The following is Sindballe’s personal account of how he attempted to make his body into a fat-burning machine, thereby giving his body the most efficient and limitless fuel available to him. It was originally seen in the Nov/Dec 2010 issue of Inside Triathlon magazine.

A good friend of mine once finished a six-hour ride in the mountains on nothing but pure water. No gels, no energy drinks—just water. And he was not out on a Sunday ride—he was hammering, riding hard on the ascents and flying down the descents. Can you do that? Or are you already thinking of how many gels and bars you would need to drag along for the ride?

While research over the last 10 years has improved our understanding of fatigue and the interplay of metabolism, heat and fluids—as well as the role our brains play in all of this—there is still a general consensus that the size of our carbohydrate stores and the rate at which we can derive energy from fat play a significant role in endurance. And while most triathletes are well versed on the carbohydrate side of this equation—how to stock and replenish glycogen stores before, during and after workouts—few understand how to tap into their ability to use fat for fuel.

When we are very fit, our glycogen stores can fuel a six-hour hard ride in the mountains, similar to the one my friend took. But after the ride, the glycogen tank is almost empty. In comparison, even a rail thin triathlete stores enough fat to fuel five Ironmans in a row.

Fat is an almost unlimited resource, but it comes with two problems: The human brain is a sugar lover, and the rate at which fat is burned for fuel is too slow to support a hard, fast Ironman effort. In other words, your body fuels itself with a combination of glycogen and fat (and a little protein), with fat being the source of fuel that lasts but which cannot be tapped quickly enough to keep you moving fast.

The problem of your brain loving sugar can be solved by taking in enough carbohydrates during exercise. And the fat burning problem can be abated by teaching your body to use fat at a faster rate—thus staving off the depletion of the glycogen tank and allowing you to go faster longer. (Once the glycogen is gone, your body can only tap into its fat for fuel, thus forcing you to slow down or bonk.)

The easiest way to improve your ability to oxidize fat—turn fat into energy—is to train for long hours on the trails or in the saddle at a relatively slow pace. Generally, you don’t want to go much faster than your Ironman pace if you’re trying to stimulate your fat oxidation capabilities. While most athletes are well aware of this, there are several diet and training tricks out there that claim to increase the quality of the training stimulus these rides and runs provide. I have researched and tried most of these tricks myself while I was an Ironman pro and now have an understanding of what does and doesn’t work.

Some have hypothesized that L-carnitine supplementation can enhance one’s fat burning capacity, as L-carnitine is one of the enzymes involved in the fat burning process. Unfortunately, there isn’t evidence that this type of supplementation works.

Other hypotheses suggest that taking in medium chain triglycerides, a readily absorbable form of fat, during exercise increases one’s ability to burn fat. But studies have shown that one would need to take in as much as 30 grams of these fats per hour to achieve any noticeable effect, according to Louise Burke’s book “Clinical Sports Nutrition.” What’s more, taking in these fats comes with a high risk of gastrointestinal stress and isn’t advisable for athletes, Burke notes.

Along these same lines, some have believe that caffeine’s performance-enhancing effect stems from it increasing the body’s ability to burn fat. But the effect actually comes from increased neuromuscular function and a reduction in perceived effort.

When it comes to increasing one’s ability to burn fat, diet tweaks have by far received the most scientific attention. In recent years, many athletes have pinpointed the Paleo, or hunter-gatherer, diet—which emphasizes natural foods and is low in carbs and rich in good fats and protein—as a possible shortcut to increased endurance. Advocates say that this diet causes the metabolism to begin using mostly fat for fuel. In 2005, I tested the Paleo diet over a six-month period. While the challenge of eating 4,000 to 6,000 calories a day from a diet composed of only fruits, vegetables, meats and nuts was outweighed by the feeling of being super healthy, I did not see any gains in my performance. I kept my endurance, but over time I felt the upper end of my power and recovery fade as I had difficulty replenishing the calories I burned. For athletes with more balanced workloads, the Paleo diet might work fine, but for an Ironman pro with 25 to 40 hours of weekly training, it became insufficient. In other words, I have doubts about its performance potential and have yet to see a study that proves it enhances the body’s ability to burn fat.

After trying out the Paleo concept, I became interested in how my diet affected which fuels—carbs, fat or protein—my body used. I discovered that it is possible to shift our metabolism to burn mostly fat, but we need to eat excessive amounts of fat to do this. That is, up to 60 percent or 70 percent of our daily calories would need to be from fat—a percentage far above what the Paleo diet recommends. Despite the shift in metabolism and fat-burning capabilities, a high-fat diet has no effect on endurance performance, a review of studies by B. Kiens and Burke in 2006 concluded. What’s more, the diet diminishes one’s ability to do intense workouts, surges and sprints as well as the ability to adapt to training if the diet is maintained over a long period of time. These results, along with high-fat diets putting one at risk for developing severe malfunctions in metabolism, suggest that this approach should be categorized as a “useless tool.”

In 2007 I was inspired by a modified version of the high-fat diet called “periodized nutrition,” in which five to six days of high-fat intake is followed by a day of carbo loading, leading into a race simulation. Your body adapts to the high-fat diet after five to six days and shifts into a fat burning machine, and many studies have found that this shift persists even after the carbo loading day. In theory, this type of diet is promising because athletes can do most of their training on a normal high-carb diet, which is optimal for intense endurance training, then switch to a high-fat diet for five to six days, carbo load for a day and then race with a full tank of glycogen and an enhanced ability to burn fat. But researchers L. Havemann et al found that these periodized diets reduced one’s ability to do intense workouts, making them less than ideal for most athletes.

Nevertheless, the scientists hypothesized that the periodized diet could be somewhat useful for those doing ultra-endurance events, such as Ironman, in which there are few surges and low-end endurance is the major performance determinant. A 2001 study by A.L. Carey et al tested the diets of eight well-trained cyclists. As part of the test, the cyclists did a four-hour steady ride followed by a one-hour time trial—a good measure of ultra-endurance performance. Those eating the periodized diet improved an average of 11 percent in power and 4 percent (two kilometers per hour!) in their time trial splits. These results were not statistically significant because of the low number of participants in the study and some variation within them, but they indicated that there might be something there for athletes involved in self-paced racing over many hours. The researchers on this study and others like it also concluded that there might be athletes who respond well to periodized diets, while others won’t.

Inspired by the research, throughout 2007 I did several tests in which I ate a high-fat diet for five to six days followed by two days of carbo loading—I felt one day of carbo loading was too short for the glycogen tank to completely refill. Subsequently, I did a solid Ironman where I felt great the entire way—something that hadn’t happened in years—when I won the Vikingman in Fredericia, Denmark, in early August. I used the same protocol leading up to the Ironman World Championship in October, where I once again felt strong on the later part of the race and made the podium for the first and only time in my career.

Despite achieving the best results of my life, I was still unconvinced about periodized diets, given there were other possible causes for my breakthroughs. I wanted proof, so in the spring of 2008 I hooked up with Danish scientist Lars Nybo Nielsen and Team Danmark to test the diet. I already had a natural fat-burning rate of 0.8 grams per minute at Ironman pace while taking in carbs during the test—higher than anyone in Carey’s study, even after his test subjects ate a high-fat diet for five to six days. The highest rate of fat burning achieved by the cyclists in Carey’s study was 0.7 grams per minute. In fact, my natural fat burning rate was on par with the highest fat oxidation rate cited by Tim Noakes in “Lore of Running.” During the days of eating a high-fat diet, I reached a fat-burning rate of 1.2 grams per minute at Ironman pace, confirming the theory that a high-fat diet shifts your body’s reliance on fat for fuel, but my rate shifted back down to 0.8 grams per minute after two days of carbo loading. It seemed the protocol had no effect on me.

One could speculate that my fat burning ability was already high and that this strategy has the greatest effect on individuals who aren’t as fit as I was or that the two days of carbo loading was too much. I should also note that such a sudden switch in diet puts you at risk for constipation and other digestive problems.
Despite the results of my personal study, I decided to use the periodized diet again before Kona in 2008, because it had been successful the year before and it might have an effect on the size of my glycogen stores that we had not yet been able to detect. Unfortunately, the risks caught up with me as my digestion literally stopped the Friday before the race until I was back home five days later. This mistake cost me dearly in the lava fields.

While Paleo, high-fat and periodized diets have little or no effect on enhancing one’s endurance, there is a less risky tweak you can implement in your training to improve your ability to burn fat for fuel—regular rides on water. I began to implement them back in 2007, and at first I could scarcely go for one-and-a-half hours before I bonked. But as my body adapted, I was able to ride three, four or five hours on water alone. This approach has a big effect in activating the genes that stimulate the production of enzymes involved in fat oxidation, as shown in a 2005 study by L.J Cluberton et al. In other words, the water-only rides might be the reason my ability to burn fat was already so high when I did the periodized diet experiment.

If you decide to implement these water-only rides in your training, remember that these rides are depletion sessions, which leave you drained, and should always be followed by recovery days. During my Ironman building period, I had to work hard on consecutive days, so I did a modified version of these water-only rides that allowed me to maximize the stimulation of fat oxidation without requiring so much recovery. In short, I would do the first two hours of every training session on water alone and then add slowly absorbable, solid carbs, such as whole grain sandwiches or oatmeal-based energy bars. This process kept my blood sugar levels in check so I avoided bonking and could keep the intensity up late in the sessions. But at the same time this allowed my body to predominantly use fat for fuel, and it stimulated enzyme production. Early in the year, when intensity was more important, I would fuel rides from the start and only do a water-only ride once a week.

My scientific team and I tested the water-only approach and found that during these rides, I would burn fat at a rate of 1.2 grams per minute—similar to the fat burning rate I achieved after high-fat eating on my periodization diet. This indicated that the training stimuli of the water-only rides were the same as those in a high-fat diet. Plus, these rides were far more practical and didn’t sacrifice my ability to do intense workouts and properly digest my food.