So let's review the physiology involved and clarify what you can do to improve your performance.

I. THE SOURCE OF MUSCLE ENERGY (what to eat)

Percent VO2max (%VO2max) is a common measure of exertion to compare your effort across all types of riding - from a fast ride on the flats to a steep hill climb. Although both triglycerides (fat) and glycogen (carbohydrates) will provide ATP for the exercising muscle, glycogen is the preferred energy source as levels of exertion (%VO2max) increase.

What keeps the muscle cell from using more fat for high level exercise? Bottlenecks, or rate limiting steps, in the intracellular metabolism of fat into ATP.

An individuals maximum rate of fat metabolism is an inherited characteristic (hardwired) into their cellular chemistry. But enzyme activity can be increased, and the upper limit of fat metabolism increased to a limited degree, by training on a high fat/low glucose diet.

The maximum rate at which our cells can produce ATP from fat (to provide energy for exercise) is about one-third the rate of ATP formation from glucose. The net effect is that fat alone as an exercise energy source sets an upper limit for maximal performance at ~ 65% VO2max.

With training you can modify the ratio of fat ATP to glycogen ATP used at any specified endurance pace (and extend your riding time before you bonk) BUT you cannot increase the maximum VO2 (%VO2max) that can be achieved on fat alone (about 65% VO2max).

Thus with endurance training athletes are better able to replace glycogen Calories with fat Calories, allowing them to exercise longer.

For those interested, the science of fat metabolism is covered in more detail in this paper, and the relationship of glycogen to fat calories as VO2max increases is nicely summarized in this graph. You will notice that the total calories from fat metabolism - stored muscle triglycerides and free fatty acids (FFA) extracted from the blood - remains relatively constant as exercise measured by VO2max increases. And extra caloric needs are met almost entirely from muscle glycogen or blood glucose.

II. WHEN TO EAT (rule of 4's)

Four days prior to the ride. Maintain a diet that replaces training Calories. As this is a short period and worries about the impact on your weight or fat stores are really insignificant, 9 gm carbohydrate/kg BW/day (or approximately 600 grams/day) should err on the high side.

Four hours prior to the event. This will be topping off the tank, so to speak. It is long enough before the ride that the carbohydrate Calories will have been digested, absorbed, and metabolized. How much? A 300 gm (1000 Calorie) complex carbohydrate meal will avoid rapid absorption with an insulin spike. Rice and pancakes are two easy choices. A high Caloric density glucose polymer sports drink is another option.

Four minutes before the ride. You are just getting on the bike and want to have some Calories working their way through the digestive system and on there way to being absorbed. You are not worried about simple sugars leading to an insulin spike. A candy bar works well.

During the ride. Here the focus should be on regular carbohydrate replacement starting immediately as you leave the start. 60 gram of carbohydrate per hour, liquid preferred (i.e. sports drink), 10% concentration optimal (equivalent to a cola drink), and taking something every 10 - 15 minutes.

Four hours post ride. Take advantage of the insulin independent restocking of muscle glycogen. Start immediately, high carbohydrate, for 3 to 4 hours.

III. TRAINING INCREASES STORED GLYCOGEN

Less well known is that training also increases total body glycogen stores. These 2 training benefits then work together to improve your endurance riding. More "fuel" (glycogen) as well as a more efficient use of these precious carbohydrate calories.

The facts:

• Your body has limited glycogen stores but an almost unlimited amount of fat. And for high VO2 activity, it is glycogen that limits the duration.
• 20% of our glycogen is stored in the liver and 80% in the muscles - where it is readily accessible by the exercising muscle.
• Endurance training increases the amount of stored glycogen. The data:
  • Subjects were compared to themselves. Muscle glycogen content was analyzed both before and then after 10 weeks of endurance training.
  • Post ride, all study participants maintained a high carbohydrate diet (10 g CHO/kg body wt/day) with a diet composition of 80% CHO, 7% fat, and 13% protein.
  • After endurance training, the participants restored their depleted glycogen reserves at a rate twice that of pre-training.
  • After training, total glycogen stores at 48 hours maxed out, on average, at 170 mmol/kg muscle compared to 95 mmol/kg pre training.

It appears that the upper limits of human carbohydrate storage are in the range or 15 kg glycogen per kg body weight or approximately 1000 grams. This is almost twice the average 500 grams in those not training, and at 4 calories per gram equate to almost 5000 potential calories.

IV. REPLACING GLYCOGEN STORES - CAN YOU OVERDO IT?

First let's review a few facts on dietary carbohydrates.

• The old teaching was "fat is bad and carbohydrates are good" to cut the risk of heart disease. We now know that high carbohydrate diets can actually increase the risks for obesity, high blood pressure, heart disease, and even cancer.
• All dietary carbohydrates are built up from single sugar molecules. Simple carbohydrates (table sugar for example) is composed of just two single sugar molecules (one glucose and one fructose). Complex carbohydrates are made from longer chains of multiple sugar molecules.
• To be absorbed (from the small intestine), all carbohydrates must first be broken down (digested) into single sugar molecules. Some complex carbohydrates are easily digested (wheat flour for instance) while others are not (fruit and vegetable fiber). A food's glycemic index measures the ease of digestion and absorption.
• Simple sugars (glucose, table sugar) are easily absorbed into the bloodstream and are often absorbed faster than they can be used in normal metabolism. This excess is shunted into fat (triglycerides) production to be stored for future needs. It is this sugar to fat conversion that is the likely culprit in the health risks of high carbohydrate diets.
• Complex carbohydrates or starches are more slowly broken down by digestive enzymes and in turn are more slowly absorbed. The rate of rise of the blood sugar after eating starches is less (their glycemic index is lower) and is spread over a longer period of time. The post eating blood sugar spike is less severe, less insulin is released, sugar calories are absorbed over a longer period of time, and the body now has more time to use them to fuel normal metabolism instead of fat production.

Your goal should be to eat just enough extra carbs (beyond your normal diet) to replace the calories expended on a long ride or on a training day.

How many calories do you expend on a ride? There are a number of formulas floating around the internet (here is one at CPTIPS).

• 5 mph = 7 Cal/mile or 37 Cal/hr
• 10 mph = 13 Cal/mile or 133 Cal/hr
• 15 mph = 23 Cal/mile or 349 Cal/hr
• 20 mph = 37 Cal/mile or 742 Cal/hr

Not all the activity calories come from glycogen. At anything less than 100% VO2max some are from fat metabolism as well.

If 1 gram of carbohydrates yields 4 calories of energy, and we trim that 30 calories per mile to 28, a reasonable upper limit for replacement is 7 grams of carbohydrate per mile ridden. So log your mileage, multiply by 7, and you have a ballpark figure as a goal for your carbohydrate replacement after a ride - remember to subtract calories from snacking and gels taken while riding.

An easy way to check if you are in replacement balance is weighing yourself every morning. Assuming you have been staying hydrated a stable weight will show you are on track.

Complex carbohydrates (starches and whole grains) are preferable to sugary drinks and gels (with the exception of during active exercise and in the immediate post ride period when the muscles are still actively clearing glucose from the blood).

Is there a better time to eat these carbs? The rate of replacement of glycogen stores are is highest in the hour or two after exercising, so a post exercise soda or high starch snack makes sense. Otherwise a bigger meal in the few hours after riding is better than early in the day. That gives the body time to move these carbohydrates into muscle storage rather having them sitting in your stomach as you set out for the day's ride.

V. HOW ABOUT CARBOHYDRATE LOADING?

It was estimated that the full 3 day program of carbohydrate loading might increase the time to exhaustion (without the use of oral supplements while exercising) by 20%.

But the glycogen depletion phase was onerous and led to the next iteration, carboloading 2.0 with an elimination of the depletion phase while 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.

Now we have evidence that suggests an adequate carbohydrate diet while endurance training maximizes glycogen reserves with the storage effect in the same range (1000 grams) as that obtained by a more traditional glycogen depletion-glycogen loading regimen. Add in the fact that any excess dietary carbohydrates (beyond those stored in the muscles and liver) are converted directly into fat, and carbo loading loses its appeal. There are no clear advantages versus the hassle of changing your diet before an event.

Thus we arrive at the strategy currently in favor - maintaining the high carbohydrate training diet, maximizing glycogen stores 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.

VI. USE GLYCOGEN SPARING SUPPLEMENTS WHILE RIDING

As a rule of thumb, the higher the level of intensity of the ride (closer to your VO2max), the simpler the carbohydrates (energy drinks, gels, and fruits) you should be using. On longer rides and at lower heart rates, more complex snacks with complex carbohydrates and a higher fat content offer other alternatives.

You should begin your oral supplements as you start the ride - don't wait until you are beginning to bonk (thinking about food is a great clue) - and eat something every 15 - 20 minutes.

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).

Here are some on the bike snack ideas.

SUMMARY POINTS

• Glucose is the preferred energy source for moderate level, endurance activity.
• Glucose is the only energy source for activities at and above 100% VO2max
• If you want to preserve internal glycogen stores (limited) for an endurance ride by maximizing the use of fat reserve calories (large) you can either
  • ride slower OR
  • supplement glycogen stores with snacks while riding.
• Use the rule of 4's to time your carbohydrate intake for an endurance ride.
• Training increases the potential to store muscle glycogen, the preferred energy source for high level aerobic activity.
  • You can maximize this potential by eating a diet high in carbohydrate calories while training.
• The endurance training benefits (riding longer at a given level of exertion or %VO2max) are derived from the combination of decreasing the ratio of carbohydrates versus fat calories used to support muscle contraction as well as increasing available total carbohydrate calories (glycogen).
• It takes 7 grams of carbohydrates per mile ridden to replenish glycogen stores.
• Dietary carbohydrates should be complex while simple sugars are best for a boost while riding.
• Take advantage of the 1 or 2 hours post ride to maximize the rate of glycogen replenishment.
• If you are eating a carbohydrate replacement diet while training, carbohydrate loading is not necessary.
• For planned rides of more than 2 hours you will need carbohydrate supplements. Start at the time you start the ride, and eat something every 15 minutes.