Active site – The molecular location, in this instance in the small intestine, where enzymes and substrates come together.


Allele – One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.


Amylase – A digestive enzyme that breaks down carbohydrates such as starch and glycogen into smaller units; the two types of amylase are salivary and pancreatic.


Amylose – The linear form of starch that consists of α-(1,4) linkages of glucose polymers; amylose starches are less rapidly digested than amylopectin starches. Amylose makes up about 20% to 30% of the structure of starch.


Amylopectin – The linear form of starch that consists of α-(1,4) glucose polymers as well as branched α-(1,6) glucose polymers; amylopectin starches are digested more rapidly than amylose starches. Amylopectin makes up about 70% to 80% of the structure of starch.


Anus – A muscular valve that controls the opening from the large intestine to facilitate waste elimination (bowel movements).


Autosomal recessive – The pattern of inheritance of CSID; a mutation in a gene on one of the 22 autosomal (non-sex) chromosomes that can lead to a genetic disorder such as CSID. Recessive inheritance means both genes in the pair must be defective to cause disease.


Biopsy – A tissue sample; in digestive diseases, the biopsy is usually painless and is taken from the inner layers of the esophagus, stomach, small, and/or large intestine during an endoscopy or colonoscopy procedure.


Bloating – A fullness or distention of the abdomen, usually caused by excess gas that accumulates in the small and/or large intestine. CSID can cause bloating; some foods such as beans, peanuts, cabbage, cauliflower, broccoli, and kale may lead to bloating from fermentation due to a high level of a certain carbohydrate in these foods. Also, improper digestion of other nutrients (fats, carbohydrates, or proteins) may cause bloating.


Brush border hydrolases – Enzymes present in the brush border or microvilli of the small intestine, which break down disaccharides and starches; examples are sucrase, isomaltase, and lactase.


Carbohydrate – Any substance that has a molecular structure based on sugar, including glucose and fructose, as well as more complex starches found in plant food sources (fruits, grains, and vegetables); all starches are carbohydrates, but not all carbohydrates are starches (for example, honey and grapes are carbohydrates, but not starches).


Colonoscopy – A procedure in which a camera on the end of a lighted tube, an endoscope, is inserted through the anus and rectum and is used to view the inside of the entire large intestine and sometimes the terminal ileum. Biopsies of the mucosal layers within the large intestine or terminal ileum of the small intestine may be taken during the colonoscopy procedure. Preparation involves the use of medications to clear out waste from the large intestine and avoiding food and drink other than clear liquids for at least 12 hours before the procedure. The colonoscopy procedure requires sedation.


Compound heterozygote – A pattern of genetic inheritance; in CSID, a person who has two different mutated alleles at a particular location on a gene, one on each chromosome 3 of a pair. The human genome contains two copies of each gene, 1 paternal and 1 maternal allele. A mutation affecting only one allele is called heterozygous. A person is homozygous if they have two identical forms of a particular gene, one inherited from each parent. However, when a person inherits a mutated maternal allele and a different mutated paternal allele, these mutations are called compound heterozygous.


Congenital – A medical term which means present at birth.


Congenital sucrase-isomaltase deficiency (CSID) – A rare, inherited disorder that is characterized by reduced or absent digestive enzymes that are required for digesting sucrose (table sugar) and some starches; gastrointestinal symptoms associated with CSID include chronic diarrhea, gas, and bloating.


Digestive tract – Muscular organs involved in eating, digesting foods, and eliminating solid food wastes, including the mouth, esophagus, stomach, small intestine, and large intestine.


Disaccharides – Sugars that require brush border enzymes for digestion; sucrose, lactose, and maltose are all disaccharides.


Disaccharidases – Brush border enzymes that break down disaccharides into monosaccharides.


Duodenum – The upper third of the small intestine, located between the lower part of the stomach and the jejunum.


Dyspepsia – A medical term for indigestion or upset stomach that describes discomfort or pain in the upper abdomen; dyspepsia is not a disease but rather a group of symptoms that often include bloating, nausea, and burping.


Endoscope – A tube with a light and a camera on the end of it that is used to view the esophagus, stomach, part of the duodenum, part of the terminal ileum, and the entire large intestine. Endoscopes usually contain a biopsy aspiration port to collect tissue samples during endoscopic procedures.


Endoscopy, upper gastrointestinal – A procedure in which an endoscope is passed through the mouth into the esophagus, stomach, and part of the duodenum. Biopsies may be taken of the mucosal layers during this procedure. The patient is typically sedated during the procedure. The only preparation is avoiding all food and non-clear liquids for at least 4 to 8 hours before the procedure.


Enzyme – A type of catalyst, which means it speeds up chemical reactions. In the case of digestive enzymes, they speed up the digestion or breaking down of food into nutritional components such as fatty acids, amino acids, simple sugars, vitamins, and minerals. Some metabolic disorders cause the decreased activity of a certain enzyme needed for processing or metabolizing specific food components, such as the situation with CSID and sucrose.


Failure to thrive (FTT) – A general term used for insufficient growth (measured by standard growth charts) or inadequate weight gain caused by a variety of medical and psychosocial conditions; does not imply abnormal intellectual, social, or emotional development.


Flatulence – Passing gas through the anus; may be accompanied by bloating, indigestion, and/or other gastrointestinal symptoms.


Fructose – A monosaccharide, simple sugar found in many plants; 1 of 3 monosaccharides (others are glucose and galactose) that are absorbed directly into the bloodstream during digestion.


Galactose – A monosaccharide, simple sugar; galactose and glucose are the monosaccharides resulting from the breakdown of lactose.


Gastroenterologist – A physician who specializes in disorders of the gastrointestinal tract and associated organs.


Genetics – The branch of biology that studies heredity and variations in organisms determined by the expression of inherited genes.


Genotype – The inherited instructions a person carries within his/her genetic code or DNA.


Glucoamylase – An enzyme that hydrolyzes the glucoside bond in starches and dextrins; breaks down starchy carbohydrates, specifically polysaccharides into glucose.


Glucose – A monosaccharide, simple sugar that is the building block of starch and maltase and a component of lactose or sucrose; glucose is the major source of energy for most cells.


Glycosidases – Enzymes that hydrolyze glycosidic bonds, such as those seen in disaccharides such as sucrose.


Glycosidic bonds – A glycosidic bond is a type of bond that joins a sugar (carbohydrate) molecule to another group, which may or may not be another sugar (carbohydrate); the type of chemical linkage between the monosaccharide units of disaccharides, oligosaccharides, and polysaccharides.


Heredity – The passing of genetically-determined traits from parents to offspring, also called genetics.


Heterozygote – A person who has two different forms of a particular gene; a heterozygote for CSID has a mutated gene for the disorder from one parent and a normal gene from the other parent; also known as a carrier.


Homozygote – A person with CSID who has two identical mutated forms of a particular gene, one inherited from each parent.


Hydrolysis – The process by which a molecule is broken down by adding water; hydrolysis is a step to break down the glycosidic bonds of disaccharides.


Ileum – The last third of the small intestine, which connects the jejunum with the large intestine.


Isomaltase – The enzyme partially responsible for the breakdown of starches; maltase-glucoamylase also plays a part in starch digestion; isomaltase is synonymous with palatinase; isomaltase breaks the bonds linking saccharides, which cannot be broken down by amylase or maltase.


Jejunum – The middle third of the small intestine, linking the duodenum with the ileum.


Lactase – Brush border enzyme that breaks down lactose, the sugar found in milk; some CSID patients are deficient in lactase, but most are not; many CSID patients have higher than normal lactase levels, presumably to compensate for the lack of sucrase-isomaltase activity.


Lactose – Disaccharide found in milk that is broken down by the enzyme lactase; also called milk sugar; made up of 1 glucose unit and 1 galactose unit.


Lactose intolerance – A deficiency in the intestinal enzyme lactase, which results in a reduced or absent ability to digest lactose. Lactose that is not fully digested and absorbed in the small intestine reaches the large intestine and is fermented by bacteria. Byproducts of the fermentation process cause symptoms of flatulence, abdominal cramps, and/or diarrhea after the ingestion of lactose (milk sugar).


Large intestine (large bowel) – A long muscular tube that connects the small intestine with the anus; primary function is to absorb water from waste before it leaves the body.


Lumen – The hollow portion of a tubular body structure, such as a blood vessel, the intestines, and the esophagus.


Malabsorption – A clinical term that describes the inability to absorb sufficient nutrients, either certain specific nutrients or all nutrients in general, from one’s typical diet.


Maltase-glucoamylase – A brush border membrane enzyme that plays a role in the final steps of small intestinal digestion of starch to glucose.


MGAM gene – The gene responsible for the expression of maltase-glucoamylase in the body; works closely with the SI gene to digest starch.


Maltase – An enzyme that breaks down the disaccharide maltose. Maltose is present in many starchy foods.


Maltose – A disaccharide that requires maltase for digestion; breaks down into 2 molecules of glucose in digestion; maltose is usually found in grains and starchy vegetables.


Metabolic disorder – A disorder in which the production or breakdown of one or more specific chemical substance(s) within the body is abnormal due to a genetic alteration in an enzyme.


Microvilli (villi) – Tiny, finger-like projections that protrude from the lining of the small intestinal wall; serve to increase the internal surface area of the intestinal walls, which assists in the absorption of nutrients.


Monosaccharides – A simple sugar composed of only one sugar molecule; glucose, galactose, and fructose are monosaccharides.


Motility – The coordinated neuromuscular activity of the wall of the gastrointestinal tract that moves gastrointestinal contents from the esophagus to the anus.


Oligosaccharides – Sugars made up of 3 to 10 sugar units.


Orphan disease – A disorder that affects a small population of patients. In the United States, an orphan disease is defined by law as having a prevalence of fewer than 200,000 individuals. In Europe, an orphan disease is defined as diseases affecting fewer than 5 individuals per population of 10,000.


Orphan drug – A drug that is used to treat a small population of patients. In the United States, an orphan drug treats an orphan disease (see orphan disease).


Osmotic diarrhea – Watery diarrhea that is caused by the presence of unabsorbed nutrients in the colon, creating an osmotic gradient for water to move into the colon. In CSID, sucrose is not enzymatically cleaved and absorbed in the small intestine, so it reaches the colon in an intact form, which drives water into the colon and causes watery diarrhea.


Palatinase – A brush border enzyme that hydrolyzes palatinose; a term sometimes used interchangeably with isomaltase; usually, palatinase is measured during a disaccharidases test of a small bowel biopsy.


Phenotype – Observable physical and/or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences; the expression of a specific trait based on genetic and environmental influences.


Polymer – A chemical compound or mixture of compounds consisting of repeating structural units.


Polysaccharides – Sugars made up of more than 10 sugar units; starch is a type of polysaccharide.


Prevalence – The total number of persons in a given population with a disease or other health-related event during a specified period of time, usually expressed as a percentage.


Ptyalin – A term sometimes used interchangeably with salivary amylase.


Registered dietitian – Registered dietitians (RDs) are experts in food and nutrition who have met both academic and professional requirements. RDs can be extremely beneficial for a patient with CSID.


SI gene – The gene responsible for the expression of sucrase-isomaltase in the body.


Sigmoidoscope – A short, flexible, or rigid endoscope designed to examine the rectum and sigmoid portion of the large intestine.


Sigmoidoscopy – A procedure in which a flexible or rigid sigmoidoscope is inserted into the rectum and through the entire length of the sigmoid portion of the large intestine for examination and possible biopsy. Preparation includes an enema and mild laxative to clear waste from the sigmoid and rectal areas. Sedation may be used during the procedure, if needed.


Small intestine (small bowel) – A long, muscular tube that connects the bottom of the stomach with the large intestine (large bowel); primary function is to complete digestion and nutrient absorption into the bloodstream.


Starch – Any of a group of polysaccharides; a complex carbohydrate found chiefly in seeds, fruits, tubers, and roots, notably in corn, potatoes, wheat, and rice; all starches are carbohydrates, but not all carbohydrates are starches (for example, honey and grapes are carbohydrates, but not starches, while bread is both a carbohydrate and a starch).


Substrate – A molecule that is affected by an enzyme(s); sucrose is the substrate for sucrase.


Sucrase – The brush border enzyme that is responsible for breaking down sucrose, a disaccharide, into glucose for the body to use as fuel; patients with CSID have little or no sucrase activity.


Sucrose – A disaccharide that requires brush border enzymes for digestion, primarily sucrase; is metabolized into the monosaccharides fructose and glucose.


Villi (microvilli) – Tiny, finger-like projections that protrude from the lining of the small intestinal wall; serve to increase the internal surface area of the intestinal walls, which promotes nutrient absorption.