Low-carb athletes burn 2.3 times more fat than carb-loaders


A fascinating new study published in Metabolism: Clinical and Experimental may revolutionize our thoughts on sports nutrition. We’ve long understood how glycogen stored in the body and carbohydrates help fuel long distance training. This is why runners, cyclists, triathletes, and other endurance athletes focus on high-carb diets and carb-loading before their events and often before each workout. The study’s findings show that this may not only be unnecessary, but it actually limits the amount of fuel you can utilize from stored body fat. The study out of Ohio State University looked at the fat burning rates of ultra-marathoners and ironman triathletes that either ate a high carb diet (59% carbohydrates, 14% protein, 25% fat) or a low-carb ketogenic diet (10% carbohydrates, 19% protein, 70% fat). As I’ve mentioned before, don’t assume all the findings in athlete studies will apply to your workouts (especially athletes of this caliber), but it is interesting because it highlights previous findings that may be wrong:

  1. Humans are capable of burning far more body fat during a workout than we previously thought possible.

  2. It may take longer to adapt to a ketogenic diet than we thought.

  3. The low-carb athletes had normal glycogen levels before exercise, they utilized their glycogen at the exact same rate, and they even replenished spent glycogen the same as the high-carb athletes.

  4. We consider the “fat-burning zone” to be at a VO2 Max below 50 percent, but the highest rate of fat burning occurred at a VO2 Max of around 75 percent.

What is ketosis?

Low-carb diets are all the rage lately (which is good), but in case you’ve never heard the term ketosis, let me go over it quickly. People assume our body’s preferred source of fuel is carbohydrates because glucose is used up first preferentially to fat, it’s harder to store than fat, and most importantly, because sugar, unlike fat, can cross the blood-brain barrier to feed your brain cells. Seeing as how sugar is actually difficult to find naturally (especially during the frozen winter months), being unable to live solely off of fat would be a serious design flaw. Fortunately, we can live just fine without carbs. That’s why you’ll hear about essential fats and essential amino acids, but you’ll never hear about essential carbohydrates.

As your body switches from a high-carb diet to a low-carb one, your glycogen levels begin to deplete. Once they’re used up, you go through a transition period (typically 48 hours but it can be longer for some people) where your body develops enzymes to create enough ketone bodies from fatty acids to replace the now lost source of fuel. Ketone bodies are able to cross the blood-brain barrier, which means they can provide all the energy every cell of your body needs. Another benefit of the ketogenic diet is that it’s much easier to “waste” body fat (something we actually want to do if we’re trying to lose weight). While your body can always mobilize fatty acids out of fat cells, if you don’t burn them up, they just get put right back into your body’s fat stores. Ketone bodies cannot be restored, so if they don’t get used up, they simply get excreted in your urine.

A lot of people struggle with the ketogenic diet for various reasons:

1. Low-carb flu. The transition period can be pretty miserable for some. As your body switches over from carbs to ketone bodies, there is a two-day period (or longer) where your brain doesn’t have much fuel to work with. This can result in flu-like symptoms such as headaches, lethargy, confusion, irritability, and even nausea. Slip ups while trying to get into ketosis or remain there can cause this adjustment period to drag on and on. Many people get sick of the unpleasant feelings and simply abandon the diet before their bodies can adapt.

2. Not enough dietary fat. You still need to eat less calories than you burn (so you shouldn’t go nuts), but percentage-wise, the majority of your calories should come from dietary fat. Just look at the study participants. A full 70 percent of their daily calories were from dietary fat. I find that many people trying a low-carb diet are actually doing a high-protein, low-carb, and low-fat diet. Your body can turn protein into sugar through gluconeogenesis, but it’s not a very efficient way to get useable fuel. The end result is you feel terrible like with the low-carb flu because you’re not generating enough brain-fuel. It’s also not a good idea to train your body to live off of protein, because then it gets very efficient at devouring your muscle tissue for fuel. If you keep your dietary fat high, you will keep the ketone-producing engines churning and your body and brain will feel properly fueled.


Fat-burning machines

The thing I found most interesting about this study was the shear amount of body fat they were burning during their workouts. It’s true they are elite athletes, but we’ve studied elite athletes many times before. All the participants in the ketogenic group were burning 50 percent more fat than the highest ever recorded amount and over 6 times greater than the 31.4 calories per pound of fat that I’ve seen referenced in many studies as the upper limit of human fat usage. The low-carb participants possessed on average of about 12 pounds of body fat meaning they should only be able to utilize about 380 calories from body fat, yet they were burning 1.5 grams of fat per minute during their 3-hour test runs for a total of about 2,430 calories (1.5 grams x 9 calories per gram x 180 minutes) from fat alone. By comparison, the high-carb group was burning .67 grams of fat per minute. While still impressive, it was much less than the keto-adapted group.

The reason for the big difference is because the study focused on athletes that have been doing the ketogenic diet for at least 5 months. They theorized that it takes longer for the body to fully adapt to the diet, which is why previous studies showed far more limited amounts of fat burned as well as glycogen stored and used during exercise.

“A 4 week ketogenic diet in elite cyclists decreased resting muscle glycogen by half and the rate of glycogen use during exercise by 4-fold. Other studies have shown that a low-carbohydrate/high-fat diet decreases resting glycogen and the rate of glycogen use during sub-maximal exercise.”

Glycogen is an important fuel for sports performance which is why many see the ketogenic diet as an excellent fat burning tool, but not ideal for athletics. This demonstration that glycogen levels can not only remain normal, but also be replenished with limited carbohydrates after exercise (only 4 grams in the low-carb group) means that a fully-adapted athlete shouldn’t see any drop off in performance. Other studies will need to look outside of endurance athletes to see if it remains true but it’s a very exciting prospect. They theorized the higher lactate levels in the low-carb group may have provided the substrates needed for glycogen replenishment.

We may be limiting ourselves

Both groups were elite athletes with nearly identical characteristics, but the high-carb group was not able to use nearly as much fuel during the test workouts. Their peak fat utilization also occurred at lower exertion levels, which meant the low-carb group had much more fuel to use when they needed it more. While there was no reported difference in perceived exertion levels, I’ve found people adjust their perceptions pretty quickly to create their own sense of normal. It’s the boiling frog analogy:

“They say if you put a frog in a pot of boiling water, it will leap out right away to escape the danger. But, if you put a frog in a room temperature pot and then gradually heat it until it starts boiling, the frog won’t become aware of the danger until it is too late.”

We simply may not understand how much better we could feel with a new sports nutrition protocol. I’ll keep an eye out for more studies on the subject as they become available, but we may be witnessing a paradigm-shift over the best way to fuel our workouts.

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