Hence, if applied to dairy cattle, this could allow the easy and efficient production of low-lactose milk to fulfill the requirements of lactose maldigesters, because a 50–70% decrease in lactose is sufficient to prevent intestinal disorders after milk ingestion.

First steps to low-lactose milk ELEANOR LAWRENCE A cheap and easily available low-lactose milk would be a welcome option for the 70% of the adult population worldwide which has a problem with lactose intolerance. Removing the lactose from milk can be done, but the process is expensive and laborious. To this end, Bernard Jost and colleagues from the Institut National de la Santé et de la Recherche Médicale in Strasbourg, France have taken the first steps towards the production of transgenic animals producing a nutritious but low-lactose milk. As they describe in Nature Biotechnology , the French researchers have produced mice that express the lactose-digesting enzyme lactase in their mammary glands. This lowers the amount of lactose in the milk by between 50 and 85% without any obvious changes to its protein and fat content. The milk retained its nutritional value as suckling mice reared on it grew and developed quite normally. Human infants can digest large amounts of milk because they produce the enzyme lactase in their digestive tract. But after weaning, there is a quite normal decrease in the production of this enzyme. In a considerable proportion of adults, the amounts of lactase produced are so small that they cannot digest even the amount of lactose one would get from a glass of milk. The undigested lactose in the small intestine causes abdominal pain, nausea and diarrhea, which can lead to severe dehydration. Lactase is normally produced only in the intestine and not in the mammary gland. To get the mammary gland to produce lactase, Jost and colleages had to construct an additional engineered lactase transgene that could be introduced into mice and be expressed only in the mammary cells. They did this by attaching the DNA sequence encoding the lactase protein to a regulatory DNA sequence that directs its expression only in a lactating mammary gland. Female mice transgenic for this new gene not only produced lactase in the milk-producing cells, but also secreted it into the milk itself, anchored in the outer membranes of the fat globules. Milk collected immediately on secretion had 50% less lactose than normal but if it was allowed to collect in the mammary gland, the level dropped even further to around 85% less lactose than usual.