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Fasting is a exercise that involves a restriction of food or drink input for any period. Fasting has been practiced for a selection of factors that range from dieting to spiritual beliefs to clinical testing. The is typically used in clinical practice because that blood glucose and lipid markers activities tests to help in the diagnosis of numerous diseases and assessing countless risk factors. Variations of fasting have been studied for their capability to improve physiological indicators related come health. Few of these components include insulin sensitivity, blood pressure, atherogenic lipids, body fat, and also inflammation. Plenty of of these studies involve those who take part in the Islamic tradition of Ramadan since participants abstain indigenous food and also drink each day from dawn till sunset for an entire month.<1> The compiled results present a variety of metabolic and also physiological adaptations that happen from fasting. From a basic perspective, this contains the alters in metabolic pathways to create energy for the body.

Issues of Concern

The impacts of fasting have actually been completely studied in populaces of healthy and balanced adult individuals. However, data worrying underweight, geriatric, and also pediatric patient is tho lacking. A notable result at the beginning periods of fasting is a tension-type headache.<2> This kind of headache has actually an etiology the is dependent on many factors and the precise cause has not been identified yet. Proposed mechanisms that might lead to a fasting headache encompass hypoglycemia, dehydration, in addition to caffeine withdrawal.<2><3> A examine has shown that the use of rofecoxib, a COX2 inhibitor, deserve to be efficient in reducing and also even stopping a fasting headache, saying that the etiology might be a product that the pro-inflammatory eicosanoid metabolic pathway.<4> Fasting should always be carry out under the supervision of a doctor or ideally in a clinical setting.


Fasting entails a radical readjust in cellular physiology and also metabolism. Blood glucose generally provides the body through sufficient energy through glycolysis. During a fast, maintain of blood glucose levels initially relies ~ above glycogen stores in the liver and also skeletal muscle. Glycogen is made up of chains of polymerized glucose monosaccharides that are used for power by the procedure of glycogenolysis. Many glycogen is save on computer in the liver, which has actually the greatest duty in the maintain of blood glucose throughout the very first 24 hours of a fast. ~ fasting for roughly 24 hours, glycogen stores space depleted resulting in the human body to utilize energy stores native adipose tissue and protein stores.<5> The drastic adjust in metabolism that adheres to glycogen depletion is mainly dependent ~ above the line of triglyceride shop in adipose tissue. Triglycerides room separated into cost-free fatty acids and also glycerol that the liver dong converts right into ketone bodies and glucose. Ketone bodies made from cost-free fatty acids with the process of ketogenesis. This ketone body travel v the body and also are reconverted ago into acetyl-CoA in ~ the tissues requiring energy. In addition to adipose catabolism, protein catabolism, through the process of gluconeogenesis, simultaneously takes ar in time of fasting.<6><7> Gluconeogenesis produces glucose native amino acids broken down from miscellaneous tissues consisting of muscle. After glycogen stores end up being depleted, the dependency of human body tissues for glucose gradually declines as ketone bodies become much more readily obtainable to metabolize.


One that the most greatly studied fasting regimens is known as intermittent fasting, which involves the restriction of caloric intake during a set duration continually. Instances of fasting regimens encompass restriction the calories for 1 complete day the end of the mainly or 2 nonconsecutive days, additionally known as the "5:2" diet. Pet studies have actually repeatedly prove a vigorous, positive an answer of various wellness indicators come intermittent fasting regimens.<8> These encompass improved insulin sensitivity and also a palliation of body fat, atherogenic lipids, blood pressure, and IGF-1. Animal models have also demonstrated a statistically far-ranging improvement in the capacity of intermittent fasting to delay the development of neurological conditions including Alzheimer’s, Parkinson’s, and Huntington’s disease.<1> Human studies of intermittent fasting likewise demonstrate promising outcomes in protection versus metabolic syndrome and also other lifestyle illness including diabetes and cardiovascular disease.<1><9> A significant cellular process that is upregulated during times of fasting includes the inhibition of the tyrosine kinase enzyme. Inhibition of this enzyme is a backbone because that the therapy of many species of cancer, and further research study is essential to evaluate whether fasting regimens have the right to be provided concomitantly through chemotherapy to boost patient outcomes.<10>

Organ systems Involved

The most immediate organ affected by a rapid is the pancreas. Throughout times of short plasma glucose, the pancreas will release much more glucagon from the alpha cells discovered in the islets of Langerhans. Glucagon will mainly impact the liver together it stores most of the glycogen in the body. Skeleton muscle is also affected by glucagon, but to a lesser extent because skeletal muscle includes a short glycogen concentration. After ~ hepatic glycogen stores space depleted, the body uses adipose tissue and also protein for energy. The liver has an active role in the line of fats together it is the main oxidizer of triglycerides. In much more extreme version of fasting, whereby fat sources have actually been expended, the body breaks under skeletal muscle because that energy. Catabolism of bones muscle gives the body v amino mountain that can be metabolized. However, this process also leader to a reduction in muscle mass.


Fasting is dependent on three types of energy metabolism: glycogen, lipid, and amino acid.


As blood glucose levels fall during fasting, the pancreas secretes increased quantities of glucagon. This activity also to reduce insulin secretion, which subsequently decreases glucose storage in the form of glycogen. Glucagon binds to glucagon receptors at the liver to cause a cyclic AMP cascade that ultimately activates glycogen phosphorylase. Glycogen phosphorylase and debranching enzyme relax glucose-1-phosphate (G1P) native glycogen branches at the alpha-1,4 and alpha-1,6 positions, respectively. Then phosphoglucomutase counter G1P to glucose-6-phosphate (G6P). The last step the this process is that G6P is hydrolyzed into glucose and inorganic phosphate by glucose-6-phosphatase.  


The malfunction of triglycerides starts with the activation of hormone-sensitive lipase (HSL). This enzyme is stimulated by glucagon, epinephrine, cortisol, and also growth hormone all of which have increased plasma levels during fasting.<11> Each of these hormones activates HSL through a different pathway. Glucagon and epinephrine bind to adenylyl cyclase (on the cabinet membrane) producing cyclic AMP. Cyclic AMP activates protein kinase A (PKA), which in turn activates HSL. Cortisol binding to glucocorticoid receptor alpha (GR-alpha) located in the cytosol that the cell. Activation of GR-alpha boosts transcription of the protein angiopoietin-like 4 (Angptl4). This protein directly stimulates cyclic AMP-dependent PKA signaling which speak HSL to begin lipolysis.<12> Growth hormone transforms on HSL with the phospholipase C (PLC) pathway. PLC activates protein kinase C (PKC) which have the right to either directly or indirectly wake up HSL. The indirect pathway involves PKC phosphorylating MAPK/ERK kinase (MEK). MEK phosphorylates extracellular signal-related kinase (ERK) which straight phosphorylates HSL.

After HSL is activated, that works v adipose triglyceride lipase to rest a fatty acid (FA) native triglyceride reducing it come a diglyceride. HSL and also monoacylglycerol lipase break turn off the various other two FA leaving a net total of one glycerol molecule to add three separate FA. Glycerol is convert to glycerol-3-phosphate and then come dihydroxyacetone (DHAP) by glycerol kinase and glycerol-3-phosphate dehydrogenase respectively. DHAP is climate metabolized in the glycolysis pathway.

Fatty acids room transformed right into fatty acyl CoA with fatty acyl CoA synthetase. Energy from fatty acyl CoA is mainly created through beta-oxidation and also ketogenesis. Omega oxidation is a minor pathway that oxidizes fatty acids right into dicarboxylic acids in the smooth absorbent reticulum. It remains a young pathway uneven mitochondrial beta-oxidation is defective. The place of beta-oxidation is dependence on the length of the fatty mountain chain; short, medium, and also long chains room degraded in the mitochondria when very-long and branched chains are degraded in peroxisomes. Every bicycle of beta-oxidation to produce 1 FADH, 1 NADH, and also 1 acetyl CoA molecule. The really last cycle of produces 2 acetyl CoA (from also chained FA) or 1 acetyl CoA and 1 propionyl CoA (from weird chained FA).

The process of ketogenesis first starts v the enzyme thiolase combining 2 molecules of acetyl-CoA into acetoacetyl-CoA. HMG-CoA synthase then adds one more acetyl-CoA to develop beta-hydroxy-beta-methylglutaryl-CoA. HMG-CoA lyase clears an acetyl-CoA group from the molecule to kind acetoacetate. Indigenous this step, acetoacetate is broken down right into acetone (by non-enzymatic decarboxylation) and beta-hydroxybutyrate (by D-beta-hydroxybutyrate dehydrogenase).

Amino Acid

During fed and fasting states, amino acids are normally used because that the synthesis of physiologically crucial metabolites. Amino acids space metabolized based on their category and only the liver have the right to degrade every amino acids. Glucogenic amino acids space made right into Krebs bike intermediates or pyruvate. Ketogenic amino acids are processed right into acetoacetate or acetyl-CoA. There are amino mountain that space categorized as being both glucogenic and also ketogenic which method that they deserve to be metabolized by either pathway.

Categorization that Amino Acids

Glucogenic: alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, histidine, methionine, valine

Ketogenic: leucine, lysine

Glucogenic/Ketogenic: isoleucine, phenylalanine, threonine, tryptophan, tyrosine

Related Testing

Fasting is performed clinically as soon as blood tests call for minimal caloric intake to help in the diagnosis of various diseases. Fasting blood glucose is an instance of a test the helps to help in the diagnosis of diabetes mellitus based on a collection threshold that determines if a patient’s insulin receptors are functioning appropriately by their capability to lower blood glucose in solution to insulin. In situations of diabetes mellitus form 2, insulin resistance results in high fasting blood glucose. Additionally, high fasting blood glucose has actually been studied as a risk aspect for the development of high blood pressure.<13>

Another test that traditionally calls for a patience to be fasting for accuracy includes triglyceride measurement on a lipid panel. Blood triglythrade are current in comprehensive quantity in the transport proteins chylomicrons and also very-low-density lipoprotein (VLDL). Chylomicrons are responsible for delivering triglycerides from spend food come peripheral tissues while VLDL is made in the liver and represents a baseline blood triglyceride level sturdy to food intake. Therefore, an exact measurement of blood triglythrade in VLDL needs a patience to be fasting to exclude chylomicron triglycerides from the measurement. Current data indicate that precise lipid measurement may be feasible in the absence of fasting although fasting because that lipid panels is still recommended by many national and also international guidelines.<14><15>


Chronic or overabundance exposure come glucocorticoids (GCs), such together cortisol, deserve to lead to insulin resistance or also muscle atrophy.<16> This form of exposure have the right to be widespread in an ext intense/prolonged versions of fasting. GCs typically relay their signal v the glucocorticoid receptor (GR) discovered intracellularly in skeleton muscle tissue. One primary activity of GR is to control transcription the target genes by either straight binding to DNA or tethering chin to various other DNA-binding transcription factors. Inappropriate regulation of these target genes leads to the pathophysiological responses of GCs.

Clinical Significance

Fasting is not only essential for clinically appropriate tests but likewise has the potential come be used as a treatment for some illness in humans. One examine (sample size of 6) has displayed that intermittent fasting, combined with the ketogenic diet, have the right to be efficiently implemented in pediatric patients through epilepsy.<17> However, existing literature top top the subject is still limited and countless studies still have to be performed to display the actual clinical efficacy of fasting as a treatment for human being neurological disorders.<18> Recent data also suggests that larger clinical trials room warranted to additional investigate the efficacy of prescribed fasting regimens because that the treatment of chronic lifestyle and also obesity-related diseases.

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<19> Most studies concerned fasting as a therapy for conditions have to be based on pet models.

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