Charles River Wheelers

Showstoppers I: How to Avoid Bonking while Cycling

2023-05-13 10:55 AM | Anonymous

By Coach John Hughes

“Bang! Bang! Maxwell’s silver hammer came down on her head.

”Bang! Bang! Maxwell’s silver hammer made sure that she was dead.”— Paul McCartney

McCartney said, “Maxwell’s Silver Hammer was my analogy for when something goes wrong out of the blue.” (

That’s bonking!  You’re riding along under a blue sky and all of a sudden your brain feels like mush. You’ll be depressed and discouraged and may also feel anxious, irritable, or confused.

Or your legs suddenly can’t turn the cranks.  This is hitting the wall. You’ll feel extremely weak and tired and you may feel dizzy or light-headed.

Both of these occur for the same reason: running out of glucose for fuel.

A showstopper is anything that makes a ride very difficult and may cause a DNF.

A Little Physiology

You are always metabolizing a combination of fat and glucose even when you are sleeping.  The more active you are the higher the proportion of glucose you are burning.  Riding below your anaerobic threshold (AT), also called lactate threshold, about 50% of your energy is coming from glucose and 50% is coming from fat. Above your AT the major source is glucose although you are still burning fat. The harder you ride above AT the more glucose per minute you are burning.

Glucose is stored in the body as glycogen. Your body can store about 1,800 calories of glycogen. (1,400 in the muscles, 320 in the liver and 80 in your blood) How much you store depends on your body size and your fitness. 

Your body has about 100,000 calories of energy stored as fat, an unlimited supply of fat.  Even the skinniest pro has enough body fat to fuel a long race.

If you are riding at 15 mph (24 km/h) you are burning about 4.5 calories / lb. / hour (10 calories / kg / hour). If you weigh 150 lbs you are burning about 675 calories / hour, about half from glucose (338 calories) and about half from fat (338 calories).  You have 1800 calories of glucose stored as glycogen so burning 338 calories of glucose per hour you’ll run out of glucose in about 5 – 5.5 hours.

If you are riding at 20 mph (32 km/h) you are burning about 7.5 calories / lb. / hour (16 calories / kg / hour). If you weigh 150 lbs you are burning about 1,125 calories / hour, primarily from glucose and you’ll run out of glucose in about 1.5 – 2 hours!

Your brain can only burn glucose for fuel and when you run out of glucose that silver hammer comes down.  At a moderate pace your muscles are burning about a 50 / 50 mix of fat and glucose. When you run out of glucose you only have half as much fuel and you hit the wall with dead legs. To compound the problem the metabolism of fat for energy requires some glucose so even your fat stores aren’t providing much energy.

Note that protein provides only about 5% of the energy for the working muscles, although it is important for rebuilding muscle damage after a ride. If you run out of glycogen your body can produce glucose from protein by a process known as gluconeogenesis, which is inefficient, i.e., the metabolic conversion of protein to glycogen requires more energy than just converting glycogen to glucose.

Bonking Prevention

Endurance training helps defer bonking and hitting the wall in two ways.  By riding at a conversational pace over many rides your body will shift to metabolizing more fat and less glucose thereby sparing glucose. (This doesn’t mean you’ll lose weight. To do that you need to consume fewer total calories than you are burning.) Endurance training also increases your muscles’ capacity to store glycogen by 20 to 50%. If you’ve been doing endurance exercise for years both of these adaptations have taken place but if you’re a relatively new roadie you can improve your fuel mix and your storage capacity with endurance riding.

These adaptations only postpone the silver hammer but don’t eliminate it.

If the gas gauge on your car starts to approach empty you get more fuel and the same applies to riding.  Rather than running out of fuel you need to start refueling during your ride.

Glycogen comes from carbohydrates, which include fruits, vegetables, dairy products, and legumes as well as the sweets, pasta and bread that we normally think of as carbs. Healthy carbs should provide 60 – 70% of the calories in your daily diet.

American College of Sports Medicine (ACSM) recommends consuming 25 to 60 grams of carbs (1 to 2 ounces or 100 to 240 calories) per hour after the first hour of exercise. This is sufficient for several hours of exercise. If you are riding for three hours or more start eating carbs in the first hour. If you are relatively small or exercising lightly 25 grams / hour is enough. If you are larger or riding at a moderate to fast pace eat up to 60 grams / hour.

Note that the recommendation is for carbs only. Gels and some sports drinks are 100% carbs; however, bars are a mix of carbs, protein and fat. Fruit and vegetables are100% carbs while carbs are only part of other foods.

The ACSM recommends up to 60 grams per hour of carbs because this is the maximum amount of one kind of carb (glucose or sucrose or fructose or maltodextrin) you can digest per hour.However recent research shows that eating a combination of types of carbs can increase your ability to digest carbs. You can digest up to 90 grams per hour (2 to 3 oz. or 240 to 360 calories). Test subjects who consumed a mix of glucose and fructose could digest more every hour than subjects who just consumed glucose. They digested more per hour because the different types of carbohydrate used different intestinal transporters. Consuming a mix of carbohydrate reduces fatigue, increases endurance and may result in reduced gastric distress. Some sports bars and drinks are made from several types of carbs — read the label to see. Or you could eat a couple of cookies and a piece of fruit.

Lab tests have shown no performance difference among carbs ingested in liquid, gel or solid form, assuming that each substance has the same caloric value. Further, sports products have no performance advantage over regular food. One of my clients was a nurse, and after consultation with the doctor for whom she worked, she raced the Race Across AMerica on pancake syrup instead of spending money on sports gel! Sports drinks and gels are easier to consume than solid food; however, you can ride just as well on food from the local grocery. Real food is cheaper and tastier. The key is to read the labels so that what you are buying and consuming is composed primarily of carbs.

Bottom line: Eat Carbs!

The principles and recommendations for eating before, during and after a ride apply to all roadies. These are explained in my eArticle Nutrition for 100K and Beyond. Although written for roadies riding 100K and farther, all roadies can learn from it. I show you how to calculate how many calories per hour you burn. I compare the nutritional value of bars, cookies and candy. Both Peppermint Patty candy and Fig Newton cookies have a higher percentage of carbs than any of the sports bars! I also discuss hydration and electrolytes. I conclude by discussing what you should eat every day to ride your best. My 17-page Nutrition for 100K and Beyond is just $4.99.Click here for details on how to order.

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