To characterize human skeletal muscle enzymatic adaptation to a low-carbohydrate, high-fat, and high-protein diet (LCD), subjects consumed a eucaloric diet consisting of 5% of the total energy intake from carbohydrate, 63% from fat, and 33% from protein for 6 days compared with their normal diet (52% carbohydrate, 33% fat, and 14% protein). Biopsies were taken from the vastus lateralis before and after 3 and 6 days on a LCD. Intact mitochondria were extracted from fresh muscle and analyzed for pyruvate dehydrogenase (PDH) kinase, total PDH, and carnitine palmitoyltransferase I activities and mitochondrial ATP production rate (using carbohydrate and fat substrates). β-Hydroxyacyl CoA dehydrogenase, active PDH (PDHa), and citrate synthase activities were also measured on whole muscle homogenates. PDH kinase (PDHK) was calculated as the absolute value of the apparent first-order rate constant of the inactivation of PDH in the presence of 0.3 mM Mg2+-ATP. PDHK increased dramatically from 0.10 ± 0.02 min−1to 0.35 ± 0.09 min−1at 3 days and 0.49 ± 0.06 min−1after 6 days. Resting PDHaactivity decreased from 0.63 ± 0.17 to 0.17 ± 0.04 mmol ⋅ min−1⋅ kg−1after 6 days on the diet, whereas total PDH activity did not change. Activities for all other enzymes were unaltered by the LCD. In summary, severe deficiency of dietary carbohydrate combined with a twofold increase in dietary fat and protein caused a rapid three- to fivefold increase in PDHK activity in human skeletal muscle. The increased PDHK activity downregulated the amount of PDH in its active form at rest and decreased carbohydrate metabolism. However, an increase in the activities of enzymes involved in fatty acid oxidation did not occur.