Blood glucose levels and liver

Blood glucose levels and hepatic glycogen

One of the main functions of the liver is to participate in the maintaining of blood glucose levels within well defined range (in the healthy state before meals 60-100 mg/dL or 3.33-5.56 mmol/L). To do it the liver releases glucose into the bloodstream in:

  • fasting state;
  • between meals;
  • during physical activity.

Blood glucose levels and hepatic glucose-6-phosphatase

In the liver, glycogen is the storage form of glucose which is released from the molecule not as such, but in the phosphorylated form i.e. with charge, the glucose-1-phosphate (this process is called glycogenolysis). The phosphorylated molecule can’t freely diffuse from the cell, but in the liver it is present the enzyme glucose-6-phosphatase that hydrolyzes glucose-6-phosphate, produced from glucose-1-phosphate in the reaction catalyzed by phosphoglucomutase, to glucose (an irreversible dephosphorylation).

glycogen(n glucose residues) + Pi → glucose-1-phosphate + glycogen(n-1 glucose residues)

glucose-1-phosphate ↔ glucose-6-phosphate

glucose-6-phosphate + H2O → glucose + Pi

Then, glucose can diffuse from the hepatocyte, via a transporter into the plasma membrane called GLUT2, into the bloodstream to be delivered to extra-hepatic cells, in primis neurons and red blood cells for which it is the main, and for red blood cells the only energy source (neurons, with the exception of those in some brain areas that can use only glucose as energy source, can derive energy from another source, the ketone bodies, which becomes predominant during periods of prolonged fasting).

Note: the liver obtains most of the energy required from the oxidation of fatty acids, not from glucose.

Glucose-6-phosphatase is present also in the kidney and gut but not in the muscle and brain; therefore in these tissues glucose-6-phosphate can’t be released from the cell.
Glucose-6-phosphatase plays an important role also in gluconeogenesis.

Glucose-6-phosphatase is present into the membrane of endoplasmic reticulum and the hydrolysis of glucose-6-phosphate occurs into its lumen (therefore this reaction is separated from the process of glycolysis). The presence of a specific transporter, the glucose-6-phosphate translocase, is required to transport the phosphorylated molecule from citosol into the lumen of endoplasmic reticulum. Although a glucose transporter is present into the membrane of endoplasmic reticulum, most of the released glucose is not transported back into the cytosol of the cell but is secreted into the bloodstream. Finally, an ion transporter transports back into the cytosol the inorganic phosphate released into the endoplasmic reticulum.

References

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000

Potassium intake and cardiovascular risk factors

Potassium intake and health

In a study published on British Medical Journal a research team has conducted a systematic review of the literature and meta-analyses on potassium intake and health in apparently healthy adults and children without renal impairment that might compromise its handling.
Eleven cohort studies (127038 participants) reporting all cause mortality, stroke, cardiovascular disease, or coronary heart disease in adults and twenty-two randomized controlled trials (1606 participants) reporting blood lipids, blood pressure, renal function, and catecholamine concentrations were included in the study.
In adult with hypertension an increased potassium intake reduced systolic blood pressure by 3.49 mm Hg and diastolic blood pressure by 1.96 mm Hg.
No effect was seen in adult without hypertension (however, the studies were of relatively short duration and did not consider the effect that increased potassium intake may have over time) and in children (there is a lack of data in children: only three controlled studies with 156 partecipants).
There was no adverse effect of increased potassium intake on blood lipids, or catecholamine concentrations in adults whereas an inverse statistically significant association was seen between its intake and the risk of incident stroke (a 24% lower risk).
In healthy adult there was no significant adverse effect on renal function.
This study suggests that, in people without impaired renal function, increased potassium intake (at least 90 mmol/day) is potentially beneficial for the prevention and control of elevated blood pressure and stroke.

How to increase potassium intake

Potassium Intake: Fruits and Vegetables: Rich in Potassium
Fig. 1 – Fruits and Vegetables: Rich in Potassium

It should be noted that an increased potassium intake can be achieved following the largely plant-based Mediterranean Diet, which is characterized by the consumption of large quantities of fresh fruit, vegetable, legumes and unrefined cereals, all rich in potassium (that is also accompanied by a variety of other nutrients).

Aburto N.J., Hanson S., Gutierrez H., Hooper .L, Elliott P., Cappuccio F.P. Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ 2013;346:f1378