So there's that soylent green movie where they eat people, right?
And I've heard the same sort of thing in other works of fiction but...
The human body can provide up to 100,000 calories if you eat one.
A human eats about 1,000,000 in a year.
As such one person would need to eat 10 people to live just one year.
Even if everyone diets, your population will be cut in half every 2 months.