It is preferable to brush your teeth before breakfast or, if doing so afterward, to wait at least 30 minutes to protect dental enamel from acidic erosion.

noli's read

This advice is based on the sensitivity of our teeth to acids present in morning foods such as coffee or fruit. The American Dental Association (ADA), in its consensus recommendations (expert opinion), advises waiting at least 30 to 60 minutes after a meal to avoid scrubbing enamel that has been temporarily weakened by acidity. A laboratory study published in the Journal of Dentistry (experimental data) demonstrates that brushing immediately after acid exposure accelerates the wear of the tooth surface. Brushing teeth before a meal offers the benefit of stimulating the production of protective saliva and depositing a preventive fluoride film. Although there is no definitive proof that one method surpasses the other in the long term, waiting 30 minutes if brushing after eating is scientifically very sound.

the sources

No named source held this one up — that absence is part of the verdict.