Glucose absorption, gastric emptying and bananas

[Nigel Leeming]

I have been trying to get my head around this one for a while. It comes from a long held urban myth that the fastest way to get energy into your system is by eating a banana. This belief/misbelief has been strengthened by watching athletes, especially tennis players at wimbledon munching on a banana between games.

A banana contains glucose, fructose and sucrose(glucose and fructose bonded) as well as starch. It also contains potassium.

Now, the glucose and potassium can slow gastric emptying because of osmotic effects, meaning the glucose does not get into the duodenum where it can be absorbed as rapidly as it might. However, the fructose helps speed up gastric emptying. Unfortunately, the fructose heads off to the liver to be digested, and seems to hold up all other digestion until it's finished.

My question is, during and post exercise, is it better to take glucose and protein rather than protein and a banana? I used to be a banana man, but have recently slid over to the glucose side.

[RatioFitness]

How much difference do you think that's actually going to make for anything? Unless you are doing a glycogen depleting workout and have less than 24 hours before you are going to do another glycogen dependent activity, you're just splitting hairs.

[Joshua Naterman]

Bananas have a fair bit of fiber, which will slow the rate of absorption. The sugar to water ratio is what will determine whether gastric emptying is delayed or not, but solid foods in general will always empty more slowly than liquid foods.

You can't have more than a 6% glucose to water solution, so 60g of glucose in 1L of a water solution is the max. Or, most likely, 10g of whey protein and 40-50g of glucose, which will be more useful to your body. This mixture will enter the blood more quickly than the banana.

However, for practical purposes I wouldn't worry too much about that. Just consume what you have available and know that you'd have to eat at least 2 large bananas to get the same amount of total sugar, and more than half would be fructose. That's too much fructose for one feeding. Mashed white potatoes with some salt and a bunch of water would be much, much better.

[Coach Sommer]

Mashed white potatoes with some salt and a bunch of water would be much, much better.

Could you explain in more detail? I have been avoiding mashed potatoes, but perhaps that will be able to change in the future.

Yours in Fitness,

Coach Sommer

[Nigel Leeming]

Thanks Joshua, I've been avoiding fructose apart from eating fruit, but when I read that a glucose/fructose mixture entered the duodenum quicker than glucose alone, hence the glucose ingested would be absorbed into the blood quicker, I began to itch again. It's something I have been trying to get my head around for some time, but there are too many variables! I hadn't considered the concentration of solution. When I'm running, I sometimes munch on dextrose(glucose) tablets, so I guess there's not much in my stomach to dissolve them as I can never be bothered to carry water. They seem to take only a few minutes to get through though, as I can feel their effect fairly soon.

[Joshua Naterman]

That entry issue is partially concentration and partially due to the fact that the fructose and glucose use different carriers so they won't interfere with each other. Unfortunately, there's only a 10% difference in time to glycogen replenishment rate and that is only a 3.6 hour (3 hours and 36 minutes) difference over a 36 hour time period. In other words, it is not a practical difference to concern yourself with. Unless you are hitting 100% glycogen deplention, which is not going to happen in anyone who is not a seriously competitive endurance athlete, this is just not a practical concern. A glucose-sucrose solution is far preferable to a glucose-fructose solution, because free fructose gets absorbed too quickly and can lead to processing issues in the liver. Fructose is only preferentially converted to glycogen if the liver is 50% or more depleted of glycogen. Small doses and slow trickles are not a big issue, which is why for many people fresh fruits (not melons) are generally not a problem. Free fructose is a problem.

Coach: This has a lot to do with glycemic index. Sucrose is a 67, white baked potatoes are an 88-90 and sometimes considered higher than that. White potatoes are also 100% glucose polymers with no fructose in sucrose or free form, and are fairly calorically dense so it is easy to get enough carbs without over-filling the stomach. The mashed potatoes are nice because they are pretty much super fine particles so you get rapid absorption. During a workout, absorption is impaired somewhat due to redistribution of blood flow to the working muscles, but a gymnastic workout is not the same as steady-state aerobic exercise and so you don't have the same degree of blood flow alteration. so the potatoes should be a non-overfilling and cheap way to steadily introduce easily digested glucose into the blood as several snacks during the workout so that you don't have to go hunting down that glucose powder. You would still want to limit the intake to what you need, but you won't need quite as much water with them because they are larger chains of glucose molecules. Fewer particles means less osmotic action, meaning less water needed in the stomach to allow for rapid gastric emptying.

At first there will be a short adjustment period of 1-2 weeks or less, while the body gets used to processing the food in the workout. You get good at what you do regularly, so it would be important to use the mashed potatoes regularly as an intra-workout or pre-workout carb supplement and start off with small doses, maybe 20-30g. After a week you can probably increase to 40-50g without any negative effect and probably have a positive effect. You can spread that out with a few spoonfuls every 20-30 minutes, which is probably a good idea because it would allow for virtually no extra break time and no heaviness in the stomach. I would imagine that you would only need 8 oz of water for every 20-30g of carbs from white potatoes for osmotic purposes. Obviously you'd need more than that to stay properly hydrated in a serious workout per hour, typically 1-1.5 liters per hour for an average sized person, but for ideal absorption speed you wouldn't need so much all at once. This should translate to something that is much more useful for your athletes. You can also use maltodextrin powder but that stuff is chalky and mashed potatoes with some salt will keep their (or your) electrolyte levels properly maintained and also give you blood sugar to burn so that workouts are able to more consistently stay intense and productive AND have a reduced recovery requirement.

Basically, because there are fewer particles per potato (in the stomach) and because there is virtually no fiber you should have an extremely high rate of gastric emptying. It is nearly identical to glucose in terms of providing sugar to the blood, and it is a very simple and tasty way to also get sodium into the blood to replace what is lost with sweating. Wearing a slightly loose band around the wrist while exercising is a great way to measure sodium loss... if it gets tight you are swelling because you are getting low on sodium. Peripheral swelling (edema) is one of the first signs of hyponatremia (low blood sodium) and also a sign of dehydration. They typically go together when exercise is involved, and water without sodium replacement can cause more problems than it solves. Compared to fruits, and probably all other whole foods, mashed potatoes are going to be one of the best whole food fast energy sources. You just have to make sure you are using it 5-15 minutes before you start your workout because it really does enter the blood quickly and if there isn't an increased need for energy when it is in the blood there will be a big insulin spike and then you will crash. Timing is important

Nigel, there is also evidence that a glucose + protein + electrolyte mixture enters more quickly, and that is fairly new research that does not have a ton of replication but in my personal experience it works much better than a glucose and electrolyte solution alone, OR gatorade (glucose/fructose with electrolytes). Also, if you can't be bothered to stay hydrated while running then you clearly can't be bothered to be the best runner you can be What distances do you usually run and in what period of time?

[Nic Branson]

I might need to try the potato method. My own workouts are very low rep and not very much in need of carbs with allergy season starting. Allergy induced asthma is a pita. But when I have someone push my SUV this might be a good change over the Gatorade they use now.

[Joshua Naterman]

I might need to try the potato method. My own workouts are very low rep and not very much in need of carbs with allergy season starting. Allergy induced asthma is a pita. But when I have someone push my SUV this might be a good change over the Gatorade they use now.

Maybe, I mean the gatorade is nice because if it's hot out they will definitely stay hydrated, and they use sucrose now so it's a trickle of fructose instead of a big rush. Putting 5-10g of unflavored whey in each liter would probably be the only improvement to be made, unless they just really like the idea of salty mashed potatoes

Gatorade is probably in need of slight watering down for most athletes, but overall it is a great drink when used properly. It is just more expensive than mashed potatoes, I think. Either way, a little bit of protein would be a good idea. As long as they are getting water, carbs and electrolytes the source isn't all that important. As long as it is a 6% solution or so of sugars everything should be fine. This is really more of an issue regarding hydration status than energy status. The net energy release doesn't change all that much, but the water intake does.

[Nigel Leeming]

Also, if you can't be bothered to stay hydrated while running then you clearly can't be bothered to be the best runner you can be What distances do you usually run and in what period of time?

Ouch, but I've been coming to the same conclusion. I'm mostly a rock climber and started running to get fitter/lighter. I usually run 10-12 miles, use about 1000 calories, and average around 6 miles per hour. Running for 2 hours I lose about a kilogram of weight. I only began measuring it recently, so I guess I'm sweating 1kg=1litre of water. It's been pretty cold out, so I don't know what I lose in the summer.

[Nic Branson]

I might need to try the potato method. My own workouts are very low rep and not very much in need of carbs with allergy season starting. Allergy induced asthma is a pita. But when I have someone push my SUV this might be a good change over the Gatorade they use now.

Maybe, I mean the gatorade is nice because if it's hot out they will definitely stay hydrated, and they use sucrose now so it's a trickle of fructose instead of a big rush. Putting 5-10g of unflavored whey in each liter would probably be the only improvement to be made, unless they just really like the idea of salty mashed potatoes

Gatorade is probably in need of slight watering down for most athletes, but overall it is a great drink when used properly. It is just more expensive than mashed potatoes, I think. Either way, a little bit of protein would be a good idea. As long as they are getting water, carbs and electrolytes the source isn't all that important. As long as it is a 6% solution or so of sugars everything should be fine. This is really more of an issue regarding hydration status than energy status. The net energy release doesn't change all that much, but the water intake does.

Typically 4% carb solution with some whey added in. Just think it might be a good option for those who do not care for gatorade at all.

[Joshua Naterman]

Definitely! Man, at 4% you will definitely have no delays in gastric emptying

There's just something satisfying about eating mashed potatoes as a workout supplement, you know?

[Nic Branson]

4% was also shown to be the easiest amount to digest and not cause discomfort. Potatoes are awesome any excuse to eat more is a good thing!

[Larry Roseman]

Definitely! Man, at 4% you will definitely have no delays in gastric emptying

There's just something satisfying about eating mashed potatoes as a workout supplement, you know?

Think you are on to something Jman. A potato bar sports supplement!

Seriously, my only problem with mashed potatoes are (lack of) taste and palatability. A banana as a sweet taste, disolves in the mouth and goes down easily. I'd agree that mashed potatoes may be a better choice, but can induce

gagging - especially eaten cold without gravy!

And trying to get kids to eat it instead of an apple or banana, well good luck. Maybe in a cone with some chocolate

syrup on top, maybe! Please, get to work on this! :wink: