LCHF: Is Low-Carb High-Fat The Best Diet For Ultra Runners?

Written By Chris Taylor

Chris is an ultra runner and adventuer. His interests include (and are limited to): ultra running, plant-based foods to eat whilst ultra running, and ultra running with dogs.

12 July 2019


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Although the benefits of a low-carbohydrate high-fat (LCHF) diet have long been touted by the likes of Arctic adventurers and extreme explorers, the concept first gained public traction in the early 2000s with the rise of fad diets (such as the Atkins and paleo). However, rather than fading into insignificance, interest in the LCHF diet and its associated theory has stuck around, eventually making the leap to endurance sports.

Eccentric scientists and bohemian athletes were the first on board, but its popularity spiked following endorsement from Tim Noakes, an influential sports scientist and author of one of the most popular running books ever written, Lore of Running. Noakes wrote in 2015: “For 33 years I followed and advocated… the current dogma that to be active and healthy, one must eat a diet low in fat and high in carbohydrate. I now believe that this advice was quite wrong. I apologise. It was an honest error.”

What’s the science?

Conventional sports nutrition advice for endurance athletes is to consume 60-65% of calories from carbohydrates, 20% from fat, and the remainder from protein. A typical LCHF diet would involve a reversal of the carbohydrate and fat percentages, while an extreme LCHF diet would be more like that followed by the Inuit population from which the concept originally emerged: 75-80% fat, 15-20% protein, and less than 50 grams per day of carbohydrate.

However, before you discard all your potatoes and head to your local butchers, it’s important to understand the role that different fuel sources play in keeping your engine running. Optimising exercise performance is not simply about filling the tank with fuel: which type of fuel is consumed, where it is stored and how quickly it can be accessed are also important matters to consider.

The basic food groups are protein, carbohydrate and fat. While protein can contribute fuel in extended bouts of exercise, its primary function lies in muscular repair. Its impact on fuelling muscle contractions is minimal, so can be put aside for our purposes here. That leaves carbohydrate and fat as our principal sources of fuel and the balance between consumption of these depends on exercise intensity.

Most runners have heard of the ‘fat-burning zone’ and are aware that fat is the primary fuel source for low-intensity exercise, such as walking and jogging. Carbohydrate enters the mix as you speed up and becomes the predominant fuel source once you’re working hard (think Zone 3 and above). You might therefore think that if you stock up on as much carbohydrate as possible, you will be able to run at tempo pace from sun up till sun down. Indeed, carbohydrate loading is more or less what sports nutritionists have been advocating since the 1970s and is what inspired the stereotype of the pasta-fuelled marathon runner that has held sway for more than half a century.

However, as anyone who has ‘bonked’ (otherwise known as ‘hitting the wall’ or ‘blowing up’) will attest, that is not the case. Unfortunately, the human body can only store a finite amount of carbohydrate as fuel (glycogen) and if you use it up without replacing it, you better be prepared to hit the deck.

Herein lies the enticing prospect of an LCHF diet for endurance athletes. Given that a well-prepared body can only store around 2,500 calories of carbohydrate and that the average runner moving as fast as possible will consume 3,000 calories of carbohydrate over the course of a marathon (depending on weight), you either have to refuel or slow down – both undesirable options for the competitive athlete. Adding further insult to this inevitable carbohydrate deficit is the fact that the average athlete carries between 30,000 and 100,000 calories of relatively unused fat stores around with them.

As such, early LCHF advocates proposed that, should an athlete be able to access these significant fat stores whilst operating at a moderately high intensity, then they’ll likely fall asleep from exhaustion long before they hit the wall. Consequently, sports scientists the world over began experimenting with various fat-adaptation protocols. The results were mixed. Although overall performance in long-distance time trial performance remained unchanged (and in some cases improved) in athletes who switched to an LCHF diet, sprint performance was often compromised. Further, studies found that it can take several weeks for the body to adjust to an almost carbohydrate-free diet.

Low Carbohydate High Fat Diet

The appeal of LCHF for ultra runners

The ultra running community, however, began to take note. In a sport which already favours fat-burning capacity due to its low intensity, a loss of sprint power is of little concern. Further, one of the greatest challenges in ultra running can be the effort of refuelling, as anyone who has tried to force their tenth flapjack down their throat after twelve hours in the mountains will tell you.

Dependency on external carbohydrate sources is mentally tiring and can be a gastrointestinal nightmare for the inexperienced. The possibility of relying instead on the consistently burning flame of your extensive internal fat stores is therefore not only physiologically efficient, but also a psychological relief – and it wasn’t long before elite ultra runners cottoned on.

Timothy Olson, a previous course record holder at Western States, and Zach Bitter, who set a U.S. track 100-mile record in 2015, have both converted to LCHF diets. According to Jeff Volek, a dietitian and researcher who co-authored an editorial alongside Noakes questioning the necessity of carbohydrates for athletes, “the benefits of low carbs really start to distinguish themselves when you get beyond the marathon, because you’re definitely running out of carbs then.” More recently a kind of periodised approach to carbohydrate and fat consumption has surfaced, in which athletes keep overall carbohydrate intake low, but increase it before and during long runs and races.

Further, a technique for increasing the use of fat stores as fuel for exercise without altering diet content, called the fasted workout, has recently become popular. By selecting certain workouts to perform carbohydrate-depleted (i.e., having a carbohydrate-free dinner and then completing your morning run before breakfast), the body is forced to utilise fat as fuel, sparing those all-important glycogen stores, and triggering the body to adapt to using fat stores as fuel in response.

However, the debate rages on. While some athletes have undeniably found success with the LCHF diet, other studies have reported an overall decrease in exercise performance and in some cases, an adaptation period so unpleasant as to be considered unhealthy. Ultimately, it depends on the individual and the type of sport, and new research must emerge before we can claim to understand definitively how the different metabolic fuels affect endurance performance.

With these caveats in mind, the LCHF certainly presents an intriguing opportunity for ultra runners to ditch their dependency on bananas and sports gels in favour of cultivating a more efficient, fat-fuelled, long-distance engine.

Disclaimer: Ultra X does not claim to offer expert nutritional advice and any significant change in diet should be made in consultation with a qualified medical practitioner.

LCHF Food Cheese Board


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