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.

the aver­age ath­lete car­ries between 30,000 and 100,000 cal­or­ies of rel­at­ively unused fat stores around with them”

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.
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.