GNG in Fasting

From (Nutrition. 1995 Mar-Apr;11(2):149-53. Glycerol gluconeogenesis in fasting humans. Baba H1, Zhang XJ, Wolfe RR.):

The contribution of glycerol to glucose production has been measured in healthy volunteers by the simultaneous primed constant infusion of 1-[13C]glycerol and 3-[3H]glucose and the determination of the rates of appearance (Ra) of glycerol, glucose, and glycerol-derived glucose.

In the postabsorptive state [sic: after you eat], glycerol Ra was 3.11 +/- 0.44 mumol.kg-1.min-1, of which 36% was converted to glucose, accounting for 4.5% of total glucose production.

After 62-86 h of starvation, glycerol Ra rose to 5.32 +/- 0.58 mumol.kg-1.min-1, and 68% of glycerol was converted to glucose. This accounted for 21.6% of total glucose production. Glycerol Ra was closely correlated with its conversion and contribution to glucose.

These findings confirm that the contribution of glycerol to glucose production is directly correlated to its release as a consequence of lipolysis and support the notion that the central physiological role of accelerated lipolysis in fasting is the provision of gluconeogenic precursor.

If you don’t want Gluconeogenesis then stop fasting (Science 25 Oct 1991: Vol. 254, Issue 5031, pp. 573-576. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR. DL Rothman, I Magnusson, LD Katz, RG Shulman, GI Shulman
See all authors and affiliations).

The rate of net hepatic glycogenolysis was assessed in humans by serially measuring hepatic glycogen concentration at 3- to 12-hour intervals during a 68-hour fast with 13C nuclear magnetic resonance spectroscopy. The net rate of gluconeogenesis was calculated by subtracting the rate of net hepatic glycogenolysis from the rate of glucose production in the whole body measured with tritiated glucose.

Gluconeogenesis accounted for 64 +/- 5% (mean +/- standard error of the mean) of total glucose production during the first 22 hours of fasting. In the subsequent 14-hour and 18-hour periods of the fast, gluconeogenesis accounted for 82 +/- 5% and 96 +/- 1% of total glucose production, respectively. These data show that gluconeogenesis accounts for a substantial fraction of total glucose production even during the first 22 hours of a fast in humans.

I am making this point tongue in cheek. Of course you want GNG. Otherwise you would produce nearly zero glucose and your body requires some glucose.

During fasting your energy comes from fat (The American Journal of Clinical Nutrition, Volume 60, Issue 1, 1 July 1994, Pages 29–36. Fuel and energy metabolism in fasting humans. M G Carlson W L Snead P J Campbell.).

Fuel and energy homeostasis was examined in six male volunteers during a 60-h fast by using a combination of isotopic tracer techniques ([3-3H]glucose, [2H5]glycerol, [1-14C]palmitate, and L-[1-13C]leucine) and indirect calorimetry.

Plasma glucose concentration and hepatic glucose production decreased by 30% with fasting (5.2 ± 0.1 to 3.8 ± 0.2 mmol/L and 11.8 ± 0.5 to 8.2 ± 0.6 µmol·kg−1·min−1, respectively, both P < 0.001) and glucose oxidation declined ≈85% (P < 0.01). Lipolysis and primary (intraadipocyte) free fatty acid (FFA) reesterification increased 2.5-fold (1.7 ± 0.2 to 4.2 ± 0.2 µmol·kg−1·min−1 and 1.5 ± 0.4 to 4.2 ± 0.8 µ mol·kg−1·min−1, respectively, both P < 0.05). This provided substrate for the increase in fat oxidation (from 2.7 ± 0.3 to 4.3 ± 0.1 µ mol·kg−1·min−1P < 0.01), which contributed ≈75% of resting energy requirements after the 60-h fast and increased the supply of glycerol for gluconeogenesis.

Proteolysis and protein oxidation increased ≈50% during fasting (P < 0.01 and P < 0.05, respectively).

We conclude that the increase in FFA reesterification with fasting modulates FFA availability for oxidation and maximizes release of glycerol from triglyceride for gluconeogenesis.

Follow that? Glycerol comes from triglyceride in GNG. Your own body fat puts sugar in your blood. It has to.

Author: Doug

I'm an Engineer who is also a science geek. I was pre-diabetic in 1996 and became a diabetic in 2003. I decided to figure out how to hack my diabetes and in 2016 found the ketogetic diet which reversed my diabetes.

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