Can Runners Benefit from a Low Carb & High Fat Diet?

Author: FITivate | Published date: June 20, 2023 | Category: Nutrition
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For the longest time, a diet that is low in carbohydrate and high in fats has been touted as an effective form of weight loss remedy, some even going to the extreme of eliminating carbohydrates entirely from their plate. This sort of eating pattern has come under much scrutiny, especially, in relation to its sustainability and possible harmful side-effects. Some swear by its efficacy, whilst others shudder at its highly restrictive requirements. In recent years, there has been growing interest as to how beneficial this fad diet would stack up for runners looking to improve their stride. Amongst other things, 2 key elements that bolstered such interests stems from the shedding of extra body mass to reduce running effort, and tapping on fats as a primary energy resource, due to its abundance in our body.

Why Fats?

Even though the body primarily uses carbohydrate as its go-to source of energy, ahead of protein and fats, there is actually a limit as to how much carbohydrate stores the body can accommodate at any one time (in the form of glycogen). Fats, on the other hand, has a larger reservoir to tap into and is more energy dense by nature. It is said that, by priming the body to more readily utilize fats for energy metabolization, performance and endurance of the individual will naturally improve. But is this all true?

Fatty food here I come… or should I?

Firstly, studies have shown that for the body to be able to burn fats primarily as the body’s fuel, it must enter a state known as nutritional ketosis. For an individual to attain this, a prolonged, consistent daily diet consisting of more than 80% fat and less than 50 grams of carbohydrates is required. This drastic change in dietary arrangement can be daunting and pose a challenge to many who attempt to adapt to it.  

Even with the successful conversion to fat fueling, the second aspect to look into will be the quality of this energy source. Fats are indeed more energy dense than carbohydrates, but the rate of energy metabolization from fats supplied to the muscles takes longer when compared to carbohydrate breakdown. It means that, as the exercise gets more intense, the quicker the individual on fat fueling “hits the wall” – this happens when the little reserves in carbohydrate store dries up, the reliance on fats makes working out more laborious.

There are, in fact, some studies which do show evidence where a high fat diet did improve the performance of those who participated in ultramarathon events. However, this is a scenario where they specifically exercised long periods (longer than marathons), and at approximately up to only 50% of their VO2 max.  To further exemplify this finding, there was one such study that tested the performance of 47 ultra-marathoners. This group, 6 weeks into a 100 mile race, were split into diets that consisted of mainly fats and protein, and those who consumed high carbohydrates as staples. Undergoing the same training regimen, those that were on a high fat diet saw better finishing times than those on high carbohydrates. However, it is also worth noting, up to 50% of those on high fat and low carbohydrate diets did not finish the race, whilst all from the high carbohydrate group completed the 100 mile.

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Immediate side effects that some may experience whilst going through a high fat, low carbohydrate diet may include acute, flu like symptoms known as the keto-flu. As the body adjusts towards the new diet arrangement, issues related to the keto-flu, which include dizziness, fatigue, headache, diarrhea etc. may set in. These symptoms might last over a few weeks, where ample rest and avoidance of intense exercises are strongly recommended. This can be counter-productive for runners preparing for their impending race. Also, such diets are best kept at bay for those who suffer from kidney, liver or pancreatic diseases.

Last but not the least, in order to achieve a healthy and effective high fat diet, the spread is extremely important. As the name suggests, a high fat diet does not translate to a free for all consumption of deep fried food, processed meat or animal fats. Rather, ingesting food that are high in healthy fats, such as oily fish, baked nuts, dark chocolate, eggs , white meat etc is essential. This brings us to the issue of sustainability, in general, these healthy, high fat foods can be considerably more expensive than an average meal. For those who are considering a long term relationship with such a diet should be prepared to shoulder the rise in food expenses.


Whilst many standby the benefits for a high fat, low carbohydrate diet in improving running performance, my opinion, based on many pro-carbohydrate studies and research, is to still adhere to balanced meal plans. Even though ultramarathoners might benefit from fat fueling, loading substantially on carbohydrates should still be the way to go for those intending to cover only marathon distances. Likewise, complex carbohydrates (whole grains, beans, fiber-rich foods) must take precedence over the sugary and starchy food fare. To end off, before embarking on any diet that deviates from the usual dietary arrangements, a certified physician or nutritionist should be duly consulted.

By : Alvin Ho

B (Eng), MBA, Certified Allied Healthcare/Fitness Professional (EIMS), Master Fitness Trainer / Fitness Nutrition, Resistance & Endurance Training Specialist (NFPT)



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