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Carbohydrate Intake During Physical Activity (long)(6 posts)

Carbohydrate Intake During Physical Activity (long)Mr Nick
Sep 25, 2003 1:27 PM
Today I was at a presentation on carbohydrate (CHO) during physical activity and I thought all the people at RBR would appreciate the information.

The presentation looked at the most recent data concerning CHO consumption during work outs and whether it is affective and when it is necessary.

The first thing that was pointed out is that the body's main energy source is muscle glycogen. This fuel source alone will deliver addiquate energy for 2-3 hours of medium to high intensity physical activity. This is important because if someone is exercising at a lower intensity, or a shorter duration, CHO intake is not necessary. It is also good information for people trying to loose weight because it confirms that they should not be consuming any CHO during their rides.

For individuals who are performing at a medium to high intensity and are planning on riding for 2-3 hours or longer, or for people who are competing and can't risk any down time the current recommendations for CHO intake is:
10-15grams of CHO per 15minutes or 40-60grams an hour.
This is equivalant to 8oz of gatorade every 15 minutes.

In clinical testing this has shown to increase physical activity time by approximetly an hour. The way the study was done was to have trained cyclists ride at 75-80% of VO2 max until fatigue with and without the CHO supplementation.

The theory behind why CHO works during exercise is that ingesting CHO will keep blood glucose at a higher level throughout the exercise thereby decreasing the need for muscle glycogen and increasing time before muscle glycogen depletion. Also having a elevated blood glucose level throughout exercise will keep an individual more mentally alert and feeling better, because drop in blood glucose is what really makes most athletes feel really bad.

The last aspect that the presentation discussed was forms of CHO. The body only abosorbs monosaccharides (glucose, galactose, fructose) and it only absorbs glucose and galactose well. Fructose takes a much longer time to be absorbed. Therefore energy drinks or energy gels that are affective contain mostly sucrose or straight glucose. It also means that there is no need to spend a lot of money on special CHO formulas. You really just need normal sugar because it is what reacts fastest in the body.

Hope some of you find this interesting, and if you have any specific questions let me know.
Well documented perhaps 20 years ago, but some clarificationKerry Irons
Sep 25, 2003 4:16 PM
This stuff has been in books like "Food for Sport" for at least 20 years, and repeated in bike magazines off and on for years. One point to make is that while it is correct to say that fructose takes longer to digest/absorb than glucose or sucrose, it's not enough longer to actually make a difference in the real world. I don't have numbers in front of me, and there is a lot of variation by individual and by circumstance, but something that takes 10 minutes instead of 5 minutes "takes twice as long" but means nothing in terms of athletic performance. The knock of fructose is that it gives some (some, not all!) people digestive problems.
Seems like fructose absorption would matterMr Nick
Sep 25, 2003 5:58 PM
The glycemic response of foods containing fructose as opposed to simple carbohydrates or starches is much lower. This quick spike in blood glucose can be very beneficial in sports situations were it has dropped very low. I also do not have exact times for glycemic response, but having a greater increase over a shorter time seems like it would be better. It would be interesting to actually get some times.
Sucrose (table sugar) is half fructose. (nm)TFerguson
Sep 25, 2003 6:28 PM
And?Mr Nick
Sep 25, 2003 9:21 PM
I was discussing glycemic response of starches and other food CHO which are primarily long strands of glucose. Sport drinks have sucrose, but the glucose component is what primarily creates the the glycemic spike, because fructose is considered to have a low glycemic index. That is why it is recommended for diabetics.
CHO intake sounds low + a questionpowergyoza
Sep 26, 2003 4:23 PM
I've seen that 40-60g CHO/hr number for med/high perf. activity elsewhere too, but other schools of thought recommend something more in the 70-90g range.

I'd think that the max CHO that one can comfortably digest should be the recommended intake. Glycogen preservation is the name of the game for us longer distance athletes.

I do ~100g/hr using homebrew. What are your CHO intake rates people?

The human body can store about 1500-2000 cal in its muscles and liver, and if we deplete it, we bonk. It takes 48hrs to get it all back. I don't like the sound of not riding well the next 1-2 days...