What's type 1 and type 2 diabetes, and gestational diabetes?

The term diabetes mellitus comes from Greek words for "flow" and "honey",
referring to the excess urinary flow that occurs when diabetes is untreated,
and to the sugar in that urine.

Diabetes mellitus (DM) comes in the following classifications (which some
will argue don't really represent the actual types very well):

   type 1 -- characterized by total destruction of the insulin-producing beta
             cells, probably by an autoimmune reaction. Onset is most common
             in childhood, thus the common (but now deprecated) term
             "juvenile-onset", but the onset up to age 40 is not uncommon and
             can even occur later. Patients are susceptible to DKA (diabetic
             ketoacidosis). There seems to be some genetic tendency, but the
             genetic situation is unclear. Most patients are lean. Always
             requires treatment by insulin. Not sex-linked. Also referred to
             as IDDM (insulin dependent diabetes mellitus).

   type 2 -- characterized by insulin resistance despite adequate insulin
             production. A large majority of patients are overweight at onset,
             and a majority are female. Most are over 40, hence the common
             (but now deprecated) terms "adult-onset" or "maturity-onset", but
             onset can occur at any age. Patients are not susceptible to DKA
             (diabetic ketoacidosis). There is a strong genetic tendency, but
             not simple inheritance. Depending on the individual, treatment
             may be by diet, exercise, weight loss, oral drugs which stimulate
             the release of insulin, or insulin injections -- and usually a
             combination of several of these. Also referred to as NIDDM (non
             insulin dependent diabetes mellitus) *even when treated with
             insulin* -- a confusing terminology which, unfortunately, is
             supported by the ADA.

   gestational -- occurs in about 3% of all pregnancies as a result of
             insulin antagonists secreted by the placenta. It is recommended
             that all pregnant women receive a screening glucose tolerance
             test (GTT) between the 24th and 28th weeks of pregnancy to detect
             gestational diabetes early if it occurs, as diabetes can cause
             serious difficulties in pregnancy. Sometimes requires insulin
             treatment. Not susceptible to DKA (diabetic ketoacidosis).
             Usually disappears after childbirth, but about 40% of patients
             develop type 2 diabetes within five years. Most authorities state
             that the typical patient is female ...

   malnutrition-related -- severe malnutrition sometimes causes diabetes --
             hyperglycemia and all the usual symptoms. The reason is unknown,
             and since this syndrome occurs almost entirely in third world
             countries, research on this form of diabetes is nearly nonexistent.

   other types -- sometimes called secondary. A catchall for forms not covered
             by the types described above. Causes include loss of the entire
             pancreas (to trauma, cancer, alcohol abuse, or exposure to
             chemicals), diseases that destroy the beta cells, certain
             hormonal syndromes, drugs that interfere with insulin secretion
             or action, and some rare genetic conditions.

These terms are not used entirely consistently. Some doctors will refer to
any diabetic using insulin as type 1, and will refer to the early onset of
type 1 diabetes as type 2 until insulin therapy is required. This usage does
not fit with most modern usage as described above (type 1 is beta cell
destruction, type 2 is insulin resistance). The situation is complicated by
the fact that early in the course of the disease it can be difficult to
determine which type is occuring, especially for patients in their 30's, the
age when the onset of both types is common.

Different patients respond very differently to what is categorized above as
the same disease. The root causes of all forms of diabetes are not
understood, and are likely more complex and varied than the simple categories
show. Type 1 diabetes likely has a few root causes, and type 2 diabetes
probably has a larger number of root causes.

There are also well documented reports of cases of diabetes with unexplained
combinations of syndromes from types 1 and 2. These are sometimes referred to
as "type 1-1/2", and the reasons are not understood.

The classification above is not completely standard, and other classifications
exist.

About 90% of diabetes patients are type 2 (some 12 million in the US), and
about 10% are type 1 (some 1 million in the US). Discussion on m.h.d tends to
run about 2/3 type 1, I'd guess. This probably reflects the fact that type 1
diabetes is harder to ignore, and that type 2 seldom strikes the younger
people who are more likely to have net access. Type 2 is *not* less serious.

"1" and "2" are often written in Roman numerals: type I, type II. Because
typography is often unclear on computer terminals, I've stuck with the Arabic
numeral version.

Diabetes accounts for about 5% of all health care costs in the US, some
US$90 billion per year.