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  Last updated: 3/7/2019

Building Blocks of all Foods

Carbohydrates (CHO) and fats are our main source of energy. As physical activity becomes more strenuous (over 50% VO2max), carbohydrates take on a more important role. For the average individual, carbohydrates provide between 40 and 60% of total daily Caloric needs.

Carbohydrates contain 4 Calories per gram when there is adequate oxygen at the cell level for complete oxidation. However, during anaerobic exercise (above 100% VO2max) - when the oxygen needs of the exercising muscles outstrips the ability of the cardiovascular system to provide enough for efficient metabolism, much less than 4 Calories/gram is available. In fact, only 1/19 as much ATP (the basic packet of energy in the cell) is released per gram of glycogen (or ingested carbohydrate) metabolized.

The basic unit of carbohydrates is a single molecule (monosaccharide) of 6 carbon atoms. Single molecules, linked together, form complex carbohydrates. Link two monosaccharides and you get a disaccharide; longer chains of single sugar molecules are referred to as polysaccharides. Glucose and fructose are the two most common dietary monosaccharides.

During digestion, complex carbohydrates must be broken into individual monosaccharides to be absorbed. After their absorption through the intestinal lining cells, monosaccharides are transported (via the blood) to cells throughout the body. Once taken up by a cell, they are either metabolized immediately to provide energy for the cell, stored in the form of glycogen, or converted into fat.As they can be absorbed without first needing to be digested, monosaccharides are the preferred source of quick energy. Glucose and fructose are the two most common dietary monosaccharides.

During exercise, the muscles use glycogen stored in the muscle and the liver. When glycogen is depleted, the muscle either switches to fat metabolism (which is less efficient than carbohydrate metabolism) or uses absorbed monosaccharides as an energy source.

During recovery after a bout of exercise, absorbed monosaccharides are the carbohydrate that rebuild depleted glycogen stores.

Along with their role in providing the energy that powers the muscle and other cells, carbohydrates have peripheral effects such as affecting our mood. It has been speculated that insulin released to metabolize sugar may also modify amino acid levels in the blood stream and in turn produce an increase in brain serotonin (a chemical which can produce a feeling of calm). Another peripheral effect may be a direct stimulation of endorphins.

Over the last few years, evidence has been accumulating that sugar, possibly via the same release of these neurochemicals, may induce a dependency (addiction) much akin to nicotine and opiates. And as excess sugar (not used by the exercising muscles) is converted into fat once glycogen stores have been filled, excess sugar intake from this addiction may play a major role in obesity and the development of atherosclerosis.


Dietary carbohydrates include one of two disaccharides sucrose (found in table or cane sugar, apples, bananas, oranges) or lactose (milk sugar found in dairy products), or starches, complex carbohydrates primarily supplied by grains.

Fiber, the structural skeleton of all fruits, vegetables, nuts, and beans is also a starch, but unlike flour, it is impervious to digestive enzymes and as a result passes unchanged into the colon where it is digested as a source of energy by our microbiome (the millions of resident bacteria).

Fiber comes in two forms.

Carbohydrates can be rated by their glycemic index (or GI). The GI reflects the rate at which oral carbohydrates are absorbed into the blood stream (and thus how rapidly they might be 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 have read about fructose as a superior alternative to glucose as a carbohydrate fuel for the athlete. Fructose is a monosaccharide, a 6 carbon sugar (hexose), that does not need insulin for transportation into the cell to be metabolized, instead being preferentially extracted from the blood stream by the liver. Does it have any benefit for the athlete as an energy source for the active muscle cell? Possible for or longer distance aerobic events, but not for anaerobic (>100% VO2max) sprints.


Historically, carbohydrate loading was touted as THE way to maximize liver and muscle glycogen stores for endurance events such as a marathon. The process began with avoidance of all carbohydrates for several days (to deplete total body glycogen stores) followed by several days of a high carbohydrate diet. The strategy was based on the observation of an increase in the rate of glycogen replacement in glycogen depleted individuals and it was estimated that a full 3 day program of carbohydrate loading might increase the time to exhaustion (without the use of oral supplements while exercising) by 20% (an estimate based on a bicycling specific study)

But the glycogen depletion phase was onerous and led to the next iteration - an elimination of the depletion phase but retaining the high carbohydrate diet. This modification was estimated to provide 90% of the benefits of the full depletion-repletion program while avoiding the digestive turmoil that carbohydrate depletion can produce.

Evidence that any excess dietary carbohydrates (beyond the amount that can be stored in the muscles and liver) are converted directly into fat has lessened the appeal of changing an athlete's diet to carbohydrate "load". We know there is an upper limit to how much total carbohydrate can be stored in the muscles and liver, and increasing the amount you present to the muscles/liver by changing one's diet will not increase it. Plus there is suggestive evidence that any extra fat in muscles may actually reduce the amount of sugar they can store.

Thus we arrive at the current strategy - maintaining the high carbohydrate training diet, maximizing glycogen store by cutting back training workloads for the three days before an endurance ride or event, and supplementing stored glycogen with oral carbohydrate replacement while exercising to extend the time to depletion.

And for shorter periods of exercise, remember that there is enough glycogen stored in the muscles to support 2 hours of vigorous cycling (which I'll define as cycling at greater than 70 to 80 % VO2max) without supplements.

If you plan of being on the bike for more than a couple of hours, be diligent about replacing those expended carbohydrate Calories. Any supplementation will protect your internal carbohydrate stores by minimizing the amount of stored glycogen being metabolized. A good target for energy drink/energy bar supplementation would be in the range of 1 gm CHO per kg body weight per hour.

But remember supplements will only increase the duration of exercise before you bonk (where a switch to fat metabolism alone will lead to a decrease in maximum performance) and NOT increase your maximum level of performance (VO2max).

I recently received this question:

Q. "Should I use the carbohydrate supplementation if I'm overweight by let say 20 lbs.? What will help my body to burn its own fat for energy?"

A. Being overweight means your total body fat stores are too great and has little to do with carbohydrate stores. Anyone, of any weight, who wishes to prolong exercise at 70 to 80% or more of VO2max beyond 2 hours can benefit from carbohydrate supplements. On the other hand, if your intent is just to lose weight, not improve performance, and you want to use exercise to lose weight, hold back on those extra Calories on the bike, and be prepared for the decrease in performance that accompanies carbohydrate depletion as the tradeoff of forcing your body to draw on fat reserves for Calories expended.

If you have been training hard, and are worried that you may have not been completely replacing your muscle and liver glycogen each day, what should you do? Oral supplements on the ride will help, but keeping your "glycogen tank" filled, so to speak, with a training diet containing adequate carbohydrates should be your goal.

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 the 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 dietary carbohydrate and exercise performance (I, II, and III) agree that 8 - 10 grams of carbohydrate per kg body weight per day (equal to about 600 grams of carbohydrate for a 70 kg rider) should be enough. So as long as you get your total of 600 or 700 grams during the day, the remainder of the 24 hour diet plan can be filled out with fat and protein. And as total Caloric needs increase, fat will help you maintain weight (stay in total Caloric balance) while the minimum of 600 to 700 grams of carbohydrate per 24 hour base will maximize muscle glycogen repletion.


In the 2 to 4 hours immediately post ride, oral carbohydrates are converted into muscle glycogen at 3 times the normal rate. And the earlier you start replacement the better as some data suggests a 50% fall in the repletion rate by 2 hours with a return to a normal glycogen repletion rate by 4 hours. Smart nutritional training will take advantage of this 4 hour window of opportunity to get ready for the next day - especially on a multiday ride.


One strategy to prolong the time to total glycogen depletion in bouts of prolonged exercise is to facilitate the use of alternative energy sources (for example energy bars and drinks). There has been interest in stimulating the metabolism of free fatty acids (FFA) as an alternative energy source for the working muscles. It has been speculated that caffeine's benefit in improving endurance performance, is related to an increase in the body's use of blood FFAs.


I mentioned the possibility of a 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 while the percentage covered by carbohydrate metabolism increases). And in maximum performance events, where metabolism is 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 is evidence that protein will facilitate the absorption of carbohydrates in the immediate post ride glycogen replacement window (several hours) and improve glycogen repletion in the muscle. 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.

One reason protein supplementation might be considered, in my opinion, would be to improve taste and in that way optimize supplement use (and 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 a 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 over time and the risk of chronic fatigue. Go high protein/low carbohydrate and you could be chronically bonked.


Although carbohydrates are the fuel of choice for the active muscle cell - for both aerobic and anaerobic activity - 6 carbon sugars in the form of glucose or sucrose also have a negative side. Why might that be? Our physiology developed when the carbohydrates available were in the form of polymers of numerous glucose molecules - 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 from glucose hitting the blood stream in large quantities as happens with the monosaccharides. And it may be the insulin surge that is the biggest contributor to the negative effects of a high sugar diet.

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:

Although fructose is absorbed with less of an insulin surge, fructose corn syrup fortified foods have also been targeted as a health hazard and may be part of the "simple carbohydrate" effect behind the epidemic of obesity, diabetes, and metabolic syndrome or "fatty liver" that we are seeing. This 60 minutes clip is a good summary of current thinking.

While exercising, blood glucose is taken up in muscle cells via insulin independent pathways (other than insulin) and 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.

A negative of complex carbohydrates is that, ingested in large amounts during exercise, they can move through the intestinal tract undigested and unabsorbed, and after entering the colon, are metabolized by the resident bacteria with an increase in gassiness or flatus.

Tooth decay is another 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 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.

It's important that you pay attention to how sugar affects you and your riding. Do you have a low spot in your energy a half hour after a sugar snack? If so, try these tips:


There are a few diets that have never made sense to me.

One was the paleo diet, an unbalanced, high protein diet. I suspect our ancestors were lucky when they killed that wooly mammoth and had a feast. But most of the time their diet had plenty of carbs from gathered grains, nuts, and seeds.

At the other extreme is the ketogenic diet. Ketosis is the body's response to a lack of carbs as it metabolizes fat in the diet or fat stores in the body.

This doesn't mean I am a fan of a high carbohydrate diet, and personally avoid simple sugars which lead to an insulin surge and in turn, the studies are now telling us, an increase in heart disease.

But there is a middle ground. As this study suggests, shift your diet too much toward avoiding carbs and you run an increased risk of atrial fibrillation, already a risk for the life long athlete.

And it may not be the carbs that are the problem, but how they are packaged. Carbs in fruits and vegetables are as good as simple sugars are bad. Keep that fruit basket filled at home for the times you need a snack.

So don't obsess over a few pieces of toast for breakfast, a whole grain bowl, or breakfast grain based cereal. And even simple sugars are fine to replace expended calories WHEN YOU ARE EXERCISING and they won't lead to that harmful insulin surge.

Common sense, supported by another medical study, tells us a balanced diet is better than the extremes of low carbs, high protein, or high fat. We need a minimal amount of carbohydrates for our long term health. But it's best in the form of a few extra servings of fruits and vegetables rather than a bigger serving of pasta or other complex carbs.

Additional Links: Harvard School of Public Health - Carbohydrates
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