Weight and athletic performance are often directly linked. In some cases, performance can be optimized at a certain body weight while in others athletes are aiming to compete in a specific weight class. Weight loss is never easy; however, it can be more challenging for athletes, as they need to maintain energy levels and muscle mass with lower caloric intakes.
In cases where athletes need to drop weight quickly (not ideal but a reality!), techniques include: severe caloric restriction, diuretics, vomiting, excessive training, saunas, etc. Unfortunately, although effective at achieving short-term weight loss, these strategies can negatively impact performance AND result in acute and long-term health problems (Turocy et al, 2011). Lately, there has been interest from athletes in the ability of a ketogenic diet to control body weight while maintaining muscle strength and size.
Can a ketogenic diet help athletes lose weight and maintain performance? Or is this a recipe for weak muscles and sluggish outcomes?
What is a Ketogenic Diet?
A ketogenic diet is very low in carbohydrates, high in healthy fats (~70%) and provides an adequate amount of protein. The idea is that this type of diet will force the body into ketosis. Ketosis is a physiological state where the body burns fats instead of carbohydrates as a energy source (Rhyu et al., 2014). Carbohydrates tend to be a quick energy source, whereas fats take longer to burn.
Historically, athletes focus on carbohydrates to maximize their ability to use this quick energy source. From a weight loss perspective, however, increased use of fat as an energy source theoretically results in a slow and steady energy use potentially leading to decreased weight and fat mass.
The jury is still out on the effectiveness of a ketogenic diet in promoting weight loss in general and most studies seem to focus on subjects with obesity NOT healthy athletes (Paoli et al, 2012).
Are Ketogenic Diets Effective for Athletes?
Firstly, it is important to note that not all athletes are the same and there is a substantial amount of individual variation, in addition to the natural variation you would expect by sport – i.e., ultra-endurance sport (running a marathon), endurance sport (running a 10 km), and an anaerobic sport (100 m dash). It is also important to note that there has not been a lot of research in the area.
Here’s what the LIMITED evidence suggests…
To start, one study looked at the effect of three weeks of weight loss achieved using a ketogenic diet versus a regular diet on physical fitness and inflammation in Taekwondo athletes. They compared two groups of 10 athletes and did not find any difference in the amount of weight lost or percent body fat between the groups. They also found that both groups lost an equal amount of lean mass (muscle). Conversely, they did report that those on the ketogenic diet had faster 2,000 m sprint times, less fatigue, and less of an increase in tumor necrosis factor-alpha (one of the biochemical markers of inflammation). The ketogenic diet did not impact short, anaerobic activities such as 100 m sprint and jumping. Importantly, however, the duration of the diet was short and the groups used were small (Rhyu et al., 2014).
A second study in gymnasts found that 30 days of a ketogenic diet resulted in decreased body weight (-1 kg) and body fat (-2 kg) and an increase in lean mass (+2.5 kg) without negatively impacting performance. Here the authors used nine elite gymnast did baseline testing a placed them on the ketogenic diet for 30 days and tested them again. Three months later they used the same athletes and tested them before and after 30 days on a Western diet. In both cases the athletes were allowed to eat as much food as they liked within the diet constraints provided (Paoli et al., 2012).
A third study using eight male cyclists and either four weeks of a ketogenic diet or a regular diet in a cross-over design (similar to that explained above) found the ketogenic diet improved body composition and increased cardiovascular performance (VO2max and oxygen uptake at lactate threshold). Conversely, they also reported lower power output when the athletes were cycling at maximum effort.
The idea is that a high-carbohydrate diet on a regular basis trains the body to use and need more carbohydrates, whereas a high fat diet would train the body to use more fat and thus need less carbohydrate. There may be some merit to the idea at lower intensities but at a certain intensity the body is going to need to switch to glucose; in these conditions, a ketogenic diet may not work as well (Zajac et al., 2014).
No Baloney’s advice? There is limited evidence to suggest that short-term use of a ketogenic diet may be beneficial, or at least not harmful, for athletes. The catch is that it needs to be done without inducing ketoacidosis – a dangerous condition typically seen in uncontrolled diabetics where accumulating ketones alter the body’s pH. Although ketoacidosis is rare in people without diabetes, it has been reported in cases involving very low carbohydrate, ketogenic diets (Freeman et al., 2014). More commonly, there is also the potential for ketogenic diets to increase dehydration, hypoglycemia, nutrient deficiencies, and possibly kidney stones among other side-effects (Zajac et al., 2014).
The performance benefits, although found to be statistically significant, are small enough that they are unlikely to make a difference to all those but the most elite athletes and do not apply to high-intensity exercise. Red flags should also be going up because the maximum duration of any of the studies was 4 weeks and the maximum number of participants was 10 – in our books that means more research is needed. Given the “playing-with-fire” nature of these diets we wouldn’t recommend them for the average athlete, unless they are being closely monitored by a health professional.