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What influence do ketogenic / low-carbohydrate diets have on training performance?

Welchen Einfluss haben ketogene / kohlenhydratarme Ernährungsformen auf die Trainingsleistung?

Low-carb and ketogenic diets are very popular. These diets have been around for a long time and share some similarities with the so-called Paleo diet (1). Scientific research shows that low-carb diets can help with weight loss and improve numerous health markers (2). However, the data on muscle growth, strength and performance is mixed (3, 4, 5).

This article will take a more detailed look at the effects of low-carb/ketogenic diets on physical performance.

What are low-carb and ketogenic diets?

The guidelines for a low-carb diet vary between different sources. In the scientific field, low-carbohydrate diets are usually classified as diets with less than 30% of calorie intake in the form of carbohydrates (6, 7).

Most average low-carbohydrate diets and diets include 50 to 150 grams of carbohydrates per day, a fairly high protein intake and a moderate to high fat intake. For some athletes, "low-carb" can also mean a carbohydrate intake of over 200 grams per day.

In contrast, a ketogenic diet or dietary pattern is much more restrictive and usually includes only 30 to 50 grams of carbohydrates per day in combination with a high fat intake (8). This extremely low carbohydrate intake allows the body to reach a state known as ketosis, in which ketones and fat become the primary sources of energy for the body and brain (9).

There are different versions of ketogenic diets, with the following three being the most common:

  • Standard ketogenic diets: these are extremely low-carbohydrate diets with moderate protein and high fat intake. These diets usually include 75% of calories in the form of fat, 20% of calories in the form of protein and 5% of calories in the form of carbohydrates (8).
  • Cyclical ketogenic diets: These diets include short periods of high carbohydrate intake (called refeeds). A typical pattern would be 5 ketogenic days followed by 2 high-carbohydrate days.
  • Targeted ketogenic diets: These diets allow you to consume carbohydrates at specific times (usually before, during and after intense workouts).

Most low-carb and ketogenic diets and diets restrict people from eating foods such as grains, rice, beans, potatoes, sweets, cereals and fruit.

An alternative approach is a cyclical carbohydrate intake, where carbohydrate-rich phases or refeeds are regularly integrated into ketogenic diet plans.

Summary: A low-carbohydrate diet usually involves a higher protein intake combined with less than 30% of calories in the form of carbohydrates. Ketogenic diets are very high in fat, include moderate carbohydrate intake and are virtually carbohydrate-free.

Low-carbohydrate diets and the body's adaptation to fat as a primary energy source:

During a low-carb or ketogenic diet, the body adapts to using fat as an energy source and becomes more efficient at this. The drastic reduction in carbohydrate intake causes an increase in the production of ketones, which are produced in the liver from fatty acids (10). Ketones can provide energy in the absence of carbohydrates, during prolonged fasting, during long periods of exercise or in people with poorly controlled type 1 diabetes (11, 12, 13). Even the brain can be partially supplied with energy by ketones (14).

The rest of the required energy is provided through a process known as gluconeogenesis, in which the body breaks down fats and proteins and converts them into carbohydrates (glucose) (14).

Ketogenic diets and ketones have many beneficial properties. Ketogenic diets are even used to treat diabetes, neurological diseases and cancer, as well as to reduce risk factors for heart and respiratory diseases (2, 15, 16).

The adaptation to using fat as an energy source can be extensive during a ketogenic diet. A study conducted with ultra-endurance athletes found that the ketogenic group burned up to 2.3 times more fat during a three-hour training session (17).

However, although low-carbohydrate and ketogenic diets can have many health benefits, there is still debate about how these diets affect exercise performance (18, 19).

Summary: In the absence of carbohydrates, your body burns fat as an energy source. This happens mainly through increased fat oxidation and the production of ketones.

Low-carbohydrate diets and muscle glycogen

Dietary carbohydrates are broken down in the body into glucose, which enters the bloodstream as blood sugar and serves as the primary source of energy for moderate to high intensity exercise (20). Over several decades, scientific research has repeatedly shown that the consumption of carbohydrates can support exercise performance, particularly in endurance sports (21).

Unfortunately, the human body can only store enough carbohydrates (glycogen) for about 2 hours of exercise. After these 2 hours, exhaustion, fatigue and declining endurance performance can occur. This is also referred to as "running into a wall" (22, 23, 24). To counteract this, most endurance athletes consume a high-carbohydrate diet, fuel their bodies with carbohydrates the day before a major sporting event and consume carbohydrate supplements or other carbohydrate-rich foods during training and competition.

However, low-carbohydrate diets contain comparatively small amounts of carbohydrates and therefore cannot help to optimize and maximize the stored glycogen reserves of the muscles.

Summary: Stored carbohydrates are an efficient source of energy for training sessions lasting up to 2 hours. After this period, energy release and endurance performance usually decline.

Low carbohydrate diets and endurance performance

Numerous studies have been conducted on the use of fat as an energy source for athletic performance (25). During exercise, fat provides more energy at lower intensities, while carbohydrates provide more of the energy needed at higher intensities. In other words, the use of carbohydrate as an energy source increases with increasing exercise intensity, while the use of fat as an energy source decreases with increasing exercise intensity (26).

Image source: The Science of Sport.

As part of scientific research, scientists wanted to find out whether a low-carbohydrate diet could change this behavior (18, 19). In their study, the scientists found that ketogenic athletes mainly burned fat at up to 70% of maximum intensity, while high-carbohydrate athletes only burned fat at up to 55% of maximum intensity. It is also worth noting that the ketogenic athletes participating in this study burned the greatest amount of fat ever measured in a study of this type (17).

Despite these positive study results, it may be that fat is not capable of producing energy to meet the demands of elite athletes' muscles (27, 28, 29). For this reason, further research with athletes is needed before concrete conclusions can be drawn.

However, studies have shown that low-carbohydrate diets can help reduce fatigue during prolonged exercise. They may also help to lose body fat and improve health without compromising exercise performance in the low to moderate intensity range (4, 30, 31).

In addition, these diets can train your body to burn more fat, which could help you conserve your glycogen reserves during exercise (17).

Summary: A low-carbohydrate diet is probably suitable for most people training at low to moderate intensity, but more research is needed before any conclusive statements can be made for athletes at a high performance level.

How carbohydrates affect muscle growth

To date, there are no studies that have shown that low-carbohydrate or ketogenic diets are beneficial for high-intensity strength or speed-based sports. This is due to the fact that carbohydrates support muscle growth and high-intensity exercise performance in several ways:

  • Promote recovery: carbohydrates can help support recovery after exercise (32).
  • Insulin production: Carbohydrates promote insulin production and secretion, which aids nutrient transport and absorption (33).
  • Provision of energy: Carbohydrates play an important role in the anaerobic and ATP energy systems, which are the primary energy pathways utilized during high-intensity exercise (34).
  • Reducing muscle breakdown: Carbohydrates and insulin help to reduce muscle breakdown, which can improve net nitrogen balance (35, 36)
  • Improve neural drive: Carbohydrates improve neural drive, resistance to fatigue and mental focus during exercise (37).

However, none of this means that your diet has to be very high in carbohydrates, as is the case with the typical Western diet. A moderate carbohydrate diet or cyclical carbohydrate intake can work well for most sports. In fact, a diet with moderate carbohydrate intake and high protein intake appears to be optimal for muscle growth and body composition in people who are already lean and active (38).

Summary: Carbohydrates play an important role in muscle growth and high-intensity exercise performance. There are no studies showing that low-carbohydrate diets are superior for this purpose.

Studies on low-carbohydrate diets for athletes

Several studies have looked at the effects of low-carbohydrate diets on high-intensity exercise performance. However, these studies show mixed results.

One study found no difference between the ketogenic and high-carbohydrate groups during high-intensity sprints. However, the ketogenic group fatigued less during low-intensity cycling, which was likely related to the body using more fat as an energy source (39).

Other studies have shown that people who follow a low-carbohydrate diet conserve more muscle glycogen and use more fat as an energy source, which could have positive effects in ultra-endurance sports (18). However, these results are less relevant for athletes who perform high-intensity training sessions or training sessions lasting less than 2 hours.

The evidence is also mixed for obese people, with some studies showing benefits of low-intensity aerobic exercise and others showing negative effects (31, 40).

Some studies have concluded that the individual response may also vary. For example, one study found that some athletes showed improved endurance performance, while others experienced drastic deterioration in their endurance performance (41).

At this time, research does not show that low-carbohydrate or ketogenic diets can improve high-intensity athletic performance compared to higher-carbohydrate diets. However, when it comes to lower-intensity exercise, low-carbohydrate diets can keep up with conventional high-carbohydrate diets and may even help to ensure that more fat is used as an energy source (31).

Summary: Low-carbohydrate and ketogenic diets do not appear to enhance high-intensity exercise performance. However, they do appear to rival high-carbohydrate diets when it comes to exercise performance at lower intensities.

Are there any benefits for athletes?

One beneficial aspect of a low-carb or ketogenic diet is that it habituates the body to burn fat as an energy source (42). Research has shown that this can help endurance athletes conserve their glycogen reserves and prevent them from "hitting a wall" during endurance activities (18, 42).

This helps you rely less on carbohydrates, which could be important for athletes who struggle to digest and consume carbohydrates during exercise. It could also be beneficial during ultra-endurance events where access to food is limited (18).

In addition to this, several studies have shown that low-carbohydrate and ketogenic diets can help people lose weight and improve their overall health (43, 44). Such fat loss can improve your fat-to-muscle ratio, which is extremely important for exercise performance, especially in weight-based sports (45, 46).

Training with low glycogen stores has also become a popular training technique known as "train with low glycogen reserves and compete with full glycogen stores" (47). This can improve fat utilization, mitochondrial function and enzyme activity, which has beneficial effects on health and exercise performance (47).

For this reason, following a low-carbohydrate or ketogenic diet for a short period of time - such as during the off-season - could improve long-term exercise performance and health.

Summary: Low-carbohydrate diets can be beneficial for some types of endurance sports. They can also be used strategically to improve body composition and health.


Low-carb or ketogenic diets may be a good choice for healthy people who exercise primarily to stay healthy. However, there is currently no solid evidence that these diets improve performance in athletes compared to high-carbohydrate diets.

However, research in this area is still in its infancy and some early results suggest that these diets may be a good choice for lower intensity or ultra-endurance training.

At the end of the day, however, carbohydrate intake should be tailored to individual needs.




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