Editing Glucose (section)

===Energy source===
[[File:Glucose catabolism intermediates de.png|thumb|upright=1.2|class=skin-invert-image|Diagram showing the possible intermediates in glucose degradation; Metabolic pathways orange: glycolysis, green: Entner-Doudoroff pathway, phosphorylating, yellow: Entner-Doudoroff pathway, non-phosphorylating]]
Glucose is a ubiquitous fuel in [[biology]]. It is used as an energy source in organisms, from bacteria to humans, through either [[aerobic respiration]], [[anaerobic respiration]] (in bacteria), or [[Fermentation (biochemistry)|fermentation]]. Glucose is the human body's key source of energy, through aerobic respiration, providing about 3.75&nbsp;[[kilocalorie]]s (16&nbsp;[[kilojoule]]s) of [[food energy]] per gram.<ref>{{citation | title = Food energy&nbsp;– methods of analysis and conversion factors | chapter-url = http://www.fao.org/docrep/006/Y5022E/y5022e04.htm | chapter = Chapter 3: Calculation of the Energy Content of Foods – Energy Conversion Factors | series = FAO Food and Nutrition Paper 77 | publisher = Food and Agriculture Organization | location = Rome | year = 2003 | isbn = 978-92-5-105014-9 | url-status = live | archive-url = https://web.archive.org/web/20100524003622/http://www.fao.org/DOCREP/006/Y5022E/y5022e04.htm | archive-date = 24 May 2010 }}</ref> Breakdown of carbohydrates (e.g., starch) yields mono- and [[disaccharide]]s, most of which is glucose. Through [[glycolysis]] and later in the reactions of the [[citric acid cycle]] and [[oxidative phosphorylation]], glucose is [[oxidize]]d to eventually form [[carbon dioxide]] and water, yielding energy mostly in the form of [[adenosine triphosphate]] (ATP). The insulin reaction, and other mechanisms, regulate the concentration of glucose in the blood. The physiological caloric value of glucose, depending on the source, is 16.2 kilojoules per gram<ref name="Schwedt">Georg Schwedt: ''Zuckersüße Chemie'' {{In lang|de}}. John Wiley & Sons, 2012, {{ISBN|978-3-527-66001-8}}, p.&nbsp;100.</ref> or 15.7 kJ/g (3.74 kcal/g).<ref>Schmidt, Lang: ''Physiologie des Menschen'', 30. Auflage. Springer Verlag, 2007, p.&nbsp;907 {{in lang|de}}.</ref> The high availability of carbohydrates from plant biomass has led to a variety of methods during evolution, especially in microorganisms, to utilize glucose for energy and carbon storage. Differences exist in which end product can no longer be used for energy production. The presence of individual genes, and their gene products, the enzymes, determine which reactions are possible. The metabolic pathway of glycolysis is used by almost all living beings. An essential difference in the use of glycolysis is the recovery of [[NADPH]] as a reductant for [[anabolism]] that would otherwise have to be generated indirectly.<ref>{{Cite journal |pmc=1220531 |pmid=10493919 |year=1999 |last1=Dandekar |first1=T. |title=Pathway alignment: Application to the comparative analysis of glycolytic enzymes |journal=The Biochemical Journal |volume=343 |pages=115–124 |last2=Schuster |first2=S. |last3=Snel |first3=B. |last4=Huynen |first4=M. |last5=Bork |first5=P. |doi=10.1042/bj3430115 |issue=1}}</ref>

Glucose and oxygen supply almost all the energy for the [[brain]],<ref>{{cite web |first = Pramod|last = Dash |url =http://neuroscience.uth.tmc.edu/s4/chapter11.html |title=Blood Brain Barrier and Cerebral Metabolism (Section 4, Chapter 11) |work =Neuroscience Online: An Electronic Textbook for the Neurosciences |publisher =Department of Neurobiology and Anatomy – The University of Texas Medical School at Houston  |url-status=dead |archive-url=https://web.archive.org/web/20161117104126/http://neuroscience.uth.tmc.edu/s4/chapter11.html |archive-date=17 November 2016 }}</ref> so its availability influences [[psychological]] processes. When [[Hypoglycaemia|glucose is low]], psychological processes requiring mental effort (e.g., [[self-control]], effortful decision-making) are impaired.<ref>{{citation | last1 = Fairclough | first1 = Stephen H. | last2 = Houston | first2 = Kim | s2cid = 44500072 | title = A metabolic measure of mental effort | journal = Biol. Psychol.  | year = 2004 | volume = 66 | issue = 2 | pages = 177–190 | pmid = 15041139  | doi = 10.1016/j.biopsycho.2003.10.001}}</ref><ref>{{citation | last1 = Gailliot | first1 = Matthew T. | last2 = Baumeister | first2 = Roy F. | last3 = DeWall | first3 = C. Nathan | last4 = Plant | first4 = E. Ashby | last5 = Brewer | first5 = Lauren E. | last6 = Schmeichel | first6 = Brandon J. | last7 = Tice | first7 = Dianne M. | last8 = Maner | first8 = Jon K. | title = Self-Control Relies on Glucose as a Limited Energy Source: Willpower is More than a Metaphor | journal = J. Pers. Soc. Psychol. | year = 2007 | volume = 92 | issue = 2 | pages = 325–336 | pmid = 17279852 | doi = 10.1037/0022-3514.92.2.325 | url = http://www.uky.edu/~njdewa2/gailliotetal07JPSP.pdf | url-status = live | archive-url = https://web.archive.org/web/20170818111037/http://www.uky.edu/~njdewa2/gailliotetal07JPSP.pdf | archive-date = 18 August 2017 | citeseerx = 10.1.1.337.3766 | s2cid = 7496171 }}</ref><ref>{{citation | last1 = Gailliot | first1 = Matthew T. | last2 = Baumeister | first2 = Roy F.  | title = The Physiology of Willpower: Linking Blood Glucose to Self-Control | journal = Personal. Soc. Psychol. Rev. | year = 2007 | volume = 11 | issue = 4 | pages = 303–327 | doi = 10.1177/1088868307303030 | pmid = 18453466 | citeseerx = 10.1.1.475.9484 | s2cid = 14380313 }}</ref><ref>{{citation | last1 = Masicampo | first1 = E. J. | last2 = Baumeister | first2 = Roy F. | title = Toward a Physiology of Dual-Process Reasoning and Judgment: Lemonade, Willpower, and Expensive Rule-Based Analysis | journal = Psychol. Sci. | year = 2008 | volume = 19 | issue = 3 | pages = 255–60 | doi = 10.1111/j.1467-9280.2008.02077.x | pmid = 18315798| s2cid = 38596025 }}</ref> In the brain, which is dependent on glucose and oxygen as the major source of energy, the glucose concentration is usually 4 to 6&nbsp;mM (5&nbsp;mM equals 90&nbsp;mg/dL),<ref name="Satyanarayana" /> but decreases to 2 to 3&nbsp;mM when fasting.<ref name="Dwyer" /> [[Confusion]] occurs below 1&nbsp;mM and [[coma]] at lower levels.<ref name="Dwyer" />

The glucose in the blood is called [[blood sugar]]. Blood sugar levels are regulated by glucose-binding nerve cells in the [[hypothalamus]].<ref name="Koekkoek">{{Cite journal |doi=10.3389/fnins.2017.00716 |pmc=5742113 |pmid=29311793|year=2017 |last1=Koekkoek |first1=L. L. |title=Glucose-Sensing in the Reward System |journal=Frontiers in Neuroscience |volume=11 |pages=716 |last2=Mul |first2=J. D. |last3=La Fleur |first3=S. E. |doi-access=free }}</ref> In addition, glucose in the brain binds to glucose receptors of the [[reward system]] in the [[nucleus accumbens]].<ref name="Koekkoek" /> The binding of glucose to the sweet receptor on the tongue induces a release of various hormones of energy metabolism, either through glucose or through other sugars, leading to an increased cellular uptake and lower blood sugar levels.<ref name="Tucker">{{Cite journal |doi=10.1016/j.physbeh.2017.09.016 |pmid=28939430 |year=2017 |last1=Tucker |first1=R. M. |title=Do non-nutritive sweeteners influence acute glucose homeostasis in humans? A systematic review |journal=Physiology & Behavior |volume=182 |pages=17–26 |last2=Tan |first2=S. Y. |s2cid=38764657 |url=https://ap01.alma.exlibrisgroup.com/view/delivery/61USOUTHAUS_INST/12149779800001831 |access-date=7 June 2020 |archive-date=29 July 2020 |archive-url=https://web.archive.org/web/20200729195754/https://ap01.alma.exlibrisgroup.com/view/delivery/61USOUTHAUS_INST/12149779800001831 |url-status=dead |url-access=subscription }}</ref> [[Artificial sweetener]]s do not lower blood sugar levels.<ref name="Tucker" />

The blood sugar content of a healthy person in the short-time fasting state, e.g. after overnight fasting, is about 70 to 100&nbsp;mg/dL of blood (4 to 5.5&nbsp;mM). In [[blood plasma]], the measured values are about 10–15% higher. In addition, the values in the [[artery|arterial]] blood are higher than the concentrations in the [[vein|venous]] blood since glucose is absorbed into the tissue during the passage of the [[capillary bed]]. Also in the capillary blood, which is often used for blood sugar determination, the values are sometimes higher than in the venous blood. The glucose content of the blood is regulated by the hormones [[insulin]], [[incretin]] and [[glucagon]].<ref name="Koekkoek" /><ref>{{Cite book |doi=10.1016/B978-0-444-53480-4.00026-6 |pmid=25410233 |year=2014 |last1=La Fleur |first1=S. E. |series=Handbook of Clinical Neurology |volume=126 |pages=341–351 |last2=Fliers |first2=E. |last3=Kalsbeek |first3=A. |title=Diabetes and the Nervous System |chapter=Neuroscience of glucose homeostasis |isbn=978-0-444-53480-4}}.</ref> Insulin lowers the glucose level, glucagon increases it.<ref name="Satyanarayana" /> Furthermore, the hormones [[adrenaline]], [[thyroxine]], [[glucocorticoid]]s, [[somatotropin]] and [[adrenocorticotropin]] lead to an increase in the glucose level.<ref name="Satyanarayana" /> There is also a hormone-independent regulation, which is referred to as [[glucose autoregulation]].<ref>{{Cite journal |doi=10.1002/cphy.c140009 |pmid=25589267 |year=2015 |last1=Bisschop |first1=P. H. |title=Autonomic regulation of hepatic glucose production |journal=Comprehensive Physiology |volume=5 |issue=1 |pages=147–165 |last2=Fliers |first2=E. |last3=Kalsbeek |first3=A.}}</ref> After food intake the blood sugar concentration increases. Values over 180&nbsp;mg/dL in venous whole blood are pathological and are termed [[hyperglycemia]], values below 40&nbsp;mg/dL are termed [[hypoglycaemia]].<ref>W. A. Scherbaum, B. M. Lobnig<!--: ''Abschnittstitel''.-->, In: Hans-Peter Wolff, Thomas R. Weihrauch: ''Internistische Therapie 2006, 2007''. 16th Edition. Elsevier, 2006, {{ISBN|3-437-23182-0}}, p.&nbsp;927, 985 {{in lang|de}}.</ref> When needed, glucose is released into the bloodstream by glucose-6-phosphatase from glucose-6-phosphate originating from liver and kidney glycogen, thereby regulating the [[homeostasis]] of blood glucose concentration.<ref name="Szablewski" /><ref name="Löffler/Petrides 195" /> In [[ruminant]]s, the blood glucose concentration is lower (60&nbsp;mg/dL in [[cattle]] and 40&nbsp;mg/dL in [[sheep]]), because the carbohydrates are converted more by their gut microbiota into [[short-chain fatty acid]]s.<ref name="Harper">Harold A. Harper: ''Medizinische Biochemie''. Springer-Verlag, 2013, {{ISBN|978-3-662-22150-1}}, p.&nbsp;294.</ref>

Some glucose is converted to [[lactic acid]] by [[astrocyte]]s, which is then utilized as an energy source by [[brain cells]]; some glucose is used by intestinal cells and [[red blood cell]]s, while the rest reaches the [[liver]], [[adipose tissue]] and [[muscle]] cells, where it is absorbed and stored as glycogen (under the influence of [[insulin]]). Liver cell glycogen can be converted to glucose and returned to the blood when insulin is low or absent; muscle cell glycogen is not returned to the blood because of a lack of enzymes. In [[Adipocyte|fat cells]], glucose is used to power reactions that synthesize some [[fat]] types and have other purposes. Glycogen is the body's "glucose energy storage" mechanism, because it is much more "space efficient" and less reactive than glucose itself.

As a result of its importance in human health, glucose is an analyte in [[glucose test]]s that are common medical [[blood test]]s.<ref>{{Cite journal |pmid=22872934 |year=2012 |last1=Clarke |first1=S. F. |title=A history of blood glucose meters and their role in self-monitoring of diabetes mellitus |journal=British Journal of Biomedical Science |volume=69 |issue=2 |pages=83–93 |last2=Foster |first2=J. R. |citeseerx=10.1.1.468.2196 |doi=10.1080/09674845.2012.12002443|s2cid=34263228 }}</ref> Eating or fasting prior to taking a blood sample has an effect on analyses for glucose in the blood; a high fasting glucose blood sugar level may be a sign of [[prediabetes]] or [[diabetes mellitus]].<ref>{{Cite web |url=http://www.diabetes.org/diabetes-basics/diagnosis/ |title=Diagnosing Diabetes and Learning About Prediabetes |website=American Diabetes Association |language=en |access-date=20 February 2018 |url-status=live |archive-url=https://web.archive.org/web/20170728020224/http://www.diabetes.org/diabetes-basics/diagnosis/ |archive-date=28 July 2017}}</ref>

The [[glycemic index]] is an indicator of the speed of resorption and conversion to blood glucose levels from ingested carbohydrates, measured as the [[area under a curve|area under the curve]] of blood glucose levels after consumption in comparison to glucose (glucose is defined as 100).<ref name="Harvey 366">Richard A. Harvey, Denise R. Ferrier: ''Biochemistry''. 5th Edition, Lippincott Williams & Wilkins, 2011, {{ISBN|978-1-608-31412-6}}, p.&nbsp;366.</ref> The clinical importance of the glycemic index is controversial,<ref name="Harvey 366" /><ref name="Satyarayana 508">U Satyanarayana: ''Biochemistry''. Elsevier Health Sciences, 2014, {{ISBN|978-8-131-23713-7}}, p.&nbsp;508.</ref> as foods with high fat contents slow the resorption of carbohydrates and lower the glycemic index, e.g. ice cream.<ref name="Satyarayana 508" /> An alternative indicator is the [[insulin index]],<ref>{{Cite journal |doi=10.1093/ajcn/66.5.1264 |pmid=9356547 |year=1997 |last1=Holt |first1=S. H. |title=An insulin index of foods: The insulin demand generated by 1000-kJ portions of common foods |journal=The American Journal of Clinical Nutrition |volume=66 |issue=5 |pages=1264–1276 |last2=Miller |first2=J. C. |last3=Petocz |first3=P. |doi-access=free}}</ref> measured as the impact of carbohydrate consumption on the blood insulin levels. The [[glycemic load]] is an indicator for the amount of glucose added to blood glucose levels after consumption, based on the glycemic index and the amount of consumed food.