to take more glucose.
  • If you ingest too many carbs, you suppress fat oxidation rates and do not spare glycogen.
  • I would start with 70 g/h and then increase the intake towards the end because glycogen stores in the muscle start to go low. (the glucose uptake starts going up)
  •  In the end, is when we see the highest exogenous carbohydrate oxidation rates.
  • If the event is long and we can tolerate it, I do not see a problem with going to 120+ g/h. (professional cyclists can go higher)
  • In stage races, they calculate energy expenditure and want to match energy intake.
  • If you have a high intake during the stage, you must reduce the energy amount ingested after the stage.
  • However, starting with 90g/h and then going higher in one-day races is advisable.

Triathlon nutrition recommendation

27:57 -

  • One mistake triathletes make is having a specific plan (e.g. 90 g/h throughout the event), but start experiencing gastrointestinal issues in the run and keep eating as planned.
  • Residual carbohydrates in the digestive system are the cause of gastrointestinal issues, so it does not make sense to eat more carbohydrates (the absorption of carbohydrates in the intestine has not finished yet, so adding more would make the situation worse)
  • It would help if you understood how you feel. If you feel okay, you can increase your intake. Otherwise, you reduce it.
  • You eat carbs during the bike, but they are still in the digestive system when you start running.
  • If you consume too many carbs at the end of the ride, you might suffer from unabsorbed carbs in the early stages of the run.
  • Therefore, you should eat much at the beginning of the bike split and decrease the intake towards the end.
  • If you start the run without residual carbs and feel good, increase the intake during the run.

The influence of athlete level and race distance on carb intake

31:29 -

  • For more than three hours, I would go to 90 g/h for most athletes. Some we would feed with 120 g/h.
  • For novice light female athletes, I would go with 60 g/h to prevent gastrointestinal discomfort because energy expenditure is not as high.
  • For events less than three hours, if you have full glycogen stores, you do not need much additional energy because you will only need it for the last 45 min. So, going between 45-90 g/h would work well.

After competition

32:44 -

  • After every session, I recommend having carbs, protein, and very little fat to initiate the recovery process faster.
  • The recommended dose of carbs is 1-1.5 g/kg, and combining different carbohydrate types becomes crucial. (glucose and fructose - juices, Fanta or coke)
  • I have many concerns with the "window of opportunity" because it does not exist.
  • If you prolong time without eating, you might not ingest enough carbs for the next day. If you finish the day at 4 pm and start eating at 5 pm, you have a limited time window when you can eat. So, getting food in as quickly as possible makes it possible to ingest enough food during the day and replenish muscle and liver glycogen stores.
  • If they do not eat straight away in training, Amateur athletes have food cravings and overeat for the rest of the day.
  • Moreover, you should understand the future sessions you will do.
  • When I look at nutrition, I evaluate it between two training sessions. (the next day starts after the first recovery meal)
  • You must understand how much glycogen you need to do the session and your recovery status.
  • For example, if I have a two-hour easy ride followed by an interval session the next day, I would need to eat many carbs for the rest of the day.
  • If you look at 24h energy intake, you would be in energy surplus from 0-24h the day before a challenging session.
  • You should also check if you want to recover fully or if it is an easy session.

Fueling for a demanding session

39:09 -

  • When calculating energy intake, I predict the burning expenditure of the session and then work backwards.
  • I distribute the carbs during the session and recovery so athletes can be in energy balance from the end of one session to the next.
  • Then, you correct it after each session.

Nutrition after an Ironman

40:40 -

  • Concerning physiology, nutrition is an essential topic after an Ironman.
  • Concerning psychology, it does not matter as much.
  • Nutrition is not only about physiology and number but also psychology.
  • We recover physiologically and psychologically from a race.
  • If "unhealthy food" helps athletes to get through the training block without cravings for specific foods, I do not see a problem with having some "cheat days".

"Train low" strategies

42:21 -

  • The goal is to have low muscle glycogen stores before the start of the training session to potentiate the response of such training sessions after it.
  • Some research has shown that this strategy could lead to performance improvements, and some evidence is that this will not affect carbohydrate availability.
  • It comes down to the type of athletes you work with and how much time they have for training.
  • Training with low carbohydrate availability means we cannot ride for 4-5 hours at a relatively high intensity. (even at the first threshold)
  • So, we lose training volume with this strategy.
  • If you are a time crunch athlete, it is a simple way of achieving similar adaptations with only a two-hour ride in the morning where they do not consume carbohydrates after the previous session.
  • On the other hand, training volume for elite athletes is crucial, so you cannot afford to reduce training volume to do low-carb training sessions.
  • Therefore, I do not recommend doing low-glycogen training sessions, knowing that most sessions will result in low-glycogen stores. (intensity and energy expenditure are high)
  • Therefore, it is helpful for time-crunch athletes but can lead to overtraining in athletes that have all time available for training.

Implementation of "train-low strategies" for time crunch athletes

46:40 -

  • Some people can struggle when they have low-glycogen availability. 
  • The most popular strategy is the sleep-low strategy. You do a demanding training session on day one, and because of the nature of the workout, you will finish with low-glycogen stores.
  • During the rest of the day, you would fold from carbs.
  • The morning after, you would do a 1h30 training session at moderate to low intensity.
  • Some people could execute this, whereas some would struggle in the afternoon. They would be useless at home because you do not have the energy to do much.
  • Many people also struggle to sleep in this state.
  • Some studies show no problems with sleeping, but I experience it when I do these strategies.
  • To do the prescribed session in the beginning, you can have carbs during the session. I studied this during my PhD and analysed ways of increasing training volume. I suggested ingesting carbs with a delay, so that fat oxidation rates remained stable.
  • So, after 30 minutes of exercise, we started adding carbs, and blood glucose levels were stable, and fat oxidation rates remained high.
  • Over time, athletes will get used to the session, and carbs will not be necessary anymore.
  • Other strategies would be to use caffeine or mouth rising.
  • I would only do it once a week.

Carbohydrate availability, REDs and bone density

51:01 -

  • Carbohydrate availability makes the most significant impact on hormones (cortisone, glucagon) in the recovery process.
  • If there are not enough carbohydrates, the body will sense that it will not have enough energy, especially carbohydrates that are the prefered fuel. Therefore, it will slow down bone formation and recovery.
  • Evidence shows that REDs come down to carbohydrate availability (not necessary overall energy availability).
  • If liver glycogen stores are constantly low, it could lead to low glucose levels, and hypoglycemia will be a problem.
  • Moreover, low liver glycogen stores mean you produce glucose from muscle tissue (start breaking down the body instead of building it).
  • A study shows that low carbohydrate availability drove low testosterone levels rather than fat availability.

Carbohydrate periodisation

53:51 -

  • It started with the implementation of "train-low" strategies.
  • You can potentiate the responses to training sessions by starting the training sessions with low carbohydrate availability.
  • For some time, the periodisation was highly polarised. (low-intensity sessions with low carbohydrate availability and the opposite for high-intensity sessions)
  • In the present, it is about ensuring you fuel for the work required.
  • You do not look only at the session's time but also the intensity. (base the carbohydrate intake more on the intention of the next training session)
  • You might want to end a session with low glycogen rather than starting a session with low glycogen. To reach this, it makes sense to give some energy during the training session, but not too much.
  • If you have a more extensive training session in one day and an easy spin on the next, it does not make sense to load your body with too many carbohydrates.

Training the gut

57:02 -

  • The most crucial aspects are tolerance and absorption.
  • We train the gut by eating many carbs on the bike or while running to train the body to absorb more carbohydrates from the intestines to the bloodstream.
  • It is only a theory, and I think we are building carbohydrate tolerance.
  • The gut does not distinguish between exercise time and rest.
  • Eating more carbs is not enough stimulus to increase the transporters for carbohydrates while eating during exercise.
  • We will increase the number of transporters by having a high carbohydrate diet while eating during exercise builds tolerance.
  • First, we need to tolerate carbohydrates to sit in the intestines to get absorbed. Then, it is about the personal capacity to absorb them.
  • Recently, I did a study of 90 vs 120 g/h of carbs during exercise, and the best participant was doing 290 W for three hours (95 % of the first threshold)
  • This participant does a lot of "low carbohydrate training", and his maximal exogenous carbohydrate oxidation rate was not high.
  • Even though he is highly trained, he cannot absorb those carbohydrates. Perhaps if he had more carbs during the day, he would have the capacity to utilise more, but it is something we still need to investigate. (trying to find a relationship and ability to oxidise ingested carbs)

Personalising carbohydrate intake from physiology responses

1:01:41 -

  • Some people say we need to individualise everything. Whereas when I work with athletes, I only look at carbohydrates. (ensuring high carbohydrate availability at all times)
  • Some people calculate how much carbs athletes burn in racing/training, but I do not know if this concept is correct.
  • You can change the number of carbs burnt by having different carbohydrate availability, glycogen store levels or nutrition strategies.
  • If you come up with a number, you do not know what to do with that value.
  • If a person burns 300g of carbs, it does not mean they will absorb 300g of carbs.
  • In my opinion, you cannot make solid predictions. Ensuring carbohydrate availability is high as possible is the most crucial part.
  • On another topic, you do not train fat oxidation rates. Instead, you increase your fitness.
  • If you did a fat max test, elite athletes with high carb diets would have higher fat oxidation rates than amateur athletes with low carb diets.
  • We train to get the Vo2max and threshold up, and only when these cannot improve anymore can you start thinking about how to increase fat oxidation rates. Otherwise, if you want to improve fat oxidation, you will induce low carbohydrate availability and risk not recovering and getting REDs.

False precision

1:06:35 -

  • We use an app to calculate the energy expenditure of cyclists, so I always look at the data and the information it provides me.
  • Data is helpful, but you need to understand its limitations and how to operate with it.
  • If you blindly follow those apps, it could lead to a disaster.

Continuous glucose monitoring to guide carbohydrate intake

1:07:19 -

  • I do not think we need such a device as it might not bring any benefits.
  • Blood glucose levels remain stable during exercise, especially if you exercise above your first lactate threshold.
  • We would see similar lactate/glucose curves if we did a lactate test.
  • The higher the intensity, the higher the glucose concentration.
  • Glucose concentrations depend on exercise intensity and carbohydrate intake.
  • This information can be essential if glucose levels drop below five mmol (hypoglycemia). However, we can prevent this by eating on the bike. (90 g/h)
  • At higher intensities, knowing blood glucose concentrations does not tell us much because it depends on many factors. For example, stress can increase blood glucose.
  • When we did the test (glucose breakfast vs glucose/fructose breakfast), my hypothesis was blood glucose concentrations would start to decline, and the longer we maintained blood glucose concentrations, the longer they would go.
  • However, no one was in hypoglycemia when measuring that parameter at the task failure point.
  • Other papers show that the higher the intensity, the less you will see blood glucose variations.
  • Of course, a drop in blood glucose means insufficient carbohydrate intake at low intensities.

Differences in carbohydrate intake between genres

1:11:00 -

  • There is probably a substantial difference that would make me change my recommendations.
  • An elite male athlete and a female athlete with the same size and fitness level will probably get the same fuel recommendation.
  • Of course, I would also be aware of the menstrual cycle and how they couple with it mentally. Women can struggle in different phases.
  • During those periods, I would try to find ways to increase carbohydrate availability during those periods.
  • However, I would not vary carbohydrate recommendations depending on the menstrual phase.

Influence of age

1:13:11 -

  • I can influence younger athletes easier. I can tell them to eat a specific carbohydrate amount, and they will follow it.
  • My approach to those athletes would be easier than someone 40-50 years old because they will have ideas in their minds. We, as nutritionists, have to find solutions to overcome them.
  • The difference is more about psychology than physiology.
  • For age groupers, carbohydrates are also crucial, and protein plays a more significant role, so the portion sizes would need to be higher with a 60-year-old athlete because of the anabolic resistance.
  • It will depend on energy expenditure and fitness, but not age-related differences.

Future research in sports nutrition

1:15:14 -

  • I would like to see if individualisation is an essential feature of nutrition (personalisation or day-to-day variability).
  • For example, measuring exogenous carbohydrate oxidation in people in one day would show significant differences. However, would we see those differences every single day? Can we make exact recommendations: e.g. 90g/h on the bike, 80 g/h on the run to prevent gastrointestinal distress?
  • We could also measure glycogen stores before races and the carbohydrates requirements to recover better.

Rapid-fire questions

1:17:15 -

What is your favourite book, blog or resource?

Exercise Physiology: Theory and Application to Fitness and Performance by Scott Powers

What is an important habit that benefited athletically, professionally or personally?

Having a routine. (waking up at the same time every single day)

Who is someone you have looked up to or who has inspired you?

I would say my boss Gareth Wallis.


Bernardo Gonçalves

Bernardo is a Portuguese elite cyclist and co-founder of SpeedEdge Performance, a company focused on optimising cycling and triathlon performance. He writes the shownotes for That Triathlon Show, and also produces social media content for each new episode.

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