Testing urea levels in the blood provides information about your health. This post covers the causes of high and low blood urea nitrogen (BUN) and what they may mean for you.
What is Blood Urea Nitrogen (BUN)?
Blood urea nitrogen (BUN) is a measure of the amount of urea in the blood .
The liver produces urea as a waste product of proteins breakdown. It binds excess nitrogen from used-up proteins and safely removes it from the body [1, 2].
Urea is created not only from dietary protein, but also from protein in your tissues .
On a normal diet, we produce about 12 g of urea each day .
The bulk of the urea, about 10 g each day, is eliminated by the kidneys .
Being a diuretic, urea helps the kidneys quickly flush water and other compounds [4, 5].
A small amount of urea (less than 0.5 g/day) is lost through the gut, lungs, and skin. During exercise, a substantial amount may be lost through sweat .
BUN Blood Test
Why Doctors Order it
A blood urea nitrogen (BUN) test is performed to:
- See if your kidneys are working normally or if kidney disease is progressing
- Check for severe dehydration
Any standard blood test will have BUN or urea numbers.
Conventional doctors will look at high or low BUN numbers and not mention anything, but these can indicate that certain processes in the body aren’t optimal.
What Your BUN Levels Mean
BUN levels represent the balance between :
- Urea production (in the liver)
- Urea breakdown, and
- Urea elimination (via the kidneys)
Therefore, BUN is an indicator of kidney health and/or liver health.
However, creatinine is a much more reliable marker of kidney function. BUN is far more likely to be affected by dietary and physiologic conditions unrelated to kidney function .
Optimal and Normal Range
Lab results are commonly shown as a set of values known as a reference range, which is sometimes referred to as a “normal range.” A reference range includes upper and lower limits of a lab test based on a group of otherwise healthy people.
Your healthcare provider will compare your lab test results with reference values to see if any of your results fall outside the range of expected values. By doing so, you and your healthcare provider can gain clues to help identify possible conditions or diseases.
In Europe, the whole urea molecule is measured, whereas in the United States only the nitrogen component of urea is measured (the blood or serum urea nitrogen, i.e., BUN or SUN) .
The BUN is roughly one-half of the blood urea .
Normal human adult blood should contain between 5 to 20 mg of urea nitrogen per 100 ml (5 to 20 mg/dL) of blood, or 1.8 to 7.1 mmol urea per liter .
To convert from mg/dL of blood urea nitrogen to mmol/L of urea, multiply by 0.357.
The range is wide because of normal variations due to protein intake, protein breakdown, state of hydration, liver urea production, and urea elimination by the kidneys .
Decreased or elevated BUN concentrations are usually seen in pregnancy [7, 8].
A BUN test is usually done with a blood creatinine test.
The level of creatinine in your blood also tells how well your kidneys are working. A high creatinine level may signal problems with the kidneys or heart, but if it’s slightly elevated, it could simply mean that your diet is high in protein .
Blood urea nitrogen (BUN) and creatinine tests can be used together to find the BUN-to-creatinine ratio (BUN:creatinine).
In most cases, it’s healthier to have a lower ratio of BUN to creatinine (10:1 to 20:1) .
High BUN Levels
The urea nitrogen test is often ordered for people who are experiencing signs and symptoms of kidney disorders. 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)
The causes shown here have been associated with elevated BUN, but this marker alone is not enough to diagnose any health condition. Do not attempt to self-diagnose based on this lab marker. Work with your doctor to determine an accurate diagnosis and to develop an appropriate treatment or management strategy.
All the following can elevate your BUN:
- Kidney disease or failure, and blockage of the urinary tract by a kidney stone .
- High-protein diet 
- Fever or infection, which increases protein breakdown. Increased protein breakdown is a common feature of an illness. Protein breakdown is stimulated by hormones (such as glucagon, epinephrine, and cortisol) and inflammatory cytokines. Protein production, on the other hand, is reduced by lowering growth hormone, insulin, and testosterone levels [2, 6].
- Inflammation or interval training, which results in protein breakdown from muscle
- Dehydration or low water consumption – I use this as a measure of hydration in clients. BUN increases as blood volume decreases .
- 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 [10, 11].
- Gut bleeding -When upper GI bleeding occurs, the blood is digested to protein. This protein is transported to the liver and metabolized to BUN [2, 1].
- Poor circulation, which results in lower blood flow to the kidneys and therefore less of an ability to clear the urea [12, 6].
- Thyroid abnormalities, which result in abnormal kidney function: hypothyroidism, and hyperthyroidism [13, 14].
- Glucocorticoids, Tetracyclines (except doxycycline) and other anti-anabolic drugs 
- Lower growth hormone or IGF-1. IGF-1 and growth hormone inhibit urea synthesis .
- Severe burns
Negative Health Effects
You may be wondering why high BUN is bad.
To start with, elevated urea on its own has some adverse effects. Urea in high concentrations can cause oxidative stress in cells .
However, high BUN is also an indicator of other underlying conditions.
High BUN indicates increased protein breakdown, which is associated with decreased immune function. A study shows that patients with elevated BUN (> 20 mg/dl) have an increased risk of infection .
High BUN is associated with increased mortality in critically ill patients [6, 17, 18, 19].
Elevated BUN is also associated with increased stroke risk in heart surgery, and adverse outcomes in atherosclerosis and heart failure patients [20, 21, 22].
Note that BUN values remain within the normal range until more than 50% of renal function is lost. Within that range, however, a doubling of the values (e.g., BUN rising from 8 to 16 mg/dl) may mean a 50% fall in kidney function .
How to Lower BUN
Most importantly, work with your doctor to treat any underlying conditions causing your high BUN levels. You may try the additional strategies listed below if you and your doctor determine that they could be appropriate. None of these strategies should ever be done in place of what your doctor recommends or prescribes.
Diet & Supplements
There are two main ways to decrease BUN:
- Drink more water
- Eat less protein
Low BUN Levels
The causes shown here have been associated with low BUN, but this marker alone is not enough to diagnose any health condition. Do not attempt to self-diagnose based on this lab marker. Work with your doctor to determine an accurate diagnosis and to develop an appropriate treatment or management strategy.
The following can excessively lower your BUN:
- Low-protein diet, malnutrition, or starvation 
- Impaired liver activity due to liver disease 
- Genetic deficiency of urea cycle enzymes 
- Higher IGF-1 and growth hormone. These inhibit urea synthesis. Growth hormone-deficient children given human growth hormone have lower urea nitrogen, and this is due to decreased urea synthesis [15, 23].
- Anabolic steroids, which decrease protein breakdown
- Overhydration, or drinking a lot of water
- Pregnancy (due to increased plasma volume)