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If you have switched to a vegetarian diet but find yourself feeling hungry, tired, and irritable, you may not be eating enough. Some people who switch overnight haven´t had time to figure out what they can eat thus they end up eating only a few different types of foods which don't provide enough calories or nutrients.
You might want to spend some time in the public library and find books about vegetarian diets where you will learn the different ways that you can substitute other nutritous foods for meats and dairy products(if you are vegan) and still be able to maintain your daily calorie requirements.
Another good option is to go online where there are several sites such as http://www.goveg.com/books.asp that will help point you in the right direction.