The general rule for employees: expenses: wholly and exclusively: example
A professional footballer is advised by his employer to increase his calorific intake. He requires an intake of 4,500 calories each day to provide enough fuel to get through training and competitive matches. His current intake is only 3,500 calories each day. A dietician advises him about the sort of food he should be eating to make up the difference. He asks for a deduction for the cost of the extra food. While he accepts that he cannot deduct his normal food intake he argues that the extra food is not to keep him alive but is wholly and exclusively eaten to provide fuel to carry out the duties of his employment.
No deduction is due. It’s not possible to distinguish between that part of his food intake that gives him energy to do his job and that part that merely keeps him alive. Different jobs require different calorific intake and it’s not possible to deduct any part of it.
Example EIM31617 deals with an expense that can be apportioned.