There are 3 main fuels used during exercise by the contracting muscle: 1) Protein; 2) Carbohydrate; 3) Fat.
A majority of text books do not acknowledge protein as a major fuel, and it likely is not. It should be pointed out that protein requirements of someone who is exercising on a regular basis are GREATER than those of a sedentary population. Is this something to worry about? Most "North American" diets contain more protein than is needed. So the bottom line is you get more than you need. Vegetarians? Again the answer is likely "yes", they also get enough protein. Even when consuming a pure protein diet there is enough protein to cover the needs of a person who regularly exercises.
Fats and carbohydrates (CHOs) are then the major fuel sources for the exercising person. The balance of the use of these fuels is dependant upon exercise intensity and duration (the two are inversly related). The general rule is that the lower the intensity the greater the energy cost of exercise can be covered by fat. Hence, the greater the exercise duration the more fat will be burned, usually because the intensity of one's workout will decrease - fatigue! The flip side then, is that during higher intensity exercise (>70% of max), one relies heavily on CHOs.
Fats are stored as adipose, body fat, and muscle fat (triglycerides). CHOs are stored as muscle and liver glycogen (long chains of glucose) and blood glucose. During a workout the early phases are characterized by a reliance on CHOs, both muscle glycogen and blood glucose. The blood glucose comes from the breakdown of liver glycogen. Again this is dependent upon intensity (see above). However, the muscle can also use fat as a fuel, the sources of this are from the inside of the muscle or from the outside - i.e., from adipose tissue. The problem is that levels of fats from adipose take a while to reach high enough levels for their use to become significant. Their concentration in the blood only reaches very high levels when the intensity of the exercise is low (i.e. 50% of max or less) and if the duration is sufficient (1 hour or more). However, when the concentration of fats from outside of the muscle is high enough, the muscle can use these instead of glycogen and delay the use of glycogen, this is critical at times since muscle glycogen is a "rate-limiting" fuel for muscle. That is when muscle glycogen runs out, or gets very low, then you feel terrible - you've bonked (see below).
BONKING/HITTING THE WALL:
Lots of people talk about the phenomenon of bonking. It hits some people harder than others, I don't know why and have never seen any good information why? However, bonking is a combination of two processes. The first is a lack of muscle glycogen (see above). The second is low blood glucose. When muscle glycogen is low the muscle runs into a fuel crisis. It cannot burn fats at a rate high enough to sustain the muscle's maximal output. The consequence is that your muscle switches to burning more fats and so you have to slow down. The crappy feeling that you experience at the same time, often characterized by nausea and disorientation, is likely a consequence of low blood sugar/glucose (hypoglycemia). The trick then is to alleviate/delay the onset of these symptoms by consuming sugar solutions, or simply by becoming so well trained that you don't have to worry (see TRAINING below). Why is low blood sugar bad? Because your brain, eye tissue, and others are able to burn only glucose. That is when the levels of glucose are low your brain runs out of fuel, so you feel awful. Your vision might become impaired also.
FATS vs. CHOs:
However, as I've said above your muscle can burn fats and if given the chance your muscle will burn whatever fuel it has in the greatest abundance, even lactate! So, if supplied with enough fat muscle can burn fat and hence, "spare" muscle glycogen. This is the idea behind many runners drinking caffeine/coffee before a race. The caffeine has effects that cause release of fats from adipose tissue and the level of fats in the blood increases. The end result is that for the early phases of the race the runner's muscle's can use fat and delay the use of muscle glycogen, hence, sparing that glycogen for later use. One should be cautioned, however, that this mechanism for increasing fat usage has only been shown with some very high doses of caffeine that are not achievable without taking caffeine pills. It also critically dependent upon the person's habitual caffeine intake ("big" coffee drinker appear not to derive as great of a benefit as non-habitual users). There are other ways to maximize the use of muscle glycogen, however.
CHO loading is a practice that many athletes use before a longer duration event to "supercompensate" their muscles with glycogen, delay it's running out (see above). The practice is of little use when the duration of the event is less than 60 minutes, since muscle glycogen will usually be able to meet the demands of such a duration. However, it should be noted that repeated bouts of high intensity exercise will also deplete one's muscles of glycogen.
There are two basic protocols for CHO loading, one is just as good as the other. However, they involve an initial bout of exercise to deplete the muscle's glycogen (under normal dietary conditions), followed by a period of high CHO diet (i.e. 70% or more of one's total calories from CHO). This period should be the 4-5 days prior to the event and should be a time when the athlete tapers their training, so as not to deplete muscle glycogen too much. The result is an overload of glycogen in one's muscles. Two notes: 1) This procedure will result, if done correctly, in most people gaining 2-5 pounds. Why? Because muscle and liver glycogen is stored with water and increasing glycogen will increase water content (i.e. increased weight is water); 2) Preliminary evidence indicates that this procedure is less effective in women. That is to say that if a female runner were to increase her CHOs to 70% (or >) of her caloric intake she may not have an increase in muscle glycogen. Why? It may relate to a gender difference in the ability to store muscle glycogen or in the amount of CHOs that 70% of the female athlete's diet represents (i.e., 70% of a 2000 calorie diet would be 1400 Cal from CHO, eating this may not be enough to increase muscle glycogen content).
When one trains or conditions by completing endurance exercise changes occur at many levels, including the muscle. The changes that occur at the level of the muscle include an increased ability to utilize fats. Not surprisingly then one's endurance is increased. How? An increased utilization of fats means less reliance on glycogen, less reliance on glycogen means you don't run out of the fuel that allows you to maintain a high rate of muscle contraction, and hence a high rate of running/exercising. Another adaptation that occurs is that your muscle uses less glucose, this is important for tissues such as brain (see above).
One of the symptoms of low blood sodium is shivering at temperatures which would normally be comfortable. This can come on very rapidly late in a race when one drinks only sodas. Sodas generally have almost no sodium in them.
The sodium/potassium ratio for blood plasma is about 34, and for sweat is about 10. The ratio for Succeed! Caps is 344/21=16.38.
The question about cutting back on caffeine is a complex one, having to do with adenosine receptors on muscle cells. Those receptors will also take up caffeine molecules because they are shaped similar to adenosine. With regular exposure to caffeine, the body will grow more receptors. So, if you want to use caffeine to stay awake at night during the race, you'll need to take it in excess of your daily dose. If you want a cup or two of coffee to do the trick, you should cut way back on your caffeine consumption for a number of weeks before the race. When the caffeine intake is reduced, the number of adenosine receptors will decrease.
In addition to some caffeine at night (be it pure and simple coffee, derivative products, or guarana capsules), most racers also use a sportsdrink with branched chain amino acids in it. BCAAs compete with the amino acid tryptophan for transfer across the blood-brain barrier. Reducing the amount of tryptophan entering the brain reduces the amount of serotonin generated, so alertness is maintained, avoiding typical "zombie" effect after 2 am.
The body will readily burn BCAAs for energy if other sources of energy are low, so without taking in additional BCAAs, the tryptophan in the blood stream has an easy path into the brain, providing substrate for serotonin production. That will make you very relaxed, sleepy and fill you with the desire to stop this nonsense and just sleep. The only danger with this chemical trick is that an hour after finishing and stopping the intake of BCAAs, there was a rebound effect and you could hardly keep your eyes open.
Dehydration and the use of electrolytes in helping prevent cramping and heat related problems - most athlete use one of the following products: Succeed! Caps, Endurolytes, or Emergen-C.
EMERGEN-C is made by Alacer and comes in .18oz packets, 36 to a box. It is an effervescent product which is mixed in a small amount of water and ingested.
Serving size: 5 grams (1 packet)
Vitamin C: 1000 mg
Special Niacin Mineral Complexes: 2 mg
Folic Acid: 25 mcg
Vitamin B12: 25 mcg
Magnesium: 60 mg
Manganese: 2 mg
Potassium: 200 mg
Sodium: 55 mg
Zinc: 2 mg
Chromium: 10 mcg
Serving size: three capsules
Calcium: 150 mg
Potassium: 75 mg
Chloride (as Sodium Chloride): 300 mg
Manganese: 5 mg
Vitamin B6: 20 mg
L-Tyrosine: 50 mg
Serving size: 1 capsule
contents to be input...
Succeed! Caps and Endurolytes are easier to use (no mixing them with water, although it can be done). Endurolytes has the ideal ratio of 2:1 Calcium/Magnesium. Calcium is absent from the Emergen-C formula. (it's important that both of these minerals be present in adequate amounts in the body, since a constant blood level calcium level is required for a normal rhythmic heartbeat, healthy nerve transmission, and strong muscle contractions.) Magnesium needs to be present as well because once the muscle contracts and calcium leaves, magnesium replaces it, then the muscle relaxes.
Potassium and Sodium are present in both formulas although the 100 mg sodium per capsule versus the 55 mg per packet of Emergen-C. Manganese is also present in both formulas which is helpful not only as an electrolyte but also as an antioxidant. According to James Balch, M.D., a deficiency of manganese can lead to excess perspiration (among other maladies). While we want to perspire, any excess can contribute to premature electrolyte depletion.
Although both zinc and chromium are present in Emergen-C they are not really electrolyte minerals. More important is a balanced mix of calcium, magnesium, potassium, sodium, and manganese. The main purpose of Emergen-C is the actual Vitamin C in it with the minerals being naturally occurring as part of the mineral ascorbate complex.
Two additional nutrients available in Endurolytes are Vitamin B6 and the amino acid l-tyrosine. Among the many things B6 does, is maintaining proper sodium/potassium balance. Tyrosine aids in the performance of both the thyroid and adrenal glands. It is also involved in maintaining the rate of metabolism and is known as a mood enhancer.
Energy Sources and Body Temperature
Research indicates that an increase in body temperature signals the body to switch from burning fat to glycogen-stored carbohydrates. Because more fat is consumed when the muscle is cooler, exercising at a moderate intensity level in a cool environment will help you become a fat-burning machine. Mechanisms of carbohydrate and fat utilization are not completely clear. Heat stress does play a prominent role. Glycogen is burned most efficiently. The body tends to respond to heat stress by relying on its glycogen stores more than its fat stores. One study examined the effects of elevated muscle temperature on muscle metabolism during exercise. They observed that during exercise, the magnitude of ATP degradation and inosine 5'-monophosphate and ammonia accumulation was higher in the heat as compared with cooler temperatures. Further research indicates that glycogenolysis (glycogen fuel burned for energy) in contracting skeletal muscle is reduced during exercise when the rise in body core temperature is attenuated. These changes in carbohydrate metabolism appear to be influenced by alterations in muscle temperature and/or sympatho-adrenal activity.
 Influence of elevated muscle temperature on metabolism during intense, dynamic exercise. Febbraio, M. A., Carey, M. F., Snow, R. J., Stathis, C. G., Hargreaves, M. American Journal Physiology 1996 Nov;271(5 Pt 2):R1251-1255.
 Blunting the rise in body temperature reduces muscle glycogenolysis during exercise in humans. Febbraio, M. A., Snow, R. J., Stathis, C. G., Hargreaves, M., Carey, M. F., Experimental Physiology 1996 Jul;81(4):685-693.
This research emphasizes how much more efficient muscle energy is produced, glycogen spared and fat burned when an athlete is cool during hyperthermic circumstances. How? Coasting on the bike, or easing up on a hammer-pace, walk breaks, sponge baths, and keeping the body wet for evaporative cooling, including reasonable hydration of fluids and electrolytes, and wearing loose breathable clothing are well-advised protocols for keeping the body cool in hot or humid conditions. The longer the event, the better performance for the "chilled" competitor.
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