One of the fitness indicators that we hope to improve in the Base period has to do with using more fat and less glycogen (carb) for fuel during long endurance events. As your body becomes better at using fat to produce energy essentially your aerobic fitness improves. This is especially important for very long, steady-state events such as half- and full-Ironman triathlons, century rides, marathons and the like.
I once coached a triathlete whose fat metabolism was so ineffective that he would have had a hard time finishing an Ironman. This was discovered during a VO2 max test by looking at his Respiratory Exchange Ratio (RER). (You can read more about this topic in a previous blog post. ) He would not have been able to take in and process enough sugar to keep the engine running that long without going very slowly. He was by far the worst athlete I’ve ever seen when it comes to fat burning.
So what influences your ability to use fat during exercise? There appear to be several. Here they are.
• Genetics. I’ve never seen any research to confirm this, but it stands to reason that how well we do at preferentially using fat for fuel must be some how related to our parents. This could be as simple as how much type I (slow twitch) muscle we have, to perhaps how abundant our aerobic enzymes are and probably many other variables. Unfortunately (or perhaps fortunately) it’s too late to do anything about this. You have what you have. Life isn’t always fair.
• Training. There are several studies and reviews that support the notion that endurance training causes a shift in metabolism toward greater usage of fat for fuel [1, 2, 3]. Trying to nail it down any tighter than that to what type of training is most effective is pretty complex. The research here is not in agreement. For example, one review of the research suggests that training below 75% of VO2 max is the best way to become a fat burner . Another review suggests it’s even lower than that – 59-64% for trained athletes and 47-52% for sedentary subjects . But to further muddy the water a third found that untrained subjects doing 4-minute intervals at 90% of VO2 max, 3 times a week for 6 weeks saw significant improvements in fat burning .
This is a critical area for athletes, especially in the Base period, so I’m currently doing an experiment with n=1 (me) to see how various chronic training intensities may affect RER. It’s a 5-month project which I’m only 2 months into. But I’m seeing some interesting results already. I’ll share them with you when it’s done.
• Pre-Exercise meal. Most of the research shows that if you eat a high-fat meal before exercise (as opposed to high-carb) endurance at low-intensity improves but high-intensity usually suffers [7, 8, 9, 10]. The bottom line here is that if you eat a lot of carbohydrate before a workout your body will have little need to use fat as a fuel. It’s probably best when doing workouts that are short (for ssome numbers to perhaps give you a reference, let’s say that means less than 3 hours on the bike, under 2 hours of running, or shorter than 90 minutes of swimming) and fully aerobic (below threshold) to eat a low-glycemic-index (LGI) meal or snack prior to the workout. LGI means foods high in fat and/or protein, some fruits (apples, peaches, pears) some grains (rye bread), legumes (soy, lima beans, lentils) and others. Such a meal or snack eaten no less than 2 hours prior to exercise will encourage your body to rely more on its fat stores thus improving metabolic efficiency.
There are lots of variables that go into how much you eat pre-exercise and how long the workout is. How much depends on when your last meal was, how much you ate and what you ate. The suggested workout durations above are more likely to work for experienced athletes who train a lot (more than 12 hours per week, perhaps).
• Pre-Exercise fasting. There is some research suggesting that fasting before a workout causes the body to use more fat for fuel during exercise thus training it to become more metabolically efficient [11, 12]. This may or may not be a good idea for you. I would definitely not recommend this before very long workouts (relative to what you are well-trained for). The risk of bonking could be very high. I think a better option is to eat a LGI meal a couple of hours before the workout and then use water only during shorter exercise sessions. As with fasting, I think this would cause the body to become better at using fat for fuel. But again, how long the workout is that you use water only is determined by the individual. Your shortest workouts should not be a problem at all. The question has to do with at what duration you use sports drinks, bars, gels, blocks or whatever. The suggested durations under “Pre-Exercise Meal” above may help you decide here. Regardless, when doing any length of workout it’s a good idea to have sugar handy, such as in a back pocket – just in case. Better safe than sorry.
• Chronic diet. Most studies show that chronically (for several days, weeks, months or perennially) eating a diet that is low glycemic index (some day I will write about the difference between this and “glycemic load,” which is probably a better standard but less understood) causes a shift in metabolism favoring the use of fat for fuel [13, 14, 15, 16, 17]. This is what Loren Cordain, PhD and I suggest in our book, The Paleo Diet for Athletes. It’s the type of diet the athletes I coach follow and it nearly always produces good results. I should point out that not all of the studies listed here show better performances – only improved fat utilization. I’ve come across only one study that found no improvement in RER with a low LGI diet .
Conclusion. There are four options suggested here to improve your body’s capacity for using fat for fuel (changing genetics isn’t an option – yet). No one knows for sure, as I suspect this has never been studied, but I believe the most effective are your chronic diet and training. I’ll have a bit more to say about training in a few months after I complete my semi-scientific, personal study. Until then I’d suggest that training in zones 2 and 3 are probably the most effective for improving fat utilization through training.
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