Why does this gym smell like ammonia?

What is that smell?

It’s that time again, the new year when people are frantically trying to keep their resolutions to exercise more and eat healthier. Gyms are flooded with people trying to work off their holiday overindulgences and the air is filled with the smell of… ammonia? What’s the deal here, why does sweat sometimes smell like highly concentrated pee? If this has not happened to you, you’re lucky because it’s pretty gross. Ask me how I know.

Here’s the thing, a couple of years ago I actually started my resolution early, before Thanksgiving. It seems I had a common problem… doing too much too fast. My first clue was when I was running all out, heart rate way up, feeling great, and then suddenly smelled like I was standing in a very wet litter box. What the heck? I had recently had mice in my kitchen and I was using a kitchen dishrag as a sweat rag so I wondered if the mice had gotten into the drawer and peed on the rags. I was instantly grossed out and quit wiping my sweat for that workout. It happened a second time about a week later as I was pushing myself to run just a little farther than I felt like I should and I realized it was ME that smelled!

Google told me this was common and happens when you’re 1) eating too much protein, 2) not eating enough carbs, 3) not drinking enough water, or 4) pushing yourself too hard. I was most interested in the last one because it seemed like that’s what I had been doing. Explanations usually said anaerobic exercise, which happens at 80-90% of your max heart rate, causes you to break down protein for energy, which releases nitrogen in the form of ammonia (NH₃). I was skeptical because the body doesn’t like to use protein for energy; it likes fat and carbs (who doesn’t??).

At the start of exercise, you’re still breathing at a slower rate and your muscles are getting the same amount of oxygen they normally get - not more. Because of this, the body uses forms of energy that do not depend on oxygen. This is called anaerobic metabolism. First, the body uses a molecule called ATP (Adenosine TriPhosphate), which gets broken down to ADP (Adenosine DiPhosphate), a free phosphate molecule, and releases energy (Figure 1). However, there’s only a little bit of ATP normally hanging around so this form of energy lasts for just a few seconds.

Figure 1: Oh yeah, breaking down ATP to get some energy.

There is another phosphate-based source of energy as well that lasts for about 10 seconds: phosphocreatine. It’s a creatine molecule with a phosphate group attached. The phosphate group can be removed and then attached to ADP to form another ATP… which can begin the process in above paragraph again but phosphocreatine gets used up too.

Figure 2: Glycogen looks pretty cool.

Next, a carbohydrate called glycogen gets broken down into glucose molecules and the body can use that for energy for about 2 minutes or so (Figure 2).

By this time you’re breathing a little heavier so more oxygen is getting to the tissues where it can be used to create energy via oxidative metabolism. This happens in organelles called mitochondria. Mitochondria are inside all human cells but they are especially concentrated in muscle cells, particularly slow twitch muscle fibers important for building endurance. Oxidative metabolism is awesome and fascinating but it doesn’t cause ammonia smelling sweat so you can read about it elsewhere (http://en.wikipedia.org/wiki/Oxidative_metabolism).

When you really get exercising hard, at about 80-90% of your maximum heart rate, your body shifts back into anaerobic metabolism. At first, this flummoxed me: how can you go into anaerobic metabolism and stay there magically now that you’re all warmed up? Doesn’t the ATP get used up? So then you would need aerobic metabolism again? Well, yes. In reality, all of these energy-generating processes are happening in varying amounts while you exercise, like dimmer switches on lights. It isn’t as if one switches off completely and the next one in line takes over, which is how it’s sometimes taught in physiology class for simplicity.

In any case, that anaerobic zone is when ammonia concentration rises and people start to smell like floor cleaner. Here’s why: you know that ADP we made earlier from ATP? The next phosphate can be broken off as well, providing more energy. That leaves us with AMP (Adenosine MonoPhosphate). A high level of AMP triggers another metabolic pathway to turn on: the purine degradation cycle. This cycle breaks AMP down into IMP and, in the process, releases dun-dun-DUN, AMMONIA (Figure 3).

Figure 3: Where the stink magic happens.

From there it’s pretty straightforward, lactate is a byproduct of anaerobic metabolism that causes the pH of your blood to decrease very slightly during hard exercise. Ammonia can diffuse across cell membranes and moves toward a lower pH… into the blood. From there it can diffuse out of the body in sweat. It’s actually pretty important to get rid of the ammonia that accumulates during exercise so the body devised this great way to do it. It just didn’t ask us whether or not we liked the smell.

So is the reason for ammonia smelling sweat due to protein breakdown? For most of you exercisers out there including myself: nope. Generally, it’s only during PROLONGED exercise, like marathon running, that proteins can be broken down to release amino acids for use as energy. In that case, yes, the runners may smell like pee from their ammonia-laden sweat and not because of an accident while running.

P.S. As it turns out, Google was also correct that very low carb diets can also lead to ammonia-smelling sweat but it would also impair performance. Low carb diets were all the rage not so long ago and I wonder how many people had this stinky problem.

P.P.S. Using Occam’s razor, I also speculate that the lingering ammonia smell around wimpier exercisers out there (me) is due to the combination of sweat and hair products. Hair products often contain urea, which bacteria can break down into ammonia. Sweat - bacteria - stink. This is only speculation though because the amount of urea in these products is really low.  

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Czarnowski, Dariusz, Górski, J., Jóźwiuk, J., & Boroń-Kaczmarska, A. (1992). Plasma ammonia is the principal source of ammonia in sweat. European Journal of Applied Physiology and Occupational Physiology, 65(2), 135–137. doi:10.1007/BF00705070

Eriksson, L. S., Broberg, S., Björkman, O., & Wahren, J. (1985). Ammonia metabolism during exercise in man. Clinical physiology (Oxford, England), 5(4), 325–36.

Meyer, F., Laitano, O., Bar-Or, O., McDougall, D., & Heigenhauser, G. J. F. (2007). Effect of age and gender on sweat lactate and ammonia concentrations during exercise in the heat. Brazilian Journal of Medical and Biological Research, 40(1), 135–143. doi:10.1590/S0100-879X2006005000044

Rahnama, S. (2005). Timing is Everything: Why the Duration and Order of your Exercise Matters. University of Michigan Medical School, MedFitness website. http://www.umich.edu/~medfit/resistancetraining/timingiseverything101705.html