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 .
It provides useful information about your health.
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 .
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
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 .
I like to see BUN levels between 7 to 15 mg/dL, which I consider a more optimal range.
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 mean your kidneys are not working properly, but if it’s slightly elevated, it could simply mean that you have a lot of muscle.
Blood urea nitrogen (BUN) and creatinine tests can be used together to find the BUN-to-creatinine ratio (BUN:creatinine).
In general, you want to have lower BUN and lower creatinine. However, in most cases, it’s healthier to have a lower ratio of BUN to creatinine (10 is ideal).
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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)
All the following can elevate your BUN:
- 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 [9, 10].
- 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 [11, 6].
- Thyroid abnormalities, which result in abnormal kidney function: hypothyroidism, and hyperthyroidism [12, 13].
- Glucocorticoids, Tetracyclines (except doxycycline) and other anti-anabolic drugs 
- Lower growth hormone or IGF-1. IGF-1 and growth hormone inhibit urea synthesis .
- Kidney disease or failure, and blockage of the urinary tract by a kidney stone .
Negative Health Effects
You may be wondering why high BUN is bad.
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 .
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
Diet & Supplements
There are two main ways to decrease BUN:
- Drink more water
- Eat less protein
Arginase converts L-arginine to urea. When you inhibit arginase, you decrease urea. The downside to this might theoretically be a buildup of ammonia.
Nickel is required for the conversion of urea-form nitrogen to ammonia. A deficiency of nickel leads to urea toxicity.
The following inhibit arginase:
Make sure to test your BUN levels to see if it is optimal for you to decrease your levels.
Low BUN Levels
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 [14, 25].
- Anabolic steroids, which decrease protein breakdown
- Overhydration, or drinking a lot of water
- Pregnancy (due to increased plasma volume)