Effects of Low-Carbohydrate and Ketogenic Diets on Aerobic Performance in Trained Athletes: A Systematic Review and Meta-Analysis.

Maximal aerobic capacity (VO2max) was preserved in the majority of studies examining low-carbohydrate and ketogenic diets in trained athletes.

• VO2max was preserved in 50.0% of the 33 included studies.
• 11.1% of studies documented improvements in VO2max.
• Studies included trained athletes with at least 6 months of structured training, aged 18-45 years.
• Diets were defined as low-carbohydrate (≤130 g·day-1 or ≤25% total energy) or ketogenic (<50 g·day-1 or <10% total energy).

Submaximal exercise economy showed the greatest sensitivity to low-carbohydrate and ketogenic diets, with half of studies documenting impaired efficiency.

• 50.0% of studies measuring submaximal exercise economy documented impaired efficiency.
• This was described as the outcome variable showing the 'greatest sensitivity' among the aerobic performance variables examined.
• Aerobic performance variables assessed included VO2max, time trial performance, time to exhaustion, and exercise economy.

Time to exhaustion demonstrated context-dependent effects, with the majority of studies showing maintained performance on low-carbohydrate or ketogenic diets.

• 69.2% of studies measuring time to exhaustion documented maintained performance.
• Effects were described as 'context-dependent.'
• Time to exhaustion was one of four primary aerobic performance outcomes assessed across the 33 included studies.

Fat oxidation was consistently increased across all studies that measured it, with increases ranging from +28% to +200%.

• All 30 studies measuring fat oxidation demonstrated consistent increases.
• The range of increase in fat oxidation was +28% to +200%.
• Fat oxidation and substrate utilization were categorized as metabolic markers alongside aerobic performance variables.
• This was described as one of the most consistent findings across the body of evidence.

A temporal analysis identified a 1-week adaptation threshold distinguishing performance impairment from maintained or improved performance.

• Studies measuring outcomes within ≤7 days of diet initiation documented performance impairment.
• Studies measuring outcomes at >1 week consistently demonstrated maintained or improved performance.
• This threshold was described as a 'critical' finding from the temporal analysis.
• The adaptation pattern was characterized as 'initial acute-phase decrements (≤7 days) followed by recovery.'

The systematic review and meta-analysis included 33 aerobic-focused studies identified through a comprehensive search of five electronic databases.

• Databases searched included PubMed, SCOPUS, Web of Science, SPORTDiscus, and Cochrane Central Register of Controlled Trials.
• Studies were required to include trained athletes (≥6 months structured training, age 18-45 years).
• Studies had to be randomized to low-carbohydrate, ketogenic, or high-carbohydrate control conditions.
• Quality assessment employed Newcastle-Ottawa Scale methodology.

The authors concluded that periodized carbohydrate strategies are supported by the evidence, balancing metabolic adaptation benefits from low-carbohydrate training phases with carbohydrate requirements during competition.

• The recommendation was described as balancing 'metabolic adaptation benefits from low-carbohydrate training phases with carbohydrate requirements during competition.'
• Traditional sports nutrition emphasizes high carbohydrate intake for endurance athletes, which served as the contrasting framework.
• The conclusion was based on findings of preserved maximal aerobic power alongside transient submaximal efficiency impairments.