Brief Overview of Low-Carbohydrate, High-Fat Diets for Endurance Athletes
Current sports nutrition guidelines from leading nutrition and exercise organizations around the globe (4,5) recommend a high-carbohydrate, low-fat (HCLF) diet for endurance athletes that broadly consists of:
~50-60% of energy intake from carbohydrates
~15-20% of energy intake from protein
The remaining (~20-35%) energy intake from fat
However, since the early 1980’s, researchers have been interested in the potential benefits of a low-carbohydrate, high-fat (LCHF) for endurance athletes. Studies investigating the effects of LCHF diets have their origins in the management of type 2 diabetes, the management of epileptic seizures, and the potential management of obesity (2). However, in 1983, Phinney and colleagues (3) were the first to test the impacts of a LCHF on endurance athletes. Researchers thereafter discovered in the next decade or so that LCHF diets did not really demonstrate any efficacy for improving endurance performance.
However, there has been a renewed interest in LCHF diets for endurance athletes, particularly the more extreme version of the LCHF diet, a ketogenic LCHF (K-LCHF) diet. A typical LCHF diet might consist of an energy intake ratio of >60% of calories from fat and <25% of calories from carbohydrates. However, a K-LCHF diet would restrict an athlete to consuming <5% of their calorie intake from carbohydrate in order to promote a state of ketosis, wherein the body derives much of its energy from fat and ketones as opposed to glucose.
In a December 2019 Science Post, I discussed in great depth the proposed mechanisms behind the LCHF diet as well as provided an in-depth review of the research literature surrounding this topic up to that point. I will, therefore, spare the in-depth overview herein and refer you to this post for more background on the topic and origins of LCHF diets for endurance athletes. If you are not familiar with LCHF diets and the proposed mechanisms of action, I would strongly suggest you read this previous post before reading further. Some of the key takeaways from the research that was available prior to 2019 was as follows:
LCHF diets had minimal evidence to document any superiority to a traditional HCLF diet for endurance performance.
LCHF diets may impair an endurance athlete’s ability to do high-intensity work in training and in racing due to an impaired ability to derive energy from glucose or glycogen (i.e., glycolysis).
There is some possibility of a LCHF diet to improve performance in ultra-endurance athletes that train and race at very, very low intensities for extremely prolonged periods of time that rarely ever do work at an intensity above 60-65% of their VO2 max (e.g., multi-day adventure racers), which is an extremely low intensity compared to what most endurance athletes race at for single-day events (e.g., trail runs, road runs, triathlons, etc.).
Since 2019, however, more research has emerged on the topic of LCHF or K-LCHF diets for endurance athletes. Therefore, it is worth revisiting this topic given its resurging popularity in some realms of the endurance world, particularly in the long-distance triathlon space (e.g., half-distance and full-distance triathlons).
What Does the Latest Research Say?
I often turn to systematic reviews and meta-analyses for summaries of evidence on topics of interest to me, and this is exactly what I did here. There was a great systematic review and meta-analysis recently published in 2021 by Cao and colleagues (2) summarizing the effects of K-LCHF diets on aerobic capacity and exercise performance among endurance athletes. This article included 10 total individual studies for analysis consisting of 139 endurance athletes, albeit these athletes were primarily male (a potential limitation). The primary outcomes assessed in this systematic review and meta-analysis were related to the impact of a K-LCHF diet on the following exercise-related variables:
Aerobic capacity (i.e., VO2 max) as assessed by a graded exercise test (GXT)
Time to exhaustion (TTE) on GXT
Maximum heart rate (HR) achieved during GXT
Respiratory exchange ratio (RER) on GXT
In brief, the authors found no significant effect of a K-LCHF diet on aerobic capacity, TTE, nor maximum HR; however, there was a large shift towards greater fat metabolism during a GXT as evidenced by a large reduction in RER among those following a K-LCHF diet (lower RER indicates greater reliance on fat oxidation). These findings are very similar to the larger body of LCHF literature in that a LCHF diet shows no real benefit on markers of endurance performance or fitness. However, a LCHF diet does show a dramatic increase in the athlete’s capacity for fat oxidation, but often at the expense of a reduced capacity for glycolysis, or the generation of energy from carbohydrates. This latter shift is not necessarily performance-enhancing as many propose it to be as the available research just doesn’t show a greater shift in fat metabolism to be associated or linked with any sort of enhancement in endurance performance or fitness markers. This recent systematic review and meta-analysis lends more evidence to support this case.
A recently published study by Burke and colleagues (1) sheds some further insight into LCHF diets and endurance performance on a more granular level, and hints at what I think the true potential of LCHF diets is for endurance athletes (discussed later on below). In this study, researchers enrolled 13 elite/professional race walkers, some of whom compete at the Olympic level. These 13 race walkers all went through 3 phases in this research study, and these phases were as follows:
Phase 1: All 13 athletes completed baseline fitness and performance tests (VO2 max testing, walking economy testing, and a 10,000-m race on a track), followed by a 5-day HCLF diet to establish a baseline dietary intake, and finally completing phase 1 with a 25-km “long” walk where fitness metrics were recorded (RER, HR, RPE, and more).
Phase 2: The 13 athletes were divided into a HCLF group (n=6) or a K-LCHF group (n=7) for 5 days followed by a repeat of baseline fitness and performance measures (VO2 max testing, walking economy testing, a 10,000-m race on a track, and a 25-km “long” walk).
Phase 3: Finally, all athletes then went back to a HCLF “restoration” diet for 5 days and finished this 3rd phase with another 25-km “long” walk where fitness metrics were recorded (RER, HR, RPE, and more).
In brief, the researchers found the following after administering this dietary intervention among these elite-level race walkers:
5-6 days of adaptation to a K-LCHF diet was sufficient to increase exercise fat oxidation rates previously seen with longer-term K-LCHF dietary interventions (>12 weeks); this indicates that full adaptation to a K-LCHF may take place quickly and potentially dispels the notion that adaptation to a LCHF diet takes a long time (>12 weeks) and is why most research to date does not demonstrate benefits in favor of LCHF or K-LCHF diets.
Increases in fat oxidation among the K-LCHF group was also seen alongside a reduction in exercise efficiency, as evident with a 5-8% increase in oxygen cost at the race walkers 10,000-m race performances; this is rather significant as long-distance endurance events are heavily reliant on being as efficient as possible, so a reduction in exercise efficiency may have a significant negative impact on performance.
Acute restoration of glycogen stores with a 1-day HCLF restoration period prior to a key performance was not enough to “outweigh” the negative impacts of a K-LCHF on 10,000-m race performance; this was evident by carbohydrate oxidation rates only reaching 61-78% of baseline values established with all athletes were on the standard HCLF diet.
High-intensity exercise performance was impaired in those on the K-LCHF diet, and acute restoration of glycogen stores was not sufficient to improve high-intensity exercise capacity to baseline values seen when all athletes were on the HCLF diet.
The findings herein from this study have now been pretty clearly replicated over and over and over again, and the moral of the story remains pretty much the same as what I stated back in my 2019 post on this topic and what I stated in the introduction of this current post, of which I will state them again with some underlining to emphasize important points:
LCHF diets had minimal evidence to document any superiority to a traditional HCLF diet for endurance performance.
LCHF diets may impair an endurance athlete’s ability to do high-intensity work in training and in racing due to an impaired ability to derive energy from glucose or glycogen (i.e., glycolysis).
There is some possibility of a LCHF diet to improve performance in ultra-endurance athletes that train and race at very, very low intensities for extremely prolonged periods of time that rarely ever do work at an intensity above 60-65% of their VO2 max (e.g., multi-day adventure racers), which is an extremely low intensity compared to what most endurance athletes race at for single-day events (e.g., trail runs, road runs, triathlons, etc.).
I would also like to add one additional take-home message that seems to be emerging from the data as more studies are done on this topic:
There seem to be a very small subset of endurance athletes that respond positively to a LCHF or K-LCHF diet, demonstrating improvements in performance despite reduced carbohydrate oxidation capacity.
In the Burke and colleagues study discussed just above (1), when looking at some of the individual-level data amongst the 13 athletes, there was one single athlete that seemed to perform better in their 10,000-m race performance. This is also a typical finding in these sorts of studies. The majority of athletes do not improve their performance and usually perform worse when on a LCHF or K-LCHF diet, but there is usually one or two athletes that actually do see small improvements.
This is where I am starting to see the potential utility of a LCHF/K-LCHF diet for endurance athletes and starts getting at the genetic variability in the response to dietary interventions. It is also for this reason that dietary protocols are not a one-size-fits-all. Yes, based on the research we currently have, a “standard” HCLF diet seems to be what most endurance athletes should be following as this diet seems to produce optimal performance across a range of endurance distances and disciplines, but there is a very, very small minority (~5%) of endurance athletes that may, just may, see some performance improvement/optimization when on a LCHF or K-LCHF diet.
However, I say this with caution as there are other risks to adopting a high-fat diet related to overall health and well-being that are sometimes not captured in all the studies done in this area, namely the increased risk of illness (e.g., upper respiratory tract infection) and reported negatively impacted mood that is sometimes seen in athletes undergoing a high-fat dietary protocol. Adequate carbohydrate availability and a reliance on glucose at rest seems to be related to optimal functioning of various bodily systems and functions, and it may be that, even if an athlete performs better in a lab or in a race, they may still experience negative health consequences as a side effect. More research indeed needs to be done in this area.
Conclusions
So, there you have it. I hope this provides a rather concise update on the state of the LCHF literature and allows you to walk away with a better understanding of this topic more broadly. As with almost anything in life, let alone sport, things are rarely black and white or cut and dry. Things tend to fall more in the middle, and dietary interventions are no different. Diet can elicit strong emotions and reactions from athletes, with some dietary principles being held in an almost dogmatic or religious light among many, but I strongly encourage you to be open-minded when reading this and when reading other nutrition-related research as nutrition can be very, very individual. While a HCLF diet may work best for most, there could certainly be some that function best on a LCHF diet. The same can be said for many other dietary approaches, from veganism, to vegetarianism, etc. Just because it works for you or worked for someone else, does not mean it works for everyone. However, I would still strongly encourage the vast majority of endurance athletes to eat a HCLF diet for optimal performance and health… of which I can cheers (over a pastry or two) to that!
References:
Burke LM, Whitfield J, Heikura IA, Ross ML, Tee N, Forbes SF, Hall R, McKay AK, Wallett AM, Sharma AP. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. The Journal of Physiology. 2021 Feb;599(3):771-90.
Cao J, Lei S, Wang X, Cheng S. The Effect of a Ketogenic Low-Carbohydrate, High-Fat Diet on Aerobic Capacity and Exercise Performance in Endurance Athletes: A Systematic Review and Meta-Analysis. Nutrients. 2021 Aug;13(8):2896.
Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism. 1983 Aug 1;32(8):769-76.
Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: nutrition and athletic performance. Journal of the Academy of Nutrition and Dietetics. 2016 Mar 1;116(3):501-28.
Vitale K, Getzin A. Nutrition and supplement update for the endurance athlete: review and recommendations. Nutrients. 2019 Jun;11(6):1289.
Happy training and racing!
-Ryan Eckert, MS, CSCS
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