"For the sake of argument and illustration I will presume that certain articles of ordinary diet, however beneficial in youth, are prejudicial in advanced life, like beans to a horse, whose common ordinary food is hay and corn"

This quote by William Banting is referring to the idea that certain foods might be useful for young people, but can be destructive to those in sophisticated life. He is using the example of a horse, whose diet plan consists of hay and corn, to highlight that beans, which might be useful for a young horse, can be hazardous to an older horse. Banting is recommending that the same principle applies to people, and that certain foods that might be advantageous for young people can be harmful to those in innovative life. He is highlighting the value of bearing in mind what we consume, and how our diet can alter as we age. Banting's quote works as a tip that we need to understand the foods we consume and how they can affect our health, particularly as we age.