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Does it make sense to talk about a carb-free diet for athletes? Even if at first sight it seems like it doesn't, in many cases a carb-free diet can be useful for athletes; in fact there is a lot of evidence of the effect of this type of diet on weight loss.

The mechanisms at the root of this effect are unclear but a series of causes may be suggested: one of these is the possible hypothesis that there is a metabolic advantage that could explain the significant effect of ketogenic diets on weight loss. The authors who espouse these lines of thinking hypothesise that the use of proteins as an energy source in ketogenic diets is a "costly" process for the body and that therefore it can lead to a "waste of calories". In the first phase of a diet with few carbohydrates, our body in fact needs around 60-65 grams of glucose per day which is extracted in small part (16%) from glycerol and for the most part from gluconeogenesis of dietary or tissue proteins. The role of energy expenditure through gluconeogenesis in ketogenic diets has been confirmed by several authors and the cost of this process has been calculated as being around 400-600 Kcal/day. However, there is no definitive data on this aspect as yet. Another factor to take into account is the specific dynamic action of the foods which is now called the thermogenic response to food. This parameter calculates the energy expenditure that our body must sustain to absorb and metabolise nutrients. This parameter calculates the energy expenditure that our body must sustain to absorb and metabolise nutrients. Taking an average from the literature, this energy expenditure amounts to 7%, 2.5% and 27% of the calories provided by CHO, fats and proteins respectively. It is obvious that by modifying the nutrient ratios we can act on this aspect of daily calorie expenditure. Another aspect that has recently emerged is the lesser influence that ketogenic diets seem to have over hunger mediators (ghrelin) and fullness (PYY, CCK, etc.); in fact, it seems that low-calorie diets cause an increase in hunger signals and reduction in fullness even months after the end of the diet, while the ketogenic diet seems to have a much smaller effect in this regard. Finally, there are preliminary findings that seem to indicate that ketogenic diets act on the metabolism by lowering the respiratory quotient (ration between CO2 exhaled and O2 consumed), thereby indicating preferential use of fats rather than sugars.

In summary, we can confirm that the effect of VLCKDs on weight loss seems to be caused by several factors:

- reduction in appetite due to the action of proteins and ketone bodies, although the mechanism of the latter is not yet clear.

- minor influence on "signals" linked to hunger and fullness compared with classic low-calorie diets.

- reduction of liposynthesis mechanisms and increased lipolytic mechanisms

- reduction in resting respiratory quotient The respiratory quotient, or RQ, represents the ratio between CO2 produced and O2 consumed (CO2/O2); the RQ of sugars is equal to 1, while for a mixture of fatty acids it is 0.7

- increase in metabolic expenditure caused by gluconeogenesis and by the thermic effect of proteins

But how do these diets affect performance?

The ketogenic diet and endurance performance

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There are few studies on the ketogenic diet and performance. In one of these, Phinney and his collaborators followed a ketogenic diet for six weeks. After a week of being on the ketogenic diet, endurance time to exhaustion on a treadmill decreased, but after six weeks of following the diet a significant increase was reported. In a further study, Phinney used experienced cyclists as subjects, for a period of 5 weeks. The stationary bicycle test on endurance time to exhaustion is performed in both the first week of the eucaloric diet and at the end of the fourth week of the ketogenic diet.

The stationary bicycle resistance test is performed at 65% of the VO2 max. The average continuous time exercising on the stationary bike to exhaustion at 65% of VO2 max was 147 minutes at the end of the eucaloric diet and 151 minutes at the end of the ketogenic diet. It should be noted that during the second test an RQ of 0.72 was measured, indicating preferential use of fats as an energy source. This historical study gives us an idea of how a ketogenic diet could increase time to exhaustion during an endurance performance. But in order to have these effects, different effects must be taken into consideration: 1) 1) the time allowed for keto-adaptation (a certain period - 2.4 weeks between the start of the diet and physiological adaptation), 2) the use of electrolytic integrators is necessary given that some foods are excluded (3/5 grams of sodium per day and 2/3 grams of potassium per day), 3) thea sufficient quantity of protein must be ingested (between 1.2 and 1.7 grams of protein per kg of body weight).

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