Most of us are familiar with the two main types of diabetes: type 1 and type 2.
Type 1 is primarily autoimmune; the autoantibodies (antibodies produced against your own cells) destroy most of the beta cells that produce insulin, so people with type 1 require insulin shots, although some people with type 1 have some insulin resistance and can be helped with type 2 drugs in addition to insulin.
Type 2 is caused by insulin resistance (by definition), although some people with type 2 have small amounts of autoantibodies, and many people with type 2 take insulin shots, especially after many years with the disease.
However, there are also other forms of diabetes, and some of these have been called type 1.5 or type 3, sometimes both. It's confusing, because different authors use the terms differently.
Latent autoimmune diabetes of adults, or LADA, is like type 1 in that it's autoimmune and patients will eventually have to use insulin, but it's like type 2 in that patients are usually diagnosed as adults and the disease progresses slowly, so many are originally told that they have type 2. Because it's sort of between types 1 and 2, it's often called type 1.5.
But some use the term type 1.5 when people have autoimmune diabetes but also have a lot of insulin resistance because of obesity. This can also be called "double diabetes," because you have both type 1 and type 2.
The term type 3 diabetes has been used in many different ways.
Some people call Alzheimer's disease (AD) type 3 diabetes because there is some evidence that people with AD are not able to take up enough glucose into the brain. That is, they may have plenty of glucose in the bloodstream, but the glucose can't cross what is called the blood-brain barrier to nourish the brain cells. There is even some evidence that nasal insulin can help with AD. Inhaled insulin can get into the brain without having to pass the blood-brain barrier.
Others call gestational diabetes type 3 diabetes. Gestational diabetes occurs because hormones in late gestation increase insulin resistance, and if the beta cells are not able to secrete enough insulin to cover the increased insulin resistance, blood glucose (BG) levels increase. Usually after the baby is born, the BG levels return to normal, but the woman is at increased risk of developing type 2 in the future.
Some people use the term type 3 for the spouses of people with diabetes.
And some call "double diabetes" type 3 as well as type 1.5.
Finally, there's another kind of type 3 diabetes that I'd never even heard of until recently. This is diabetes caused by malnutrition, and it is not uncommon in areas of the world where getting sufficient nutrition is difficult. Some call it malnutrition-related diabetes mellitus, or MRDM, and not much is known about it.
The body seems to think that the pancreas is not the most important organ we have, so when nutrients are in short supply, it sends most of them to the brain and other vital organs. Children born to mothers who lived through a famine while the children were in the womb often develop diabetes as adults because their pancreases are not large enough to cope with any insulin resistance. In the case of MRDM, the malnutrition is in the growing children, not the mother.
Because there are so many different usages of type 1.5 and type 3 diabetes, I think it's high time settled on definitions everyone could agree with. Maybe it would be best to forget the numbers altogether. Here is one suggestion:
Type A diabetes (autoimmune, type 1)
Type R diabetes (insulin resistance, type 2)
Type G diabetes (gestational)
Type L diabetes (LADA)
Type AR diabetes (double diabetes)
Type S diabetes (spousal)
Other forms of diabetes not mentioned could also be included
Type M diabetes (MODY; or maturity-onset diabetes of the young)
Type C diabetes (caused by chemical damage to the pancreas)
Type O diabetes (caused by an operation; not surgical to avoid confusion with spousal)
These are just suggestions. What the diabetes community eventually decides on is not important. But it's high time we get rid of confusion about type 3.