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Fat Burning

By Monique Ryan

Long distance triathletes have logged plenty of long rides, built their run, and have heavy blocks of training booked for the weekends ahead. During heavy training cycles, fuel is at a premium and coaxing our muscles into a higher level of fat burning is an appealing potential performance-enhancing benefit. After all, fat is a seemingly unlimited fuel supply. Burning more fat would spare our relatively limited muscle glycogen stores and getting leaner provides its own performance boost. Let’s take a look at the latest research on fat burning. 

Where’s the fat?
We are keenly aware of our most abundant fuel store of adipose fat, a 5,000 to 10,000 calorie supply even in lean triathletes. But newer techniques in fat measurement are unlocking the potential of intramuscular fat or IMTG, fat droplets that reside in the muscle fibers themselves and which provide 2,000 to 3,000 calories of fuel. Training intensity regulates burning of these fat fuels.

At very low exercise intensities of 25% to 40% VO2 max, adipose fat releases fat into your bloodstream and is the main fat fuel supply. Increasing intensity to 40 to 65% VO2 max, often called the “fat-burning” zone, keeps the adipose fat burning, but also turns on IMTG burning. Total fat burning is at 50% of calories, with carbohydrate supplying the other half of your fuel needs. Higher training intensities at 70% to 80% VO2 max, turn down fat burning a notch, as blood glucose and muscle glycogen best meet the demand for fast fuel. 

Hold the carbs?
Currently there is some debate as to the benefits and drawbacks of limiting carbohydrates before and during low to moderate intensity training sessions. But in trained triathletes, training on water alone could have clear performance limitations, while providing carbohydrate fuel provides measured performance benefits.

In regard to pre-exercise fuel, what and how much you eat is often a matter of practicality. Early morning training sessions impose restraints on both feeding and digestive time, with many triathletes swimming, and running on low liver glycogen stores. Data shows that consuming carbohydrates before exercise does reduce fat burning during exercise, but that this effect lasts only about 60 minutes, after which fat burning is similar to when carbohydrate was not consumed. Because early morning liver glycogen stores are low, a small carbohydrate snack may boost blood glucose levels, helping you to focus and concentrate on your workout. 

Avoiding carbohydrate during training is likely not the best answer to increased fat burning. One study completed by Jeff Horowitz, PhD at the University of Michigan in Ann Arbor measured the effects of carbohydrate ingestion during exercise on fat metabolism. Subjects were fed a high glycemic carohydrate at 30, 60, and 90 minutes during a 2 hour ride at 40 and 65% VO2 max, or received no carbohydrate at all. “At the higher exercise intensity carbohydrate feeding did significantly decrease fatty acid availability late in exercise, but there was no effect on how much fat was used for energy,” said Horowitz. “We also found that the carbohydrate feeding maintained blood glucose availability for fuel, and muscle glycogen use was the same whether they ate during exercise or not.” 

Horowitz maintains that restricting carbohydrate during training can induce adaptations in muscle to increase their capacity to use fat as fuel, but several studies have reported that this does not improve endurance performance. “The best performance results are typically obtained when the athlete is able to train harder and longer with adequate carbohydrate for fuel,” he said. Completing training sessions at the desired duration and intensity also increases your energy expenditure, working in favor of losing body fat. Endurance training itself is likely the most important factor in increasing IMTG metabolism, with highly trained athletes using more fat as fuel than moderately trained or sedentary counterparts. 

Aside from experiencing fatigue, reduced training pace, and hunger, longer training sessions with no carbohydrate intake can also stress your immune system. Stress hormones such as cortisol are elevated during and after hard training, and several studies indicate that carbohydrate consumption during exercise weakens this increase, because blood glucose levels are maintained. “We are not analyzing data in which cyclists trained for three hours at 65 to 67% VO2 max, and compared carbohydrate intake to a placebo. We found that parts of the immune system showed less dysfunction and inflammation with the carbohydrate consumption,” said David Nieman, PhD of Appalacian State University. “We still need to determine if this translates into less infection.”

Refueling your fat
Triathletes are well aware that carbohydrate intake after training starts the process of muscle glycogen replenishment. Now emerging data indicates that fat content of your post-exercise diet can influence the rate at which IMTG are replenished. IMTG stores are significantly depleted after training sessions lasting over three hours, and inadequate fat intake replenishment during blocks of high volume workouts could potentially limit your ability to train.  

Focus on carbohydrate and protein intake several hours after training to facilitate glycogen replenishment and muscle repair. But be sure to add healthy fat sources to subsequent meals and snacks. Aim for half a gram to one gram of fat per pound of body weight in the 24 to 48 hours after long training sessions, especially if you plan to complete higher volume workouts in the following days. Meeting both carbohydrate and fat intake after these longer workouts takes planning and plenty of calories. 

Periodizing your fat intake
Louise Burke, PhD, of the Australian Institute of Sport and other researchers have conducted a series of fat adaptation studies designed to stress the body to up-regulate fat burning and improve performance during endurance exercise. They have investigated a dietary periodization protocol in which endurance athletes adapt to a high fat diet for 5 to 6 days and then switch to rest and a high carbohydrate diet to restore muscle glycogen. This type of nutritional manipulation is a strategy designed to optimize both fat and carbohydrate oxidation during training.

Their studies demonstrated an enhanced capacity for fat oxidation, even with a carbohydrate loading phase, high carbohydrate pre-event meal, and intakes of substantial amounts of carbohydrate during the exercise test. However, despite these metabolic changes, no statistically significant improvement in performance was measured during an endurance performance test. 

Their ongoing research and that from another lab, found that this nutritional periodization protocol may actually even down-regulate carbohydrate metabolism. In a 100k cycling protocol that included several features of a real-life race, such as high intensity efforts experienced during a break, uphill climb, or sprint to the finish, subjects suffered an impairment in performance and were not able to maintain these short periods at higher exercise intensities. 

Burke presented more recent and unpublished research at American College of Sports Medicine meeting in June 2006, “We had sixteen well-trained cyclists and triathletes complete four weeks of matched training on a moderate or high carbohydrate diet. The difference in the carbohydrate diets was from additional carbohydrate consumed during training sessions,” she said. Subjects consumed 1.5 g carbohydrate per/kg body weight (about 105 g carbohydrate in a 150 lb. cyclist) per hour, and trained about two hours daily. Subjects completed a performance ride before and after the four-week training block. “We saw a significant increase in the ability to oxidize carbohydrate from the drink in the group who trained with carbohydrate, whereas in the control group there was no difference,” said Burke.
The bottom line on fat burning
Many experts believe that training in simulated race conditions is the best preparation, including consuming carbohydrate during training. Real life may sometimes get in the way of being optimally fueled with carbohydrates before and during certain types of workouts, but don’t let this happen as often closer to race day. Keep the final goal in mind – being optimally trained for race day by getting in plenty of quality workouts. Going without carbohydrate during training may burn a bit of fat, but it could hurt your immune system and the longer and harder training efforts that make you a faster triathlete.

Monique Ryan, MS, RD is the author of Sports Nutrition for Endurance Athletes, 2nd edition (VeloPress 2007). Click here to view more about the book or purchase. She was a member of the Athens 2004 Performance Enhancement Teams for USA Triathlon and USA Cycling Women’s Road Team. She is owner of Personal Nutrition Designs and offers her sports nutrition “E Program” at