Why did Miss Green go to see her doctor?

Miss Green was very fat. She went to see the doctor.
   The doctor said,’ You need a diet, Miss Green, and I’ve got a good one here. He gave her a small book and said, ’Read this carefully and eat the things listed on page 11 every day. Then come back and see me in two week’s time. ’
   Miss Green came again two weeks later, but she wasn’t thinner; she was fatter. The doctor was surprised and said. ’Are you eating the things listed on page 11 of the small book?’
   Yes, doctor,’ she answered.
   The next day the doctor visited Miss Green during her dinner. She was very surprised to see him.
   ‘Miss Green,’ he said, ‘why are you eating potatoes and bread? They aren’t in your diet.’
   ‘But, doctor,’ Miss Green answered. ‘I ate my diet at lunch time. This is my dinner.’