Grain-Free Food = Stinkier Poop?

I visited my sister in her new house yesterday.  She has two adult male cats (both neutered).  I didn't notice a bad smell when we first went in, but when we moved from one end of the house to another and passed the litterbox, I was nearly bowled over by the smell.  I asked if it had been awhile since they'd cleaned the box (my sister can be lazy about it) and she said her boyfriend just did it a couple days ago, but ever since they switched them to a grain-free food, it had gotten much worse.  

She fired up two incense sticks and opened a window, and it still smelled like an open sewer!

I volunteer in a shelter, I've owned cats my whole life, so I'm pretty immune to the normal levels of "eau de cat"...this was just awful.  I feed my cats Nutro Max cat (wet and dry), so it's not grain-free.  I don't know what brand she uses, but I've heard that when people do a low carb diet, their waste smells really bad too.  

Does this happen with all grain-free, or does she just need to find a new brand?  

Sounds like one of her cats may have an allergy to an ingredient in the food.

In my experience, there is less smell with grain free, both for cats and dogs, and generally a much lower volume of feces as well.

I can get a really foul stench from Jade if she gets too much chicken in commercial food, she's not allergic to it, but she does have a low tolerance to cooked chicken.

And your sister (or her boyfriend) need to be scooping that litter box at least once every day.

Were they just recently switched to another food?  It might be adjustment, might be the food doesn't agree with them, or might just be that it needs to be cleaned, "a couple days ago" could be quite a variance and with two cats it should be cleaned at least once a day, letting poop sit around is going to stink.  FWIW my cat has been on grain free kibble, grain free canned and now is on raw and there's been no noticeable odor. 

I agree with GoingPostal, if she switched to the food without doing it gradually it can smell really really bad. I had the same thing happen about 3 weeks ago when we had a kitty that wasn't eating and had to try and coax her with different foods, the smell of her stool afterwards was really stinky, but after about a week the bad smell cleared up.

It also depends on the food brand. Not every grain-free food is SUPERIOR. Cats are unable to digest many fruits and vegetables and in some brands the list is over 20 different fruits and veggies and sometimes the foods even include dairy products, grasses and herbs! That is when you know they are added either to appeal to humans or as cheap fillers.

If she's been feeding the food for awhile and there is still that smell, it is likely the cat is having a hard time digesting the food and it could be due to a sensitivity or allergy.

I think I'd advise your sister (and her boyfriend) to scoop the litter box at least once EVERYDAY. Wash and disinfect the box once a month. It's possible it has nothing to do with the food. This is a box shared by 2 cats. 

Hmmm, let's think - how could that possibly have been smelly?

2 cats sharing "a" litter box that was just cleaned a couple of days ago...  

Seriously though, transitioning too rapidly could lead to some GI issues.

However, if it were a relative of mine, I'd ask something like: "How many times do you use the toilet between flushes?"