Why Does Sweat Turn Shirts Yellow?

Have you ever gone through your laundry and noticed unsightly yellow stains in all of your white shirts? If your sweat isn't yellow, what's causing this?

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Sources:

https://www.livestrong.com/article/10...
https://www.thompsontee.com/blog/the-...
https://www.businessinsider.com.au/wh...
https://www.nivea.com.au/skin-care-ti...
https://health.howstuffworks.com/skin...
http://c.ymcdn.com/sites/www.aocd.org...
https://www.gizmodo.com.au/2014/08/ho...
https://www.sweathelp.org/home/unders...
https://health.howstuffworks.com/well...
http://cenblog.org/artful-science/201...