by Aditi Chawla
The sugar known as lactose is the main carbohydrate component of milk, which is the only food source for newborns. The enzyme lactase is found in the small intestine, and is responsible for cleaving lactose into glucose and galactose, which can be absorbed into the bloodstream and used as a source of energy. An estimated 65% of adults are lactase non-persistent (or lactose intolerant) and downregulate the production of intestinal lactase after weaning (PMID 19034520, PMID 14616060). In others, lactase activity persists throughout adult life. Those who are lactose persistent (or lactose tolerant) are able to drink milk without any of the side effects experienced by people who are lactose intolerant.
It is not surprising that lactose intolerance results in the avoidance of dairy products. The fermentation of undigested lactose by bacteria in the colon can cause diarrhea, abdominal pain, flatulence, bloating and cramps. It is thought that variation in the gut flora may account for some of the differences in symptoms between people (PMID 19034520).
Family and twin studies have shown that genetics plays a role in lactose intolerance. In recent years, the rs4988235 (13910C>T) variant has been associated with lactase persistence as well as lactase enzyme activity in Caucasians (PMID 11788828, PMID 17659699, PMID 17311063, PMID 17706627, PMID 12914565). The rs4988235 variant, in the MCM6 gene, has been proposed to act as a switch regulating expression of the nearby lactase gene (PMID 12915462, PMID 14724821). Individuals with one or two T alleles make sufficient lactase to digest lactose. Thus, people with the C/T or T/T genotype at rs4988235 are more likely to be lactose tolerant, while those with the C/C genotype are more likely to be lactose intolerant.
While rs4988235 almost completely explains the frequency of lactase persistence in Caucasians, it does not account for the frequency of lactase persistence worldwide, including populations in sub-Saharan Africa and Asia. In those populations, other variants have been found to be associated with this trait (PMID 20144208, PMID 17159977, PMID 19947896).
Your Pathway Fit® test can help you determine your genotype at the rs4988235 variant, and if you are lactose intolerant, you should make sure that you are getting enough calcium from non-dairy or lactose-free sources (PMID 20225268). On the other hand, if you are lactose tolerant, be aware that dairy products can be high in calories, fat, or both. A recent study of 17,374 Finnish people showed that those with the lactose tolerant genotypes (C/T or T/T) were more likely to have a higher BMI (body mass index) than those with the C/C genotype (PMID 20015952). Thus, you should manage your dairy intake appropriately.