If you've ever had a blood test to check your kidney function, you may have heard of creatinine. For those of us concerned with kidney health, we are intimately acquainted with the term. Yet, even with creatinine on our radar, many of us still don't know how it works, what the numbers mean, and how it all relates to our kidneys.
In short, creatinine is a waste product produced by our muscles and excreted by our kidneys (or, in some cases, our single kidney). But did you know that our muscle mass can impact our creatinine levels and, therefore, how doctors measure our kidney function? Let's dive into the science behind it.
First things first, let's talk about creatinine.
Creatinine is a molecule that's made when our muscles break down creatine, a substance used to produce energy during muscle contractions. Our kidneys filter creatinine out of our blood and into our urine. When our kidneys aren't functioning properly, creatinine can build up in our blood.
How does muscle mass affect creatinine levels?
The more muscle mass we have, the more creatinine our body produces. This means muscular people will naturally have higher creatinine levels than those who are less muscular—even if they have normal renal function. This is why doctors take into account a person’s muscle mass when interpreting their creatinine levels to determine kidney health.
How do doctors measure kidney function, anyways?
The most common way is by using the glomerular filtration rate (GFR), which measures how well our kidneys filter waste products from our blood. Our eGFR (that is, estimated glomerular filtration rate) is calculated using a formula that takes into account age, sex, and serum creatinine levels.
Serum creatinine, by the way, is the amount of creatinine found in our blood. Doctors get a peek into our levels by administering a blood test. To determine the amount of creatinine in our urine, we would have to undergo a 24-hour urine test—a long, inconvenient process when compared to the two minutes it takes to stick a needle in a vein and draw some blood.
But, as research shows, there are limitations to using serum creatinine levels to estimate GFR, particularly in people with higher muscle mass—like weightlifters, powerlifters, sports professionals, and endurance athletes. And it is especially restrictive for bodybuilders like myself who are muscular and have one kidney.
In fact, a 2020 Lancet study found that using serum creatinine levels to estimate GFR underestimated kidney function in people with higher muscle mass, leading to misdiagnosis and potentially unnecessary treatments. Another study, published in 2010 by the Indian Journal of Clinical Biochemistry, discovered that kidney health can be overestimated in patients with reduced kidney function when relying just on normal serum creatinine levels to make a diagnosis—a finding affirmed by a 2022 Journal of Nephrology report. When doctors estimate GFR based on serum creatinine alone, they assume a constant rate of creatinine production, which may be inaccurate not only for people with higher muscle mass, but also for people who are less muscular or may have a muscle-wasting disease.
That's because there is no constant rate of creatinine production.
Outside of kidney function, many different variables can impact our serum creatinine levels on any given day. Dehydration, certain medications, a high-protein diet, intense exercise, heart disease, high blood pressure, diabetes. A blood test is a snapshot in time, so our creatinine may seem worse on paper if, say, we decided to show up to our routine check-up dehydrated after running 10 miles. It doesn’t mean, though, that our actual kidneys are worse for the wear.
Of course, this all begs the question: what can be done to address this issue? One emerging approach is to test cystatin C, another waste product that's filtered out by the kidneys but isn't influenced by muscle mass. Cystatin C is a small protein produced by all cells in the body and broken down at a constant rate. Because muscle mass doesn’t significantly impact cystatin C levels, there is growing interest among doctors, researchers, and scientists to use it as a marker to determine kidney function. After all, several studies have found that, when comparing the accuracy of creatinine- and cystatin C-based formulas in estimating GFR, cystatin C was shown to be a more accurate marker of kidney health—particularly in people with low muscle mass, as well as older adults. What’s more, according to 2021 Scientific Reports paper, the cystatin C formula alone can predict mortality better than estimating GFR in combination with creatinine levels. Together, this research suggests that cystatin C may be a valuable tool for more accurately measuring renal function and predicting health outcomes, both in people with or at risk for kidney disease and for people with higher muscle mass who want to eat protein and lift heavy.
In the end...
Muscle mass does have an impact on creatinine levels and can affect how doctors perceive kidney function. Which is why it is not only important, but also necessary for doctors to consider a person’s muscle mass when interpreting their creatinine levels and measuring their GFR. When doctors perpetuate outdated research and apply blanket rules to unique situations, they do a grave disservice to their patients who want to be healthy, stay fit, and do what’s best for their bodies. And they open the door for those patients to seek out coaches who have no clue about the kidneys other than the half-truths perpetuated by biased fitness experts.
This is especially infuriating for fitness enthusiasts concerned with kidney health. The lack of information makes the fitness journey even more frustrating and overwhelming. That’s why for bodybuilders with one kidney—especially those of us with a history of cancer or kidney disease—having a more accurate understanding of our kidney function is critical to how we train and what we eat. Knowing our precise GFR (or as close to precise as possible) will inform the type of fitness program and nutrition protocol we create. And when we have all the right information, we can optimize our performance without sacrificing our health.
Disclaimer: I am not a doctor, and I cannot make determinations about your kidney function. So, if you are concerned with your kidney health and creatinine levels, please do consult your healthcare team first. But make sure to advocate for yourself and seek a second medical opinion if something doesn’t sit right with you.