Dietary proteins contribute little to glucose production, even under optimal gluconeogenic conditions in healthy humans

Abstract

Dietary proteins are believed to participate significantly in maintaining blood glucose levels, but their contribution to endogenous glucose production (EGP) remains unclear. We investigated this question using multiple stable isotopes. After overnight fasting, eight healthy volunteers received an intravenous infusion of [6,6-²H₂]-glucose. Two hours later, they ingested four eggs containing 23 g of intrinsically, uniformly, and doubly [¹⁵N]-[¹³C]-labeled proteins. Gas exchanges, expired CO₂, blood, and urine were collected over the 8 h following egg ingestion. The cumulative amount of dietary amino acids (AAs) deaminated over this 8-h period was 18.1 ± 3.5%, 17.5% of them being oxidized. The EGP remained stable for 6 h but fell thereafter, concomitantly with blood glucose levels. During the 8 h after egg ingestion, 50.4 ± 7.7 g of glucose was produced, but only 3.9 ± 0.7 g originated from dietary AA. Our results show that the total postprandial contribution of dietary AA to EGP was small in humans habituated to a diet medium-rich in proteins, even after an overnight fast and in the absence of carbohydrates from the meal. These findings question the respective roles of dietary proteins and endogenous sources in generating significant amounts of glucose in order to maintain blood glucose levels in healthy subjects.

Trial registration: ClinicalTrials.gov NCT01154582.

Figures

FIG. 1.

Plasma levels of AAs: total (black circles), indispensable (black triangles), and gluconeogenic (open circles) during the 8 h after ingesting four eggs containing 23 g of doubly [15N]-[13C]–labeled proteins. Data are means ± SD (n = 8 at each time point). * indicates a significant difference from the initial value: *P < 0.05.

FIG. 2.

Transfer of dietary AAs into plasma AAs (A) and proteins (B) in human subjects during the 8 h after ingesting four eggs containing 23 g of doubly [15N]-[13C]–labeled proteins. Data are means ± SD (n = 8 at each time point). * indicates a significant difference from the initial value: *P < 0.05.

FIG. 3.

Cumulative transfer of dietary AAs to urinary urea (A) and expired CO2 (B) and dietary AA total deamination and oxidation in human subjects (C) after ingesting four eggs containing 23 g of doubly [15N]-[13C]–labeled proteins. Data are means ± SD (n = 8 at each time point). * indicates a significant difference from the initial value: *P < 0.05.

FIG. 4.

Plasma glucose concentrations (A), total EGP rate (B, black circles), and contribution of dietary AAs to this production (B, open circles) in human subjects during the 8 h after the ingestion of four eggs containing 23 g of doubly [15N]-[13C]–labeled proteins. Data are means ± SD (n = 8 at each time point). * indicates a significant difference from the initial value: *P < 0.05.

FIG. 5.

Plasma levels of insulin (A) and glucagon (B) in human subjects during the 8 h after ingesting four eggs containing 23 g of doubly [15N]-[13C]–labeled proteins. Data are means ± SD (n = 8 at each time point for insulin and n = 3 at 1.5 h; n = 4 at 5 h; n = 5 at 0, 1, 6, and 7 h; n = 6 at 0.5, 4, and 8 h; n = 7 at 3 h; and n = 8 at 2 and 2.5 h for glucagon). * indicates a significant difference from the initial value: *P < 0.05.

FIG. 6.

Nutrient oxidation during the 8 h after the ingestion of four eggs containing 23 g of doubly [15N]-[13C]–labeled proteins. Lipids (open triangles), carbohydrates (CHO; black circles), and proteins (open diamonds). Data are means ± SD (n = 8). The effect of time was significant for lipids (P = 0.01) and carbohydrates (P = 0.007). * indicates a significant difference from the initial value: *P < 0.05.

FIG. 7.

Possible sources of the glucose produced. We observed that 50 g of glucose was produced but that only 4 g was synthesized from dietary AAs. Endogenous AAs could have significantly contributed to glucose production since 27 g was oxidized. The complete dietary fat oxidation would generate 28 mmol of glycerol and lipolysis 8 mmol. Fat oxidation could then not be a major contributor. Finally, residual glycogen could have been the major contributor under our experimental conditions. *, measured; **, calculated; ***, speculated from the literature.