How long does it take for fructose to "do something"?

Hello,

Is there any data on how long it takes to get “energy in your muscles” when you take in fructose? My understanding…based on nothing…is that we’re talking hours vs glucose which feels like it’s doing something in 15-20 minutes. Is this something we “know”?

Joe

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This is just one study, I’m sure there are others if you Google them.

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Fructose is absorbed more slowly than glucose. It takes about 1-2 hours to be converted into glycogen and provide energy to muscles, compared to glucose which can take effect in 15-20 minutes. This difference is well-documented in nutritional studies. Fore more info visit this useful article. Stay healthy.

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That’s pretty much in agreement with the study above that showed 2-3 hours when exercising. Is there any reason to take in fructose if riding less than, say 2 1/2 hours? Let’s assume we’re not doing another workout later in the day nor are we in a stage race. Just “normal” daily training.

Joe

Other than a pre-ride banana or two, I minimize fructose during my workouts unless they are 3+ hours. When I do 90-120g/hour bottles at 30-60g fructose, I feel a boost around 3 hours in. That has been consistent and repeatable. So that aligns with some of the studies. Given all the carbs I eat in the kitchen, trying to “make it up on the bike” and cram calories in on sub 2.5 hour workouts, at 250-280W ftp, only results in weight gain despite attempts to modulate carbs/calories in the kitchen. YMMV.

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Going from memory here, but isn’t there a relationship between fructose and the ability to take / absorb / utilize more glucose in the short term too? (Being lazy, haven’t gone looking for the studies)

I generally mix at 1:0.8 all the time and just don’t worry about it, because that’s the ratio I use when I do gels. I just don’t fuel much or at all for shorter or easier efforts.

My FTP is 340, and I have to watch out for weight gain, even when I’m doing 12-15+ hours a week. I pretty much never have an issue getting enough calories except maybe on one given day when I’m doing a 6+ hour ride.

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Yeah, mine has been the same. I haven’t felt any difference in 3 hour rides using straight maltodextrin vs. adding fructose at 1:2 or 1:1, even though the calorie intake has been higher with the +fructose rides. Not sure about longer rides yet but it makes sense…gonna have to do a little more experimentation.

Joe

I think it’s good absorption in the intestines. But the bottleneck for utilize is the liver metabolizing fructose into something the rest of the body can use (glucose or fat). It’s something about 5-carbon ring structure in fructose vs 6-carbon ring structure for glucose.

Warm take?

Instantly

Because your body will recognize that you ingested energy and will now allow you to use reserves. This has been tested before when athletes just had the flavor of sugar and spit it back out, they performed better.

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I like this take. But does our body also instantly know that 30% of the sugar you took is fructose? Or is that irrelevant?

There has been tons of discussion on this very topic on this forum.

On the one hand, your body has enough energy in its glycogen stores for a <3-hour ride. So a lot of people argue that fueling on the bike is not necessary. In my experience, fueling adequately on the bike has been a huge net positive with significant power gains:

  • My nutrition off-the-bike is much more stable as I only need to compensate for a much smaller calorie deficit. When I did not fuel properly on the bike, I used to be famished for breakfast and eat half a loaf of bread (I train almost exclusively in the morning). That was a problem when I took a break or had a rest week as my appetite was still the same as if I were training hard. I tended to overeat.
  • My performance on the bike has improved significantly. Not sure whether this is because of lower RPEs, better nutrition overall or some other factor.
  • It makes it much easier to get into the habit of fueling appropriately. That’s particularly important for longer outdoor rides. If you are not in the habit of fueling, you won’t feel any cues and I always got into a hole by hour 3 or 4. And then it was too late for any quick fixes. For mellow endurance rides, I could persevere, but it was definitely less enjoyable. Add hard climbs and less enjoyable quickly became miserable.
  • Recommended average carb intake has increased with time.

I have had other members vigorously disagree with this, but this is my take. One forum member who is an expert is @Dr_Alex_Harrison. Search for posts by him, they are a treasure trove. He also has a nutrition app that might be helpful.

My understanding is that this has nothing to do with detecting specific types of sugar, just that you are giving your body a signal that you are replenishing yourself. I remember the trio mentioning a study repeatedly where athletes either just rinsed their mouthes with a sweet solution or used artificial sweeteners.

Even though athletes in these studies have not consumed any energy, it seems that their bodies wrote a few more checks as they were tricked into believing that more carbs were on the way. That’s my understanding anyway.

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Not what I meant. Pretty sure it impacts absorption of glucose. (Edit - posted below, impacts total carb oxidation, unsure about absorption)

So, even if it takes a while to utilize the extra fructose, I think it enables more short term glucose utilization.

My understanding is that because glucose and fructose absorption happen on different pathways, you increase total carb absorption by spreading the load over two absorption channels.

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Referring to this. Total exogenous carb oxidation is impacted.

However, when fructose is co‐ingested with glucose during exercise (at 50% maximum power output (W max)), exogenous carbohydrate oxidation rates can increase up to 1.75 g/min (Jentjens & Jeukendrup, 2005), which is substantially higher than the rates reported following ingestion of glucose (polymers) or fructose only.

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Another one, liver glycogen repletion rates:

In contrast, liver glycogen repletion rates are approximately doubled with ingestion of glucose–fructose (sucrose) mixtures over isocaloric ingestion of glucose (polymers) alone. Furthermore, glucose plus fructose (sucrose) ingestion alleviates gastrointestinal distress when the ingestion rate approaches or exceeds the capacity for intestinal glucose absorption (~1.2 g/min).

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From that very paper…

" In addition to providing fuel to the working muscle, ingesting carbohydrates during exercise has also been suggested to prevent the depletion of endogenous (muscle and liver) carbohydrate stores. Due to the higher capacity for carbohydrate absorption and the predominant hepatic metabolism of fructose (Fig. ​(Fig.1),1), it could be speculated that the combined ingestion of glucose and fructose further prevents the lowering of endogenous carbohydrate stores during exercise, particularly in the liver. We recently performed a study investigating the effects of ingesting either 1.7 g/min of glucose or 1.7 g/min of sucrose during 3 h of exercise (at 50% W peak) on liver and muscle glycogen concentrations (Gonzalez et al . 2015). We observed that neither type of carbohydrate was able to prevent the lowering of muscle glycogen concentrations during exercise. However, ingestion of either type of carbohydrate fully prevented liver glycogen depletion. This suggests (at least during submaximal exercise at 50% W peak) that carbohydrate ingestion can prevent liver glycogen depletion during exercise, but that fructose co‐ingestion is not more effective than glucose (polymers) ingestion alone in preventing exercise‐induced glycogen depletion. Whether the same holds true for exercise sessions performed at higher exercise intensities remains to be elucidated."

My take on this is that comparing 100% glucose vs. 50% glucose + 50% fructose, for 3 hours at 1/2 of your VO2 max power there is no difference. Which fits with the study in post #2.

Again, we’re not talking about “not fueling workouts” or stage racing or two grueling workouts in one day or athletes having trouble recovering from their workouts (just can’t get enough carbs in later in the day), the question is how fast does the fructose in your 2:1 (or whatever) ratio mix getting used in a “normal” day of training. So far it looks like it’s at least 2-3 hours.

Joe

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I think this is a great question that has prompted a really interesting discussion. For this specific case - your glyocgen levels are topped off, the ride is less than 2.5 hours and it’s not an all-out race - 60 grams of glucose an hour may be absolutely fine for you.

There are cases where we do not need to maximise carnohydrate oxidation, and this is one such example.

However, we do not understand everything about individual differences in oxidation rates. So I would encourage you to do as you suggest and experiment to find what works best for you under different circumstances. This will likely change over time.

Dr Podlogar expressed a similar sentiment on the podcast recently. You can check it out the conversation here.

On the other hand, there are many cases where we should try to maximise carnohydrate oxidation. In these cases (a long race or ride, when our glcyogen stores are suboptimal, a high absolute power output) the evidence points toward ingesting a mixture of glucose and fructose to increase the amount of carbohydrate our muscles can use.

There are other factors (size, diet, ambient temperature, to name a few) that may influence the amount of carboydrate we need, can tolerate, absorb and oxidise. Therefore, fueling on the bike is going to differ from person to person, so I encourage you to experiment to find what works for you.

Questions like these would be great questions to ask Dr. Podlogar when he’s next on the podcast. You can submit your questions for future podcasts here: Ask a Cycling Coach Podcast - TrainerRoad :+1:.

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I’ve listened to him on a couple of podcasts, including the first one on TR. He is also associated with the Nduranz company and they have a calculator. Lets put the following into the calculator:

  • Workout: Race or Workout
  • Intensity: High
  • Duration: 3 hours

we get the following recommendations for cold, warm, and hot conditions:

3 hour recommendations, any temp, race or high intensity workout

Weight <60kg 60-75kg 75+kg
Carbs between rides 6-9g/kg 6-9g/kg 6-9g/kg
Carbs 1st Hour 45g 45g 68g
Carbs 2nd Hour 68g 90g 90g
Carbs 3rd Hour 68g 90g 90g

Example for 3 different weights, workout yesterday, high intensity workout or race today

Weight 60kg 70kg 80kg
Carbs off the bike over 24 hours 360-540g 420-630g 480-720g
Carbs 1st Hour 45g 45g 68g
Carbs 2nd Hour 68g 90g 90g
Carbs 3rd Hour 68g 90g 90g

45g is about 2 medium bananas. Eat two while walking out the door like I do (90kg) and I’ve satisfied 1st hour recommendations for high intensity workout or hard group ride, assuming I ate carbs off the bike.

45g breakdown (Nduranz 1Nrgy Unit)

  • 25g maltodextrose
  • 20g fructose

68g breakdown (Nduranz 1Nrgy Unit)

  • 38g malto
  • 30g fructose

90g breakdown (Nduranz 1Nrgy Unit)

  • 50g malto
  • 40g fructose

3 hour easy workout, any temp

Weight <60kg 60-75kg 75+kg
Carbs between rides 4-6g/kg 4-6g/kg 4-6g/kg
Carbs 1st Hour 28g 28g 28g
Carbs 2nd Hour 28g 45g 45g
Carbs 3rd Hour 28g 45g 68g

Example for 3 different weights, workout yesterday, high intensity workout or race today

Weight 60kg 70kg 80kg
Carbs off the bike over 24 hours 240-360g 280-420g 320-480g
Carbs 1st Hour 28g 28g 28g
Carbs 2nd Hour 28g 45g 45g
Carbs 3rd Hour 28g 45g 68g

OK.

All that and now about me.

  • 90-ish kg
  • do a lot of 1.75 to 2 hour workouts

Put that into a typical week for myself with a 3 hour ride on Saturday, all others 1.5-2 hours so I’ll use 2 hour recommendations:

Day Monday Tuesday Wed Thur Fri Sat Sun
Intensity High Easy Med Off High Easy Off
Carbs 24 hours before 450-540g 270-450g 270-450g 450-540g 270-450g 270-450g 270-450g
Carbs 1st Hour 45g 0g 45g - 45g 28g -
Carbs 2nd Hour 68g 28g 68g - 68g 45g -
Carbs 3rd Hour - - - - - 68g -

I ride in afternoon, before dinner. Dinner can easily hit 100-150g carbs. Breakfast easily 120g carbs. Lunch only counting rice its 100g. So thats 320-370 on the low-end, at or just below the off-the-bike 24 hour recommendations. A couple bananas pre-ride and I’m at 370-420g on the low-end.

Saturday 4 hour easy endurance ride would be 5-6g (450-540g @ 90kg) the 24 hours before, then during the 4 hour easy endurance ride: 45g first hour, 45g second hour, 68g third hour, and 90g fourth hour. (FWIW I’ve experimented and prefer front-loading 90g, 90g, 45-60g, 45-60g).

Day Sat
Intensity Easy
Carbs 24 hours before 450-540g
Carbs 1st Hour 45g
Carbs 2nd Hour 45g
Carbs 3rd Hour 68g
Carbs 4th Hour 90g

Saturday 5 hour ride would be 6-7 (540-630g @ 90kg) the 24 hours before, then during the 5 hour easy endurance ride: 45g first hour, 68g second hour, 90g third hour, 90g fourth hour, and 90g fifth hour.

Day Sat
Intensity Easy
Carbs 24 hours before 540-630g
Carbs 1st Hour 45g
Carbs 2nd Hour 68g
Carbs 3rd Hour 90g
Carbs 4th Hour 90g
Carbs 5th Hour 90g

Hope that helps someone.

@SarahLaverty two questions for the upcoming podcast… 1. Does Dr Podlogar generally agree with the Nduranz calculator recommendations? 2. I’ve heard what sound like really high recommendations on TR podcast, if Dr Tim answers yes to question 1, can TR explain their position?

Thank you.

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I really enjoy listening to him discuss his current curioisities and recent research. I always learn SO much. Did you listen to Jonathan’s most recent conversation with him on the Ask A Cyclcing Coach podcast?
Really interesting stuff. I’ll share the link below.

Huh! I’ve never seen this before! Very cool. I’ll need to check it out.

This is great! :raised_hands: The science should guide you but you really have to experiement to figure out what works best for you. I’m glad you have found what works for you!

Go ahead and submit your questions at the link I shared above! That’s the best way for us to keep things organised.

Does he talk about fructose metabolism or is this a recommendation for a “generic carb mix”? I’m not sure I understand the point of all of that (?)

Joe