The vast diversity of life on Earth would not exist without the specialized protein molecules known as enzymes. Digestion, along with many other essential biological processes, depends on biochemical reactions that can occur at adequate rates only because enzymes lower the energy required to initiate these reactions. One of the most familiar human digestive enzymes is lactase, which must be present in the small intestine to ensure proper assimilation of the sugar molecules in dairy products.
Dairy products contain significant quantities of lactose, commonly known as milk sugar. Lactose cannot be directly absorbed by the digestive tract because it is a compound sugar -- only simple sugars, or monosaccharides, can pass through the membranes of the cells lining the small intestine. Lactose is classified as a disaccharide because it is composed of two monosaccharides, glucose and galactose. For successful digestion of lactose, the digestive system must break down the lactose molecule into its constituent monosaccharides, and the essential agent in this process is the lactase enzyme.
The primary source of lactase is the intestinal tract. The interior of the small intestine is populated by epithelial cells that can synthesize lactase. The synthesized lactase molecules remain attached to hairlike projections called villi, where they readily mix with partially digested food as it passes through the small intestine. The situation is not quite this simple, however, because individuals vary widely in their ability to synthesize lactase. Essentially all infants produce abundant quantities of intestinal lactase to digest their primary food -- mother's milk. But many individuals gradually lose the capacity for lactase synthesis as they age, resulting in poor digestion of dairy products and processed foods containing lactose.
A Fungal Enzyme Factory
The basic disorder caused by inadequate lactase production is known as lactose intolerance. The various unpleasant gastrointestinal symptoms associated with lactose intolerance are caused by the accumulation of lactose in the intestines and by digestive bacteria that consume undigested lactose and release irritating byproducts. An alternative source of lactase is a fungal organism known as Aspergillus oryzae. In the 1980s, researchers discovered that they could use these fungi to mass-produce lactase enzymes, which were then added to certain dairy products intended for lactose-intolerant individuals.
Many lactose-intolerant individuals can safely consume fermented dairy products such as yogurt, and a common explanation for this is that the fermenting bacteria accomplish a form of predigestion by consuming lactose. Thus, some of the lactose is already broken down before it reaches the small intestine. The benefits of fermented dairy, however, are also attributable to lactase content -- some of the bacterial species that create yogurt and other fermented dairy products can synthesize lactase.
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