Below are descriptions of many popular biomarkers measured in research, sorted alphabetically.

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6-sulfatoxymelatonin (aMT6s)

6-sulfatoxymelatonin (aMT6s) is excreted in urine and can be measured as a surrogate biomarker for melatonin. Melatonin has a half-life in humans of just 10 to 60 minutes, and more than 90% of circulating melatonin is deactivated by the liver. Through a series of processes, melatonin is converted into 6-sulfatoxymelatonin. The advantage of measuring 6-sulfatoxymelatonin is the ability to assess pineal function by urine sampling, since urine contains only traces (~1%) of non-metabolized melatonin. Urinary aMT6s levels closely mimic the melatonin profile of the general population, and first morning urine samples can be used to evaluate the total melatonin synthesis of the prior night. Measuring 6-sulfatoxymelatonin in urine provides a non-invasive, integrated parameter to assess circadian rhythms over a period of time (Graham, 1998; Basket, 1998). Learn more about 6-sulfatoxymelatonin here: www.alpco.com/melatonin.

 

Adiponectin (ADP)

Adiponectin (ADP) is a fat-derived protein that increases glucose use and plays an important role in protecting against insulin resistance, diabetes, and atherosclerosis. Low ADP levels are an indicator in the development of type 2 diabetes, obesity, and cardiovascular disease. While there are several isoforms of ADP in circulation, High Molecular Weight (HMW), Total ADP, and the ratio of HMW to Total ADP are commonly measured. Studies show that HMW value is the most accurate predictor of insulin resistance and metabolic syndrome. In addition, adiponectin yields many therapeutic benefits including increasing insulin sensitivity, reducing inflammation and atherosclerosis, and in certain circumstances decreasing body weight  (Arunkumar, 2017; Hara, 2006).

Alpha-1 Antitrypsin (A1A, AAT)

Alpha-1 Antitrypsin (A1A) is a linear glycoprotein predominantly synthesized in the liver, but also by intestinal macrophages, monocytes, and epithelial cells. Research studies show A1A is an acute phase protein that can be used as a possible indicator of inflammation. Elevated fecal A1A is a potentially strong indicator of excessive gastrointestinal protein loss associated with protein losing enteropathy (PLE). PLE can be caused by many gastrointestinal diseases, including regional enteritis, gastric cancer, Whipple intestinal lipodystrophy, and allergic gastroenteropathy. A1A resists degradation in the intestines due to its anti-proteolytic activity (Florent, 1981; Mayo Clinic Laboratories, 2020).

Anti-gliadin IgA and Anti-tissue Transglutaminase (tTG) IgA

Gluten proteins are major storage proteins in the endosperm of wheat, barley, and rye grains. Wheat gluten is comprised of gliadin monomers and glutenin polymers. Gluten intolerance is implicated in celiac disease, wheat allergy, and non-celiac gluten sensitivity (NCGS). Patients are often highly sensitive to very small amounts of gluten.  In patients with celiac disease, the enzyme tissue transglutaminase (tTG) removes an amino group from gliadin peptides. Research suggests IgG and IgA antibodies are produced against native gliadin in patients with NCGS who ingest food containing gluten. Antibodies against deamidated gliadin peptides and tissue transglutaminase are likely produced in patients with celiac disease. Anti-gliadin antibody tests and anti-tTG antibody tests may be useful in detecting gluten-related disorders and differentiating celiac disease from NCGS. Measuring antibodies against gliadin and tissue transglutaminase may also allow for monitoring of adherence to a gluten-free diet (Balakireva, 2016; Barbaro, 2018; Rubio-Tapia, 2013; Vogdani, 2013; Armstrong, 2011).

Asymmetric Dimethylarginine (ADMA)

Asymmetric dimethylarginine (ADMA) is a naturally occurring chemical found in plasma and a potential biomarker of insulin resistance in skeletal muscle. ADMA is a byproduct that occurs as proteins break down and modify in the body. ADMA inhibits the production of nitric oxide (NO), a key chemical involved in normal endothelial function and cardiovascular health (Lee, 2018).

 

β-defensin 2

β-defensin 2 is an inducible antimicrobial peptide produced by epithelial cells involved in protecting intestinal barrier function. Studies report elevated levels of β-defensin 2 in cases of dysbiosis, an imbalance of normal gut bacteria associated with irritable bowel syndrome. Research also demonstrates that β-defensin 2 is upregulated in response to inflammation in cases of inflammatory bowel disease, with much higher levels observed in UC than CD. A study of necrotizing enterocolitis (NEC) in neonates shows an increase in β-defensin 2 in some infants, with a lack of increase indicating a deficiency in immune activation (Schwiertz, 2018; Raimondi, 2008).

Bile Acids (BAs)

Bile acids (BAs) are steroid hormones that act as signaling molecules to regulate cholesterol, glucose, lipids, energy homeostasis, and gut permeability. Bile acids are stored in the gallbladder as a component of bile and are released into the intestines following the ingestion of fats. Studies suggest an increase in the secretion of bile acids in stool may occur due to bile acid malabsorption (BAM), increased bile acid synthesis in irritable bowel syndrome with diarrhea (IBS-D), or non-alcoholic fatty liver disease (NAFLD) (Mantis, 2011; Brandtzaeg, 2013; Corthesy, 2013; Chang-Keun, 2013).

 

C4d

The complement system is an essential part in innate immunity.  C4d protein is a product of two complement pathways, the lectin and classical pathways.  C4d is a biomarker for activation of these pathways, and a potential biomarker for conditions where antibodies can cause tissue damage, such as in systemic autoimmune diseases, antibody-mediated disease, and pregnancy (Cohen, 2012).  Learn more about C4d here: www.alpco.com/complement-system.

Calprotectin

Calprotectin is a calcium- and zinc-binding protein produced by neutrophils. Levels of the protein correlate with neutrophil infiltration and inflammation in the intestines. Measurement of fecal calprotectin offers a non-invasive way of differentiating patients with inflammatory bowel disease (IBD) from patients with irritable bowel syndrome (IBS). The current recommendation for an upper limit of normal calprotectin is 50 μg/g, with levels above 50 μg/g suggesting inflammatory bowel disease. Diagnosis of IBD is typically confirmed via colonoscopy. Preliminary studies have suggested monitoring calprotectin levels can predict relapse and assist with treatment optimization (Manz, 2012; McMahon, 2018; Heida, 2017).

C-peptide

C-peptide is produced in the pancreas when proinsulin splits. C-peptide is often measured in lieu of insulin as it is made 1:1 and does not break down as quickly. C-peptide’s half-life is ~30 minutes compared to about three minutes for insulin, making c-peptide an attractive biomarker for indirectly estimating glucose-stimulated insulin secretion, understanding ß-cell function, and identifying ß-cell functional mass. Measuring c-peptide level is analogous to measuring insulin level and allows investigators to distinguish between type 1 or type 2 diabetes. Recently, c-peptide has shown therapeutic potential for the treatment of diabetes-associated microvascular complications. Further, in studies of animal models of type 1 diabetes, c-peptide administration significantly improved nerve and kidney function (Bhatt, 2014; Sima, 2001).

 

Eosinophil-derived Neurotoxin (EDN)

Eosinophils are proinflammatory white blood cells developed in the bone marrow and released to the lamina propria within the GI tract. Research indicates that in the presence of Inflammatory Bowel Disease (IBD), more typically with ulcerative colitis (UC) than Crohn’s disease (CD), there is an increase in the number of eosinophils in the gut mucosa. Additionally, IBD may increase the release of granule proteins, including EDN. Studies also show that stool-derived EDN is elevated in infants and young children with non-IgE food protein allergies, including food protein-induced proctocolitis (FPIAP), food protein-induced enterocolitis syndrome (FPIES), and food protein-induced enteropathy (FPE). and may be helpful in the diagnosis of severe GI food allergies (Calvani, 2121; Kaetzel, 2014; Puollis, 2022; Van Der Slyys, 1998).

 

Fecal-Occult-Blood-(iFOB, FIT, Hemoglobin)

Fecal occult (hidden) blood refers to non-visible blood in stool detectable through immunochemical or chemical testing. Hemoglobin molecules in red blood cells are comprised of four heme groups, each attached to a globin chain. Immunochemical FOB tests (iFOB/FIT) measure the globin component of hemoglobin. Unlike chemical, guaiac-based FOB tests that react with heme, immunochemical tests are specific for blood from the lower GI and do not require dietary restrictions prior to testing. iFOB/ FIT testing screens for the presence of benign or pre-malignant polyps that can be removed before developing into colorectal cancer. The test may also assist with the identification of asymptomatic early-stage colorectal cancer (CRC) (American Cancer Society, 2020; Doubeni, 2020).

Fetuin-A

Fetuins are proteins made in the liver and secreted into the blood.  Fetuin-A is a major carrier protein of free fatty acids (FFA). Fetuin-A is a marker for insulin resistance.  Higher Fetuin-A levels are associated with impaired insulin sensitivity and glucose tolerance, and may indicate that prediabetic patients are moving toward developing type 2 diabetes (Trepanowski, 2015).

 

Ghrelin

Ghrelin is a hormone produced and released primarily by the stomach when it’s empty, and by the small intestines, pancreas, and brain. Ghrelin enters the blood and signals the brain to feel hungry, thereby stimulating appetite, increasing consumption, and promoting fat storage: it is the only circulating hormone with this effect. Ghrelin plays a key role in regulating calorie intake and body fat levels, and its levels indicate nutritional status and body fat stores. Higher levels correlate to feeling hungry while lower levels correlate to feeling full. Ghrelin has broad therapeutic potential for conditions including eating disorders, neurogenerative disorders, GI disorders, and metabolic syndrome (Austin, 2009; Collden, 2017).

Glucagon

Glucagon is a peptide hormone made by α-cells inside pancreatic islets when blood sugar levels are lower than normal, typically in response to fasting or exercise. Glucagon works with the liver to convert sugar into glucose which is released into the blood to support increased energy expenditure and help maintain energy homeostasis. In addition, glucagon reduces food intake and may have a role in appetite regulation. Glucagon is essential for the maintenance of normal blood glucose in diabetes, and its measurement helps doctors monitor patient glucose control. Elevated glucagon levels also indicate the presence of a glucagonoma, which is a rare pancreatic tumor that secretes glucagon: Type 2 diabetes is present in ~80% of glucaganoma cases (Drucker, 2006 and 2015; Tan 2013; Sandhu, 2021).  Learn more about glucagon here: https://www.alpco.com/download/researching-the-relationship-between-glucagon-and-energy-homeostasis-ebook.

Glucagon-like peptide-1 (GLP-1)

GLP-1 is an incretin hormone, produced in intestinal L cells, that stimulates insulin release from pancreatic ß-cells following food consumption. Studies show that GLP-1 has regulatory effects on glucagon secretion, however those effects are debated. Previously, it was accepted that GLP-1 inhibits glucagon secretion but the underlying mechanisms were not well-described. More recently, studies demonstrate that GLP-1 can either stimulate or inhibit glucagon secretion depending on glucose levels or physiological need. Low levels of GLP-1 are an indicator for type 2 diabetes. GLP-1 reduces body weight and food intake and slows gastric emptying. GLP-1 also promotes weight loss and is therefore a therapeutic target for diabetes and obesity. In humans, the majority of GLP-1 in circulation is the active GLP-1 (7-36) form (Zhang, 2019; Drucker, 2018; Nadkarni, 2014). Learn more about GLP-1 here: https://www.alpco.com/choosing-the-relevant-glp-1-species.

Glucose-dependent insulinotropic polypeptide (GIP)

GIP is an incretin, a signaling hormone primarily produced in the small intestine in response to glucose, protein, or fat ingestion. GIP stimulates insulin release from pancreatic ß-cells following food consumption, encouraging energy storage in adipose tissue. GIP receptors are also present in α-cells, where GIP stimulates glucagon release and contributes to insulin secretion through paracrine signaling. GIP is an indicator for high-fat diet (HFD)-induced obesity and insulin resistance (Campbell, 2013; Baggio, 2007). Learn more about incretin hormones here: https://www.alpco.com/incretins-gip-and-glp-1-in-diabetes-and-obesity.

 

Helicobacter pylori (H. pylori)

Helicobacter pylori (H. pylori) is a gram-negative, spiral-shaped bacterium that evolved to survive in the harsh environment of the human stomach. An estimated 50% of the global population is infected with H. pylori, with the highest incidence in developing countries. Most infected individuals are unaware and show no clinical symptoms. It is estimated that 17% of individuals infected with H. pylori develop peptic ulcers, while 1% develop gastric cancer. H. pylori infection is 100% treatable with antibiotics. H. pylori screening is highly recommended along with follow-up testing to confirm eradication in cases of infection. (Bowen, 2017; Weaver, 2010; Mouzaki, 2016).

 

Insulin

Insulin is a hormone produced in the pancreas by ß-cells inside the Islets of Langerhans when proinsulin splits. Insulin signals the body to store glucose, the main type of sugar in the blood and the body’s major source of energy, thereby lowering blood sugar levels. Insulin binds to specific receptors on the cell wall, initiating a series of complex processes that permit cells to take in and use just enough glucose to complete necessary functions. Levels of insulin correlate to many conditions including insulin resistance, diabetes, hypertension, and metabolic syndrome. Measuring insulin allows researchers to distinguish between type 1 and type 2 diabetes, to predict diabetic risk and engage patients in prevention programs, and to monitor the progress of prediabetes and insulin resistance (Wilcox, 2005).  Learn more about insulin resistance here: https://www.alpco.com/insulin-resistance-research-review.

 

Lactoferrin

Lactoferrin is an iron-binding glycoprotein produced by neutrophils. The concentration of lactoferrin correlates with neutrophil infiltration and inflammation in the GI tract. Fecal lactoferrin has been used as an aid in distinguishing IBD from IBS in adults and children, with values above 7.25 µg/g indicating IBD. Diagnosis of IBD is typically confirmed via colonoscopy. Additionally, fecal lactoferrin is a timely indicator of changes in mucosal inflammatory activity in response to therapy and may be useful in monitoring IBD treatment response (Abraham, 2018; Yamamato, 2015; Rubio, 2019).

Leptin

Leptin is a hormone produced by fat cells that signals the body to stop eating. Leptin reduces hunger and helps regulate energy balance. Leptin tells the brain how much fat is stored in body cells. Generally, increased leptin levels correlate to an increase in fat, and decreased leptin levels correlate to lower body fat. However, about 5% of obese people have very high levels of leptin indicating a lack of sensitivity to the hormone, a condition known as leptin resistance. As a therapeutic target, leptin shows value in reducing caloric intake, increasing weight loss, and improving insulin function (Austin, 2009; Samuel, 2016).

Lysozyme

Lysozyme acts as an antibacterial defense in the GI tract. Research demonstrates that granulocytes, macrophages, Paneth cells, Brunner’s Glands and normal colonic crypt cells can secrete lysozyme. The protein functions by hydrolyzing peptidoglycan in the cell wall of gram-positive bacteria. Adherence of pathogenic bacteria can cause elevated levels of fecal lysozyme when compared to healthy controls. Studies also indicate that active/inactive CD, active UC, and non-inflammatory bowel disease may be associated with elevated lysozyme levels (Han, 2015; Pang, 2014; Fasano, 2011; Malickova, 2017).

 

Melatonin

Melatonin is an endogenously expressed hormone and a key modulator of seasonal and circadian biorhythms. Circadian rhythms are in-born, 24-hour cycles the influence many behaviors and physiological processes such as the sleep-wake cycle and how organisms anticipate and adapt to their environments. The rise and fall of circulating levels of melatonin correlate with circadian rhythm, making it a useful biomarker for the identification and monitoring of circadian rhythm disorders. These include disorders related to jet lag, night shift work, delayed sleep phase syndrome (DSPS), advanced sleep phase syndrome (ASPS), and aging (Scheer, 2005; Charrier, 2017). Learn more about melatonin here: www.alpco.com/melatonin.

Myostatin

Myostatin is a small protein produced and released by skeletal muscle cells when muscles contract. Skeletal muscle is the largest insulin-sensitive organ. Myostatin restrains skeletal muscle growth. Elevated myostatin levels are associated with obesity and with development of insulin resistance and type 2 diabetes. Investigators study how myostatin inhibits muscle growth and the resulting effect on glucose uptake. Myostatin is also a therapeutic target since inhibiting it may improve muscle strength and insulin sensitivity (Amor, 2019; Eilers, 2020).

 

Obestatin

Obestatin is a hormone secreted from the stomach and present in the spleen, mammary glands, breast milk, and plasma. Obestatin signals fullness, suppressing appetite, decreasing food intake, and reducing weight gain. Circulating obestatin levels generally have an inverse association with obesity and diabetes. Obestatin levels have also shown a negative correlation with c-peptide and anti-insulin antibodies in type 1 diabetes onset, potentially indicating islet dysfunction (Cowan, 2016; Prodam, 2014).

 

Pancreatic-Elastase

The digestive functions performed by the pancreas are referred to as exocrine functions. Acinar cells in the pancreas generate various enzymes needed to properly digest lipids, proteins, and carbohydrates. Once released from the pancreatic duct, digestive enzymes mix with bile to break down food and release nutrients. Exocrine pancreatic insufficiency, or EPI, involves the malabsorption of nutrients due to insufficient production or release of pancreatic enzymes. Pancreatic elastase (PE or PE-1) refers to a family of chymotrypsin-like elastases (CELAs), digestive proteases produced by acinar cells in the pancreas. PE is highly stable in the intestinal tract and the concentration of pancreatic elastase in stool correlates well with overall pancreatic enzyme output. PE levels above 200 µg/g are considered normal. Levels between 100 and 200 µg/g indicate moderate EPI and levels below 100 µg/g indicate severe EPI. Early detection and daily enzyme replacement therapy can mitigate malnutrition and growth delays sometimes associated with EPI (Lindkvist, 2013; Columbia University, 2018; Lohr, 2016).

Peptide YY (PYY)

PYY is a hormone secreted by the small intestines into the bloodstream in response to food consumption. PYY signals the brain that it is full, slows down digestion, and decreases appetite for about 12 hours. The amount of PYY released depends on the type and amount of food eaten. High PYY levels are associated with conditions that lead to weight loss like eating disorders, IBD, and some cancers. Low PYY levels are associated with increased appetite and weight gain, and correlate to obesity, making PYY a potential therapeutic target to treat the condition (Helou, 2008; Karra, 2008).

Proinsulin

Proinsulin is a precursor molecule produced by pancreatic ß-cells, primarily in response to glucose, that is cleaved equally into insulin and c-peptide. Elevated levels of proinsulin are associated with insulinomas and insulin resistance while proinsulin to insulin ratios help elucidate ß-cell function. Proinsulin levels are also an independent marker of cardiovascular risk. Some studies suggest that intact proinsulin has potential as an early indicator of insulin resistance, where levels of intact proinsulin and intermediates (des-31,32) become disproportionately high in subjects with glucose intolerance. Further, they suggest measurement of fasting intact proinsulin may aid in therapy selection and monitoring in type 2 diabetes progression (Kim, 2000; Guettier, 2013; Pfutzner, 2011).  Learn more about proinsulin here: https://www.alpco.com/immunoassays-for-intact-and-total-proinsulin.

 

Renin

Renin is a hormone made by the kidneys that helps control blood pressure.  When blood pressure drops too low or the body lacks sufficient salt, renin is released into the bloodstream triggering a chain reaction called RAAS (renin-angiotensin-aldosterone system) that creates a hormone called angiotensin and signals your adrenal glands to release another hormone called aldosterone. Renin is explored as a potential biomarker of cardiovascular risk (Volpe, 2013).

Retinol Binding Protein (RBP-4)

Retinol binding protein is a specific transport protein for retinol (Vitamin A) which takes it from the liver to tissues. Increased levels of RBP-4 may be a marker for early detection of type 2 diabetes (Steinhoff, 2021).

 

Secretory IgA (sIgA)

Secretory Immunoglobulin A (sIgA) is the most abundant antibody type found in the intestinal lumen and is central to the GI tract’s immune functions. Research studies demonstrate sIgA can control inflammation and regulate the immune response against pathogens, toxins, and microflora in the GI tract. Evidence also suggests that sIgA levels can increase in response to pathogens, toxins or food allergens and often decrease during active inflammatory bowel disease (Acharya, 2014; Orihara, 2017; Corthésy, 2013).

 

Terminal Complement Complex (TCC, sC5b-9)

The complement system plays an essential role in acute and chronic inflammation, in immune response to infections, and in autoimmune disease. Terminal Complement Complex (TCC) is a well-published marker for complement studies in basic and translational research, clinical evaluation of treatment, and safety studies for medical devices and drugs. TCC is produced when any of the three complement pathways is activated. Measuring TCC is useful whenever complement system activation is suspected, as it gives an overall view of activity independent of pathway (Harboe, 2011). Learn more about  TCC here: www.alpco.com/complement-system.

 

Vasopressin(AVP/ADH)

Vasopressin, also known as arginine vasopressin (AVP) or anti-diuretic hormone (ADH), is produced by the brain in response to either high plasma sodium levels or decreased blood volume. It regulates hydration by ensuring cells are surrounded by the right amount of water to function optimally. Vasopressin also helps maintain body temperature, blood volume, and the proper flow of urine from the kidneys (Hannon, 2016).  Learn more about vasopressin here: https://www.alpco.com/vasopressin.

 

Zonulin Family Peptides (ZFPs)

Zonulin family peptides (ZFPs) are involved in the regulation of tight junctions that prevent the movement of bacteria, viruses, allergens, toxins, and other potentially harmful molecules across the intestinal barrier. Studies show that zonulin family peptides and increased intestinal permeability, or “leaky gut,” may be involved in the progression of autoimmune disorders such as celiac disease and type 1 diabetes. Research indicates that inflammatory bowel disease and Parkinson’s disease may involve elevated levels of fecal ZFPs. In most cases, the increase in the level of ZFPs and intestinal permeability precede disease or disease recurrence (Diaconu, 2017; Sahil, 2009; Rokkas, 2017).   References

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Handout: Featured Biomarkers from A to Z Download