Low-carbohydrate dietary interventions have gained scientific interest due to their potential effects on body weight, glucose regulation, and energy metabolism. Carbohydrate restriction may also influence biochemical and molecular pathways involved in nutrient sensing and metabolic regulation. The aim of the present study was to investigate physiological, metabolic, biochemical, and molecular responses to a 21-day low-carbohydrate high-protein dietary intervention in healthy young adults. Four healthy university students (two males and two females, aged 20–26 years) participated in the study. Participants maintained carbohydrate intake below approximately 10 g per day during the intervention. Body weight, body composition, blood pressure, fasting blood glucose, and blood ketone concentrations were assessed, while clinical blood biomarkers and amino acid profiles were analyzed using external laboratory services. Gene expression of FOXO3, SIRT1, and BDNF was investigated using quantitative polymerase chain reaction (qPCR), with GAPDH used as the reference gene. Fasting glucose concentrations were lower and blood ketone concentrations were higher during the dietary intervention compared with baseline. When the first seven intervention days were compared with the final seven days, mean fasting glucose decreased from approximately 4.41 to 4.12 mmol/L, while mean ketone concentration increased from approximately 1.24 to 2.46 mmol/L. However, neither difference reached statistical significance. Body weight decreased in all four participants, and numerical decreases were also observed in BMI, visceral fat, and blood pressure. Circulating concentrations of the branched-chain amino acids leucine, isoleucine, and valine increased numerically, although these differences were not statistically significant. Clinical biomarkers showed considerable inter-individual variability, with mixed responses in lipid and other metabolic markers. FOXO3 expression remained largely unchanged, while SIRT1 showed a numerical increase. Reliable quantitative BDNF data were not obtained and were therefore excluded from the final quantitative comparison. Overall, the results showed metabolic adaptation to severe carbohydrate restriction, particularly through lower fasting glucose concentrations and increased ketone production. However, the small sample size, short intervention duration, absence of a control group, and inter-individual variability limit the conclusions that can be drawn. Larger and more controlled studies are required to determine whether the observed physiological, biochemical, and molecular changes are reproducible and persist during longer-term carbohydrate restriction.