Fat for Fuelling? No Thanks. Why Carbs are still King

Long-term studies have shown that consuming carbohydrates can improve endurance, particularly in events lasting longer than 45 minutes in duration. But the concept of using fat as the primary fuel for endurance has emerged and grown in popularity in recent years, alongside a backlash against carbohydrate based fueling.

There has been a wedge driven between those who fervently practice Low Carb, High Fat diets (LCHF) and those who are confidently pro-carb. Research into this subject has further muddied the waters, as studies have found that the body’s ability to burn fat as fuel can quite quickly be trained to very high levels. However, as endurance athletes we are ultimately concerned with performance, and the real question we need to ask is does using fat for fuel make us faster?

The earliest research into carbohydrate fuels

Since 1925, studies have discussed the potential benefits of carbohydrate consumption for athletes. During the Boston Marathon, runners who carb-loaded the night before and then ate sugar (sweet tea and confectionery) during the race almost all ran faster.

This study was the first to prove that eating carbohydrates is beneficial if you are participating in a long, challenging endurance event.

Nearly a century later, the researchers' conclusion that consuming additional carbohydrates before and during an endurance event will increase performance and reduce fatigue still holds true.

Using carbs and fat as fuel for exercise

In general, your energy mix changes according to your relative intensity of exercise. Your body typically relies on some combination of fat and carbohydrate for energy. The amount of fat you burn will vary depending on the intensity. The higher the level of effort, the more your body relies on carbohydrate for fuelling.
Low-intensity exercise utilizes fat more than high-intensity exercise because fat requires a lot of oxygen to be broken down into fuel for your muscles (oxygen is more abundant in low-intensity exercise). Fat is incredibly energy-dense, meaning we will never run out of it in a sports context since we store so much of it in our bodies. There are 100,000 calories stored as fat in even the leanest people!

On the other end of the spectrum, carbohydrates are necessary when exercising hard since they can release their energy faster and without the need for as much oxygen as fat. When exercising at high intensities the process to convert fat to energy simply takes too long to rely on, so the body switches to its quick-burning stores of carbohydrate.

There's a considerable tradeoff here, though. Even after eating, an average person has only about 2,000 calories of carbohydrate stored in their body (as glycogen), and 90-120 minutes of challenging exercise will burn that all up pretty quickly.

Despite the complexity of fatigue in endurance sport, it has been known for decades that running out of glycogen - often called "bonking” or "hitting the wall" causes a pretty rapid and catastrophic collapse of performance. This is similar to your car running out of petrol. Those of you who have experienced this will understand entirely what is meant!

Carbs, then, are a good source of quick fuel for working hard, but they are scarce and must be resupplied frequently if you plan to sustain your exercise intensity beyond the point when glycogen is depleted.

Are fats a viable fuel source for endurance exercises?

The science of adapting human physiology to 'learn' to burn more fat through dietary manipulation has been examined in many studies.

The basic theory behind LCHF diets for athletes is fairly straightforward. We carry around plenty of energy in the form of fat stores, but we generally can’t burn it quickly enough to fully fuel anything more intense than a mild jog. If you adapt to an LCHF diet, you can double or even triple your maximum rate of fat burning.

In theory that might allow you to fuel long-distance events mostly with fat, reducing the need to cram in carbohydrates during the event and eliminating the risk of bonking. Several studies on endurance athletes training on a LCHF diet have shown no difference in performance in sub maximal time to exhaustion tests compared to high carb diets, indicating that fat can power steady-state endurance performance.

Studies have also shown that the fat adaptation process can allow for muscle glycogen (carbohydrate) stores to be more effectively protected or “spared” during low intensity exercise. However, the research is also very clear that fat adaption blunts your carbohydrate burning ability, and reduces your “exercise efficiency” (the amount of oxygen you use at a given effort level or pace).

To put it another way, the trade-off for becoming a better fat burner is that you become worse at utilising carbohydrate, and even at modest paces ie. marathon pace, the intensity is high enough that your overall efficiency is significantly impaired. Furthermore, your ability to perform at higher intensities (sprinting, hard intervals) is significantly diminished.

What do elite endurance athletes do?

With the majority of studies demonstrating that high-carb fuelling is the most effective way to fuel endurance athletes in the real world, most elite athletes eat high-carb diets to support their training and racing.

Whilst there is a huge amount of conjecture online about the benefits of low-carb diets, in reality there are very few examples of elite endurance athletes practicing a truly low-carb strategy, especially in challenging training periods or races. Carbs are vital for these men and women.

At the highest level of sport, the least successful concepts are usually weeded out very quickly. Top level athletes are always looking for a slight edge, however small, and LCHF has been tested by many athletes without gaining significant traction in practice.

So, are carbs better than fat as an endurance fuel?

The science is pretty conclusive that fueling hard endurance efforts with adequate carbohydrates still offers the best nutritional approach to maximizing endurance performance, as it did in 1925 when the Boston Marathon researchers presented their findings. Although there is clear evidence that the body can adapt to a high fat diet to become glycogen sparing, the key takeaway for athletes is that this adaptation comes at the expense of carbohydrate metabolism, meaning your body becomes worse at using carbohydrate when it needs it. In any event where you need to respond to changes in pace, surge, make the break, power up a hill, sprint to the finish line, or even just work at a high intensity for a sustained period, your performance is going to be compromised if you’re relying on fat for fuelling.

So, if all other factors are equal, are fat-adapted athletes faster or more effective in endurance sports? The answer, according to the research, is a big fat no.