It seems that low-carbohydrate, high-fat (LCHF) diets are of greater interest among athletes (1). More and more elite and amateur endurance athletes are claiming that there is an advantage to ditching the traditionally accepted high-carbohydrate, low-fat (HCLF) dietary strategies of the past in favor of higher-fat diets. Unfortunately, the realm of dietary intake for optimal performance is very gray with few “black and white” answers. The discussion of carbohydrate and fat intake for endurance performance is no different. Dietary intake can vary widely between athletes and is influenced by sport, personal preference, food intolerances and the training cycle of an athlete. Athletic success in the realm of endurance sport has been achieved by individuals following both a LCHF approach as well as a HFLC approach. To make matters even more confusing, there isn’t even consencus among those most qualified to be providing sports nutrition advice, with some Sports Dieticians favoring LCHF diets for optimal performance and others relying on the “tried and true” benefits of the more traditional HCLF diet.
This topic area is of great interest to me as I have had personal experience with both high-carbohydrate diets and low-carbohydrate diets. As a competitive amateur triathlete, I am always looking for ways to safely and effectively improve my performance. Alongside proper sport-specific training, optimal rest and recovery, and effective mental preparation, dietary habits represent a rather significant target area for performance improvements.
Therefore, the purpose of this training topic post is to discuss what the research says regarding LCHF diets vs. HCLF diets for endurance sport performance and to provide some general, albeit practical, recommendations for daily carbohydrate intake as well as carbohydrate intake during training sessions. Before I begin to dig deeper into the available research, let me first begin by differentiating between LCHF and HCLF diets.
High-Carbohydrate, Low-Fat (HCLF) diets, or the more “traditional” dietery approach for endurance athletes, typically consists of a carbohydrate intake of ~45-65% or more of total daily caloric intake (~5-12 grams/kilogram of body weight/day) and a fat intake of 20-35% of total daily caloric intake (9,10)
What does the Research Say?
You would think that there would be a plethora of research surronding this topic with the rapidly increasing popularity of LCHF diets among athletes (1). Interestingly, there is very little research out there and little evidence available that actually supports a LCHF diet for endurance sport performance (1,3,4,5,6). The majority of the research studies conducted have been in cyclists and triathletes, and most have found that cycling and running endurance remains largely unchanged and that high-intensity exercise performance (i.e., sprinting or hard, short efforts) decreases when consuming a LCHF diet (1,3,4,5,6) In most studies, researchers will typically have subjects consume both a LCHF diet (60-70% of caloric intake from fat) and a HCLF diet (65-75% of caloric intake from carbohydrates) in random order and perform a variety of exercise tests (primarily cycling tests) on them after following each dietary protocol.
One of the few studies (6) that found a LCHF diet to be superior to a HFLC diet had five well-trained cyclists consume, in random order, a HCLF diet (74% of caloric intake) or a LCHF diet (70% of caloric intake) for two weeks. Cycling exercise to exhaustion at 60% of VO2 max (maximum aerobic exercise capacity) was longer after consuming a LCHF diet when compared to the HCLF diet. However, performance on the other two higher-intensity tests (time to exhaustion at 90% VO2 max and 30 second all-out sprint) were not improved with a LCHF diet.
This seems to be the case with much of the research comparing LCHF diets with HCLF diets. LCHF diets tend to have a small, positive effect or no effect at all on lower-intensity cycling endurance (for example, cycling at 60% VO2 max to exhaustion). However, a LCHF diet typically loses its traction when examining higher-intensity exercise performance. For example, one study in seven female cyclists showed that cycling time to exhaustion at 80% of VO2 max was reduced by 47% in a LCHF diet (59% of caloric intake from fat) when compared to a HCLF diet (8). Other studies have also shown that cycling sprinting performance declines when consuming a LCHF diet (1).
One of the theries behind the proposed benefits of a LCHF diet is that this type of dietary manipulation allows an endurance athlete to alter the type of fuel that they rely on when exercising. Consuming a LCHF diet should improve an athlete’s ability to utilize fat during exercise, in turn allowing them to rely less on their limited carbohydrate stores during exercise and moreso on their nearly limitless stores of fat (even the leanest of athlete still have a nearly limitless store of fat to draw upon during exercise). This should subsequently allow athletes to go much farther without “bonking” or “hitting the wall”, which is that unpleasant feeling of complete fatigue, exhaustion, dizziness, and lethargy that is associated with depletion of carbohydrate stores during prolonged, intense exercise. In order to prevent this from occuring, athletes consuming a more traditional HCLF diet will need to consume carbohydrates during prolonged exercise in order to keep maintain normal blood sugar levels once carbohydrate storage becomes depleted.
To some extent, this theory makes sense at first. Humans do have a very limited store of carbohydrates to fuel exercise and a nearly unlimited store of fat to draw upon to fuel exercise. Humans also rely heavily on fat to fuel exercise at lower-intensity exercise, with the reliance on carbohydrates to fuel performance increasing with an increasing exercise intensity. In other words, fat utilization is highest during low-intensity aerobic exercise, such as an easy cycling, running, or swimming session. However, as the speed or pace of that training session increases, so does that athlete’s reliance on carbohydrate metabolism to fuel their performance. The reason for this increase in carbohydrate metabolism with increasing exercise intensity is simply due to the speed at which carbohydrate metabolism can provide energy to the working muscles. Fat metabolism is a much slower process than carbohydrate metabolism, making fat the ideal fuel choice when the exercise intensity, and therefore, energy demand, is low. It has been shown that peak fat metabolism during exercise occurs around 45-65% of VO2 max, which is a relatively low exercise intensity (7). However, when the demand for energy increases during more intense exercise, carbohydrates are a much more effective fuel source as they will deliver energy to the muscles much faster than fat metabolism will.
It has been clearly demonstrated that LCHF diets change the body’s ability to utilize fat as an energy source. Athletes following a LCHF diet will indeed increase their ability to utilize fat at any given exercise intensity (1,3,4,5,6). However, this also comes with a decreased ability to utilize carbohydrates during exercise, leaving an athlete with a reduced capacity to perform higher-intensity exercise (1). This may explain why research shows a decrease in performance of high-intensity exercise tests (for example, cycling to exhaustion at 80-90% of VO2 max or during high-intensity sprints) when following a LCHF diet, but unchanged performance during lower-intensity exercise tests (for example, cycling to exhaustion at 60% of VO2 max).
You may be thinking that this doesn’t matter for an endurance athlete as they typically perform at lower exercise intensities over a prolonged period of time when compared to other high-intensity, stop-and-go style sports (think of basketball, baseball, hockey, the 100m sprint, etc.). Even endurance athletes, however, compete at intensities in both training and competition that far surpass the intensity at which peak fat metabolism has been shown to occur (7). Essentially, this means that an improved ability to metabolize fat during exercise (as has been shown to occur with LCHF diets) would be beneficial if endurance athletes trained and competed at intensities low enough that they didn’t need to rely on effective carbohydrate metabolism to fuel higher-intensity performance. Endurance athletes, however, require the ability to effectively metabolize fat and carbohydrate in order to support efforts in high-intensity training sessions as well as in competition. This capacity to quickly and efficiently metabolize both fat and carbohydrate to fuel performance has been termed “metabolic flexibility”, and is important for endurance athletes to be able to perform at both lower and higher intensity exercise (1).
So it seems, based on available research, that a LCHF diet may not be the optimal choice for most endurance athletes as the improvement in fat metabolism may not outweigh the potential risks of reducing one’s capacity to metabolize carbohydrates during exercise and competition. However, this doesn’t mean that a LCHF diet won’t prove to be beneficial for some athletes.
As I mentioned previously, there is very little research that has been done comparing LCHF diets vs. HCLF diets for sports performance, and the research that has been done suffers from many limitations that make it difficult to draw sound conclusions, including very small samples chosen for the studies (typically 5-16 subjects), the inclusion of primarily well-trained athletes in these studies, and a focus on cycling performance measures (1,3,4,5,6,8). This makes it hard to completely throw the LCHF diet out the window as there simply isn’t enough research yet to discount this dietary strategy for endurance performance.
Additionally, there may be cases in which a LCHF diet may be beneficial for performance in some athletes. A recent literature review on this topic suggests that a LCHF diet may be beneficial for athletes wishing to reduce body fat as this type of diet has been shown to be slightly more favorable for reducing body fat when compared to a HCLF diet of similar caloric intake (1). This may simply be due to the fact that a LCHF diet improves the body’s ability to metabolize fat during rest and during exercise, but it is difficult to tell exactly why this is the case as there isn’t enough research available.
Furthermore, there may be a sub-group of athletes in which a LCHF dietary approach may be beneficial, including ultra-endurance athletes that compete at a relatively low exercise intensity for a very long period of time (1). For example, athletes competing in multiple-day running or hiking competitions in which the majority of competition takes place at <65% of VO2 max may benefit from an improved ability to metabolize fat as much of their exercise during competition takes place below the intensity at which peak fat metabolism occurs. Howver, even these endurance athletes will still need to consume carbohydrates during competition at the very least as carbohydrate metabolism will still be required in order to support their performance, albeit at a relatively much lower level than an endurance athlete competing in a single-day event like a short-course triathlon or a marathon.
Finally, a LCHF dietary strategy may make sense for athletes that are sensitive to carbohydrates (1). A higher-fat approach may be necessary if consuming high amounts of carbohydrates causes unneccesary gastrointestinal discomfort. Gastrointenstinal distress is a rather significant issue for many endurance athletes, and so if an athlete is sensitive to carbohydrates, it may simply make sense to consume a higher-fat diet.
Summary & Practical Recommendations
To summarize, there truly is no”perfect” or “ideal” dietary approach for endurance athletes. Although the current research seems to indicate that a more traditional, HCLF dietary approach is better suited for overall endurance performance, research often doesn’t take into account individual differences between athletes. Therefore, while most athletes may perform best on a moderate- or high-carbohydrate diet, some may perform best on a low-carbohydrate diet, and some may perform best on something in between these two approaches. At the end of the day, diet is very personal and can vary widely between athletes. Scientific research can often give us insights into what seems to be best for large groups of people, but it is up to each individual athlete to find what works best for them. If an athlete does wish to experiment with a new dietary approach or wishes to tweak their diet in some way, I would suggest doing so under the guidance of a Registerd Dietician, preferably a Sports Dietician, as these professionals are most qualified to help guide an athlete’s dietary choices. For those interested in the current sports nutrition recommendations for daily carbohydrate, fat, and protein intake as well as carbohydrate intake during exercise, see Table 1 and Table 2 below.
Table 1 describes the current recommendations for each of the macronutrients for both general health/well-being as well as fitness and general sports performance (9).
Table 2 describes the currently accepted sports nutrition recommendations for athletes (10).
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