The Short Version
Most major dental organizations and professionals, including the ADA and AAE, recommend brushing before breakfast to remove overnight bacteria and coat teeth with fluoride before acid exposure. However, the claim overstates the certainty: the only peer-reviewed comparative study found post-breakfast brushing reduced cavity-causing bacteria more effectively (though it was preliminary), and the advantage of pre-breakfast brushing is largely conditional — post-breakfast brushing is mainly problematic only if done immediately after acidic foods. The preference is real but not as clear-cut as the claim suggests.