In the United States, a single cow during warm seasons needs anywhere from 2-25 acres of arable land (actually, in parts of the world this number could be higher, but lets make this easy) to feed on (depending on the region, most of central Indiana 2 acres is suitable, say in the Dakotas 25 acres of unimproved prairie is needed).
Let's average this, 13.5 acres per cow, let us be extra generous and slash that in half, 6.75 acres, now let us be generous again and assume only 3.25 acres more used only for saving grass for the cold season — 10 acres per cow per year.
It takes about 2 years for a cow to reach slaughter weight. The average grass-fed cow will yield about 530lbs of meat. Assume 1lb consumed per day per human.
The Earth had a population of 6.974 in 2011. That's 6.974 billion pounds of meat a day for the BP diet.
You need 5.091 TRILLION pounds of beef every 2 years for the world to eat BP diet at 1lb a day per person. With the Earth at 5.04bn acres arable for us to use, you can feed 252 million cattle to slaughter weight. Every 2 years, you can produce 133.56 billion pounds of beef, assuming no deaths.
Ought oh!!! Bulletproof Diet isn't sustainable either! Even if you drop it to 1/4lb per day per human, that's 1.27275 TRILLION pounds of beef needed every 2 years while you can produce 133.56 billion pounds... that's still nearly 10x what you can provide.