Figure 2.

a–d, Time courses of plasma lactate concentrations (a), glucose concentration (in millimoles per liter) (b), plasma lactate ¹³C fractional enrichment (FE) (c), and glucose ¹³C fractional enrichment (d) from the same volunteer for the two different infusion protocols A (black symbols) and B (white symbols). As aimed, plasma lactate level is either (protocol A) maintained close to a physiological level (∼1.5 mmol/L) with a ¹³C FE close to 33% or (protocol B) doubled (∼2.5 mmol/L) with a ¹³C FE close to 50%. Even if the plasma glucose level remains steady at euglycemic values, as the circulating [3-¹³C]lactate is metabolized by the liver, ¹³C atoms are progressively incorporated into the plasma glucose through gluconeogenesis.