What I'm going to share that I know about carbs really is applicable to endurance events. When I speak about those, I'm talking about events that last longer than 2 or 3 hours.
First, you should only eat solid food no less than 3 hours before the event begins.
I've read this in so many sources that I can't even count, including peer-reviewed journals, elite professionals, and sports publications. After your pre-race meal, all nutrition should be in liquid form. There are several reasons which I'll get to as I blather on about this.
Some researchers believe you should probably not have ANY carbs within an hour of the race. There's a lot of information to support the idea that carbs taken within and hour or so right before a heavy event can have negative effects on performance, including a rapid rise in blood sugar, which releases too much insulin, process the sugar super fast, and can cause a sugar crash. It can also interfere with how your body burns its fat for fuel. I used to grab a gel right before hitting the water. I wonder if this is why on occasion, my swim sucked so much. This might be something you have to experiment with.
How much you need.
When we're at rest, we usually have enough carbs stored and available to fuel up to 3 hrs of exercise. All this is stored in the blood, muscle, and liver. When you eat your pre race meal, which again should be at least 3 hours prior to the race, it should have about 75-100 grams of carbohydrates. After this, endurance athletes need 40 to 75 grams of carbohydrate per hour, depending on who you ask, (one estimate I found was 1.2 – 1.5 grams of protein per kilograms of weight; divide your weight by 2.2 and then multiply by 1.3 to get a good estimate).
How much you need also depends on how hard you're working, your body size, level of fitness, et cetera, and it also depends on what you're doing. That last part is really important when planning for a long course triathlon. For instance, cyclists can drink lots of liquids, while runners might not be able to handle that much fluid.
If your event lasts longer than 2 hours your carbs should be be in liquid and semi-solid forms, especially for running. Some racers can tolerate solids when cycling, but if you do, you should stop eating solids three hours before beginning the run.
What form carbs need to be in.
For the best, fastest and most effient use of nutrients, with less chance of stomach distress, liquids are best. Getting your carbs in liquid form is best for absorption, rehydration and it means you get a steady flow of sugars than you would in solid form. It's also the easiest and most convenient way to get a calorie and nutrient dense fuel.
Solid food can't beat liquid food supplements. When you eat too much solid food, your body has to re-route the energy and blood that your hard-working muscles need to digest the food. This taxes your body and can result in a feeling of bloating and/or nausea.
What to eat.
When carbs are absorbed, your body converts some of it to stored fuel for your muscles (glycogen) and some for immediate use for your muscles (glucose). Most of the sugars we eat are simple sugars, and they're either short-chain or long-chain molecules.
Short chained molecules are "simple" sugars. They include sucrose (table sugar and corn syrup), fructose (fruit sugars and honey), glucose, and/or dextrose. You'd think they would be best, since "simple" sounds like something that would be easier for our bodies to break down, wouldn't you?
But, no, it doesn't work that way. The shorter the chain length a carbohydrate source the harder it is for your body to absorb it. This is because it have the right chemistry to match up with your stomach has in it. So, when you eat these simple sugars, even if they're in the right concentration to give you the calories you need each hour, they usually sit in your stomach undigested, causing stomach upset.
In order to be absorbed better, they have to be diluted a whole bunch by water, BUT, if they're dilluted, then they don't give you enough carbs for the fuel that you need.
So, you look to longer chained carbs, called "complex sugars" are absorbed better. They include starchs and other sugars, such as the famed "maltodextrin." These are so well absorbed that you can even make a super high concentration of carbs in a fluid and still be able to absorb it well. Now, you might have a problem with maltodextrin if you're really sensitive to corn products, because although it can be made from any starch, most of it in the US is made from corn.
To complicate matters, signs and symptoms of hyponutremia, electrolyte imbalance, and carb problems can all include nausea, dizziness, and vomiting, so it's really important to experiment in training and under similar conditions to what you'll experience on the course.
So, in summary: liquid fuel that includes long-chain carbs, or sports gels, making sure you can drink enough to stay hydrated and get the amount of carbs you need per hour. (don't forget electrolytes, either)
To summarize this in a personalized fashion (including what I've learned about hydration and electrolyte in my previous posts), here's the worst mistakes I made this year, using my two half irons as an example:
When I did the Olahoma Redman, I drank a few bottles of electrolyte fluid on the bike, but not nearly enough to keep me hydrated. Certainly not enough to overcome the fluid loss on the swim, nor the fact that it was really windy out.
On the bike, I ate some shot blocks, but probably nowhere near what I needed to continue to fuel my carb needs.
When I got to the run, I'd been racing for nearly 6 hours. On the run, I ate two gels. That's it. Now at this point I was not only really dehydrated, I was running out of fuel. When your fuel load gets really low, one of the things that can happen, according to the Lore of Running, is that your brain tries to protect itself (brains use a lot of fuel) by making you want to quit. I basically stumbled drukenly to the finish line, with a body temp of 95.6 and was in the med tent with an IV in my arm pretty soon after.
At Soma, I did things differently. I did have a gel before I started, along with a bunch of water. On the bike, I drank several bottles of hydration fluid (Perpetuum) made of electrolytes and maltodextrin. I drank it continuously. When I ran out, I drank bottles of water, and for every bottle of water I took a gel and an electrolyte cap.
On the run, I sipped gel and about a cup of water every mile, and took some electrolytes ever hour or so. If my stomach started feeling sloshy, I lowered my intake of water just a bit, and when I started to feel nauseated (my body likes to get nauseated when it has too much sugar in it) I skipped one of my sips of gel.
I finished Soma a little over an hour earlier and in much better shape than I did the Redman.
So that's what I know about carbs. Again, I want to stress the importance of testing these principals in training more than once, using similiar conditions as you expect on race day.
good luck, and have a great race!
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