The question about feeding has come up with some swimmers preparing for long swim (more than 2 hours). Something like, “I start to feel suddenly quite tired after an hour or so, and I think I need some food to keep going. What should I eat and drink? 

This ‘suddenly quite tired’ feeling is what is colloquially called “hitting the wall” or “bonking”, and it is not fun at all. Before answering that question about what to eat, we need to realize that though bonking is a common experience, it is not a necessary one, if you train your metabolism to take a different route.


When You Bonk

Let’s see what’s happening in the body when you bonk. Then I will explain how conditions in the metabolism could be set up to have a different experience.

“Hitting the wall” is a common expression heard among marathon runners, and more than half of all non-elite marathon runners report having “hit the wall” during a marathon regardless of how hard they trained. This phenomenon usually happens around mile 20 to 22. The runner’s pace slows considerably and the legs feel like lead. Tingling and numbness are often felt in the legs and arms, and thinking often becomes fuzzy and confused. “Hitting the wall” is basically running out of available energy.

The runner’s primary fuel source during prolonged exercise are carbohydrates and fats. Fats might seem to be the logical first choice of fuel for endurance events – they are ideally designed to be energy dense, and stores are virtually unlimited. Unfortunately, fat metabolism requires a constant supply of oxygen, and delivery of energy is slower than than provided by carbohydrate metabolism. 

Most runners are able to store 2,000 to 2,200 calories of glycogen in their liver and muscles, which is enough to provide energy for about 20 mi (32 km) of moderate-pace running. Since the body is much less efficient at converting fat to energy, running pace slows and the runner suffers from fatigue. Furthermore, carbohydrates are the sol fuel source for brain function. Physiology, not coincidence, dictates why so many marathon runners hit the wall at around the 20 mi mark. 

Chapter 2 of Physiology Of Sports and Exercise, Sixth Edition, Kenny/Wilmore/Costill – Human Kinetics

Fuel Preference

The body will naturally seek out carbohydrates first, and then seek fats as fuel. It is easiest to process carbohydrates, but that supply is extremely limited. It takes a bit more time to convert, but the supply of fats is practically unlimited. When the runner or swimmer’s body starts with carbs, runs out and then has to switch to mostly fats, this is when that terrible feeling of sudden fatigue and mental haze hits. But, that wall can be avoided if the athlete’s body is trained to seek out fats from the beginning while working at race pace intensity.   

An example in that textbook describes a lean 65 kg male who was measured to have about 2,100 kcal of glycogen in the blood and liver and about 72,000 kcal of fat under the skin and around the internal organs, all available to be tapped as fuel. It so happened that some years ago I went on a 21 day fast to see what would happen in my body and mind. I just drank water, and occasional very diluted 100% fruit juice and very occasional diluted bullion tea (for some minerals). Guess what? Already a fairly lean person, I lost some pounds I didn’t need to lose, yet my health was fine and felt remarkably good, especially in the third week. I was pleasantly surprised how my body could adapt and my mind was radiantly clear. There would probably be no reason for you to try something like that, but you too would be surprised at how far your body can really go on what it already has stored inside.  


Limited Fuel vs Limitless

However, in our modern, calorie-abundant world full of convenient (and often junky) carbohydrate foods, most people out there have a metabolism that is conditioned to prefer carbohydrates as fuel. But the body can only store about 2100 kCal of glucose/glycogen in the system, and it can burn through 600-700 kCal per hour or more during aerobic exercise. 

If the body is first feasting on easy glucose/glycogen stores, it will be reluctant to put in the extra effort to switch to seeking out and burning more fat. That terrible feeling is trying to coerce you to keep stoking it with glucose so it doesn’t have to make the switch. The switch to fat-orientation takes a while to kick in and takes more energy in the process (as pointed out in the quote above). The higher the effort level and the more your body-in-exercise is conditioned for glucose, the quicker you will deplete the stores and hit the wall. But, if the body was already seeking out and burning more fat from the beginning, you would have a totally different experience, and there would be no wall to hit. 

Now, it should be pointed out that the exercising body is burning some amount of fat and even when it is carbo-oriented – it’s not an all-or-nothing situation until we get to the most extreme anaerobic effort level. Some bodies, trained or naturally so, seek out more fat already. But every body in aerobic exercise feeds  with some mixture of the two, which increases to more carbs as intensity increases. Maybe some other time I will attempt to summarize how this works, but not here. Let me just say that, while exercising, whether you naturally burn a lot of fat or a little compared to carbs, you have the option to improve that and avoid hitting the wall. And it will come mostly by your nutrition habits, not by exercise.


Option 1 – Stay Carbo-Oriented, Snack Periodically

So, if you keep your metabolism carbohydrate-oriented, then you may find that you’ll have to supply 600-700 kCal of carbohydrates an hour to keep up with the demand to prevent hitting the wall. 

If you would like to stay with this approach – since it is what most bodies are already oriented for and what most cultural diets emphasize – you would do well to get your calories from whole-food, plant-based sources (meaning whole grains, nuts, fruits and veggies), and you may experiment carefully with specialized processed foods like special energy foods and drinks made especially for endurance athletes. One of my swimmer friends uses diluted baby food!

You can get the calories you need if you feed in small amounts a couple times per hour, and start this before you feel hungry. The bad news is that, after a few hours your digestive system may revolt. It will be having a hard time breaking down all that food, competing for scarce water and energy, while you are working so hard to keep up the effort. Although your body will still be running low on energy, you may lose your appetite, or get cramps, because the digestive system can’t keep up – then your body will really hit the wall (which I experienced on an 11-hour bike race when I was in college).

The vast majority of athletes out there know of no other way to fuel so they just deal with it – some are lucky with a natural fat-orientation, or they may be clever with their foods and have little problem while others suffer with gastro-intestinal distress. 


Option 2 – Train To Be Fat-Oriented, Snack Much Less

If you want to condition your metabolism to prefer fats, the great news is that the body (using a lean body as an example) has over 72,000 kCal of accessible fat laying around – it is practically inexhaustible for endurance events. Literally, you could go for days.

The less-pleasing news is that you have to train your body to prefer fat for fuel and to quit providing easy carbs through a particular process. It would take some weeks and it is going to require a general lifestyle diet shift, but you can avoid the threat of hitting the wall with this approach. In this case, once adapted, you can get by in a race with less than 80 kCal of carbs per hour – much less than the former approach. This means the digestive system will not be at so much risk for being distressed.

And for those snacks, you don’t need to eat fats, you’ve got more than enough laying around inside; you take in carbs because you are just ‘topping off’ the glycogen stores in the body (remember, the brain can’t feed on fats, so it will be tapping that glucose supply), because they are not being used up rapidly.


Metabolic Efficiency Training

Where did I get this crazy idea? It’s not mine and it’s not new. Some of you may know of it by other names. 

I’ve been studying it in a book called Metabolic Efficiency Training, by Bob Seebohar – a resource recommended to me by TI Master Coach and ultra-distance triathlete Shane Eversfield. Shane has considerable experience and success with this – doing triathlon and running events that span not merely hours but days.

One of the remarkable points of this book is that if you want to create and maintenance of a fat-oriented metabolism the recipe is about 25% exercise and about 75% nutrition plan. What you eat greatly determines what your metabolism will come to prefer. 

Metabolic Efficiency Training…

“…is about teaching your body to use more of its almost unlimited fat stores and preserve its extremely limited carbohydrate stores. The most important lesson of metabolic efficiency and daily nutrition centers on the control and optimization of blood sugar. When blood sugar is controlled through food, specifically eating sources of protein, fat and fiber at almost every feeding, it minimizes the amount of insulin that is secreted from the pancreas. One of the roles that insulin has in the body is to manage high levels of blood sugar. However, when blood sugar is uncontrolled and insulin is high, it inhibits the process of lipolysis, the breakdown of fat. (p.39)



This is not a ‘how-to’ post, but more of a ‘you should seriously consider this’ kind of post. If you’d like to learn more, look up the book. It’s a fairly short, very logical, persuasive book, with data and charts to explain the concepts and many case studies to show you how it works on a practical level. 

If attracted to the concept, it may still seem complicated enough that those without inclination or experience in nutrition may feel the need to seek out a sports nutrition specialist who is trained in this particular metabolic efficiency approach. But I do think you can get the basic principles from it and start shifting your diet and training plan in this advantageous direction. 

For distance swimmers and runners and triathletes competing over two hours, with either route you take, you do need to snack periodically during long training sessions and races. And for this you should aim for whole (close to their natural, most-easily digestible state), clean, low-digestive-stress foods, which happens to be plants. 

You will want to start snacking in small amounts before you feel hungry so that your body is processing that into energy and is topping off your fuel tank before it gets too low. 

You are going to need to supply more water to support that digestion. So take a good swallow with each snack… but not too much. Try adding a pinch of salt to your water to improve it’s quenching power and water retention in your tissues.

For many reasons, not just related to sports, I do advise you to stay away from processed sugars and processed foods that have sugar in them (in all the forms sugar is disguised in). Though the brain craves it, sugar is hard on the body, and adds increased stress at the cellular level. Yes, your body is converting clean carbohydrate to glucose anyway, but the form in which the food comes really matters in the overall digesting processing and its impact on your body. Please resist the craving. In contrast, whole-foods are a package of naturally combined materials that together create a better digestive situation, with more nutrition and lower stress. If you need sweet stuff, eat fruit instead. 


Are You Persuaded?

There is a serious performance advantage with a fat-oriented metabolism. Your body produces more steady energy – no bonking. There is less inflammation after. There is quicker recovery. You don’t overwhelm the digestive system with food while exercising for hours. As a matter of fact, you may not even feel the need to snack at all for efforts up to 3 hours (but you may still need to top off those glucose stores). 

The more efficient your metabolism gets through nutrition and fitness training, the higher the intensity you can sustain while still tapping mostly fats. The possible downside is that you may have less ‘turbo’ boost beyond that high intensity level, but for long distance endurance athletes, that’s an easy trade off. Just realize that you have to condition your body to do this, and maintain that metabolic preference. It doesn’t come without an investment to get this way. 

But, consider the great payoff – more steady, longer lasting power supply for racing, and for ordinary daily tasks. No bonking and fuzzy thinking a couple hours after eating. And you get a body that wants to burn fat rather than store it. That bodes well for your overall health, for the rest of your life, after and beyond the athletic training. 


PS – After writing this and thinking a bit more, I wonder how prolonged exposure to cold water, like on a channel swim, would affect a body’s preference for fat metabolism when having to work extra hard to stay warm, considering fat metabolism requires more energy, more oxygen to process. I imagine most who make long, cold water swims are normal carb-oriented athletes, so it would be interesting to hear from a fat-oriented athlete who then put that metabolism to a test in a much colder situation.

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