CSID (Congenital Sucrase Isomaltase Deficiency): A rare genetic digestive enzyme deficiency in which affected individuals lack two separate enzymes found in the small intestine, sucrase and isomaltase. A small percentage of these individuals also lack the ability to break down lactose.
Sucrase helps break down a disaccharide sugar--called sucrose--into two other simple sugars (glucose and fructose). Sucrose has many different aliases in the food industry world: table sugar, cane sugar, cane juice, palm sugar, palm juice, brown sugar, saccharose.....and there are probably more than even that! But don't be tricked, they all indicate that sucrose is present in a product. Sucrose is commonly found in desserts (such as cookies, cakes, pies, and ice creams--to list a small few), in addition to most fruits.
Isomaltase helps break down a disaccharide starch, called maltose, into two other glucose molecules. Maltose has several pet names on ingredient lables as well. Basically, if you see the word "malt" anywhere on the label (i.e. malted barley flour, maltodextrin, etc.) know that maltose is present. Maltose has got to be found in nearly every grain product you can think of (bread, tortillas, pasta, chips, and so on.). It even sneaks its way into other food groups you'd never automatically think to check, such as artificial sweetners (listed as maltodextrin).
CSID is a fairly rare autosomal recessive disorder. The SI gene exists on chromosome 3, and helps with the instructions for producing sucrase and isomaltase. There are a select unlucky group of people who inherit one bad copy of the SI gene on chromosome 3 from their mother, and another bad copy from their father--becoming what is called a homozygote for the CSID mutation gene. These people show the typical symptoms that eventually lead to a diagnosis of CSID. Those who are heterozygotes/carriers of CSID (possesing one normal SI gene on chromosome 3 and one mutated), don't usually show the full blown symptoms of CSID, although they can mimic them to a lesser degree. From what I understand, it's not that common to have even one chromosome 3 with the CSID mutation. This is exactly what makes CSID more rare--the likelihood of two people who each carry the mutation to meet up isn't an everyday thing, much less for them to have a child who only has a 1 in 4 chance of getting two bad gene copies. It is interesting to note, however, that the SI mutation responsible for CSID has been pinpointed with higher frequency in people who have descendents from Greenland, Alaska, or Canada (screen your dates accordingly)!
People with CSID who ingest sucrose and maltose suffer from a wide variety of symptoms. The symptoms that drive most people to seek a diagnosis include stomach cramps, excessive bloating, gas, and diarrhea that is severe enough to interfere with normal life activities, and often leads to dehydration/failure to thrive if left unchecked. This is not a complete list though; I've heard accounts of those who actually suffer constipation, or get rashes around the mouth, and so on. Research I have done indicates that there are at least seven different mutations in the SI gene that lead to CSID, which is why different people seem to have different levels of starch tolerance (all are still unable to tolerate any sugar) and hence have the different symptoms/levels of them.
The only true cure for CSID is to stop ingesting sucrose and maltose entirely, although supplemental sucrase enzymes (called Sucraid) can be taken to assist in eating small amounts of sucrose (less than 25g per meal). No enzyme treatment has yet been FDA approved for isomaltase. However, there are several different individuals who have reported taking Kirkman Isogest and finding an increase in tolerance for starch products.
The most reliable method for diagnosing CSID is through a endoscope guided small intestine biopsy. Tissue from several locations in the small intestine are taken with the endoscope, and then sent to a pathology lab for evaluation of four intestinal disaccharide enzyme activities: sucrase, maltase, lactase, and palatinase (which is linked to the maltase activity if I recall correctly). The numbers resulted from the biopsy give an indication of a person's tolerance level to sugars, starches, and dairy products. Other methods include hydrogen breath analysis and stool pH levels, but have proved to be falsely negative in up to 1/3 of people who actually have CSID. A blood test became approved in 2012 that searches for any of the known mutations on chromosome 3 that would indicate CSID; however the only place that does this testing is the University of Washington Molecular Development laboratory, and is probably still more costly to most than getting the biopsy performed (plus it does not provide actual enzyme levels for determining tolerance). However, if one wanted to know the exact CSID mutation they carried, this would be the way to go. Perhaps in the future the small intestine biopsy and blood test will be routinely coupled together....
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