For decades, sports nutrition has revolved around carbohydrate (more detail). Although this substrate has its benefits, of course, it has a major limitation: its scarcity.
We barely accumulate a few hundred grams of carbohydrate, while our fat reserves are immense: we have enough energy in the form of fat to complete dozens of marathons.
Including some period of ketogenic diet could improve our ability to access this large fat store, reserving glycogen for high-intensity efforts and improving overall performance (as well as losing more fat).
But on the other hand, the reduction of muscle glycogen could impair higher intensity activities, which rely mainly on this fuel.
Your energy systems
To better understand the impact of ketosis on athletic performance we need to take a quick review of our energy systems and their relationship to each type of training.
Our body requires a constant supply of ATP, the energy currency of life, and this demand for energy skyrockets with physical activity.
Muscles have a small reserve of ATP in its pure state, available for only a few seconds. To produce new ATP we use three different pathways, corresponding to our three energy-producing systems.

  1. Phosphagen system: This is the fastest, and uses the creatine phosphate (or phosphocreatine) present in the muscle to synthesize new ATP (detail). The amount of creatine phosphate stored in the muscle is small, so this system only contributes relevant energy during the first seconds of intense activity. When we perform a hundred-meter sprint or a couple of heavy squat repetitions we are mainly using this system. Creatine allows us to increase these reserves, hence its benefits.
  2. Glycolytic system: It uses muscle glycogen as an energy substrate. In addition to ATP, it generates lactate, which accumulates in the blood and is associated with increased fatigue (detail). This system produces more ATP than the phosphagen system, but it remains an inefficient pathway, unable to support exercise beyond a few minutes at high intensity.
  3. Aerobic system: This is the slowest system, but also the most cost-effective from an energetic point of view, as it produces a large amount of ATP. Unlike the previous
    systems, it requires oxygen (hence its other name, oxidative system), and is carried out
    in the mitochondria of the cells. It uses fatty acids as its main substrate, and since we
    have tens of thousands of calories in the form of fat, this system could sustain physical
    activity for days.
    Since the production of ATP from phosphocreatine is only useful for short efforts, when
    performing physical activity of a certain duration we basically use a combination of fat and
    glycogen.
    The specific percentage we use of each type of fuel depends on multiple factors, such as our
    metabolic flexibility and the intensity of the physical activity. On average, 70% of our energy at
    rest comes from fat, a percentage that decreases as intensity increases.
    The amount of fat used also depends on the available glycogen. If the muscles have less
    glycogen, they will use more fat, and vice versa.

  1. After training, the carbohydrate in the meal facilitates the reloading of muscle glycogen, but
    even in the total absence of carbohydrate the body has mechanisms to recover some muscle
    glycogen, for example by converting lactate into glucose.
    With these concepts clear, we will better understand the effect of a ketogenic diet in different
    types of training.
    Strength
    Maximal strength and power are highly dependent on phosphocreatine stores, which are not
    impacted by a ketogenic diet. For this reason most studies do not observe loss of maximal
    strength or power when following a ketogenic diet. Let’s review some examples.
    One study subjected elite gymnasts to a ketogenic diet for 30 days, with the following
    macronutrients: 22g of carbohydrate, 200g of protein and 120g of fat. During the 30-day study
    they did not lose strength, but lost more fat than those on a standard diet.
    A similar study, this time with men and women, obtained the same results. In the short term, a
    ketogenic diet did not produce worse performance in several tests of maximal strength or
    power: grip strength, vertical jump, bench press and squat (one repetition).
    And finally, a study in powerlifting athletes concludes that several months of ketogenic diet
    produces greater fat loss without harming the marks.
    Muscle mass
    To gain muscle mass we need to increase training volume (more repetitions per set), and in
    this case we will rely more on muscle glycogen than on phosphocreatine. Since a ketogenic
    diet reduces glycogen levels, it stands to reason that it will be less effective in gaining muscle
    mass. In addition, the ketogenic diet reduces insulin levels, a hormone with anabolic potential.
    Although protein stimulates insulin, carbohydrate has a greater effect.
    And indeed, that is what we see.
    For example, in a study of 18 overweight, untrained women, the high-carbohydrate diet group
    gained more muscle than those on a ketogenic diet, but lost less fat.

  1. Another study compared the outcome of a 10-week strength training program in 25 young
    men, who were divided into two groups with different diets but the same calories. One group
    ate a high-carbohydrate diet and the other a ketogenic diet. Both groups gained muscle and
    lost fat, but not to the same degree. The ketogenic diet group gained 2.4% muscle mass,
    compared to 4.4% in the high-carbohydrate diet group. However, those on the ketogenic diet
    lost more fat.
    In addition, several studies indicate that beta hydroxybutyrate reduces the oxidation of
    leucine, the main signaling amino acid of protein synthesis, preventing muscle degradation and
    explaining in part why ketogenic diets are not as limiting as previously thought when it comes
    to gaining muscle mass.
    In short, it is possible to gain strength and muscle mass on a ketogenic diet, but it is generally
    less effective than a carbohydrate-rich diet. That said, the differences are relatively small, and
    in return it helps you lose more body fat while gaining muscle.
    It is no coincidence that the ketogenic diet has been used in the bodybuilding world during the
    most aggressive definition cycles.
    On the other hand, in many sports it is not so much the absolute strength or muscularity that is
    important, but the muscle/fat ratio, and a relatively short period (3-6 weeks) of ketogenic
    dieting improves in most cases this ratio.
    A study in Crossfit athletes divided the 27 participants into two groups. One followed a
    conventional diet (almost 200 grams of carbohydrate daily) and the other a ketogenic diet (44
    grams of carbohydrate/day). Six weeks later, no differences were found in performance in
    different physical tests, but there were significant differences in body composition, with those
    who had followed a ketogenic diet losing an additional 3 kilos. This additional fat loss can
    compensate for a lower level of muscle glycogen.
    In taekwondo practitioners, a three-week period of ketogenic diet improved sprint time and
    delayed the onset of anaerobic fatigue, probably because of the greater fat loss.
    Summary and conclusions
    The ketogenic diet produces metabolic adaptations that impact performance through different
    pathways. It increases fat oxidation on the one hand but reduces glycogen stores on the other.
    This is reflected differently in each type of sporting discipline, in addition to the need to
    consider its effect on body composition. Ketosis usually improves the muscle/fat ratio,
    counteracting some of the possible negative effects of glycogen loss. For this reason, ketogenic
    diets do not usually impair performance significantly, but facilitate fat loss.

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