OK, so now you know what the building blocks are for good nutrition. Now, let's look at how much of everything you need for your body to work properly.
The DRV (Daily Reference Value) recommended for protein is about 50 grams. If you want to get your daily requirement of protein, you'd have to eat between 12 and 14 tablespoons of peanut butter. If you were eating only peanut butter, though, you'd be eating more than that. You'd have to eat about 21 tablespoons of peanut butter to reach 2,000 calories a day (which is the least amount of calories you'd want to consume in a day, unless you're on a diet). You can see that if you only ate peanut butter, you would be consuming almost twice the recommended DRV of protein. So, you're getting (more than) enough protein -- but what about the other stuff your body needs?
You know that peanut butter makes you thirsty, so let's assume you are supplementing your singular diet with eight to 10 glasses of water a day, so that you don't become dehydrated. Protein - check; water - check. But what else are you getting from the peanut butter? What about fiber, vitamins, minerals, and fats?
As it turns out, your diet of peanut butter would supply 21 grams of fiber -- very close to the DRV at 25 grams. It would also provide enough Vitamin E and more than twice the RDI (Reference Daily Intake) of Vitamin B3 (Niacin), but you would be receiving less than one third the RDI for Vitamins B1 and B2 (Thiamin and Riboflavin). Furthermore, supplementing your diet with the required RDI of Vitamins A, C, D and K would be necessary. Night blindness, scurvy, rickets and poor blood clotting or internal bleeding have been associated with deficiencies of these vitamins, respectively. You would not be getting enough carbohydrates, either. The DRV of carbohydrates for a 2,000-calorie diet is 300 grams. The peanut butter would only supply 88 grams -- less than a third of the recommended amount.
While your intake of copper and magnesium would be in-line with the DRV, you wouldn't be getting enough calcium, iron or potassium. OK, so you might be thinking that you'll simply take a multivitamin -- problems solved. Not really -- let's take a look at fat.
The DRV for fat is 65 grams for a 2,000-calorie a day diet. There are 168 grams of fat in 21 tablespoons of peanut butter. So that's more than two and a half times the DRV. Yikes! While a certain amount of fat is necessary for a well-balanced diet, a number of serious conditions, such as heart disease and certain cancers, have been linked to over-consumption of fat. Given that, you can see that a peanut-butter-only diet is not ideal.