Why does a vegetarian need to plan his diet to ensure the intake of sufficient protein?

When you take meat out of your diet, you are taking out a major source of protein. This protein needs to be replaced with vegetarian protein for optimal health, or you may feel weak and crave sweets (hypoglycemia). This is the main challenge of being a vegetarian.

It's not hard and doesn't take a lot of planning; just make sure you eat a lot of beans (including soy), whole grains, nuts and seeds, and a variety of foods in general. All foods have at least some protein them, otherwise they would never have been alive!

If you eliminate/minimize refined foods such as sugar and white flour and add whole foods, you will be replacing junk calories with nutritious food, which will include protein.

Some vegetarians also eat dairy products and/or eggs--these have a lot of protein in them.