Urea is an important measure of kidney health, liver health, and protein turnover. A urea blood test is often ordered as a part of a comprehensive metabolic panel, which gives a broad overview of a person’s metabolic and overall health. How do doctors use this test? Read on to find out.
What is Urea?
Urea is a waste product that the liver makes when it degrades proteins, both from the diet and from tissue protein turnover [1, 2, 3].
On a normal/average diet, we produce about 12 g of urea each day . The bulk of it, about 10 g each day, is eliminated by the kidneys .
A small amount of urea (less than 0.5 g/day) leaves the body through the gut, lungs, and skin. During exercise, a substantial amount may be lost through sweat .
Blood urea levels represent the balance between urea production (in the liver), urea breakdown, and urea elimination by the kidneys .
Therefore, urea can be a useful indicator of kidney health and liver health. It is also used to check for severe dehydration.
Doctors often order the urea test for people who are experiencing signs and symptoms of kidney dysfunction. These symptoms can include:
- Frequent urination
- Discolored urine (bloody, dark, or foamy)
- Joint pain
- Bone pain
- Back pain
- Muscle cramping
- Restless legs
- Trouble sleeping
- Poor appetite
- Swelling (especially in the extremities)
Urea Normal Range
In Europe, the whole urea molecule is measured. Normal human blood should contain between 1.8 – 7.1 mmol/L .
In the United States only the nitrogen component of urea is measured (the blood urea nitrogen, i.e., BUN), which is roughly one-half of blood urea. The normal range for BUN is between 5 – 20 mg/dL .
To convert from mmol/L into mg/dL, divide by 0.357.
The range is wide because there are many factors that affect blood urea levels :
- the amount of protein in the diet
- protein breakdown
- state of hydration
- liver urea production
- urea elimination by the kidneys
Urea can both decrease and/or increase in pregnancy [5, 6].
Low Urea Levels
Low urea levels are often not of great concern. However, in some cases, they point to underlying issues:
- Low-protein diet, malnutrition, or starvation 
- Impaired liver activity due to liver disease, often linked to alcohol abuse [3, 7]
- Overuse of anabolic steroids, which decrease protein breakdown 
- Overhydration, or drinking too much water 
- Growth hormone use. Growth hormone-deficient children given human growth hormone have lower urea, and this is due to decreased urea production [9, 10]
- Genetic deficiency of urea cycle enzymes 
If low urea levels are combined with worrying symptoms or other abnormal lab markers, your doctor will consider all of these factors to make an accurate diagnosis.
Urea can also be decreased in pregnancy, but it is not used to determine whether someone is pregnant because urea levels tend to be highly variable during this time .
Factors that Increase Urea Levels
Low urea levels may indicate an underlying health problem that requires medical attention. Rather than trying to artificially increase urea, work with your doctor to diagnose and treat any condition that may be causing low urea. You may also try the following strategy if your doctor determines that it is appropriate.
Low urea levels may mean that you are not consuming enough protein. If this is the case, you may need to increase your consumption of protein-rich foods like lean meats or beans .
High Urea Levels
High urea levels can result from serious underlying health conditions and diseases. In addition, elevated urea may increase oxidative stress in cells [11, 12].
High urea indicates increased protein breakdown, which is associated with decreased immune function. In a study of 26,000 critically ill patients, those with elevated urea were more likely to suffer infections and to die while hospitalized .
High urea was also associated with increased mortality in ill patients in three other studies of several hundred patients each [4, 13, 14, 15].
Elevated urea has also been associated with increased stroke risk in heart surgery (5498 subjects), and adverse outcomes in atherosclerosis (1521 subjects) and heart failure patients (225 patients) [16, 17, 18].
Blood urea levels tend to increase as we age .
Levels can also rise in pregnancy, but they are not used to determine whether someone is pregnant because urea levels are highly variable during this time .
Causes of High Urea
Causes shown here are commonly associated with high urea. Work with your doctor or other health care professional for an accurate diagnosis.
- Kidney disease or failure, and blockage of the urinary tract by a kidney stone 
- Dehydration/low water consumption – Urea increases as blood volume decreases 
- High protein diets 
- Fever or infection, which increases protein breakdown. Increased protein breakdown is a common feature of illness. Protein breakdown is stimulated by hormones (such as glucagon, epinephrine, and cortisol) and inflammatory cytokines [3, 4]
- Strenuous physical exercise, which results in muscle stress and protein turnover 
- Stress – An inappropriate increase in the activation of the sympathetic, renin-angiotensin-aldosterone, and vasopressin systems elevate BUN, which is often seen in heart failure. Cortisol will also increase protein breakdown and elevate BUN [21, 22]
- Gut bleeding – When upper GI bleeding occurs, the blood is digested to protein. This protein is transported to the liver and metabolized to BUN [3, 2]
- Poor circulation, which results in lower blood flow to the kidneys and therefore less of an ability to clear the urea [23, 4]
- Thyroid conditions, which result in abnormal kidney function: hypothyroidism, and hyperthyroidism [24, 25]
- Anti-anabolic drugs such as glucocorticoids and tetracyclines (except doxycycline) 
- Lower growth hormone or IGF-1. IGF-1 and growth hormone inhibit urea synthesis 
- Inborn errors of metabolism (genetic urea cycle disorders) 
Factors that Decrease Urea Levels
High urea levels may indicate an underlying health problem that requires medical attention. Rather than trying to artificially decrease urea, work with your doctor to diagnose and treat any condition that may be causing high urea. You may also try the following strategies if your doctor determines that they are appropriate.
Your doctor may check if any environmental or dietary factors could be causing high urea levels. Among these are dehydration and a very high protein diet. If one or both of these are determined to be the underlying cause, you would likely be advised to:
- Drink more water (stay properly hydrated)
- Eat less protein
Because people with high BMI are more likely to experience kidney dysfunction, weight loss may help improve kidney health and lower urea levels [26, 27, 28].
A few supplements have also been found to decrease urea and possibly improve kidney function in animals. However, no clinical studies exist to support the use of these supplements to combat high blood urea or kidney dysfunction. Talk to your doctor before adding any new supplements, and never use these in place of what your doctor prescribes.