“My health checkup said my creatinine is high, but what’s actually the problem with that…?”
“Lately my legs have been swollen and I’ve been constantly tired. Could my kidneys have something to do with it…?”
“I’ve been on a high-protein diet, and I’m starting to worry about the strain on my kidneys…”
The kidneys aren’t an organ we usually think much about, are they?
But in fact, the kidneys rarely show symptoms even when they’re damaged, so much so that they’re called a “silent organ”.
It’s not at all unusual for kidney function to keep declining without any noticeable symptoms, only for someone to discover that chronic kidney disease (CKD) has already progressed by the time they notice.
The people who should be especially careful are the ones who let their guard down, thinking, “I’m still young” or “Nothing hurts.”
In this article, I’ll explain in detail the specific damage that high-protein diets and too much salt do to your kidneys, and the early warning signs your kidneys send out.
I’ll also share things you can start paying attention to today to protect your kidneys.
- 1 The Real Reason the Kidneys Are Called a “Silent Organ”: Why Warning Signs Rarely Appear
- 2 The Little-Known Strain High-Protein Diets Put on Your Kidneys
- 3 How Too Much Salt Slowly Destroys the Kidneys
- 4 Your Kidneys Are Sending an SOS! 7 Warning Signs to Watch For
- 5 If You Notice Kidney Warning Signs: Tests and When to See a Doctor
- 6 Frequently Asked Questions About Kidney Warning Signs and Diet
- 7 Protect Your Kidneys to Build a Body That Stays Active 10 Years From Now
The Real Reason the Kidneys Are Called a “Silent Organ”: Why Warning Signs Rarely Appear
The kidneys are bean-shaped organs, one on each side of the body.
Their main job is to filter waste products and excess salt and water out of the blood and get rid of them as urine.
At the same time, they regulate the body’s fluid levels and electrolyte balance and release hormones involved in controlling blood pressure. They’re actually an important organ quietly supporting your whole body’s health behind the scenes.
You could think of the kidneys as the body’s “water treatment plant.”
They take in dirty blood that has circulated through the whole body nonstop, send only clean blood back into the body, and flush the waste out as urine.
Did you also know that they’re responsible for a hormone that tells the bone marrow to make red blood cells, and for activating vitamin D, which keeps bones strong?
Just as a whole town suffers when its water treatment plant shuts down, when kidney function declines, the effects spread throughout the body, including swelling, anemia, and weaker bones.
| What the kidneys do | When their function declines |
|---|---|
| Filter waste products and excess salt and water out of the blood and get rid of them as urine | Swelling |
| Release a hormone that tells the bone marrow to make red blood cells | Anemia |
| Activate vitamin D, which keeps bones strong | Weaker bones |
The Kidneys Rarely Show Symptoms Even at Half Their Function
The biggest reason the kidneys are called a “silent organ” is that almost no noticeable symptoms appear until kidney function falls below 50% of normal.
Because you can function with just one kidney, even if they take some damage, the remaining function covers for it.
It’s a bit like a twin-engine airplane that can keep flying on one engine even if the other one stops.
That might sound impressive at first, but the flip side is that “you can’t notice when something’s wrong.”
In Japan, the number of people with chronic kidney disease (CKD) is estimated at around 13.3 million, and most of them are thought to be at a stage with no symptoms.
Having few pain-sensing nerves is also one of the reasons the kidneys weaken so quietly.
Unlike the stomach or intestines, they don’t send signals like “it hurts” or “I feel heavy,” so there’s almost nothing to tip you off.
Once Kidney Function Declines, It’s Hard to Recover
What makes it even more problematic is that kidney cells, once damaged, have a hard time regenerating.
They don’t have strong self-repair abilities like the liver does, so as damage builds up, function becomes harder and harder to get back.
That’s exactly why it’s so important to shift your mindset from “deal with it once symptoms show up” to “protect them before symptoms show up.”
When kidney function drops significantly, it’s not uncommon for people to eventually need dialysis or a kidney transplant.
To avoid ending up there, paying early attention to small warning signs and your checkup numbers is the surest way to protect your kidneys.
The Little-Known Strain High-Protein Diets Put on Your Kidneys
Lately, “high-protein diets” have been getting a lot of attention, driven by the strength-training boom and weight-loss goals.
They certainly help build muscle and support fat burning, but they can also be a way of eating that puts a heavy burden on the kidneys.
Processing Protein Is the Kidneys’ Job
When protein is broken down in the body, nitrogen-containing compounds such as “ammonia” are produced.
Ammonia is harmful to the body, so it can’t just be left as it is.
These compounds are converted into “urea” in the liver, and eventually filtered out by the kidneys and excreted as urine.
In other words, the more protein you eat, the more extra work you give your kidneys’ filters.
For healthy kidneys this may not be a problem, but people whose kidney function has already declined, or who are genetically prone to weak kidneys, need to be especially careful.
Excess Pressure on the “Glomeruli”
Inside the kidneys are clusters of tiny blood vessels called “glomeruli,” and this is where the blood gets filtered.
If you keep eating a high-protein diet, blood flow to the glomeruli increases and the pressure inside rises, which can lead to a condition called “glomerular hypertension” that strains the glomeruli.
If it continues for a long time, the glomeruli can get damaged, leading to proteinuria (a condition in which protein leaks into the urine).
This is one of the early signs of chronic kidney disease.
- Guideline for daily protein intake: about 0.8 to 1.2 g per kg of body weight (when kidney function is normal)
- Even less if kidney function has declined (requires a doctor’s guidance)
- Be especially careful if you take large amounts of protein supplements
- Drinking enough water can help your body get rid of urea
By the way, back when I was on a high-protein diet, I was drinking two protein shakes a day while also eating loads of chicken breast.
At the time I didn’t think about the effect on my kidneys at all, but looking back now, I may have been taking quite a risk.
After I started fasting, I also began to rethink how I was getting my protein.
Tips for Getting Along Well With Protein
Protein is an important nutrient that builds muscle, skin, and immunity, so there’s no need to cut it drastically.
What matters is not eating a lot at once, but spreading it across your meals throughout the day.
Try not to rely too heavily on protein powder, and make a point of getting protein from everyday foods like fish, soy products, and eggs.
If you exercise, properly replacing the fluids you lose through sweat and keeping up your urine output will also help protect your kidneys.
How Too Much Salt Slowly Destroys the Kidneys
Alongside high-protein diets, another major cause of kidney damage is eating too much salt.
In today’s diets, where processed foods, eating out, and instant foods are part of everyday life, it’s very common to be eating too much salt without even realizing it.
Salt Raises the Blood Pressure in Your Kidneys
When you eat salt (sodium), your body pulls in water to lower the concentration of sodium in the blood.
This increases blood volume, which raises blood pressure.
And when high blood pressure continues, it causes chronic damage to the blood vessels and glomeruli in the kidneys.
Because the kidneys constantly receive a high flow of blood in order to filter it, they’re an organ that takes a direct hit when blood pressure is high.
Precisely because a huge amount of blood flows into them every day, they bear the full brunt of blood pressure.
In fact, about 40% of chronic kidney disease cases are said to be caused by high blood pressure, and limiting salt is the most basic step in protecting the kidneys.
To compare it to a hose, it’s like how turning the water pressure up too high damages thinner hoses more easily, until they eventually burst.
I also explain the relationship between the kidneys and blood pressure in detail in 【Resistant Hypertension】The eating habits of people whose blood pressure medication isn’t working: the key to improving blood pressure is your diet, so be sure to read it as well.
Watch Out for “Hidden Salt”
The scary part is that foods you don’t even think of as salty often contain a lot of salt.
- One serving of cup noodles: about 5 to 7 g of salt (about 70 to 110% of the daily target)
- One umeboshi (Japanese pickled plum): about 2 g
- Ham or sausages (100 g): about 1.5 to 2.5 g
- Store-bought dressing (1 tablespoon): about 0.5 to 1 g
- One bowl of miso soup (soup made with miso, a fermented soybean paste): about 1.5 to 2 g
The average daily salt intake of Japanese people is said to be around 10 g, while the targets are less than 7.5 g for men and less than 6.5 g for women.
It’s not uncommon for people to be unknowingly eating nearly twice as much salt as they should.
Tips for Cutting Salt You Can Start Today
When people hear “cutting salt,” they tend to imagine “bland food,” but with a little creativity you can keep meals tasty.
- Use a strong dashi (Japanese soup stock) made from katsuobushi (dried bonito flakes) or kombu (kelp) to cut down on salt and soy sauce
- Add contrast to flavors with the sourness of lemon or vinegar, or the aroma of ginger or pepper
- Leave the broth from noodle dishes and soups instead of drinking all of it
- Dip into soy sauce or sauces from a small dish rather than pouring them over food
- When buying food, check the “salt equivalent” on the nutrition label
Even a tongue used to strong flavors will gradually start to taste the natural flavors of ingredients more easily after two to three weeks of lighter seasoning.
Don’t try to change everything at once; try the ones that seem doable, one at a time.
Your Kidneys Are Sending an SOS! 7 Warning Signs to Watch For
The kidneys rarely show symptoms, but even so, they sometimes send “something might be wrong” signals to various parts of the body.
If several of the following signs show up together, consider whether your kidneys might be the issue.
- Swelling in the ankles or eyelids…A sign your kidneys can’t get rid of excess fluid. Your face may also look puffy when you wake up
- Persistently foamy urine…Possible proteinuria. When protein leaks into urine, the foam is slow to disappear
- Brownish or reddish urine…Possible blood in the urine. The blood vessels in your kidneys may be damaged
- Frequent fatigue and sluggishness…When waste isn’t removed and builds up in the blood, it can lead to whole-body fatigue
- Loss of appetite and nausea…As kidney function declines, waste builds up and can affect the digestive system
- Itchy skin…When the kidneys can’t remove waste, it can build up in the skin and show up as itching
- Poor concentration and brain fog…Cases where anemia or waste buildup affects the brain
If you have several of these signs, or if your health checkup flagged abnormal creatinine, eGFR, or urine protein, consider consulting a nephrologist (kidney specialist).
Even a vague feeling that “something’s off with my body” can surprisingly be a signal from your kidneys.
Look at Warning Signs “in Combination”
Swelling and fatigue often happen for reasons unrelated to the kidneys, such as lack of sleep or after a salty meal.
What matters is not jumping to conclusions from a single sign, but checking whether several signs are happening at the same time and lasting a long time.
For example, if “puffy eyelids in the morning,” “foamy urine,” and “tiring easily” have been overlapping for several weeks, getting tested once will give you peace of mind.
Comparing the color and foam of your first urine of the morning each day makes changes easier to notice.
People Who Should Pay Extra Attention to Kidney Warning Signs
If the following apply to you, your kidneys are said to be more vulnerable to damage.
- You’ve been told you have high blood pressure, diabetes, or dyslipidemia
- Someone in your family has kidney disease or is on dialysis
- You’re on the heavier side and carry fat around your belly
- You smoke
- You take painkillers on a regular basis
Diabetes and high blood pressure in particular are classic conditions that gradually damage the tiny blood vessels in the kidneys, so if your numbers are on the high side, make sure to get your kidneys checked at least once a year as well.
If You Notice Kidney Warning Signs: Tests and When to See a Doctor
3 Numbers to Check on Your Health Checkup
You can learn quite a bit about the state of your kidneys from your regular health checkup results.
- Creatinine: the amount of waste produced after your muscles work; the number goes up when your kidneys can’t get rid of it all
- eGFR: a measure of the “kidneys’ filtering ability” calculated from creatinine, age, and sex; the lower the number, the weaker their function
- Urine protein: a test that checks whether protein, which normally barely shows up in urine, is leaking out
Because creatinine is affected by muscle mass, it can come out higher in people who do strength training.
If a number concerns you, don’t judge it on your own; talk to a doctor and, if needed, get your urine or blood retested.
When to See a Doctor Early
In cases like the following, even if your symptoms are mild, it’s reassuring to consult an internal medicine doctor or a nephrologist.
- Your health checkup flagged protein or blood in your urine
- You were told your eGFR or creatinine numbers are abnormal
- Swelling or foamy urine has continued for several weeks
- Your urine output has suddenly dropped, or its color is clearly abnormal
If swelling suddenly gets much worse or you’re barely passing any urine, seek medical care right away.
Be Careful Not to Overuse Painkillers or Supplements
Some over-the-counter painkillers can reduce blood flow to the kidneys if you take too many.
If you’re taking painkillers almost every day for headaches or back pain, talk to a doctor or pharmacist.
Also, supplements you’re taking in large amounts because they “seem good for you” can sometimes put a strain on your kidneys.
If your health checkup flags your kidney numbers, it’s important to tell your doctor about every medication and supplement you take.
A Few Tips for Getting Tested
If you take a urine test the day after intense exercise or while you have a fever, protein may temporarily show up in your urine.
Creatinine is also said to come out higher if you did a hard strength workout or ate a lot of meat the day before.
Avoiding intense exercise and overeating the day before and getting tested while living as you normally do makes it easier to see the true state of your kidneys.
Even if a result comes back abnormal once, a retest sometimes shows no problem, so don’t panic; listen to your doctor’s explanation.
Frequently Asked Questions About Kidney Warning Signs and Diet
For healthy people with normal kidney function, a moderate amount of protein powder is considered fine.
However, for people whose kidney function has already declined, a high-protein diet may speed up the decline, so caution is needed.
Even for people whose checkups show “nothing abnormal,” continuing to take large amounts of protein powder every day can be a risk in terms of strain on the kidneys.
It’s a good idea to be mindful of how much and how often you take it so you don’t overdo it.
Proper hydration is important for the kidneys.
Drinking enough fluids increases urine output, helping your body get rid of waste efficiently.
However, if your kidney function has already declined, excess fluid can actually build up in the body and cause swelling or strain on the heart, so follow your doctor’s instructions.
For healthy people, making a habit of drinking about 1.5 to 2 liters of water or tea a day is a positive step.
The basics of a diet that’s gentle on the kidneys are “less salt, moderate protein, and caution with potassium.”
- Lightly seasoned, low-salt dishes (target: less than 7.5 g per day for men and less than 6.5 g for women)
- Make a point of eating green and yellow vegetables, root vegetables, and mushrooms (if your kidney function is reduced, ask your doctor how much potassium is right for you)
- Moderate amounts of high-quality protein such as fish and tofu
- Go easy on drinks high in sugar or fructose
- Keep processed and instant foods to a minimum
Research has found that short fasts can actually have a positive effect on the kidneys.
During the time you’re not eating, the kidneys’ filtering workload goes down, giving them “time to rest.”
In addition, the weight loss and lower blood pressure that come with fasting may indirectly reduce damage to the kidneys.
However, if your kidney function has already declined, the changes in electrolyte balance caused by fasting can become a problem, so you need to consult a doctor.
Protect Your Kidneys to Build a Body That Stays Active 10 Years From Now
The kidneys are an organ you “only notice once they’ve been damaged.”
That’s exactly why paying attention now, while you have no symptoms, makes a huge difference to the quality of your body 10 or 20 years from now.
3 Actions You Can Take Starting Today
- Check your health checkup numbers…Look at creatinine, eGFR, and urine protein. If your eGFR is below 60, be careful
- Consciously cut back on salt and processed foods…Start by limiting processed foods and eating out to two or three times a week
- Create time without food (fasting time)…It’s important to give your kidneys and other organs “time to rest”
The third one in particular, “creating fasting time,” is also the simplest way to practice fasting.
I feel that just narrowing your eating window, as with 16-hour fasting, can greatly reduce the strain on your kidneys.
I’ve summarized the basics of how to fast in 【The Complete Beginner’s Guide】Types, benefits, and how to do intermittent fasting, so take a look.
- During your fasting window, don’t forget to sip water or tea frequently and keep up your urine output.
- Besides diet, quitting smoking, getting enough sleep every day, and keeping up light walks help stabilize blood pressure and blood sugar and protect the blood vessels in your kidneys.
None of these habits require any special equipment, so try adopting whichever ones you can, one at a time.
Since I started fasting, I’ve felt my internal organs as a whole become more at ease, and I have a sense that my organs, including my kidneys, have been getting back to working the way they should.
Because the kidneys won’t speak up for themselves, we have to make a conscious effort to protect them.
I’d be glad if you started by looking back on what you ate today.
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