The elevated blood glucose levels in diabetics is a symptom with a few possible causes. In Type 1 diabetics for example, the insulin producing beta cells are attacked by the body's own immune system. It sees them as foreign bodies. In Type 2 diabetics the beta cells stop working for other reasons, including internal deposits of fat blocking the
Islets of Langerhans in the pancreas. There are also other metabolic disorders which are factors.
Lactose intolerance is a very wide term covering many conditions. All children should be able to drink, mainly, because they need mother's milk in infancy. However, in most humans throughout the world, the ability to produce the necessary enzyme
lactase is switched off in the teens. Adults generally cannot drink milk because they lack the enzyme lactase required to digest the beta carbohydrate
lactose which is found in milk. For some unknown reason however, some can drink half a glass of milk without being sick whereas others get sick licking an ice cream.
However, the cattle dairying cultures in northern europe, india and sub saharan africa and the camel drinking nomadic cultures in arabia, all have high frequencies of specific genes which allow
lactase persistence, ie where lactase continues to be produced in adulthood. A different gene however is responsible in each of these four cultures. In europe, it is a C to T transition at 13910 Kb. The european variety is a mutation, a copy error where a cytosine nucleotide substituted by a thyamine nucleotide. It is thought to have occured around 5000 years ago. Mediterranean cultures have much lower instances of the LAC P gene as it is termed, than north europe.
In children, many things including milk allergy were termed lactose intolerance but now that the genetic components are better understood, it seems to be either an early switching off of lactase production or of insufficient amounts being produced. In eastern Finland for example, lactase production is switched off much earlier than in southern europeans. Superficially, a lack of lactase is similar to the lack of insulin but the causes are very different and the outcomes are very different. In infancy it is of course a major problem but once a child can eat solid foods, it is less so and, as you can see from the chart above, most of the world cannot digest milk in adulthood.
The Vinland Sagas tell of the vikings in North America giving the Skraelings milk to drink. The Skraelings, the native americans, became sick and thought the norse had poisoned them. Hostilities followed.
Beware of vikings bearing dairy products!
The Battles in Vinland 1020 AD