Acidic foods and drinks, such as oranges, grapefruit and fruit juices that are often eaten at breakfast time, soften the enamel on your teeth.
Brushing immediately afterward wears the enamel away, and can cause dental erosion, which may lead to pain and extreme sensitivity in the teeth, and also lead to cosmetic problems.
The saliva in your mouth neutralises the acidity and restores its natural balance. However research has shown that this can take up to an hour.
Their tips for avoiding dental erosion are:
brush teeth before breakfast if you have fruit or fruit juice, or
wait one hour after eating or drinking anything acidic before
use a straw when drinking acidic drinks to reduce contact with teeth
drink water and milk between meals in preference to juice and fizzy drinks
chew sugar-free gum - this will produce more saliva to help cancel
out acid in your mouth
finish a meal with cheese or milk to help neutralise any acids
The NHS says the same:
Brush your teeth in the morning before breakfast and last thing at night before you go to bed (and ideally at least an hour after your evening meal).
Brushing your teeth straight after a meal can damage your teeth, especially if you've had fruit, fizzy drinks, wine or any other food that contains acid. This is because tooth enamel is softened by the acid and can be worn away by brushing. Waiting an hour gives your saliva chance to neutralise the acid.