How about...

O, 'tis of an old boxer, I must bring him in.
He plugs two cookers a week, and thinks it no sin.
Swears to the health-nuts that they take the fats down,
He swears you'll lose weight yet you'll gain many pounds.
Singing...

O, now bout an aged model, young beauty from her gone.
Now she sells hair removal cream, makes foliciles less strong.
'Tis but 9.99 a bottle, with a robe thrown in for free,
They don't mention it smells so bad you'll want to jump into the sea.
Singing...

O, now for fat-fighting pills, the fad of the day.
You may eat whatever you want, whatever you may crave.
Get a months worth for a just grand total of 29.99,
Results not typical, them things don't work, it's a friggin' crime.
Singing...

Maybe something along those lines?