The use of the ketogenic diet for athletic performance – endurance and strength/power – remains a controversial topic in the nutrition and fitness industry. Whether the ketogenic diet improves or impairs performance is heavily disputed. Reaching a conclusive answer is dependent upon the individual, their goals and the way in which the ketogenic diet is utilised.
Let’s assess some of the research, rather than focusing on the anecdotal claims which usually take centre stage.
Potential benefits for endurance athletes
Research is mixed. Some studies show performance to be improved following the ketogenic diet or superior to the control group (1), some show maintenance of performance levels (2,3,4), and some show reduced performance at higher intensities (5,6).
In studies that show impaired performance at higher intensities (5,6), the individuals follow the ketogenic diet for < 4 weeks. It’s indicated by multiple studies (1,2,3,7,8) that a minimum of 12 weeks in nutritional ketosis is required for adaptations to occur, termed being ‘keto-adapted’.
When athletes are keto-adapted, they show improved performance outcomes and different adaptations, such as the capacity to resynthesis glycogen similarly to non-keto athletes (3). From spending time in nutritional ketosis, it is likely that the body adapts and builds the metabolic machinery to become more effective and efficient in this state.
Enhanced fuel reserves (aka avoid hitting the wall)
Keto-adapted endurance athletes may benefit from increased capacity to utilise fat as fuel both at rest and at higher exercise intensities (3). A recent study showed that keto-adapted athletes oxidise fat at a rate more than twice that of a high carb athlete (3), indicating the body’s adaptation to better accessing and utilising fat for fuel.
As a keto-adapted athlete your energy resources are therefore greater, with approximately 30,000 calories of energy available from fat stores to access, versus only 2000 calories of energy in glycogen stores. This avoids the need for glycogen stores to become so depleted during longer races and therefore the need to continually refuel, which may contribute to improved performance.
Despite being keto-adapted, the athletes are shown to be able to replenish and resynthesise glycogen stores as quickly as non-keto athletes (3).
Delayed central fatigue
At the later point in long endurance races, central fatigue sets in and altered brain function may result in confusion/depression. Adapting to a state of nutritional ketosis may delay central fatigue.
The presence of ketone bodies may increase levels of the amino acid leucine during exercise. Leucine then may compete with the amino acid tryptophan, which is the precursor to serotonin to cross the blood-brain barrier. This subsequently may reduce the formation of serotonin which is associated with the onset of fatigue (9).
Note: there isn’t any scientific research to support this (that I can find!). There is research to show that the oxidation of leucine is reduced (10) which could theoretically mean that there are higher levels therefore available to compete with tryptophan.
Potential benefits for strength and power athletes?
Research is mixed, some studies show improvements (1), some show maintenance (4,7,8,11), some show a reduction in performance (12). A range of factors will contribute to the varied outcomes: adaption period, type of diet and training as well as individual response.
Potential benefits in general
Athletes on the ketogenic diet show greater fat loss and maintenance/increase of lean muscle versus their comparative group (1,4,7,8,11,13).
A combination of factors are likely to contribute to the greater fat loss: following a restricted diet, increased satiety from a change in macronutrient ratios, increased metabolic flexibility and training regime.
A combination of factors are likely to contribute to the maintenance/increase of lean muscle observed: protein intake, training regime and potential protein sparing effects of ketone bodies (detailed below).
The athletes on the ketogenic diet always have their protein intake doubled versus their previous levels and are at a higher level than the comparative group. Protein activates mTOR (mammalian target of rapamycin) which stimulates muscle protein synthesis (14,15).
The volume of training is also not tightly controlled for which may not only contribute to an energy deficit and fat loss but also a greater stimulus for muscle protein synthesis.
Note: there is the scientific research to indicate that the ketogenic diet results in favourable body composition – protein intake plays a key role in this.
Protein sparing effects
The state of nutritional ketosis is an adaptive mechanism for survival and it is logical that part of this process may involve conserving protein (16). Research in human and animal models indicates that the presence of ketone bodies may promote muscle protein synthesis (10, 17) and prevent the breakdown of muscle (18).
It is proposed that for the keto-adapted athlete, when exercising, the presence of ketone bodies may result in them preferentially being oxidised in place of branched chain amino acids (BCAAs), and subsequently sparing them (9).
The potential protein sparing effects of ketone bodies may contribute to the maintenance/increase in lean body mass seen when athletes lose fat on the ketogenic diet (1, 1,4,7,8,11,13).
Note: although there is some research to support this and it makes theoretical sense, we need more current studies to understand this.
The ketone body β-hydroxybutyrate (BHB) may have anti-inflammatory properties (19), decrease the production of ROS (reactive oxygen species) (20) and increase antioxidant defences (21). This potentially may enhance the recovery process, facilitating athletes resuming training quicker.
Note: there isn’t any scientific research to show this yet, only anecdotal claims.
Improved gastrointestinal symptoms
Approximately 20-50% of athletes experience GI symptoms (for example an irritable gut, diarrhoea, vomiting etc.) which tend to increase with the intensity of exercise (22). Levels of inflammation may be a mechanism that contributes to this (23) and therefore may potentially be improved by the ketogenic diet.
Reports of improved GI symptoms may be due to intaking less food around training (due to relying on fat stores), especially sports gels and snack bars which commonly include various ingredients which may aggravate the gut and result in irritable bowel syndrome (IBS) like symptoms (24).
Note: there isn’t any scientific research to show this yet, only anecdotal claims. Obviously if you’re reducing food intake around exercise and avoiding foods that aggregate the gut, then this will be beneficial for any GI problems!
What about the drawbacks?
During the adaptation period training and performance is likely to suffer and some studies indicate that higher intensity exercise performance may be compromised by the ketogenic diet (5,6). For exercise that requires glycolytic activity, such as high intensity exercise and weight training, some carbohydrate intake timed appropriately to support this type of training is likely to improve performance versus no intake of carbohydrate.
It is argued that the ketogenic diet may result in adaptations that result in downregulation of certain enzymes that are necessary for glycolytic activity (5, 25). The body does respond and adapt to the nutrients that you provide it, as well as training.
Note: the strategic incorporation of some carbohydrate may keep the body primed to utilising carbs, reduce downregulation of glycolytic enzymes and support glycolytic performance.
Negative health effects
Are detailed in The drawbacks of the ketogenic diet. Whether the ketogenic diet works for you or not will be completely individual and it is indicated to be a success for approximately one third of people that try it, so don’t worry if it doesn’t!
Check out 10 mistakes an athlete makes when going low carb /keto for mistakes to avoid! During the adaptation period, you may experience GI issues from switching to a high fat diet and reduced fibre diet. As an athlete you are also going to be at increased risk of electrolyte imbalance/dehydration.
Other considerations, such as cholesterol and thyroid hormone levels, need to be considered in conjunction with your overall health status, training volume and diet. No diet is a pass to being “healthy” – it requires a small amount of effort and a bit of balance!
Low energy availability
In general, approximately 22-55% of athletes suffer from low energy availability (LEA) (26). The drawbacks of the ketogenic diet details LEA further. LEA means that once your body has utilised the energy it needs to stay alive and carry out exercise the remaining energy available is insufficient. Subsequently there is inadequate energy to carry out processes such as immune and menstrual function, bone growth, cardiovascular function etc.
LEA affects males as well as females, and common initial symptoms can include: increased occurrence of colds/flu/illness, tiredness/fatigue, frequent stress fractures, reduced sex hormones and irregular/absent menstrual cycles.
Although there are not direct studies evaluating the effect of the ketogenic diet on low energy availability, studies do show that athletes lose weight despite being encouraged to eat adlibitum (13). This may be due to the satiating effects of the ketogenic diet as well as it being more restrictive.
Note: as an athlete / someone who trains regularly, planning to ensure that you meet your energy requirements is key, especially if you start the ketogenic diet.
My takeaway – always personalise what you do and create a customised and periodised nutrition strategy that enhances your performance, recovery and life! And make sure that you enjoy it!
What are your thoughts athletes? What are your experiences with the ketogenic diet?