Originally Posted by
gregf83
I didn't see anything in those articles about utilization of protein during aerobic exercise. The primary sources of energy are carbohydrates and fat.
Yes, you are right. While Carmichael says,
It’s important to realize that you burn carbohydrates, protein, and fat simultaneously whenever you exercise, regardless of the intensity
that article doesn't back that up with data.
But this one does:
http://www.hammernutrition.com/knowl...wledge-section
Researchers measured the protein-calorie cost of exercise to be between 6% and 15% of the total energy expense each hour. Proteins metabolized from lean muscle stores are rate-limited (regulated) by the release of specific enzymes. For example, Leucine, a branch chain amino acid (BCAA), oxididation rate is controlled by the enzyme, Branched-Chain Oxo-Acid-Dehydrogenase (BCOAD). This enzyme is relatively low at rest (4-7%), but during exercise, dephosphorylation releases it to as high as +25%. After exercise, both BCOAD and Leucine oxidation are attenuated, though total BCOAD is up-regulated.[2] Some research utilize higher amino acid catabolism, (+15%) during endurance exercise.[3] It is conceded that as pace increases from slow aerobic to faster anaerobic, the % of amino acids burned also increase. However, for this article, the slower pace conservative 6% rate will be applied.[4]
It is true that many cyclists lose muscle mass during long, intense rides. The above link explains the mechanism. You are also right that carbohydrates and fat are the primary energy sources, however that does not mean that protein is
not burned. We all go calorie-negative during anything above an easy ride. During those rides, protein is burned. That said, my experience is that it is not necessary to lose muscle mass if one doesn't want to.
The other thing that's going on is that many cyclists have discovered that the lighter they are, the faster they climb, and not just lighter because of low body fat, but lighter in terms of less muscle mass. As some of us here are fond of pointing out, cycling does not require a lot of muscle mass. Lance had to lose 7 kilos of protein to be competitive during his come-back TdF.