This is due to acetone, a by - product of acetoacetic acid which may be smelled on a person's breath. (lowcarbalpha.com)
It is well established in the «Art and Science of Low Carbohydrate Living» (Phinney, Volek) that when one has become «keto - adjusted» over the course of 4 - 6 weeks of low carb / high fat and protein living, that the type of keytones secreted in the urine, namely acetoacetic keytones, reduce in concentration. (ketodietapp.com)
[emphasis added; note also that «acetone bodies» and / or acetoacetic acid are what are commonly referred to today as «ketone bodies.»] (freetheanimal.com)