BUN (blood urea nitrogen) shows how much urea is in your blood, expressed as the amount of nitrogen the urea contains. Urea forms when protein from food and from your own body is broken down. A high value may come from things like not getting enough fluids, eating a lot of protein, or reduced kidney function, and a low value from things like not getting enough protein. BUN alone can’t tell you which one applies, but reading it together with creatinine and other markers gives you more clues.
In Japan’s shared reference intervals (JCCLS), the reference range for BUN is 8–20 mg/dL for both men and women. Ranges can differ a little between clinics and labs, so please also check the range printed on your results (more in “Reference range”).
What BUN (blood urea nitrogen) reflects
Protein in food is broken down into amino acids, absorbed, and used as material for muscle, enzymes, and more. Inside the body, too, old proteins are broken down and rebuilt every day. From leftover amino acids and broken-down proteins, the part containing nitrogen comes off, producing ammonia.
Because ammonia is harmful to the body, the liver turns it into urea through the urea cycle (also called the ornithine cycle). Urea travels in the blood to the kidneys, where it’s filtered and leaves the body in urine. BUN is the measured amount of nitrogen in this urea. On test reports, it may also be labeled “UN” or “urea nitrogen.”
Because of this process, three things overlap in BUN:
- How much protein is being broken down (how much you eat, and the breakdown of your body’s own protein)
- How well the liver makes urea
- How well the kidneys clear urea, and your body’s hydration
The one that’s easy to overlook is the third: hydration. Some of the urea filtered by the kidneys is reabsorbed in the renal tubules and goes back into the blood. The less water in your body and the more concentrated your urine, the more urea goes back, so even in the same person, BUN moves up and down with hydration.
Reference range
In Japan’s shared reference intervals (JCCLS), BUN is 8–20 mg/dL for both men and women. Creatinine, which is often read alongside it, is 0.65–1.07 mg/dL for men and 0.46–0.79 mg/dL for women. Some clinics and labs use their own ranges, so please also check the range printed on your results.
What a high BUN can mean
Not enough fluids
BUN can come out on the high side on days when you’ve sweated a lot, after exercise or a sauna, or when you went to your blood draw without drinking much. The same goes for times when diarrhea or vomiting has gone on for a while. In these cases, creatinine doesn’t move much, and BUN tends to rise first (with more severe dehydration, creatinine rises too).
Eating a lot of protein
If you eat a lot of meat, fish, eggs, protein shakes, and the like, more nitrogen is broken down, and BUN tends to rise. A study of healthy young men reported that a week on a diet with 2.4 g of protein per kg of body weight raised BUN compared with 1.2 g per kg.
More breakdown of the body’s own protein
With a fever or a serious infection, after a major injury, while taking steroid medications, or when the thyroid is overactive, the body breaks down more of its own protein, and BUN can rise. In the first few days after sharply cutting back on food, BUN can also rise temporarily as muscle protein is used for energy. If the restriction goes on for a long time, BUN more often heads the other way and drifts down, since you’re taking in less protein.
Bleeding in the digestive tract
When there’s bleeding in the stomach, duodenum, or elsewhere, the blood that enters the intestine is digested and absorbed like protein from food, and becomes material for urea. On top of that, the body has less blood, so less blood flows to the kidneys. Together, these tend to raise BUN and push up the BUN/creatinine ratio. That said, a ratio that isn’t high doesn’t mean there’s no bleeding. If you keep having blackish stools or feel lightheaded, please see a doctor without waiting for your next test.
Lower kidney function
When the kidneys can’t clear enough urea, BUN rises, and creatinine generally rises with it. But because BUN is easily swayed by food and fluids, kidney function itself is assessed with eGFR (estimated glomerular filtration rate), calculated from creatinine, and with urine tests. A slightly high BUN alone doesn’t mean you have kidney disease. Reduced heart function or certain medications can also lower blood flow to the kidneys and raise BUN.
What a low BUN can mean
Not eating enough protein
This is one of the more common reasons for low BUN in healthy people. If you eat small amounts, if your meals center on staples such as rice, bread, or noodles and on vegetables, with little meat, fish, eggs, or soy foods, or if you’re restricting what you eat, less nitrogen is broken down, and BUN tends to come out low.
Drinking a lot of fluids, or pregnancy
When you drink a lot and make more urine, less urea is reabsorbed into the blood in the renal tubules, so BUN can come out low. During pregnancy, the kidneys filter about 50% more, so both BUN and creatinine often come out lower.
A major drop in the liver’s ability to make urea
When liver function drops sharply, ammonia isn’t converted into urea as it should be, and BUN goes down. But this applies only in limited situations, such as advanced cirrhosis. In those cases, albumin, platelets, bilirubin, and other markers usually change as well, so there’s no need to suspect reduced liver function just because BUN is slightly low.
Reading the BUN/creatinine ratio, and its limits
Creatinine is made in muscle at a steady pace, and compared with BUN, it’s less affected by food and fluids. So the ratio of BUN to creatinine offers a clue for gauging whether a change in BUN is coming from the kidneys. However, in people with a lot of muscle, people taking creatine supplements, and people who ate a lot of meat before the blood draw, creatinine can come out higher, making the ratio lower. In people with less muscle, and in women, the ratio tends to come out higher.
You find the ratio by dividing BUN by creatinine. Using the middle values for healthy people in the shared reference intervals, it comes to about 15 for men and about 19 for women. Traditionally, 20 has served as the guide for whether the ratio is high, but that likely doesn’t apply as is to healthy people in Japan, especially women. “Higher” and “lower” in the table below describe the tendency compared with these values for each sex.
| BUN | Creatinine | BUN/creatinine ratio | What it may suggest |
|---|---|---|---|
| High | Within the reference range | Higher | Not enough fluids, eating a lot of protein, the body breaking down its own protein, bleeding in the digestive tract |
| High | High | Sometimes higher, sometimes typical | Reduced kidney function, or possibly less blood reaching the kidneys because of severe dehydration, reduced heart function, bleeding, or the like (see a doctor soon) |
| Low | Within the reference range | Lower | Not getting enough protein, drinking a lot of fluids, pregnancy, a major drop in liver function |
This is only a rough guide, and some studies report that the ratio isn’t very good at telling causes apart. Rather than deciding by the ratio alone, look at the BUN and creatinine values themselves, together with other markers. If both are high, or a pattern that concerns you keeps showing up, please talk with your doctor.
How I read the numbers
Where you sit within the reference range
The reference range of 8–20 mg/dL is wide, and even within it, a 9 and a 19 may reflect different things going on in the body. In the data on healthy people that the shared reference intervals are based on, the middle value is about 13 mg/dL for men and about 12 mg/dL for women.
For BUN, the precision nutrition optimal ranges I use in my counseling are 15–17 mg/dL for men and 14–17 mg/dL for women. These are precision nutrition guides, not medical reference ranges. Being below the optimal range doesn’t by itself mean you’re short on protein, either. For people whose BUN is on the low side, around 10, even though that’s within the reference range, I first look back with them on how much protein they eat, and I also ask whether their digestion has been feeling off for a while (BUN can’t show how well you digest and absorb food). For people near 20, I first check their hydration on the day of the blood draw and how they’re getting their protein.
For creatinine, which I read alongside BUN, the optimal ranges are 0.5–1.0 mg/dL for men and 0.4–0.8 mg/dL for women. These, too, are precision nutrition guides, not medical reference ranges. Because creatinine varies with muscle mass, falling outside the optimal range doesn’t by itself tell you how well your kidneys are working.
Reading it alongside other markers
I almost always read BUN together with the following markers:
- Albumin and total protein: If BUN is on the low side and albumin is also in the lower part of its range, I consider the possibility that protein is falling short.
- GGT (γ-GTP): Drawing on ideas from precision nutrition, I take BUN and GGT both running low as one clue for considering whether protein may be falling short. This, too, isn’t a medical standard, and both being low doesn’t by itself mean your nutrition is lacking. I explain how to read GGT in the GGT (γ-GTP) article.
- Sodium, hemoglobin, and albumin: If these are all on the high side and BUN is high too, I consider the possibility that you were short on fluids on the day of the blood draw and your blood was more concentrated.
- Creatinine and eGFR: If creatinine is above the reference range along with BUN, or eGFR is below 60, that’s a cue to have a doctor check your kidney function.
Rather than deciding from a single result, I look at the change since last time and how the values move when measured the same way.
What you can revisit in everyday life
Keep your routine and fluids the same before each blood draw
Sweating heavily or exercising hard the day before, or going to your blood draw having had almost nothing to drink, can add up and make BUN come out high. Drinking fluids as you normally would, from the day before through the day of the draw, makes it easier to avoid an apparent rise from low fluids. If your doctor has given you instructions about how much to drink because of a heart or kidney disease, please follow those instructions first.
For tests that ask you to skip meals, follow the clinic’s instructions, and try to test under the same circumstances each time. That makes it easier to compare with your last result.
Look back on how much protein you eat
In the Dietary Reference Intakes for Japanese (2025 edition) from Japan’s Ministry of Health, Labour and Welfare, the recommended protein intake for ages 18 to 64 is 65 g a day for men and 50 g a day for women. The recommended amount is the level at which almost no one falls short; it isn’t an upper limit. The same report also sets a target of getting 13–20% of your daily energy from protein (14–20% for ages 50 to 64).
If your BUN is on the high side and you’ve been having protein shakes or eating high-protein meals, try adding up your daily total once. If yours is on the low side, start by looking back on whether each meal includes at least one of these: meat, fish, eggs, soy foods, or dairy. If you’re seeing a doctor for kidney disease, talk with your doctor or a registered dietitian before changing the amount.
While you’re losing weight, it’s easy to fall short on both energy and protein.
If you’re concerned
If your BUN keeps coming out outside the reference range, if BUN and creatinine are both high, or if you’ve been told your eGFR is low or that you have protein in your urine, please talk with your regular doctor or a kidney specialist (nephrologist). If you have blackish stools, vomit blood, or feel very tired or lightheaded, please see a doctor soon.
If you take medication, please don’t stop it or change the dose on your own; talk with the doctor who prescribed it. When you go, bring your past test results, too. That makes it easier for your doctor to see how your numbers have changed over time.