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Training & Nutrition Learn how to develop a training schedule that's good for you. What should you eat and drink on your ride? Learn everything you need to know about training and nutrition here.

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Old 08-06-06, 08:13 PM   #1
branman1986
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Caloric & Carbohydrate digestion/absorption

I've heard from books & others that the max absorption of carbs in the human body is like 1 gram per minute or 60 grams per hour. Also a max of about 250-300 calories per hour...

What about larger people? Will they have a higher carb & calorie absorption rate due to larger organs(intestines)? I guess I don't mean larger as in more overweight people, but taller people who should have larger organs...

I read to eat about half a carb bar while on the bike per half hour(or a pack of gel), which is approx 20-25 grams of carbohydrate. I'm 6'6", 205lbs - Do I need more? Should I be eating a half bar every 20 minutes? I've signed up for a bunch of centuries and I'm worried about maintaining proper energy levels...

Thanks in advance!
Brandon
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Old 08-06-06, 10:02 PM   #2
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everyone is going to be different, not just on height / weight, but also metabolism. i can ride for about 2 hours before i start needing carbs, after which i crash very quickly if i don't get them. i think half a powerbar per half an hour sounds like enough, maybe even more than enough (at least for a 155 pounder like myself). i would suggest going for a ~60 mile ride around a week before one of your centuries and seeing if half a bar per half hour is adequate for you. better than finding out the hard way if it isn't the day of your century.

i'll bet with your weight, it should be just about right, as long as you make sure to not drink straight water, but something like gatorade or propel with a little bit of sugar and electrolyte crap in it. i know powerbars are around 230 calories, which may not be enough for an hour of riding, so it would be good to get some from fluids as well. in my opinion, better more calories from sugar in drinks and less from food since digesting tends to sap power and make me bloaty and sick on the climbs!

this is all advice given from my experience, i'm not a nutritionist or anything... to summarize my rambling, make sure you know your requirements before the day of your first upcoming century ride!
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Old 08-07-06, 10:08 PM   #3
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Quote:
Originally Posted by branman1986
I've heard from books & others that the max absorption of carbs in the human body is like 1 gram per minute or 60 grams per hour. Also a max of about 250-300 calories per hour...

What about larger people? Will they have a higher carb & calorie absorption rate due to larger organs(intestines)? I guess I don't mean larger as in more overweight people, but taller people who should have larger organs...

I read to eat about half a carb bar while on the bike per half hour(or a pack of gel), which is approx 20-25 grams of carbohydrate. I'm 6'6", 205lbs - Do I need more? Should I be eating a half bar every 20 minutes? I've signed up for a bunch of centuries and I'm worried about maintaining proper energy levels...

Thanks in advance!
Brandon
I think you're roughly in the right ballpark.

The real questions are:

1) What digests well for you?
2) What do you like enough to continue eating?

For me, I've had much better results getting the bulk of my calories from liquid (accelerade in my case), but it's a very personal thing.

You will also likely get "food fatigue" after 3 hours or so - the food you start with will be unpalatable - so you'll likely want to bring some variety.

My experience is that most people eat too much rather than too little.
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Old 08-08-06, 09:12 AM   #4
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Body weight does not necessarily correlate with organ size within the same species, particularly intestine length. I know, I have done the studies myself. The liver however does correlate to body weight.

You really need to experiment with your own body. If you can eat 400 calories an hour with no discomfort then that is what works. These rules of thumb are good as a starting point, but there are always deviations from the so called norm.

I have also found, that practice helps. Yes, you need to practice eating. You can't go from riding on an empty stomach to doing a century with food every hour and think you won't get sick.
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Old 08-14-06, 11:46 AM   #5
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Caloric requirements are determined by lean body mass- muscle primarily- and this is determined by calculation using your height (Ideal vs actual body weight). Variance in intestinal absorption is negligible along the range of body sizes in the average human. Your small intestine is about 21 feet long- 3-3.5 times your height- this is plenty of absorptive surface for the carbohydrate and elctrolyte loads you are referring to.

The previous comment about the osmotic load from a hugh concentration is well put. In addition to slowing gastric emptying, it could also make you feel pretty lousy.....
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Old 08-15-06, 09:02 AM   #6
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Bicycling magazine has a good article on carbs and oxidation rates here.

A breakdown of the popular bars and gels here.
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Old 08-23-06, 10:32 AM   #7
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I race mainly longer MTB events; 100 milers, 12 and 24 hour solo races, etc. I use an almost 100% liquid diet and I mix multi-hour bottles that work out to about 240-250 calories per hour. In warmer temperatures I will use less. For instance, at a 12 hour event last month the temperature was mostly in the 90's during daylight and I only consumed on average 100 calories per hour.
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Old 08-23-06, 11:43 AM   #8
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The reason we eat while riding is to keep from depleting our stores of muscle and liver glycogen. Running out is not good, it's called "hitting the wall" or "bonking".

Some numbers to ponder:

A. Everyone has 1500-2000 Calories of stored glycogen (training increases glycogen storage).
B. We use 35-45 Calories per mile, a combination of fat and carbs.
C. On a long ride of several hours, energy comes from about half fat and half carbs.

Using these facts, it's easy to predict how much food you may need for a long ride. Let's say you at nothing, how far could you go (I'll use the worst case numbers, untrained person working hard):

Bonk Distance = (1500 stored Calories) / (0.5 * 45 Calories/mile) = 66 miles

To make it to 100 miles, we'll need to consume:

(100 - 66 miles) * (0.5 * 45 Calories/mile) = 765 Calories of carbs

As you can see, you don't need that much food to complete a century. The common advice of 250-300 Calories per hour is a maximum that you can digest, not how much you need to complete a century.
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Old 08-23-06, 04:46 PM   #9
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Sorry to beat this to a pulp...if gastric emptying is the throttle on calorie digestion, and I burn more calories per hour just by being bigger...do I at least start out with a larger bank of stored calories?
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Old 08-23-06, 06:29 PM   #10
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Quote:
Originally Posted by branman1986
Sorry to beat this to a pulp...if gastric emptying is the throttle on calorie digestion, and I burn more calories per hour just by being bigger...do I at least start out with a larger bank of stored calories?

I believe a bigger guy will have more muscle to store more glycogen, but the greater the muscle mass, the higher the burn rate, so it still comes out to about 2 hours worth of muscle and liver glycogen, maybe a little longer if you are a "highly trained" endurance athlete.
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Old 08-24-06, 08:38 PM   #11
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I've heard from books & others that the max absorption of carbs in the human body is like 1 gram per minute or 60 grams per hour. Also a max of about 250-300 calories per hour...
Don't be confused. Yes there are limits to one's ability to utilize food stuffs while exercising.

Understand this, your ability to "uptake" the caloric value of food stuffs while exercising is inversely proportional to the intensity of your exercise. In other words, the harder you exercise, the less capacity you have to transfer energy-substrates from your gut to your muscles.

You are on the right track Grasshopper, just don't get hung up on numbers.
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