Here’s my second article for the Richmond, VA “Miles and Minutes” magazine. My professor and I co-wrote it. You get it here before it’s on the shelves! I’m not completely convinced that fat loading isn’t the way to go for endurance athletes, but this is what the science says at the moment. Hopefully we’ll get some more complete, longer term studies done in the future which will give us more insight. And without further ado…
The Skinny on Fat Loading
Have you noticed a growing number of articles and blog posts about “fat adapted” athletes? There’s one about Tim Olsen, an ultra-marathoner, who won the Western State 100 in 2012. Then there’s Ben Greenfield who finished the 2013 Ironman Triathlon World Championships in just under 10 hrs. More recently the LA Lakers were reported to be on a “Paleo” diet. All these athletes have one thing in common; they abandoned the time-tested method of “carb-loading” and replaced it with “fat-loading.” But before you start dreaming about reaching your ever-elusive PR by replacing your oatmeal and banana breakfast with one of bacon deep fried in butter, you might want to check out what the research actually shows.
Some studies report interesting findings regarding the exercise performance of subjects instructed to eat more fat in their diets. Participants improved their time in a walking test, and some cyclists’ endurance improved slightly in a timed event but these studies are riddled with serious limitations. Lower intensity exercise, like walking, depends less on glycogen stores and more on fat use. Furthermore, as a person adapts to training, fat usage improves, no matter what the diet consists of. In the study with the cyclists, some did do mildly better, while others did worse, but most had no change at all. The timed cycling event also bears little resemblance to real world situations, let alone racing.
Some sources will tell you it takes 2 weeks to become “fat adapted” while others state it could take as long as 3 months. Most of these “sources” seem to be simply repeating anecdotal information they read on a forum or blog. Unfortunately, there are simply no scientific studies that tell us how long it takes to become “fat adapted.” While it’s reasonable to think this diet might work after reading to some of the arguments, upon further evaluation it really isn’t all it’s hyped up to be.
The most pervasive argument compares glycogen vs. fat “fuel tanks.” Consider a 110 lbs. woman with 15% body fat. Roughly speaking, she has about 16.5 pounds of fat. Those 16.5 lbs. translate into almost 30,000 calories, or enough fuel for her to run almost 300 miles! On the other hand this same woman only has about 2,000 calories of stored carbohydrate i.e. glycogen, barely enough to make it through a 20 mile run. Armed with these facts, if you’re a long distance runner, it seems logical to tap into the larger fat fuel tank. Studies do show higher fat burning after eating a high-fat (65-70% of energy) diet. Unfortunately, it doesn’t translate into performance improvements. While the body begins to metabolize fat more efficiently, it loses the ability to use glycogen at a time when carbohydrate requirements are essential.
Generally speaking, endurance or ultra-endurance sports are regarded as a “slow burn” exercise; a sub-maximal aerobic exertion that doesn’t need to use glucose, justifying the researchers observations in the lab tests. However, the strategic activities that occur during races: the breakaway, the push through an uphill section, or the sprint to the finish, are high intensity exertions that require glucose. The evidence shows that this sprint ability in real world situations is actually hindered by “fat adaptation.”
So what if we combine fat and carb loading to get the best bang for our buck; a full tank of fat for the long open road, and glucose for the turbo when you need it? Studies have tested getting athletes to become “fat adapted” and subsequently carb load in hopes to achieve this hybrid performance boost. But despite 5-6 days of increased fat consumption and then a day of carb loading, no clear performance benefits were observed.
The idea of fat adaptation is still being pursued; some say the studies were too short in duration and the subjects didn’t have enough time to fully “fat-adapt.” Until long-term studies are done, we won’t know for know for sure if athletes like Ben Greenfield and Tim Olsen are anomalies or if there’s any actual benefit. While we know there is some additional fat burning when someone eats more fat and low-carb, this is just because our bodies are smart enough to use available fuels, but it doesn’t mean it’s a better fuel for performance. Glucose is still the “hotter burning” fuel, so for a quick burst, like sprinting, running up a running up a hill, or passing in a race, carbs are the body’s preferred fuel. Furthermore when you’re coasting through miles 12-18 at marathon pace, fat is burned more efficiently when carbohydrate is also available. Consequently when we burn more fat, we don’t necessarily run faster and we actually lose the ability to shift into overdrive. If you’re gunning for that PR, train harder, enjoy your food and stick with what we know works for sure; a mixed fuel diet that’s high in carbohydrate.