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I can jump in here. I lost 16kg on a ketogenic diet.

My gut is a lot happier as evidenced by more regular and better (firm, soft, not runny) bowel movements.

There is a definite smell (breath) associated with ketosis. It varies by person but it is often noticeable.

I asked my wife to track it as it's very hard to notice it yourself and she says that my system produces a slightly metallic smell the isn't objectively bad, just different to normal.

I have Muslim friends and during Ramadaan there is a noticeable smell which is subjectively bad from some of them.

A friend who works as a nurse in a very low income area says that she can detect someone who isn't eating normally by the smell - very important to spot if that person is also pregnant.

It's a long way of saying: yes there is a smell, it varies by person.

[meshuggah]

During Ramadan Muslims are not allowed to drink water, right? That probably means their mouth is very dry during the fasting time meaning there's more than usual amount of odor-causing bacteria in their mouth.