Lactase is a protein that functions as an enzyme, playing a fundamental part in the digestion of milk sugar, lactose. This enzyme is solely responsible for breaking down lactose, a complex sugar found in all mammalian milk. The ability to process lactose is particularly important for infants, for whom milk is the primary source of nutrition and energy. Without sufficient lactase activity, the human body cannot effectively absorb the main carbohydrate component of milk.
The Biological Function of Lactase
The lactase enzyme is produced exclusively by specialized cells called enterocytes, which line the small intestine. It is embedded in the “brush border,” the dense layer of microscopic, finger-like projections known as microvilli, which maximize the surface area for nutrient absorption. Lactase acts as a catalyst for hydrolysis, splitting the disaccharide lactose molecule into its two constituent monosaccharides: glucose and galactose. These simple sugars are readily absorbed into the bloodstream for use as energy. If this breakdown does not occur, the intact lactose molecule cannot pass through the intestinal wall, leading to malabsorption.
Genetic Factors Determining Lactase Presence
The variation in lactase activity among adults is determined by a genetic distinction between lactase persistence and non-persistence. In most human populations and nearly all other mammals, the production of lactase naturally declines significantly after the weaning period. This reduction in enzyme activity is the default human condition, known as lactase non-persistence or adult-type hypolactasia.
Lactase persistence is a trait that allows the continued, high-level expression of the enzyme into adulthood. This ability results from single nucleotide polymorphisms (SNPs), which are small changes in the DNA sequence located far upstream of the LCT gene. For example, in people of European descent, the primary mutation involves a change from Cytosine (C) to Thymine (T) at position C/T-13910. This genetic change created an enhancer element that keeps the LCT gene “switched on” throughout life, without altering the lactase protein itself.
The trait evolved relatively recently, within the last 10,000 years, coinciding with the cultural development of dairy farming. This is an example of gene-culture coevolution, as the genetic change provided a strong selective advantage to populations relying on fresh milk for nutrition.
Primary vs. Secondary Deficiency
It is important to distinguish between this genetically programmed reduction, known as primary lactase deficiency, and secondary lactase deficiency. Secondary deficiency is an acquired condition resulting from damage to the small intestinal lining, which may be caused by illnesses such as acute gastroenteritis, Celiac disease, or Crohn’s disease. Unlike the permanent genetic form, secondary lactase deficiency can often be reversed once the underlying intestinal condition is successfully treated.
Symptoms and Diagnosis of Lactose Intolerance
When lactase activity is insufficient, undigested lactose travels from the small intestine into the large intestine, where it is subjected to bacterial fermentation. The bacteria residing in the colon break down the unabsorbed sugar, producing byproducts, including short-chain fatty acids. This fermentation process also generates significant amounts of gas, specifically hydrogen, methane, and carbon dioxide. These gases lead to common physical symptoms, such as abdominal pain, cramping, bloating, and flatulence. Furthermore, the presence of the unabsorbed lactose creates an osmotic effect, drawing excess water into the colon. This increase in water volume is the primary cause of diarrhea associated with the condition.
Diagnosis Methods
Diagnosis of the condition typically relies on two main tests, the most common of which is the hydrogen breath test. After consuming a measured amount of lactose, the patient’s breath is sampled at regular intervals over several hours. A substantial rise in the concentration of hydrogen in the breath, usually defined as an increase of more than 20 parts per million over the baseline, indicates that unabsorbed lactose has been fermented.
An alternative method is the lactose tolerance test, which measures the body’s ability to absorb the breakdown products of lactose. The patient drinks a lactose solution, and blood samples are drawn multiple times to monitor blood glucose levels. If the lactase enzyme is deficient, the lactose is not digested into glucose, and the blood glucose level will fail to rise significantly, typically by less than 20 milligrams per deciliter.
Practical Strategies for Managing Deficiency
Managing this enzyme deficiency involves a combination of dietary adjustments and the use of supplemental enzymes. Exogenous lactase enzyme supplements, available over the counter, are designed to replace the body’s missing enzyme. These supplements should be taken immediately before consuming any food or drink containing lactose, allowing the enzyme to break down the sugar before it causes symptoms.
Many individuals can tolerate certain dairy products without supplements. Hard, aged cheeses like Parmesan, Cheddar, and Swiss contain only trace amounts of lactose because the aging process naturally ferments the sugar. Yogurts and kefir often contain live and active cultures that act as an internal source of lactase, pre-digesting the lactose in the product. Commercial lactose-free milk is also available, which is regular dairy milk with the lactase enzyme already added.
A concern when restricting dairy is the risk of inadequate intake of calcium and Vitamin D. To counteract this, it is recommended to consume fortified foods like plant-based milks and juices. Non-dairy sources of calcium, such as canned fish with soft bones and dark leafy greens, should also be sought.