What Is the Difference Between Diabetes Insipidus and Diabetes Mellitus?

Diabetes affects the way your body turns food into energy. Diabetes insipidus causes thirst due to dehydration from constant urination while diabetes mellitus causes thirst due to high glucose levels in the blood.

The term "diabetes" refers to a group of medical conditions that affect how your body turns food into energy. There are several types related to metabolism, and about 1 in 10 Americans have some form of diabetes. However, diabetes mellitus and diabetes insipidus function in very different ways.  

Diabetes mellitus is a chronic condition that alters how your food is metabolized. People with diabetes either have a pancreas that does not make enough insulin, or their bodies cannot properly utilize the insulin that the pancreas makes.

Type 1 diabetes mellitus, also called "childhood-onset diabetes," is caused by an autoimmune reaction, affecting the pancreas so that the organ can't produce enough insulin. Insulin is a hormone that controls how blood sugar is used for energy in the body. Without enough insulin, blood sugar levels build up and cause many medical complications. In this form of diabetes, too little insulin is produced.

Type 2 diabetes mellitus is caused when the body's cells develop insulin resistance, failing to respond normally to insulin triggers. The pancreas continually makes more insulin to overcome the resistance, but it cannot keep up. This also results in a rise in blood sugar throughout the body. In this form of diabetes, too much insulin is produced with too little effect.

All forms of diabetes eventually trigger an overproduction of urine by the kidneys. In diabetes mellitus, overproduction results from damage caused by high blood sugar. 

Diabetes insipidus, though, is caused when, despite normal blood sugar, the kidneys cannot balance the amount of fluid in the body. The problem is due to the hormone vasopressin rather than blood sugar, insulin shortage, or insulin resistance. 

This condition leads to the excessive production of urine. Though the glucose levels in the body are normal, the kidneys cannot reduce urine. The kidneys would usually produce 1 to 3 quarts of urine a day; however, those with diabetes insipidus can make as much as 20 quarts a day. This imbalance leads to polyuria — or the need to urinate frequently.

Though the general symptoms of both diabetes insipidus vs. diabetes mellitus may be similar, they are produced by different causes, and there are differences in the results.

People with both types of diabetes are often thirsty. Diabetes insipidus causes thirst due to dehydration from constant urination. This keeps electrolyte levels in the body low, and those affected are commonly tired. Diabetes mellitus causes thirst due to high glucose levels in the blood. The body needs more hydration to get rid of the extra sugar. Fluctuation of high and low glucose levels can cause tiredness as well. Dehydration and high glucose levels can also lead to blurred vision.

Symptoms unique to diabetes mellitus include sudden weight loss, constant hunger, tingling of hands and feet, frequent infections, and wounds that are slow to heal. All of these symptoms can be explained by abnormal glucose levels in the body. Diabetes insipidus causes light-colored urine due to high water concentrations in the kidney. The kidneys cannot balance fluid amounts due to the irregularity of vasopressin.

Diabetes insipidus is caused by one of two problems related to vasopressin. The hypothalamus, located in the brain, does not produce enough of the hormone, or the body cannot adequately respond to the hormone.

Diabetes mellitus can be diagnosed with blood tests or an oral glucose tolerance test. A fasting blood glucose test is a blood test that can be performed after a person has fasted for 8 hours. This test is considered positive if the reading is 126 mg per dL or more. A random blood glucose level of 200 mg per dL without fasting is also considered diagnostic of diabetes. Another blood test that does not require fasting is a hemoglobin A1c or glycated hemoglobin test. A reading on two occasions of 6.5 percent or more is considered positive for diabetes. 

An oral glucose tolerance test is done after a patient has fasted for 8 hours. It is different because the patient is orally given a 75-g glucose load. Then, a blood test is given 2 hours later. If the blood glucose level after 2 hours is higher than 199 mg per dL, it is considered diagnostic for diabetes mellitus.

Diabetes insipidus can be diagnosed with blood tests and other lab tests. A urinalysis will be able to tell if your urine has too much water in it. Increased levels of sodium in blood work can also indicate diabetes insipidus. The doctor may also give a water deprivation or stimulation test. Water is not drunk for several hours; sometimes, an artificial form of vasopressin may be given to stimulate a reaction. Magnetic resonance imaging (MRI) may also be given to look for damage to the hypothalamus — which can cause diabetes insipidus. 

Neither type of diabetes is curable, but they can be treated. Diabetes mellitus, depending on the type, can be controlled with diet and exercise. There are a series of specialists that can help to keep you healthy. This may include an endocrinologist, podiatrist, eye doctor, and diabetes educator. 

The doctor may prescribe oral medication or insulin injections to manage diabetes mellitus. Blood sugar levels must be measured often, and regular doctor appointments kept. Stress management and adequate sleep are essential in managing blood sugar levels.

Dehydration prevention is the best way to manage diabetes insipidus. Treatment will also depend on the type of diabetes insipidus you have. Central diabetes insipidus is often treated with desmopressin — a man-made hormone that replaces the missing vasopressin in the body.

Nephrogenic diabetes insipidus can be treated, or may go away depending on the cause. Medication changes or nutrient balance could be enough to resolve the symptoms. Dipsogenic diabetes insipidus does not yet have an effective treatment. Sucking ice or candy can help with reducing thirst. Sodium levels are monitored to make sure they do not drop too low. Gestational diabetes insipidus can be treated with desmopressin. This type of diabetes only occurs during pregnancy, and usually will go away after the baby is born.  

Diabetes insipidus is usually treated by a nephrologist or an endocrinologist. Those with this condition can live normally and avoid major medical complications by closely following their doctor's orders.