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CYCLING PERFORMANCE TIPS |
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CYCLING PERFORMANCE TIPS |
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The basic building blocks of all carbohydrates are single sugar molecules (monosaccharides or simple sugars) made up of 6 carbon units. These can be linked together as complex carbohydrates (made up of multiples of the 6 carbon units). The linking of two monosaccharides results in a disaccharide, while long chains of sugar molecules are referred to as complex carbohydrates or polysaccharides. During digestion, these complex carbohydrates are cleaved into single 6 carbon molecular units, absorbed, and transported to the cells in the blood. These sugar molecules are either metabolized immediately to provide energy for the cell or stored in liver and muscle cells as glycogen to be used for future energy needs.
Monosaccharides, the single sugar molecules, deliver energy to the body quickly as they do not need to be broken down (digested) into smaller pieces before absorption takes place. Glucose and fructose are the two most common monosaccharides in our diet.
After absorption and transport to the cell, they can be stored as glycogen, a complex carbohydrate polymer of numerous glucose molecules.
During training or competitive events, the body draws heavily from muscle glycogen for its energy supply. As glycogen reserves fall, there is an increasing dependence on absorbed glucose circulating in the blood stream. And for recovery, simple sugars (monosaccharides and disaccharides) replenish glycogen stores more quickly than complex carbohydrates.
The Caloric value of carbohydrates is dependent on the level of exertion. Almost always exercise is aerobic and there is more than enough oxygen present at the cell level for efficient metabolism to occur. However, when the level of exercise outstrips the ability of the cardiovascular system to provide adequate oxygen for efficient metabolism (one becomes anaerobic) only 1/19 as much ATP will be produced per gram of glycogen (or ingested carbohydrate) metabolized.
Besides providing energy, sugars may affect our mood. There is some evidence that eating sugar may stimulate endorphins, and insulin released to help metabolize sugar may modify the amino acid levels in the blood stream resulting in an increase in serotonin in the brain - a chemical which can make you feel calm.
DIETARY CARBOHYDRATES - simple vs complex, liquid vs solid
Most dietary carbohydrates are in the form of the two monosaccharides sucrose (found in familiar table or cane sugar, apples, bananas, oranges) and lactose (milk sugar found in dairy products), or complex carbohydrates (starches) which are primarily supplied by grains. Before they can be absorbed from the intestinal tract, all disaccharides and complex carbohydrates must first be digested and converted back to a monosaccharide or single sugar form.
For many years it was believed that a liquid carbohydrate concentration of 2.5% (glucose or glucose polymer molecules) was the maximum tolerated without slowing stomach emptying and causing nausea. However a recent study in cyclists demonstrated normal gastric emptying with a 6 - 8% solution, and nausea occuring only when concentrations were pushed above 11%. Interestingly, the old standbys, such as apple juice and cola drinks have a sugar concentration of 10% and, although the glucose polymer sports drinks can provide more Calories per quart at the same overall concentration, in controlled studies there has been no demonstrated performance advantage of these complex carbohydrates over simple sugars such as glucose alone. The major benefit of the polymers is the absence of the sweet taste and nauseating properties of high concentration isocaloric glucose drinks, minimizing this barrier to maintaining a high fluid intake.
Carbohydrates can also be rated by their glycemic index (or GI). The GI is a measure of the rate at which oral carbohydrates are absorbed into the blood stream (and thus are available as an energy source for exercising muscle). The higher the glycemic index, the more rapidly the blood sugar will respond to what is eaten. Simple (or single molecule) sugars are generally the most quickly absorbed, but some complex (multiple molecule) carbohydrates can elevate the blood sugar almost as quickly.
FRUCTOSE You will often hear about fructose as an alternative to glucose for the athlete. Fructose is a 6 carbon sugar (hexose) that does not need insulin for its transport into the cell and is preferentially extracted from the blood stream by the liver (versus the muscle cell). Does it have any benefit for the athlete as an energy source?
Burelle Y et al (Int J Sport Nutr 1997 Jun;7(2):117-27) looked at the metabolism of glucose versus fructose eaten as a preexercise meal from 180 to 90 min before exercise in 6 subjects. They found that glucose provided more available energy than fructose, and concluded that for a PREEXERCISE meal, glucose should be favored over fructose. Although it does not appear to have any advantage as a preexercise carbohydrate, what about the role of fructose as a glycogen sparing drink during exercise? Massicotte D et al (J Appl Physiol 1989 Jan;66(1):179-83) compared the oxidation of 13C-labeled glucose, fructose, and glucose polymer ingested (1.33 gm/kg) during cycle exercise (120 min, 50% max O2 uptake) in six healthy male subjects. Oxidation of the ingested glucose and glucose polymer (72% and 65 %, respectively, of the 100 gm ingested) were similar and both were SIGNIFICANTLY GREATER than oxidation of the exogenous fructose (54%). And, as expected, internal carbohydrate utilization was significantly lower with glucose (184g), glucose polymer (187g), and fructose (211g) than with the water (control, 230g) ingestion. Thus it appeared that fructose had no advantage (and perhaps even a disadvantage) to glucose as an immediate carbohydrate energy source and glycogen sparing drink when ingested DURING exercise.
This was confirmed by Gautier JF et al. (J Appl Physiol 1993 May;74(5):2146-54). They measured the metabolic fate of fructose in a carbohydrate drink and concluded that when ingested repeatedly during moderate intensity prolonged exercise, fructose is metabolically less available than glucose, despite a high rate of conversion to circulating glucose.
Although fructose ALONE has no advantages to glucose ALONE, there was a single study that suggested it was of some benefit when used in combination with fructose in a sports drink. Adopo E et al (Appl Physiol 1994 Mar;76(3):1014-9) studied the effects of a combination 50 gram fructose/50 gram glucose drink compared to a pure 100 gram glucose supplement. The cumulative amount of exogenous carbohydrate metabolized in the combined carbohydrate drink was 21% greater than that observed when 100g of pure glucose alone was ingested. They speculated that this might be related to differing routes for absorption and metabolism of exogenous glucose and fructose, resulting in less competition for oxidation when a mixture of these two hexoses is ingested than when an isocaloric amount of glucose alone was ingested. They concluded that, from a practical point of view, these data may provide experimental support for using mixtures of carbohydrates in the energy supplements for endurance athletes.
Carbohydrate loading, which traditionally involves avoiding all carbohydrates for several days, then forcing carbohydrates for the 2 or 3 days immediately prior to the event to maximize internal carbohydrate (glycogen) stores is not essential. A high carbohydrate diet alone (without the preceding carbohydrate depletion phase) will provide 90% of the benefits of the full program and avoid the digestive turmoil that the changes in diet that go with carbohydrate depletion and loading can produce.
When should one consider using use carbohydrate loading? There are two relevant facts that to remember. First is that there are enough carbohydrates stored in the muscles to support 2 hours of vigorous cycling (which I'll define as cycling at greater than 70 to 80 % VO2max). The other is that as one increases exercise intensity above 50% VO2max, there is a shift from fat metabolism towards carbohydrate metabolism to provide the Calories being expended. Thus if you are planning to cycle for more than 2 hours, carbohydrate loading is a strategy to consider for increasing the time you can cycle at greater than 70% VO2max before "bonking". (Another strategy is to eat carbohydrates regularly from the time you start the ride to supply the Calories being expended and minimize the amount of stored carbohydrate being metabolized.) But the increase in glycogen stores from carbohydrate loading WILL only increase the duration of exercise to the bonk, NOT increase maximum performance (VO2max) during that time interval. I recently received this question; "Should I use the carbo loading technique if I'm overweight by let say 20 lbs.? What will help my body to burn it's own fat to use for energy?" As being overweight is mainly an issue of total body fat stores, and has very little to do with carbohydrate stores, the answer is that anyone, of any weight, who wishes to prolong exercise at 70 to 80% or more of VO2max beyond 2 hours can benefit from carbohydrate loading. On the other hand, if the intent is just to lose weight, not improve performance, one should actually be carbohydrate depleted, forcing the body to draw on fat reserves for the Calories burned rather than the usual combination of carbohydrate and fat stores.
There has been some controversy as to what constitutes a high carbohydrate diet. It is not uncommon to see comments that as much as 60 to 70% of an athlete's total Calories need to be carbohydrate Calories to maximize performance. But as an Calories expended in training increase, it is more and more difficult to replace expended Calories with a diet of more than 50% carbohydrates. And fat, at 9 Cal/gram, is needed to avoid weight loss. So what is the answer??
Perhaps it is better to look at the total grams of carbohydrate eaten per day rather than the percentage of total diet as carbohydrates. We know that you will replace almost 100% of your muscle glycogen with 10 grams carb per kg body wt eaten over 24 hours. So as long as you get your 600 or 700 grams, the remainder of the 24 hour diet can be filled out with fat and protein. And as total Caloric needs increase, fat will help you maintain weight (stay in Caloric balance) while the 600 to 700 grams of carbohydrate per 24 hour base will prevent chronic muscle glycogen depletion.
A recent Canadian study suggested that the carbohydrate loading effect might be sex specific as a group of men increased their time to exhaustion by 45% while the comparable women's group had no change. They speculated that women may rely more on fat than glycogen for their energy source.
In the 2 to 4 hours immediately post ride, orally ingested carbohydrates will be converted into muscle glycogen at 3 times the normal rate - and the earlier the better as some data suggests a 50% fall in the repeltion rate by 2 hours and a return to a normal repletion rate by 4 hours. Smart nutritional training will take advantage of this window of opportunity.
PROTEIN AND CARBOHYDRATES
There is some evidence that protein may help the absorption of carbohydrates in the immediate post ride window (several hours) that maximizes glycogen repletion in the muscles. But the most important part is not the protein, but maximizing carbohydrate intake during this time.
A recent study (J Appl Physiol 2001 Aug;91(2):839-46) looked at glycogen resynthesis rates in eight male cyclists who performed two experimental trials separated by 1 wk. After glycogen-depleting exercise, subjects received either CHO (1.2 gram/kg/hour) or CHO+Pro (1.2 g CHO/kg/hr + 0.4 g Pro/kg/hr during a 3 hour recovery period. Muscle biopsies were obtained immediately, 1 h, and 3 h after exercise. Although there had been prior reports of increased glycogen synthesis with protein supplements when 0.8 gm CHO/kg/hr were studied, using this larger CHO intake did NOT result in increased muscle glycogen synthesis. Again, the amount of carbohydrate is the key to maximizing glycogen repletion.
Can I substitute protein for carbohydrates in my training program? The simple answer is no. Although protein is necessary in a balanced training diet, inadequate carbohydrate and Caloric intake to meet the energy requirements of your regular daily training will lead to glycogen depletion and the risk of chronic fatigue.Go high protein/low carbohydrate and you'll be chronically bonked.
NEGATIVE EFFECTS OF CARBOHYDRATES Tooth decay is a proven hazard of a high carbohydrate diet - especially when the teeth are continuously exposed to a sugar solution such as with sipping pop or chewing sugared gums. The ingestion of a diet high in simple sugars can cause wide swings in blood sugar levels as the body releases insulin to promote cell uptake and metabolism of the sugar being absorbed into the blood stream (more rapicd with simple sugars than complex carbohydrates). These swings may:
Complex carbohydrates, although absorbed more slowly (a lower glycemic index) lessening demands on the pancreas to release insulin and minimizing large blood sugar swings, can, when eaten in large amounts, move through the intestinal tract unabsorbed. Then after entering the colon, they are metabolized by the resident bacteria leading to an increase in gassiness or flatus.
THE BOTTOM LINE
Pay attention to how sugar affects you and your riding. Do you physiologically "crash" a half hour after your sugar snack? If so, try these tips:
