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  Last updated: 10/21/2015

Building Blocks of all Foods

Carbohydrates (CHO) provide most of the Calories for normal daily activities, becoming even more important as a fuel source during exercise. Carbohydrates contain 4 Calories per gram, and provide between 40 and 60% of the Calories in a normal American diet.

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 occurring 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.


You may hear about fructose as a superior alternative to glucose as a carbohydrate supplement 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? For longer distance aerobic events, yes, for anaerobic events, probably not.


Carbohydrate loading traditionally involves avoiding all carbohydrates for several days to deplete muscle glycogen stores and then forcing carbohydrates for the 2 or 3 days immediately prior to an event to take advantage of a rebound increase in glycogen uptake to maximize internal carbohydrate (glycogen) stores in the muscles and liver. However the depletion phase is not essential as a high carbohydrate diet alone will provide 90% of the benefits of the full program and avoid the digestive turmoil that the changes in diet that carbohydrate depletion and loading can produce. It has been estimated that carbohydrate loading will increase the time to exhaustion (without the use of oral supplements) by 20%. This was confirmed in a bicycling specific study.

In the several days before an event, ingested carbohydrates are preferentially stored in the muscles, while those eaten in the several hours before exercise (an event) appear to go preferentially to liver stores.

When should one consider using use carbohydrate loading? First, two relevant facts to keep in mind:

Thus if you are planning to cycle for more than 2 hours, carbohydrate loading is one strategy for increasing the time you can cycle at greater than 70% VO2max before "bonking" (dropping back to the 50% VO2max level). The other is to be diligent about replacing expended carbohydrate Calories with carbohydrate drinks while riding (at a rate of 1 gm CHO per kg body weight per hour). These supplements will help to protect your internal carbohydrate stores by minimizing the amount of stored carbohydrate being metabolized per hour. The increase in glycogen stores from carbohydrate loading WILL only increase the duration of exercise to the bonk (where the switch to fat metabolism will lead to a decrease in maximum performance) and NOT increase maximum performance (VO2max).

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 must 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?? Several studies on carbohydrate loading and exercise performance (I, II, and III) show that 8 - 10 grams of carbohydrate per kg body weight per day (equates to about 600 grams of carbohydrate for a 70 kg rider) should be enough. Interestingly this is not much more than the average CHO intake of an active cyclist of 6 gms CHO/kg body weight/day. So as long as you get your total of 600 or 700 grams during the day, 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 minimum of 600 to 700 grams of carbohydrate per 24 hour base will maximize muscle glycogen repletion.

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 stores than glycogen for their muscle energy supply.

This question may help explain the concept:

Q.On the Cycling Performance Tips web site, you state that in the 4 day interval prior to the event you should increase carbohydrates to 9 grams per Kg of body weight. Does that mean you should increase total calorie intake by that much and the amount of protein and fat stay at the same level as the baseline diet? Or do the amount of fat and protein decrease so that total calories stays the same as the baseline diet? If protein and fat should decrease, by what proportion?

A.The idea is that you want to maximize your muscle carbohydrates (as glycogen) - to be provided by the carbohydrates you eat (that is the 9 grams of CHO per kg BW per day). The presence (or absence) of fat and protein does not influence the uptake of these carbohydrates into the muscle/liver glycogen stores.

Generally any fat and protein you eat in excess of your metabolic needs (that is fat and protein not metabolized immediately for energy) is stored as fat in fat cells. So what you want to do is have excess carbohydrates in your diet (beyond those needed to satisfy your daily energy or Caloric needs) to be available to be stored as glycogen in your muscles and liver. And any which is not stored as glycogen will be converted into fat and stored in fat cells. So although you can be short on CHO in your diet, and not maximize glycogen stores, you really cannot over eat them (you just gain weight).

This bicycling specific study provides some ideas as to how to go about it and does, I think,answer your question. As you can see, they supplemented their normal diet with extra CHO Calories - and proved that it provided benefit. The riders normal diet contained about 6 gm CHO/kg/day and with the supplement they moved up to an average of 10.5 gm CHO/kg/day (even more than the 9 gm I had pulled from other literature).

So, I would suggest that you estimate the CHO in your regular diet per day (making the assumption that you are in balance - that is Calories you eat = Calories you metabolize and thus your weight is stable). I would leave the protein and fat the same and add to the CHO to bring your TOTAL CHO per day to 9 (or 10 if you'd like) gm CHO/kg BW/day for the 3 days prior to the event.

This will provide a few extra Calories in your diet, but the risk of weight gain is minimal. Let's look at the numbers. Assume you are like the group referenced in the above study. You would be adding ~ 4 gm CHO/kg BW/ per day to your base line diet x 3 days. If you weighed 70 kg, that would be 4 gm CHO x 70 kg x 3 days = 840 gm extra gms of CHO x 4 Cal per gm = 3360 Calories. Which is about 1 pound. I suspect the extra CHO will suppress your appetite to some degree, you will most likely eat less fat, and thus this 1 pound is a maximum weight gain.


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 repletion 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.

CAFFEINE WILL HELP PRESERVE GLYCOGEN STORES During prolonged exercise, the onset of fatigue correlates closely with the depletion of muscle glycogen stores. The metabolism of free fatty acids (FFA) as an alternative energy source for the working muscles will decrease the use of muscle glycogen for aerobic activity. It is speculated that caffeine's effect to improve endurance performance, is related to an increase in blood FFAs as an alternative energy source.


I mentioned a possible sex difference (men versus women) in carbohydrate metabolism a few paragraphs above. What are the facts?

As you increase your exercise level (towards 100% VO2max) the proportion of the total energy expenditures covered by fat metabolism diminishes (and the percentage covered by carbohydrate metabolism increases). And in maximum performance events, where metabolism becomes anaerobic (greater than 100% VO2 max.), fat metabolism basically ceases and only carbohydrates are used by the muscle cell as an energy source.

A study from 1990 demonstrated that this shift occurs later (that is a higher percentage of fat is metabolized for fuel by the muscles for any level of activity) in women than in men. To quote: "...during moderate-intensity long-duration exercise, females demonstrate greater lipid utilization and less carbohydrate and protein metabolism than equally trained and nourished males." The same difference was confirmed to exist at a higher exercise level of 75% VO2max in a second study.

We also know that women appear to more resistant than men to carbohydrate loading - at least at a relatively moderate level of carbohydrate intake (55-60 to 75% of total energy intake for a period of 4 days). This would then put them at a performance disadvantage if they are not careful to eat appropriately in the 4 days prior to an event. A third study demonstrated increased glycogen storage IF women consciously increase their Caloric intake in these 4 days. Thus a focus on the grams of CHO/kg BW/day eaten may be a more appropriate strategy than focusing on % CHO Calories in the daily diet.

Finally, in the post exercise carbohydrate reloading phase, close attention to the number of Calories ingested resulted in a similar level of glycogen repletion in men and women.

The bottom line of these studies is that women may (my conclusion) be more adapted to long distance endurance activities (with their preferential use of fat as energy for muscle activity). And if my memory is correct, I think this has been shown to be the case in ultramarathons (100 miles). On the downside, they suggest that women need to be particularly focused on the amount of carbohydrate (in grams per kg BW) that they eat in the pre and post event phase or suffer a disadvantage to those that do so (men and women alike).


There has been suggestive evidence that protein might facilitate the absorption of carbohydrates in the immediate post ride window (several hours) and thus improve glycogen repletion in the muscles. However a study in 2001 (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.

The final word, in my mind, is a review of 26 studies, published in 2014. The conclusion: "When carbohydrate is delivered at optimal rates during or after endurance exercise, protein supplements appear to have no direct endurance performance enhancing effect. " And in addition, they expanded that conclusion to include supplements while riding as well as in the post ride recovery period: "...when carbohydrate supplementation was delivered at optimal rates during or after exercise, protein supplements provided no further ergogenic effect, regardless of the performance metric used." My conclusion is that the most important part of maximizing glycogen repletion after a ride (or minimizing glycogen depletion while on the bike) is not the protein in a supplement, but maximizing carbohydrate intake during this time.

The one reason that protein supplementation might be considered, in my opinion, would be to improve taste and thus optimize supplement use (and thus maximize Calories replaced) both during and after a ride. Especially for those riders who do not tolerate very sugary drinks.

Can you 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.


Although carbohydrates are the fuel of choice for the muscle cell for both aerobic and anaerobic activity, 6 carbon sugars in the form of glucose or sucrose have a negative side as well. Why might that be? Our physiology developed when the carbohydrates available were in the form of polymers of numerous glucose molecules - what are called complex carbohydrates. These are absorbed more slowly and are metabolized to glycogen (the body's storage form of carbohydrates) over a longer time. As a result they do not provoke an insulin surge when the glucose hits the blood stream in large quantities as happens with the 6 carbon sugars. And it may be this insulin surge that is the biggest contributor to the negative effects of our high sugar diet. Although fructose is thought to be absorbed with less of an insulin surge, fructose corn syrup fortified foods have been discussed more often in the last few years (this 60 minutes clip is a good summary of current thinking) and may be part of the "simple carbohydrate" effect that is behind the epidemic of obesity, diabetes, and metabolic syndrome or "fatty liver" that we are seeing.

When you are not exercising, 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 rapid with simple sugars than complex carbohydrates). These swings:

While exercising, and blood sugar is moved into the cells via mechanisms that include pathways other than insulin, the negative effects are most likely blunted. But during the non exercise phase of your day, eating complex carbohydrates would be preferred. Absorbed more slowly (with a lower glycemic index) this form of carbohydrate is less stimulating to the pancreas to release insulin and, as a result, minimize large blood sugar swings and their negative consequences as speculated on above. One negative is that, when ingested in large amounts, they tend to move through the intestinal tract unabsorbed and, after entering the colon, are metabolized by the resident bacteria leading with an increase in gassiness or flatus.

Tooth decay is another 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. Sipping on sugar "energy drinks" is also a risk factor - one of the reasons you might want to have 2 water bottles - one for your energy drink and another with pure water to rinse your mouth for that final swallow.


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:

Bottom line: Go for that simple sugar snack if it's a quick burst of energy you need, but for sustained energy eat a bagel or other complex carbohydrate.

Additional Links: Harvard School of Public Health - Carbohydrates
Questions on content or suggestions to improve this page are appreciated.

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